Optimize use of recycled aggregate in high-durability structural concrete: an experimental study

Optimize use of Recycled Aggregate in HighDurability Structural Concrete: an Experimental Study Carla Teresa Vintimilla Molina ADVERTIMENT La consulta d’aquesta tesi queda condicionada a l’acceptació de les següents condicions d'ús: La difusió d’aquesta tesi per mitjà del repositori institucional UPCommons (http://upcommons.upc.edu/tesis) i el repositori cooperatiu TDX ( h t t p : / / w w w . t d x . c a t / ) ha estat autoritzada pels titulars dels drets de propietat intel·lectual únicament per a usos privats emmarcats en activitats d’investigació i docència. No s’autoritza la seva reproducció amb finalitats de lucre ni la seva difusió i posada a disposició des d’un lloc aliè al servei UPCommons o TDX. No s’autoritza la presentació del seu contingut en una finestra o marc aliè a UPCommons (framing). Aquesta reserva de drets afecta tant al resum de presentació de la tesi com als seus continguts. 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Optimize use of Recycled Aggregate in HighDurability Structural Concrete: An Experimental Study Doctoral Thesis submitted in fulfilment of the requirements for the Degree of Doctor of Philosophy in Construction Engineering by Carla Teresa Vintimilla Molina Thesis Advisors Dra. Miren Etxeberria larrañaga Thesis by compendium of publications Universitat Politècnica de Catalunya, UPC BarcelonaTech Department of Civil and Environmental Engineering Barcelona, March 2025 Optimización del uso de áridos reciclados en hormigones estructurales de alta durabilidad: Estudio experimental Doctoral Thesis submitted in fulfilment of the requirements for the Degree of Doctor of Philosophy in Construction Engineering by Carla Teresa Vintimilla Molina Thesis Advisors Dra. Miren Etxeberria larrañaga Thesis by compendium of publications Universitat Politècnica de Catalunya, UPC BarcelonaTech Department of Civil and Environmental Engineering Barcelona, March 2025 i "You may encounter many defeats, but you must not be defeated. In fact, it may be necessary to encounter the defeats, so you can know who you are, what you can rise from, how you can still come out of it." - Maya Angelou ii iii Abstract Currently, the construction industry faces the challenge of adopting more sustainable practices, with the use of recycled aggregates in structural concrete production emerging as a key strategy. This doctoral research evaluates the viability and efficacy of structural concrete incorporating high volumes of fine and coarse recycled aggregates, specifically recycled concrete aggregates (RCA-type A) and mixed recycled aggregates (MRA-type B). The recycled aggregate concretes were subjected to exposure conditions ranging from XC1 to XC4 classes, extending to more severe environmental conditions such as XS1. The study was conducted in several experimental phases. In the first phase, the physical, chemical, and mechanical characteristics of recycled aggregates were evaluated. All concrete mixtures were designed with a compressive strength of 30/37 MPa and a cement, CEM II A/L 42.5R, content of 300 kg/m³. The second phase involved a comprehensive analysis of the physical, mechanical, and durability properties of concrete mixtures with varying proportions of RCA-type A and MRA-type B, using effective water–cement ratios of 0.48 and 0.52. This phase aimed to determine the maximum replacement percentage of recycled aggregates that could be incorporated without compromising the mechanical performance of the concrete. Results confirmed the feasibility of integrating up to 60% coarse RCA (CRCA) and 20% fine RCA (FRCA) in structural concrete mixtures, achieving mechanical properties comparable to natural aggregate concrete (NAC). The analysis was extended to the use of MRA-type B, validating good mechanical performance with up to 40% coarse MRA (CMRA) and 15% fine MRA (FMRA) without compromising structural performance. The third and fourth phases expanded the study, focusing on the effects on concrete durability using the limits established in previous phases. Recycled aggregate concrete (RAC) and NAC mixtures were produced with similar compressive strengths using effective water–cement ratios of 0.47 and 0.51, respectively. These phases utilized different cement types, such as CEM II A/L 42.5 R, CEM II A/S 42.5 N/SRC, and CEM III/B 42.5 N-LH/SR, to evaluate drying shrinkage, chloride permeability, and accelerated carbonation of the various mixtures and the influence of cement type. After validating those mixtures with up to 50% CRCA and 20% FRCA maintain their structural integrity under conditions susceptible to carbonation and chloride-induced corrosion, in the final phase, with additional studies on natural carbonation and chloride profiles, the concrete produced up to 60% CRCA and 20% FRCA was validated. The findings of this research indicate that concrete mixtures with high percentages of recycled aggregates, RCA and MRA, not only meet current regulatory standards but, in some cases, exhibit enhanced properties compared to those of NAC. These results iv support their application in structures with extended service life expectations. Furthermore, it is concluded that recycled concrete exhibits properties similar to NAC when working with the same compressive strengths. This doctoral work underscores the sustainability of recycled concrete as a viable alternative, promoting environmentally responsible construction practices without compromising structural integrity in demanding environments. Keywords: coarse and fine recycled aggregates, recycled concrete aggregates (RCA-type A), mixed recycled aggregates (MRA-type B), supplementary cementitious materials, CEM IIAL, CEM IIAS, CEM IIIB, mechanical properties, drying shrinkage, concrete durability, carbonation, chloride penetration. v Resumen La industria de la construcción enfrenta actualmente el desafío de adoptar prácticas más sostenibles, destacándose el uso de áridos reciclados en la producción de hormigón estructural como una estrategia clave. Esta investigación doctoral evalúa la viabilidad y eficacia del hormigón estructural que incorpora elevados volúmenes de áridos reciclados finos y gruesos, específicamente áridos reciclados de hormigón (RCA-tipo A) y áridos mixtos reciclados (MRA-tipo B). Los hormigones reciclados sometidos a condiciones de exposición que abarcan desde las clases XC1 a XC4, extendiéndose a condiciones ambientales más severas como XS1. El estudio se desarrolló en varias fases experimentales. En la primera fase, se evaluaron las características físicas, químicas y mecánicas de los áridos reciclados. Todas las mezclas de hormigón se diseñaron con una resistencia a compresión de 30/37 MPa y un contenido de cemento CEM II A/L 42.5R de 300 kg/m³. La segunda fase involucró un análisis exhaustivo de las propiedades físicas, mecánicas y de durabilidad de las mezclas de hormigón con proporciones variables de RCA-tipo A y MRA-tipo B, utilizando relaciones agua-cemento efectivas de 0.48 y 0.52. Esta fase tuvo como objetivo determinar el porcentaje máximo de reemplazo de áridos reciclados que podría incorporarse sin comprometer el rendimiento mecánico del hormigón. Los resultados confirmaron la viabilidad de integrar hasta un 60% de RCA grueso (CRCA) y un 20% de RCA fino (FRCA) en mezclas de hormigón estructural, logrando propiedades mecánicas comparables a las del hormigón con áridos naturales (NAC). El análisis se extendió al uso de MRA-tipo B, validando un buen rendimiento mecánico con hasta un 40% de MRA grueso (CMRA) y un 15% de MRA fino (FMRA), sin comprometer el desempeño estructural. La tercera y cuarta fases ampliaron el estudio, centrándose en los efectos sobre la durabilidad del hormigón utilizando los límites establecidos en fases anteriores. Se produjeron mezclas de hormigón con áridos reciclados (RAC) y NAC con resistencias a compresión similares, utilizando relaciones agua-cemento efectivas de 0.47 y 0.51. Estas fases emplearon diferentes tipos de cemento, como CEM II A/L 42.5 R, CEM II A/S 42.5 N/SRC y CEM III/B 42.5 N-LH/SR, para evaluar la retracción por secado, la permeabilidad a cloruros y la carbonatación acelerada de las diversas mezclas, así como la influencia del tipo de cemento. Tras validar que las mezclas con hasta un 50% de CRCA y un 20% de FRCA mantienen su integridad estructural bajo condiciones susceptibles a carbonatación y corrosión inducida por cloruros en la fase final, con estudios adicionales sobre carbonatación natural y perfiles de cloruros, se validó el hormigón producido con hasta un 60% de CRCA y un 20% de FRCA. xii xiii CONTENT: Abstract ……………………………………………………………………………………….iii Resumen ……………………………………………………………………………………….. v Resum ……………………………………………………………………………………….vii Acknowledgments ...................................................................................................................ix Contributions ............................................................................................................................ x 1. Chapter 1. Introduction. ......................................................................................... 1 1.1. Research Scope ......................................................................................................... 1 1.2. Objective ................................................................................................................... 2 1.3. Motivation ................................................................................................................ 4 1.4. Hypothesis ................................................................................................................ 4 1.5. Structure of the thesis................................................................................................ 4 2. Chapter 2. State of the art ...................................................................................... 7 2.1. Background .............................................................................................................. 7 2.2. Recycled Concrete Aggregate (RCA-Type A) .......................................................... 7 2.3. Mixed Recycled Aggregate (MRA-Type B) ............................................................ 14 3. Chapter 3. Materials and Test procedure. ........................................................... 18 3.1. Introduction ............................................................................................................ 18 3.2. Materials ................................................................................................................. 19 3.2.1. Cement and Chemical admixtures ........................................................................... 19 3.2.2. Natural Aggregates ................................................................................................. 20 3.2.3. Recycled concrete AggregatesType A .................................................................. 21 3.2.4. Mixed recycled AggregatesType B ....................................................................... 23 3.3. Test procedure ........................................................................................................ 25 4. Chapter 4. Fine and coarse recycled aggregates-Type A in concrete production. ……………………………………………………………………………………….30 4.1. Objective ................................................................................................................. 31 4.2. Concrete production ................................................................................................ 31 4.3. Results and discussion ............................................................................................ 34 4.3.1. Physical properties .................................................................................................. 34 4.3.2. Mechanical properties ............................................................................................. 35 4.3.2.1 Compressive strength .............................................................................................. 35 4.3.2.2 Splitting tensile strength ......................................................................................... 38 xiv 4.3.2.3 Elastic modulus ....................................................................................................... 40 4.3.3. Drying shrinkage ..................................................................................................... 42 4.3.4. Durability properties. .............................................................................................. 47 4.3.4.1 Sorptivity ................................................................................................................ 47 4.3.4.2 Water penetration under pressure ............................................................................ 49 4.4. Conclusions ............................................................................................................ 50 5. Chapter 5. Durability of Structural Concrete with Recycled Aggregates Type A: Evaluation in Phase 1. ........................................................................................................... 52 5.1. Objective ................................................................................................................. 53 5.2. Concrete production ................................................................................................ 53 5.3. Results .................................................................................................................... 55 5.3.1. Compressive strength .............................................................................................. 55 5.3.2. Drying Shrinkage .................................................................................................... 58 5.3.3. Chloride Ion Penetration ......................................................................................... 63 5.3.4. Carbonation Resistance ........................................................................................... 65 5.4. Conclusions ............................................................................................................ 68 6. Chapter 6. Durability of Structural Concrete with Recycled Aggregates Type A: Analysis in Phase 2. .............................................................................................................. 70 6.1. Objective ................................................................................................................. 72 6.2. Concrete production and test procedures ................................................................. 71 6.2.1. Concrete production ................................................................................................ 71 6.3. Results and Discussion............................................................................................ 73 6.3.1. Compressive strength .............................................................................................. 73 6.3.2. Drying Shrinkage .................................................................................................... 76 6.3.3. Chloride Ion Penetration ......................................................................................... 80 6.3.4. Chloride Penetration Depth ..................................................................................... 82 6.3.5. Carbonation Resistance ........................................................................................... 86 6.3.6. Accelerated Carbonation ......................................................................................... 86 6.3.7. Natural Carbonation Resistance .............................................................................. 88 6.3.8. Knat vs KnatTHEO ................................................................................................. 91 6.3.9. Carbonation Analysis at 50 and 100 years ............................................................... 92 6.4. Conclusions ............................................................................................................ 94 7. Chapter 7. Fine and coarse recycled aggregates-Type B in concrete production. ……………………………………………………………………………………….97 xv 7.1. Objective ................................................................................................................. 98 7.2. Concrete production ................................................................................................ 98 7.3. Results and discussion ........................................................................................... 101 7.3.1. Physical properties ................................................................................................. 101 7.3.2. Mechanical properties ........................................................................................... 103 7.3.2.1 Compressive strength ............................................................................................ 103 7.3.2.2 Splitting tensile strength ....................................................................................... 107 7.3.2.3 Elastic modulus ..................................................................................................... 109 7.3.3. Drying shrinkage .................................................................................................... 112 7.3.4. Durability properties .............................................................................................. 117 7.3.4.1 Sorptivity ............................................................................................................... 117 7.3.4.2 Water penetration under pressure. .......................................................................... 119 7.4. Conclusions .......................................................................................................... 120 8. Chapter 8.General conclusions and future research lines. ............................... 122 8.1 General conclusions. ............................................................................................. 122 8.2 Future Research Lines ........................................................................................... 127 9. References ............................................................................................................ 128 xvi FIGURES: Figure 3. 1 Image of all the fractions: raw aggregates ............................................................. 20 Figure 3. 2 Aggregate particle size distribution ...................................................................... 20 Figure 3. 3 Image of all the fractions: recycled aggregates ..................................................... 21 Figure 3. 4 Image of all aggregate fractions: Mixed recycled aggregate (MRA) ................... 23 Figure 3. 5 Aggregate particle size distribution ...................................................................... 24 Figure 3. 6 Comprehensive Framework of Physical, Mechanical, and Durability Testing for Recycled Aggregate Concrete ................................................................................................ 29 Figure 4. 1 Water Absorption in a) Phase 1 (effective w/c ratio 0.48) and b) Phase 2 (effective w/c ratio 0.52) ........................................................................................................................ 35 Figure 4. 2 Relative compressive strength at 28 days in a) Phase 1 and b) Phase 2. ................ 37 Figure 4. 3 Ratio of the experimental value to respect to numeral value of splitting tensile strength and modulus of elasticity a) phase 1 and b) phase 2. ................................................. 40 Figure 4. 4 Drying shrinkage development in phase 1 a) drying shrinkage CRCA1; b) drying shrinkage CRCA1&FRCA1; c) mass loss CRCA1; d) mass loss CRCA1&FRCA1..................... 43 Figure 4. 5 Drying shrinkage development in phase 2 a) drying shrinkage CRCA2; b) drying shrinkage CRCA1&FRCA2; c) mass loss CRCA2; d) mass loss CRCA1&FRCA2 ................... 44 Figure 4. 6 Relative ratio of shrinkage at 91 days a) Phase 1 (effective w/c ratio of 0.48) b) Phase 2 (effective w/c ratio of 0.52) ................................................................................................. 45 Figure 4. 7 Analysis of shrinkage estimation, the ratio of experimental results/numerical estimation following SC-BOE and EC-02 for the concretes produced in Phase 1. .................. 47 Figure 4. 8 Sorptivity values at 28 days in a) Phase 1 and b) Phase 2 ..................................... 48 Figure 4. 9 Water penetration under pressure in Phase 1 and Phase 2. .................................... 50 Figure 5. 1 Relative compressive strength at all concrete ages produced with cements: a) type CEM IIAL; b) type CEM IIAS; c) type CEM III-B ................................................................ 57 Figure 5. 2 Drying shrinkage development at 91 days: a) CEM II/AL, b) CEM II/AS, c) CEM III/B ....................................................................................................................................... 58 Figure 5. 3 Mass loss development at 91 days: a) CEM II/AL, b) CEM II/AS, CEM III/B. .... 59 Figure 5. 4 Shrinkage estimation analysis depicted through ratios of a) experimental results/ numerical estimation SC-BOE and b) experimental results/numerical estimation EC-02 ....... 61 Figure 5. 5 The ratio of chloride ion penetrability (determined in charge pass) for all concretes with respect to maximum value of 4000 Coulombs: a) 28 days and b) 56 days ...................... 64 Figure 6. 1 Relative compressive strength at all concrete ages with cements: a) Type CEM II/AL, b) Type CEM II/AS, c) Type CEM III/B ................................................................................ 75 Figure 6. 2 Drying shrinkage value development and mass loss at 91 days and their standard deviation: (a) and (d) CEM II/AL, (b) and (e) CEM II/AS, (c) and (f) CEM III/B. ................ 77 xvii Figure 6. 3 Shrinkage estimation (a) experimental results/numerical estimation (SC-BOE); (b) experimental results/numerical estimation (EC-02). ............................................................... 79 Figure 6. 4 The ratio of chloride ion penetrability (determined in charge passed) for all concretes concerning maximum value of 4000 Coulombs: a) 28 days and b) 56 days ........................... 81 Figure 6. 5 Chloride content per cement weight (%) at different depths of samples: (a) CEM II/AL, (b) CEM II/AS, (c) CEM III/B. ................................................................................... 82 Figure 6. 6 Ratio of the non-steady-state diffusion coefficients (Dnss) of RACs with respect to NAC-0.51. .............................................................................................................................. 86 Figure 6. 7 Ratio of the RAC Kacc to the NAC-0.51 Kacc. .................................................... 87 Figure 6. 8 Environmental conditions for natural carbonation. ............................................... 89 Figure 6. 9 Ratio of the RAC Knat to the NAC-0.51 Knat. ..................................................... 90 Figure 6. 10 Visual comparison of the carbonation depth in natural (1 year) and accelerated conditions (91 days) for concretes with different cement types. .............................................. 92 Figure 6. 11 Comparison of the carbonation depth relative to exposure class XC3 for all concretes (a) 50 years, (b) 100 years. ...................................................................................... 94 Figure 7. 1 Water absorption (WA) and Accessible porosity in (a) Phase 1 (effective w/c ratio = 0.48) and (b) Phase 2 (effective w/c ratio = 0.52) ........................................................... 103 Figure 7. 2 Relative compressive strength at day 28: (a) Phase 1 (effective w/c = 0.48) and (b) Phase 2 (effective w/c = 0.52) ...................................................................................... 105 Figure 7. 3 Compressive strength ratio vs Total w/c ratio ................................................ 106 Figure 7. 4 Ratio of the experimental value to the theoretical value of the splitting tensile strength: (a) Phase 1 (effective w/c = 0.48) and (b) Phase 2 (effective w/c = 0.52)............. 109 Figure 7. 5 Analysis of modulus of elasticity estimation: (a) Ratio of experimental value/theoretical SC-BOE (b) Ratio of experimental/theoretical EC-02 ............................ 111 Figure 7. 6 Drying shrinkage values in Phase 1: (a) Drying shrinkage MRCA1&FMRA1, (b) Mass loss MRCA1&FMRA1 ................................................................................................... 113 Figure 7. 7 Relative shrinkage ratio at day 91: (a) Phase 1 (effective w/c = 0.48) and (b) Phase 2 (effective w/c = 0.52) ................................................................................................ 113 Figure 7. 8 Drying shrinkage value in Phase 2: (a) MRCA2 & FMRA2 drying shrinkage (b) MRCA2 & FMRA2 mass loss ........................................................................................ 114 Figure 7. 9 Analysis of shrinkage estimation and the ratio of experimental results to theoretical estimates, per SC-BOE and EC-02, for concretes produced in Phase 1 .............................. 117 Figure 7. 10 Sorptivity values at day 28: (a) Phase 1 (effective w/c = 0.48) and (b) Phase 2 (effective w/c = 0.52) ................................................................................................... 118 Figure 7. 11 Relationship between Sorptivity and WA capacity. ........................................ 119 Figure 7. 12 Water penetration under pressure: (a) Phase 1 (effective w/c = 0.48) and (b) Phase 2 (effective w/c = 0.52) ............................................................................................... 120 xviii TABLES: Table I. 1 Summary of Publications and Scientific Contributions ......................................... x Table 2. 1 Legislation on the use of recycled concretes aggregates. ...................................... 9 Table 2. 2 Legislation on the use of mixed recycled aggregates. .........................................15 Table 3. 1 Composition of admixtures as a percentage of the total weight ........................... 19 Table 3. 2 Composition of cement as a percentage of the total weight. ............................... 19 Table 3. 3 Properties of natural aggregates studied ........................................................... 21 Table 3. 4 Constituents of type A CRCA-2 (8/20 mm) aggregates .................................... 22 Table 3. 5 Properties of natural and type A RCA aggregates studied. ................................. 22 Table 3. 6 Clasification of Type B CMRA-2 (8/20 mm) aggregates. .................................. 24 Table 3. 7 Physical, mechanical and chemical properties of NA and MRA specimens. ........ 25 Table 3. 8 Concrete properties analyzed .......................................................................... 26 Table 4. 1 Mix proportions of Phase 1 concrete. All the concretes were produced with an effective water/cement ratio of 0.48. ............................................................................... 32 Table 4. 2 Mix proportions of Phase 2 concrete. All the concretes were produced with an effective water/cement ratio of 0.52 ................................................................................ 33 Table 4. 3 Physical properties of concretes produced in the laboratory ............................... 34 Table 4. 4 Compressive strength in all concretes. ............................................................. 36 Table 4. 5 Splitting Tensile strength and Modulus of elasticity .......................................... 39 Table 5. 1 Mix proportions of concretes were produced with CEM IIAL, CEM IIAS and CEM IIIB ............................................................................................................................. 54 Table 5. 2 Compressive strength and its standard deviation (between brackets values) in all produced concretes. ...................................................................................................... 55 Table 5. 3 Chloride ion penetrability and the standard deviation (described in brackets) determined in Charge pass in coulombs. ......................................................................... 63 Table 5. 4 Carbonation depth, their standard deviation, and the accelerated and theorical natural carbonation coefficient of all concretes ........................................................................... 65 Table 5. 5 Carbonation depth after lifespan of 50 and 100 years. ....................................... 67 Table 6. 1 Mix proportions of concretes were produced with CEM II/AL, CEM II/AS and CEM III/B ............................................................................................................................ 72 Table 6. 2 Compressive strength and its standard deviation (values in brackets) in all of the produced concrete. ........................................................................................................ 73 Table 6. 3 Chloride ion penetrability and standard deviation as determined in charge passed in coulombs. .................................................................................................................... 80 Table 6. 4 Chloride concentration at the concrete surface (Cs) and non-steady state diffusion coefficient (Dnss). ........................................................................................................ 84 xix Table 6. 5 Carbonation depth and accelerated carbonation coefficient of all concrete .......... 85 Table 6. 6 Carbonation depth and natural carbonation coefficient of all concrete ................ 90 Table 6. 7 Relationship between Natural Carbonation and Accelerated Carbonation in Concretes with NA and RCA ........................................................................................................ 91 Table 6. 8 Carbonation depth after lifespan of 50 and 100 years. ....................................... 93 Table 7. 1 Mix proportions of the Phase 1 concretes. All concretes were produced with an effective water-to-cement ratio of 0.48 ......................................................................... 100 Table 7. 2 Mix proportions of the Phase 2 concretes. All concretes were produced with an effective water-to-cement ratio of 0.52. ......................................................................... 100 Table 7. 3 Physical properties of concretes produced in the laboratory. ............................ 102 Table 7. 4 Compressive strength of all concretes produced (increase in compressive strength in %) ............................................................................................................................. 104 Table 7. 5 Splitting tensile strength and modulus of elasticity (MRAC/NAC ratio) ............ 108 xx Chapter 1 1 1. Chapter 1. Introduction 1.1. Research Scope The construction industry faces significant sustainability challenges, exacerbated by the intensive consumption of natural resources, substantial CO₂ emissions, and the extensive generation of construction and demolition waste (CDW), which impacts the entire lifecycle of building materials from production to disposal. These challenges are driven by increasing urbanization and construction activities demanding vast amounts of sand and gravel—non-renewable resources whose overexploitation has led to severe environmental issues. Moreover, government restrictions on resource extraction are creating supply shortages and escalating material costs, underscoring the urgent need for more sustainable alternatives [1,2]. Concrete manufacturing, which predominantly uses cement and natural aggregates, significantly contributes to resource depletion, pollution, and a high carbon footprint, highlighting the critical need for sustainable practices within the sector [3,4]. Cement, as the primary binding material, constitutes approximately 13% of concrete’s weight and typically 10–15% of the total volume of a concrete product [5,6], with aggregates comprising 70 to 80% of the volume of concrete. The reduction of ordinary Portland cement content and the replacement of natural aggregates with recycled aggregates in concrete production represent innovative technologies that have garnered considerable interest in the construction industry, particularly due to the growing focus on sustainability [1,3]. While the use of coarse recycled concrete aggregate (CRCA) has been extensively explored and recommended in various countries due to its benefits in structural concrete production, the integration of fine recycled concrete aggregate (FRCA) has been limited. This limitation is attributed to the adverse effects that FRCA can have on the properties of both fresh and hardened concrete, which have restricted its use in structural applications [7–9]. Despite the known challenges associated with recycled aggregates, there are issues such as variability in composition and performance typically lower than that of natural aggregates due to issues like micro-cracks and old mortar adhesion [2]. Europe, and particularly Spain, shows a substantial total consumption of natural aggregates; approximately 2.7 billion tonnes annually across Europe and 136.9 million tonnes in Spain specifically for construction purposes [10]. However, the utilization of recycled aggregates remains limited, reaching only 3.5 million tonnes in 2021 [11]. In response, there is an increasing interest within the industry in employing both fine and coarse recycled concrete aggregates (RCA) and mixed recycled aggregates (MRA) in concrete production [12,13], which is crucial for reducing the environmental impact of CDW, which constitutes about one-third of total waste generated in the European Union [14,15]. Recycling CDW can prevent 1.4 times the emissions of disposal scenarios and save up to 85 times the energy consumption [16]. Chapter 2 8 Spain, as OFICEMEN [23] and ANEHOP [24] stated 14.93 million tons of cement and 25.8 million m3 of concrete were consumed in 2021. Additionally, ANEFA [25] described that total natural aggregate (NA) consumption for construction reached 136.9 million tons over the same period. Furthermore, in 2018, of the equivalent to 2277 million tons of construction and demolition waste (CDW) accounted 35.4 % of total waste generated in the EU [26,27]. Therefore, it is imperative to reduce the volume of natural resources consumed and increase the use of recycled aggregates produced from treated CDW. The guidelines of the European Commission [28] encourage assuming a circular and sustainable model incorporating waste into production processes to reduce raw material and over-exploitation. In addition, many institutions are developing an EU-wide sustainable policy to tackle climate change [17]. Projects in France, such as RECYBETON [18] and the Paris Protocol [19], have developed recommendations for the use of recycled concretes aggregates (RCA) in structural and non-structural concretes. Although the use of RCA in structural concrete production has a positive environmental impact [29,30], recycled aggregate concrete (RAC) must meet all structural concrete code requirements to ensure its safety and durability as a structural material [31]. RCA, produced by crushing original concrete, has higher porosity, lower density, more abrasion loss, and more crushability than NA due to its attached mortar. RCAs made using even high-quality RCA, always exhibit variation in fresh and hardened properties compared to NAC [29,31–37]. Adhered mortar attached to RCA affects the mechanical and durability properties of RAC [38–40]. The production of RAC with coarse RCA (CRCA) has been broadly studied [33,41–44]. However, it is known that between 30 % and 50 % of the RCA produced in the recycling process are fine aggregates (FRCA) [45]. Plaza et al.[46] concluded that using CRCA and FRCA instead of NA not only reduces CO2 emission in concrete manufacture but also mitigates environmental impacts caused by stockpiling waste. Therefore, the demand on using FRCA in concrete structures is high, but in comparison to fine NA stricter quality control is necessary [47]. In addition, the lack of welldeveloped quality control guidelines limits its wider use. Due to the growing need to introduce FRCA and increase the replacement percentages for CRCA, some countries have enacted legislation limiting the use of RCA. Table 2. 1 shows, according to different standards, the limitations of the type ARCA to be used in structural concrete. Type A-RCAs are classified according to size (CRCA and FRCA) and physical properties (density and absorption). The maximum percentage allowed to be replaced of natural aggregates for concrete production and concretes strength are also described. The Spanish Structural Concrete Code (SC-BOE) [20] allows for a maximum of 20% replacement of CRCA for concrete with a strength class of C40/50. The structural concrete standards of Belgium, Italy, Portugal, France and Switzerland allow the use of different percentages of CRAC in a specific type of concrete [26,48]. The UK standard limits both CRCA and FRCA fractions to 20% for concrete class C40/50 production. The standard requirements in China, Japan, and Denmark accept concrete production with 100% CRCA and FRCA when RAC achieved specific hardened state properties, and the RCA was also of a high quality [26,48]. According to UNE EN 206 (Future changes) [18,49], the use of FRCA together with CRCA will be allowed in concrete production, given specific Chapter 2 9 limitations on exposure classes. The Eurocode 2 (EC-02) [50] allows for up to 40% replacement of the total volume of aggregates using Type-A (FRCA and CRCA) recycled aggregates. Table 2. 1 Legislation on the use of recycled concretes aggregates.[18,20,48,49] Country Aggregate type A Max. replacement (%) Fraction Strength class Density (kg/m3) Absorptio n (%) Structural Concrete Code (SC-BOE) RCA 20 CRCA C40/50 - 7 Belgium PTV 406– 2003/ NBN B 15–001 RCA 50,30,20* C30/37 2100 9 ItalyNTC-2008 RCA 1 RCA 2 30 60 15 CRCA C30/37 C25/30 C45/55 - - Portugal LNEC - E471 RCA 1 25 CRCA C40/50 2200 7 FranceNP 15-545 RCA 1 60.30,20 CRCA No limit - - RCA 2 40,15 - SwitzerlandMB -2030 RCA 1 100 CRCA Outdoor ≥ 25/30 - - China GB/T-2573 RCA-Type1 100 CRCA No limit 2450 3 RCA-Type2 30 CRCA C40/50 2350 5 RCA-Type3 30 CRCA C25/30 2250 8 RCA-Type1 100 FRCA C40/50 2450 3 RCA-Type2 30 FRCA C25/30 2350 5 United KingdomBS 85002 RCA 20 CRCA+ FRCA C40/50 2200 - Japan JIS-5021/ JIS-5022 RCA-HQ 100 CRCA+ FRCA C45/55 2500 3(C) 3.5(F) RCA-MQ 100 CRCA+ FRCA C35/45 2300(C) 2200(F) 5(C) 7(F) Denmark DS 2426/DCA No.34 RCA 1 100 CRCA C40/50 2200 n.a RCA 2 100 CRCA+ FRCA C40/50 2200 n.a UNE EN 206 RCA 50,30 CRCA No limit - - UNE EN 206 (Future changes) RCA 60,50,40,30 CRCA No limit - - 30,20,10 FRCA - - The Eurocode 2 (EC-02) RCA 401 CRCA+ FRCA No limit - - 1Replacement of the total volume of aggregates Notes: The values are for RCA (recycled concrete aggregate). HQ: high quality. MQ medium quality. All of the standards listed in the table apply to structural concrete. *Replacement ratio by exposure class Chapter 2 10 Due to the greater amount of old mortar and residual cement particles in FRCA in comparison to CRCA, some hesitation persists regarding the simultaneous incorporation of FRCA in structural concrete applications [44,51]. Furthermore, FRCA has a much higher water absorption capacity (6– 12 %) than natural sand (0.5–2.7 %) [37]. Despite these weaknesses, the un-hydrated cement of the original concrete available in RCA may play a positive role in its use in structural concrete. Additionally, the specific surface of RCA aggregates improves the binder/recycled aggregate interface [52,53]. CRCA and FRCA (Type-A recycled concrete aggregate) exhibits higher porosity than natural aggregates (NAs) due to adhered mortar and micro-cracks that form during crushing. Thus, the water absorption capacity increases, affecting the amount of water available for mixing. This can lead to issues related to the loss of the concrete’s workability in its fresh state and a decline in the mechanical and durability properties in the long term [47,54,55]. RAC concrete is typically associated with lower workability than NAC concrete of the same composition [56]. After an exhaustive review and analysis of the workability of RAC produced with FRCA, Nedeljković et al. [57] concluded that researchers had offered many reasonable explanations for the complex flow behaviour of RAC through a combination of experiments and theories. However, they described no universal approach to obtaining and maintaining satisfactory workability of mortars/concretes with FRCA. Understanding the differences between natural aggregate concrete (NAC) and RAC is the basic premise for further studying the behaviour of RAC materials and structures [58]. It is generally believed that the compressive strength of RAC decreases as the amount of RCA replacement increases [59], which makes concrete more porous and less dense [60]. However, RAC produced using up to 30 % CRCA and 20 % FRCA has achieved similar compressive strength as NAC [51,61]. Gao and Wang [62] described a compressive strength decrease when a higher percentage of FRCA was employed in concrete production. Some researchers suggest that concrete produced using up to 30 % FRCA as a replacement for natural sand achieves adequate properties [63,64]. Leite and Santana [53] determined that concretes produced with 20 % and 40 % FRCA and 100 % coarse NA achieved a similar compressive strength of 41.3 MPa and 39.5 MPa, respectively. Several researchers have also described that concretes produced with up to 50 % FRCA achieved adequate properties [44,65]. In addition, Sim and Park [66] determined that RAC produced with up to 60 % FRCA in substitution of natural sand and 100 % CRCA could achieve a compressive design strength of 40 MPa. However, using 100 % FRCA caused a strength decrease of 33 %, which was similar to results obtained by other researchers [46,57,64,67]. Nevertheless, according to Berredjem et al. [68], concrete produced with 100 % CRCA and FRCA achieved a compressive strength of over 25 MPa. However, the tensile strength of concrete produced with CRCA and FRCA can be improved, with respect to NAC, due to the improvement of the interface transition zone in concretes containing RCA [46]. In contrast, some researchers [53,59,62,69] have reported that the splitting tensile strength decreases when FRCA is employed and that the drop increases with a higher percentage of FRCA. The elastic modulus of RAC is lower than that of NAC and decreases as the RCA replacement percentage increases due to the increased amount of attached mortar [41,70]. Chen et al. [71] determined that RAC produced with 20 % to 100 % CRCA and natural river sand suffered a decrease Chapter 2 11 in modulus elasticity of 10.8 % to 16.4 %. According to Bendimerad et al. [31], the use of 30 % FRCA and natural gravels achieved a similar value for the elastic modulus as concrete produced with a 100 % replacement of CRCA. The durability of RAC is influenced by the interaction between two main factors. On the one hand, the improvement in the interfacial transition zone (ITZ) between the RCA particles and the new mortar can contribute to increased durability. On the other hand, the higher porosity of RCA tends to increase the total porosity of the concrete, which may reduce its durability. Whether the effects of an improved ITZ or increased porosity predominate will explain the enhanced or diminished durability performance of RAC compared with NAC [72]. Furthermore, variations in the water-to-cement ratio and the type of cement used significantly impact the durability of RAC. Limited investigations have been carried out studying the durability characteristics of concrete made with fine recycled concrete aggregate (FRCA) and coarse recycled concrete aggregate (CRCA) [40,46,51,64,68]. However, it is known that, in general, the durability of RAC is lower than the durability of NAC concrete because durability is influenced by the connectivity of the porous network, water content the type of supplementary cementitious materials (SCM) used [40,44,68,73,74]. The porosity of RAC rises with an increase in the replacement ratio of RCA, and it increases even more when FRCA is employed [75]. According to Limbachiya [76], there is a slight effect in durability when the RCA replacement of NA < 30 %. However, the use of CRCA and FRCA increased the absorption capacity of concrete [59]. According to Correia et al. [7], the water capillary absorption capacity of RAC was greater than NAC, and it was significantly higher when CRCA and FRCA were used [8]. Zhao et al. [77] reviewed key durability indices and testing methods, emphasizing the importance of chloride diffusion and freeze-thaw resistance. Similarly, Bu et al. [78] investigated the effects of recycled fine aggregates on permeability and carbonation resistance, underscoring that adjustments to the water–cement ratio can enhance the durability of RAC. The corrosion of reinforcement is a major and complex pathology affecting reinforced concrete structures [79]. The reinforced concrete used in marine infrastructures such as bridges and piers is vulnerable to chloride-induced corrosion, which shortens its service life [80,81]. Seawater chlorides penetrate the concrete, causing significant damage when levels exceed a critical threshold, which is measured using chloride profiles at various depths [82]. Moreover, SCMs such as blast furnace slag (BFS) can enhance the sustainability of RAC by lowering carbon dioxide emissions and promoting circular economy benefits, potentially offsetting some of RAC's durability limitations by improving its overall environmental impact [83,84]. The use of structural RAC in chloride-containing environments has sparked debate among researchers [12]. Some authors indicate that the increase in RCA content could lead to a higher diffusion of chlorides in RAC due to its high porosity [85,86]. However, some studies have also demonstrated that by reducing the amount of adhered cement mortars, the resistance of RAC to chloride ion penetration can be improved, particularly when RCA originates from higher-strength concrete [87]. The penetration of chloride ions is a major contributor to the corrosion of steel reinforcements. The results of numerous studies [9,44,88–91] conducted on this topic have revealed Chapter 2 12 the following: the diffusion coefficient of chloride ions exhibits a linear increase with the proportional increase of recycled aggregate use; FRCA influences more than the CRCA in concrete diffusion coefficients; and, similar to NAC, chloride ion migration can be reduced by decreasing the water-to-binder ratio or incorporating SCM such as fly ash, silica fume or blast furnace slag (BFS) [44]. Kirthika and Singh et al. [92] observed that using up to 30% FRCA demonstrated an improvement in resistance to chloride penetration compared with NAC. According to Li et al. [85], following the ASTM C1202 classification, while the concrete produced with up to 50% of RCA achieved low chloride ion penetrability, the RAC produced with a higher percentage than 50% was classified with medium penetrability, consequently needing the use of SCM to improve the resistance to chloride ion penetration. As confirmed, BFS cement enhances chloride penetration resistance in concrete due to its ability to immobilize chloride ions [93,94]. This enhancement is achieved through physical and chemical mechanisms, through chloride ion adsorption on the C-S-H surface [93] and the formation of Friedel's salt due to the higher aluminate content in the BFS cement [93,94]. Researchers [72,95] have indicated that the influence of RCA on chloride penetration resistance is significantly lower than that of factors such as the w/ratio and the use of SCMs. Carbonation in concrete is a critical phenomenon that significantly affects the durability and lifespan of concrete structures. Carbonation in concrete is a physicochemical process in which CO2 penetrates the cement paste and reacts with Portlandite, forming calcite and reducing the concrete pH from 13 to 8-9. The carbonation rate is influenced by the permeability and moisture content of the concrete [40,44]. As a result of carbonation, steel reinforcement loses its protection, and corrosion begins when adequate oxygen and water levels are present [44]. Extensive research has been conducted on the carbonation resistance analysis of RAC [79,88,96,97]. According to the literature [9,79,98,99], the carbonation depth of RAC increases with the level of natural aggregate replacement. Different factors can influence the carbonation depth of the RAC. These factors include the replacement ratio of recycled aggregates, the origin and quality of the RCA, the crushing technique employed for RCA production, the cement type and quantity used in concrete production, the curing process, and the use of superplasticisers to reduce the watercement ratio [44,74,99]. Conclusions regarding the impact of RCA on the carbonation resistance of concrete can be ambiguous and conflicting. According to Pedro et al. [100], the carbonation depth increases as the concrete's compressive strength decreases. Certain researchers [73,99] have argued that RAC mixtures produced with coarse CRCA exhibit similar or even higher carbonation resistance than NAC due to aged adhered mortar. Zeng et al. [101] suggested that the optimal replacement percentage of natural aggregates (NAs) with RCA is 50%, which prevents a decrease in carbonation resistance. Etxeberria et al. [97] also reached a similar conclusion when employing a 50% replacement of uncarbonated CRCA. Loti et al. [89] found that RAC produced with up to 50% coarse RCA met the current European standards, thus supporting its use in structural application when up to 50% of CRCA is employed in concrete production. Evangelista and Brito et al. [9] noted that concrete produced with the total replacement of fine NA with FRCA showed low carbonation resistance. However, the use of up to 30% FRCA and the use of SCM (fly ash) with a higher percentage of FRCA resulted in adequate properties for structural concrete. In addition, despite RCA’s higher porosity, the adhered mortar provides additional reactive content against CO2, which Chapter 2 13 can enhance carbonation resistance [102]. However, Pedro et al. [103] determined that when the quality of mortar adhered to RCA was lower than that of a new cement paste, the carbonation rate increased as higher proportions of RCA were used to replace natural aggregates. However, more investigation is needed to evaluate the influence of FRCA recycled aggregates on carbonation resistance and, in general, on concrete durability. Furthermore, it was also demonstrated [76,79,104,105] that RAC using up to 50% CRCA, with a reduction in the effective water–cement ratio without increasing cement amount, could achieve the same strength and carbonation depths as NAC, using superplasticizers to preserve workability. Moreover, with similar compressive strength to that of NAC, RAC showed lower carbonation than NAC when 20–50% replacement was applied [106]. In order to evaluate the carbonation rate of concrete, besides considering its water-to-cement ratio and compressive strength, the employed cement type and additives, such as fly ash or slag, must be considered [107–110]. Furthermore, the service life of concrete structures against carbonation strongly depends on the type of cement used in concrete production [110,111]. The carbonation depth of concrete mixtures produced using SCM was higher due to the reduction of Portlandite during cement hydration, reducing Ca availability [109,111] and, consequently, causing less resistance to carbonation. Although SCMs reduce alkali reserve usually leads to a reduction in pore size, they can decrease the permeability of cementitious matrices [112]. However, carbonation not only lowers the overall pH but may also result in the coarsening of the pore structure, potentially diminishing its durability and susceptibility to various forms of degradation, including chemical and physical attacks [109]. Consequently, the carbonation concrete's service life decreases as more SCM is used to replace clinker [110,113,114]. However, using limited mineral admixtures in RAC production can improve carbonation resistance. The RCA produced using CEM IIAS achieved a higher carbonation resistance than that produced with CEM IIAL due to the addition of available CaO in the slag cement. In addition, the use of up to 50% of CRCA had little influence on the carbonation depth [115]. In addition, the higher drying shrinkage value of recycled aggregate concrete (RAC) than that of natural aggregate concrete (NAC) is due to the presence of adhered mortar and porous material in RCA [116]. The shrinkage value also depends on various factors, including the compressive strength, environmental conditions, and the properties of cement and additions [117]. Early stage shrinkage is particularly critical as it contributes substantially to the final shrinkage magnitude, elevating the risk of cracking in the later stages of concrete’s lifecycle [118,119]. Furthermore, the shrinkage increases when a higher percentage of RCA is employed [120–122]. Gonzalez and Etxeberria [123] studied the drying shrinkage of RACs produced using CRCA with different origins. They concluded that the CRCA produced from a lower-strength parent concrete achieved the highest drying strength value.Research by Domingo-Cabo et al. [124] revealed that RAC RAC) exhibits drying shrinkage similar to natural aggregate concrete (NAC) when using a 20% replacement ratio of recycled concrete aggregates (RCA). However, with increasing replacement ratios of 50% and 100%, drying shrinkage in RAC increases by 20% and 70% respectively. According to Zhang et al. [125], the total replacement of NA with RCA (fine and coarse) increased the drying shrinkage by more than 100 % Chapter 2 14 (102.0 %–116.9 %). The increase in the RAC shrinkage value is due to the high water absorption of RCAs, which are porous and contain old cement paste [44]. Vintimilla and Etxeberria [126] determined that all concretes with up to 60% CRCA achieved shrinkage values similar to NAC. In addition, they concluded that the use of FRCA increased the shrinkage value when compared to concrete made only with CRCA. Nevertheless, the concretes produced with up to 60% CRCA and 20% FRCA also obtained adequate values ranging up to -800µε, following American Concrete Institute (ACI) standards [122]. Simsek et al. [127] conducted a study to evaluate the influence of using 20%, 40%, 60%, 80%, and 100% FRCA or CRCA in the substitution of natural aggregates. They concluded that after 90 days, the RAC with up to 20% FRCA achieved adequate properties. Moreover, several recent studies [126,128] have confirmed the existence of a slight influence of FRCA on structural concrete performance, however not being detrimental and consequently technically viable for their use. Recent studies confirmed the feasibility of using FRCA in structural concrete with a minimal impact on overall performance [126,128]. The type of cement used in concrete production also influences the drying shrinkage value. It has been determined that concretes produced using Portland clinker-based cement have very high strength, increases the hydration heat and as a consequence, leads to higher drying shrinkage [129]. In contrast, the early-stage shrinkage caused in SCM cement significantly contributes to final shrinkage, raising the risk of concrete cracking in later stages [118,119]. 2.3. Mixed Recycled Aggregate (MRA-Type B) In response to sustainability concerns, the construction industry has developed a string interest in using fine and coarse mixed recycled aggregates (MRA) in the production of concrete [12,13]. Construction and demolition waste (CDW), a major source of recycled aggregate (RA), constitutes approximately one-third of the total waste generated in the European Union [14,15]. However, RA consumption in Spain is limited, reaching only 3.5 million tonnes in 2021 [11]. The use of MRA in construction is critical to lowering the environmental impact of CDW [15], as 70% of the volume of CDW is classified as mixed CDWthe primary raw material used to produce MRA [11]. According to EN 933-11, type B MRAs consist of masonry-based material, with ceramic particles comprising less than 30% by mass and Portland cement-based fragments comprising less than 90% by mass [15]. Current research indicates that most European countries have not enacted legislation regulating the use of MRA in structural concrete. Consequently, MRA is primarily used for low-utility purposes, such as landscaping and paving [26]. This situation can be explained by inadequate standards or specifications, as well as a lack of in-service evidence, insufficient financial incentives and government support, and the notion that concrete made with RA has less desirable properties than NA concrete (NAC) [26,130]. The use of MRA in structural concrete design needs to be addressed in national and international concrete codes and standards (Table 2. 2). Design regulations have been established for nonstructural concrete in the C20/25 to C30/37 strength classes, and the current regulations permit the use of 5% to 50% coarse MRA (CMRA) and up to 20% fine MRA (FMRA) in replacement of natural Chapter 2 15 aggregates, depending on the quality, design strength, and exposure class. DIN 4226-100 [130] stipulates limits of 15% for water absorption capacity and 0.5% for the composition of contaminants in type B MRA. Total replacement of coarse NA is permitted only in non-structural concrete in Switzerland and Denmark, while a 100% replacement ratio −with a design strength of C25/30− in structural applications is allowed in Australia, and replacement is limited to 35% in C30/37 concrete in Germany. Future UNE EN 206 specifications [131] will allow FMRA in combination with CMRA, subject to specific limitations per exposure class. The Spanish structural concrete code (SC-BOE) [20] does not permit the use of MRA-type B Table 2. 2 Legislation on the use of mixed recycled aggregates. [26,46,132,133] Country Aggregate Type B Max. replacement (%) Fraction Strength class Density (kg/m3) Absorption (%) UNE EN 206/EN 12620 MRA 50,30* Coarse C30/37 ≥1700 - Belgium PTV 406–2003/ NBN B 15–001 MRA-Type B 20 Coarse C20/25 (N.S) 1900 ≥1600 9 ≤ 18 Portugal LNEC - E471 MRA 20 Coarse C35/45 ≥2000 7 RILEM RMA 20 Coarse No Limit ≥2400 ≤ 3 FranceNP 15-545 MRA 30,5 Coarse No limit - - GermanyDIN 4226-101, DAfStb MRA-Type B 35,25* Coarse C30/37 ≥2000 ≤ 15 AustraliaAS 1141.62/HB 155:2022 MRA (Class 1B) 100 Coarse C25/30 ≥1800 ≤ 8 SwitzerlandMB -2030 MRA 100 Coarse (N.S) Outdoor ≥ 25/30 n.a n.a Norway (NS EN 12620:2008) RESIBA (2002) RMA ≥1500 ≤ 20 Denmark DS 2426/DCA No.34 MRA 100 20 Coarse Fine C20/25 (N.S) ≥1800 n.a Brazil (NBR 15.116) MRA 20 Coarse/ Fine (N.S) n.a n.a ≤ 12 ≤ 17 Japan (JIS A 5021, JIS A 5022, JIS A5023) RMA n.a Coarse (N.S) n.a ≤ 7 Low quality Fine (N.S) n.a ≤ 13 UNE EN 206 (Future changes) MRA 40,30,25,20, 15* 15,10,5* Coarse Fine C30/37 - - N.S.: non-structural * Replacement ratio per exposure class. n.a: Not Available Chapter 2 16 Ceramic particles and the attached mortar in cement paste constitute the primary factors that determine the water absorption (WA) capacity of MRAs. The high WA of MRA makes it difficult to control the effective water-to-cement ratio in cement paste [134,135] and the workability of concrete. Several researchers [41,136] described that the effective absorption capacity of RAs is around 70% of their absorption capacity, requiring that amount of water be added to the mixer at concrete production. Moreover, Etxeberria et al. [41] recommend using RA with high humidity (at 70−80% of their absorption capacity) to control the effective water-cement ratio, workability and hardened properties. Several studies have investigated the impact of using MRA in replacement of NA on the physical, mechanical and durability properties of recycled aggregate concrete (RAC) [15,55,137–141]. According to Cantero et al. [142], the effective porosity of concrete produced with MRA (MRAC) increases linearly with an increase in the content ratio of the CMRA in the concrete; NAC achieved 7.5% porosity, while MRAC produced using 20% and 100% CMRA achieved 9.5% and 15.7% effective porosity, respectively. Mas et al. [143], Cantero et al. [144] and Etxeberria and Vegas [145] report that MRAC can achieve up to 26% higher WA capacity than NAC. Several studies have determined that MRAC can achieve mechanical properties (e.g., compressive strength and flexural strength) and durability properties similar to those of NAC [137,146,147]. Cantero et al. [148] reported that concrete with up to 50% CMRA achieved compressive and flexural strength similar to that of NAC; however, a 7% decrease was observed when 100% CMRA was employed. Medina et al. [149] observed that concrete produced with 25% and 50% CMRA achieved a 15–20% lower strength than NAC and thus concluded that up to 50% CMRA is acceptable for housing construction. Nonetheless, it is recommended that the replacement rate for CMRA, when used in building structures, should not exceed 30% [146,150]. The current literature suggests that significant reductions in strength properties are highly prevalent when incorporating fine RAs into concrete [151]. Morales et al. [152] demonstrate that hollow concrete blocks produced with 50% and 100% MRA (0/8mm) achieve 27.03% and 42.65% lower compressive strength than conventional blocks. Meng, Wei, and Yang [150] report that MRAC produced with 100% fine and coarse MRA, using different w/c ratios, achieved a 50% lower compressive strength and 24.8% to 55.5% lower tensile strength than NAC at 28 days of curing. Fine fractions contain higher cement mortar amounts than coarse fractions, which yields much higher WA, resulting in concrete with lower strength [51] and poorer durability performance [55] when produced with the same effective w/c ratio. Consequently, the use of recycled sand obtained from CDW is generally not permitted in the production of structural cementitious materials in most countries [55]. However, several researchers [126,148] have shown that highly durable structural concrete can be achieved if a limited percentage of cleaned coarse and fine recycled concrete aggregate (RCA) and MRA are used. The durability properties of RAC decline as the percentage of RA employed increases due to the high WA capacity of RA [51,153]. Several studies have also found that the water penetration depth increases linearly with an increase in the MRA replacement ratio [126,142,144,154]. In contrast, Bravo et al. [155] report that concrete produced with 10–100% fine or coarse MRA achieves a 21– 52.6% lower sorptivity value than NAC. Chapter 2 17 In addition, the adhered cement pastes and ceramic particles in RAs can lead to excessive creep and drying shrinkage levels in concrete [55]. Gayarre et al. [156] performed an analysis of the shrinkage characteristics of concretes comprised of varying percentages (20%, 35%, 50%, 70%, and 100%) of coarse and fine recycled brick aggregate (RBA). They concluded that the shrinkage of this RAC was higher than that of the other RACs due to its high WA capacity and the low modulus of elasticity of the RBA. Furthermore, the concrete with 35% RBA was adequate for structural purposes. However, the use of 100% RBA produced an increase in the shrinkage value from 45% to 100% vis-à-vis NAC, depending on the water dosage in the mixes. Vintimilla and Etxeberria [105] validated the shrinkage values of concrete produced for structural purposes with simultaneous substitution of up to 50–60% coarse recycled concrete aggregates (CRCA) and 10–20% FRCA, which have 11–56% higher shrinkage values than NAC. In contrast, Bravo et al. [157] note that although the incorporation of MRA into concrete increases the total shrinkage, a consensus is yet to be reached on the extent of the increase. In conclusion, using coarse and fine MRA in the production of concrete supports sustainable construction practices. This study aims to determine the maximum percentage of CMRA and FMRA for producing structural concrete − with a characteristic compressive strength of 30 MPa (C30/37)− that is suitable for exposure to XC1 to XC4 environments, in which carbonation-induced corrosion may occur due to the level of humidity in such environments. Two experimental phases were conducted: Phase 1 and Phase 2. In Phase 1, all the concretes were produced with an effective w/c ratio of 0.48, while an effective w/c ratio of 0.52 was employed in Phase 2; 300 kg of cement was used in both phases. In Phase 1, 20%, 30%, 50% and 100% CMRA, in combination with 5%, 10%, 15% and 25% FMRA, were used in replacement of NA. In Phase 2, due to a high w/c ratio, lower MRA percentages of 20%, 30%, 40% and 50% CMRA, in combination with 5%, 10% and 15% FMRA, were employed in replacement of NA in producing the concretes. NAC was also produced in these two phases. The physical, mechanical, and durability properties of the MRACs produced were determined experimentally and compared to the corresponding values achieved by the NAC. In addition, the criteria stipulated in Eurocode 2 [50] and SC-BOE [20] were used to validate the structural MRACs produced. Chapter 3 24 0 20 40 60 80 100 0.01 0.1 1 10 100 Percentage passing (%) Sieve size (mm) FNA (0/4) CNA-1 (4/10) CNA-2 (8/20) FMRA (0/4) CMRA-1 (2/10) CMRA-2 (8/20) Upper limit Lower Limit Figure 3. 5 Aggregate particle size distribution The MRA components, characterised by the CMRA-2 fraction, meet EN 933-11 specifications (Table 3. 6). Per the EN 206 standard, the MRA used in this study were categorised as type B (RC50, RCU70, Rb30-, Ra5-, FL2and XRg2-). Based on this classification, the combination concrete (RC) and natural stone (Ru) components exceeded 50%, while the ceramic content was below 30%. Table 3. 6 Classification of Type B CMRA-2 (8/20 mm) aggregates. Type Concrete, concrete products, mortar (Rc) Unbound mortar, nature stone (Ru) Mansory (Rb) Asphalt (Ra) Glass (Rg) Other (x) CRCA-2 43.43% 24.31% 28.38% 2.24% 0.19% 1.46% EN 12620 Rc+Ru >50% ≤ 30% ≤ 5% ≤ 2% The physical, chemical and mechanical properties of the MRA employed in producing the concrete are described in Table 3. 7. The obtained values were categorised following EN 12690 specifications. All the properties achieved the limits established in the SC-BOE [20] (Table 3. 7), except the WA of CMRA, which was higher than 7%. However, the average WA capacity of the combined NA and CMRA (used in concrete production) remains below the 7% threshold required by regulations. Chapter 3 25 Table 3. 7 Physical, mechanical and chemical properties of NA and MRA specimens. Property (standard) Specification FMRA CMRA-1 CMRA-2 SC-BOE (0/4) (2/10) (8/20) Density (kg/dm3) EN 1097-6[162] 2.28 2.15 2.08 ≥1.7* Water Absorption (%) EN 1097-6[162] 6.67 8.75 9 <7 Humidity (%) 6.31 6.44 4. 86 Fines equivalent (%) EN 933-8[163] 95.5 >70 Los Ángeles coefficient (wt%) EN 1097-2[163] 39.15 <40 Flakiness índex (wt%) EN 933-3[164] 9.72 <35 *Property defined in EN 206 The MRA specimens exhibited inferior performance, lower density and higher porosity and WA than the NA specimens (Table 3. 7). These limitations can be attributed to the adhered mortar and ceramic particles in MRA [51,126,142,169]. The dry density of the different fractions of type B aggregates was between 2.02 kg/dm3 and 2.26 kg/dm3, which is similar to the values reported in previous studies [170]. However, this range of values is greater than the 1.7 kg/dm3 stipulated for type B aggregates per UNE EN 206. The MRA was characterised by a normal density exceeding 2.0 kg/dm³, making it suitable for use in concrete. The WA characteristics of CMRA in particular, FMRA are commonly documented as critical indicators that impact the properties of RAC [171]. Although CMRA typically exhibits a WA capacity ranging from 4.49% to 10% [12,148,171], FMRA can achieve a WA capacity of 5.6% to 20% [150,159]. In this study, the WA capacity of the Type B MRA fractions ranged from 6.61% to 9%. Saiz Martinez et al. [170] also analysed MRA specimens with WA capacities of between 4.65% and 8.75%. The WA capacity of the CMRA in this study was higher than the 7% threshold established by the SC-BOE [20] for RCA, while FMRA demonstrated the lowest WA capacity among the MRA specimens. MRA aggregates in this study achieved acceptable sand equivalent, Los Angeles coefficient and flakiness index values for concrete production. Furthermore, the levels of the soluble chloride and sulfate ions remained below the maximum values stipulated by the SC-BOE (Table 3. 7) [20], which resulted from the water washing of the CDW at the recycling plant. In addition, all MRA fractions were utilized with elevated moisture content. 3.3. Test procedure In all produced concretes, the properties determined were physical (density and absorption), mechanical (compressive and splitting tensile strength, and modulus of elasticity), plastic and drying shrinkage, and durability (capillary water absorption and water penetration under pressure, chloride permeability, chloride profile, accelerated, and natural carbonation resistance). Table 3. 8 lists all of Chapter 3 26 the tests conducted, with detailed descriptions of the types and number of specimens employed in each test, as well as the age of testing and the specifications followed, and the test used in each chapter. Table 3. 8 Concrete properties analyzed Property Standard Test age (days) Specimens Specimen size (mm) Charper Fresh state Slump 4,5,6,7 Hardened state Density and Absorption EN 12390-7 28 3 100x100x100 4,7 Compresive strength EN 12390-3 7,28,56 9 100x100x100 4,5,6,7 Splitting Tensile strength EN 12390-6 28 3 h=200; ϕ =100 4,7 Elastic modulus EN 12390-13 28 2 h=200; ϕ =100 4,7 Drying Shrinkage EN 12390-16 1-91 2 75x75x280 4,5,6,7 Sorptivity-Capillary water absorption EN ISO 12148 28 3 100x100x100 4,7 Water Penetration EN 12390-8 28 3 h=200; ϕ =100 4,7 Chloride Permeability ASTM C1202 28,56 2 h=50; ϕ =100 5,6 Chloride Profile EN 12390-11 90 2 100x100x100 6 Acelerated Carbonation Resistance EN 12390-12 90 2 100x100x300 5,6 Natural Carbonation Resistance UNE 83993-1 360 2 100x100x400 6 The measurement of plastic shrinkage strain was carried out by recording length changes every minute for 12 hours after casting the concrete. LVDTs were installed on the mold, secured to a steel plate in direct contact with a free-moving Teflon plate, ensuring precise measurement of linear length changes. The test specimens, consisting of two square prisms (400 × 100 × 100 mm) cast in steel moulds, were maintained under controlled conditions at 23 ± 2°C and 50 ± 5% relative humidity throughout the testing period. The LVDTs were connected to a data acquisition system to continuously record the data. For the hardened concrete tests, all samples were stored in a humidity chamber at 20 ± 2°C with a relative humidity of ≥ 95% until the testing day. • The concrete’s compressive strength was determined using a 3000-kN capacity loading machine. The compressive strength was determined at 7, 28, and 56 days following the UNEEN 12390-3 [172] specifications. For each testing age, three cubic specimens measuring 100 x 100 x 100 mm were utilized. Chapter 3 27 • Splitting Tensile strength and Elastic modulus were determined at 28 days of curing on cylindrical samples with a diameter of 100 mm and a height of 200 mm following the EN 12390-6 and EN 12390-13 specifications, respectively. • The drying shrinkage of all the produced concretes was determined following the EN 1239016 [173] specification. Each concrete mixture used two specimens of 75 × 75 × 280 mm. After a 24 h casting, they were demoulded. Their initial lengths and weights were measured and the two specimens of each concrete were placed in a controlled climatic chamber (temperature of 20 ± 2℃ and relative humidity of 50 ± 5%). Length and weight measurements were recorded at intervals of 1, 7, 14, 28, 56 and 91 days. • The capillary water absorption (including sorptivity) was assessed using 100 × 100 × 100 mm cubic specimens following the ISO 15148:2002(E) specification. In order to carry out the test, the bottom faces of specimens were submerged in 5 mm water (the lateral surfaces were impregnated with impermeable resin). The cumulative water absorbed by the specimen was recorded at different time intervals up to 48 h. Sorptivity is the slope of the regression curve of the quantity of water absorbed by a unit surface area versus the square root of the elapsed time from the initial instant (t=0 min) to 120 min. The described results are the average of three measurements. • Water penetration depth under pressure was determined in cylindrical samples with a diameter of 100 mm and a height of 100 mm following EN 12390-8. Once the specimen was placed in the apparatus, a water pressure of 500 ± 50 kPa was applied to the circular surface for 72 ± 2 h. Afterwards, the specimen was split in half along a plane perpendicular to the surface on which the water pressure had been applied, and the depth of penetration was measured. • The chloride permeability in the concrete was assessed following the ASTM C1202 [174] "Standard Test Method for Electrical Indication of Concretes Ability to Resist Chloride Ion Penetration". Two cylindrical concrete samples of 200 mm in length were employed for each mixture, and from those, two discs specimens of 100 mm in diameter and 50 mm in thickness were obtained. Two-disc specimens, one of each sample, were used to determine the concrete's chloride ion penetration after 28 and 56 days of curing. The chloride penetrability of produced concretes was quantified by measuring the total charge (in Coulombs) passed during a 6-hour testing period. A potential difference of 60 V was applied across each side of the specimen, which was immersed, one side in solutions containing sodium hydroxide (NaOH) and the other side in sodium chloride (NaCl). • The chloride profile was determined by analyzing the chloride content at different depths across eight layers (0–1 mm, 1–3 mm, 3–5 mm, 5–7 mm, 7–10 mm, 10–13 mm, 13–16 mm, and 16–20 mm) in the concrete specimens. Cubic specimens measuring 100 × 100 × 100 mm were prepared following the EN 12390-11 standard. After curing, the specimens were exposed to a 3% NaCl solution for 91 days under controlled conditions at 23 ± 2°C and 50 ± 5% relative humidity. The chloride concentration in each layer was measured using titration with a 0.02N silver nitrate solution. Chapter 3 28 • The accelerated carbonation method following the UNE-EN 12390-12 [175] specification was employed in order to assess the carbonation resistance of produced concrete mixtures. Each concrete mixture used two prismatic 100 x 100 x 300 mm samples. All concrete specimens underwent a curing process in a humidity chamber for 28 days, followed by a 14-day preconditioning period under laboratory conditions (CO2 concentration of 425 ppm, 20 ± 2°C, and 50-55% relative humidity, RH). Subsequently, the samples were stored in a chamber with an environment consisting of 3% CO2, 57% HR and at 20°C. The carbonation depth of each specimen was measured at specific intervals of 0, 14, 28, 56, 70, and 91 days of exposure to the chamber. In order to determine the carbonation depth, a solution containing phenolphthalein indicator was applied to the freshly fractured surface of the concrete. The solution contained 1 g of phenolphthalein dissolved in 70 g of ethanol and 30 g of water following the UNE-EN 14630 [176] specification. • The natural carbonation resistance was evaluated using two prismatic specimens with dimensions of 100 × 100 × 400 mm, measured at intervals of 0, 30, 90, 180, and 365 days following the UNE 83993 standard. The specimens were initially cured for 4 days in a humidity chamber and subsequently stored in a suitable plastic box placed in an outdoor environment to allow natural exposure to carbonation conditions. The depth of carbonation was assessed using a solution prepared with 1 g of phenolphthalein, 70 g of ethanol, and 30 g of water, as specified in the UNE-EN 14630 [176] standard In all experimental phases, concrete mixtures were produced in a vertical axle mixer. The materials were always added manually in the same order: first, the aggregates (from coarser to finer), and after they were mixed for 30 seconds, the cement was added. While the solid components were mixing for 1 min, water was added, followed by chemical admixtures. The complete mixture was then mixed for 1 min more. Once the concrete was produced, concrete specimens were fabricated after the slump test was determined. The concrete specimens were kept in moulds for 24 h. After demoulding, they were stored in the humidity room at 20 ± 2 ℃ with relative humidity ≥ 95 % until 1 h before testing time. All the test elements were kept in the same conditions before testing. Chapter 3 29 Figure 3. 6 Comprehensive Framework of Physical, Mechanical, and Durability Testing for Recycled Aggregates. Chapter 4 30 4. Chapter 4. Fine and coarse recycled aggregates-Type A in concrete production Limiting the maximum fine and coarse recycled aggregates-Type A used in structural concrete Authors: Carla Vintimilla, Miren Etxeberria Journal: Construction and Building Materials Publication Date: Volume 380, 30 May 2023, 131273 DOI: https://doi.org/10.1016/j.conbuildmat.2023.131273 ABSTRACT The manufacture of concrete using recycled concrete aggregates (RCA) is an alternative used to reduce the exploitation of natural resources and landfilling of construction waste. This paper discusses the suitability of producing structural concrete to be exposed to an XC1-XC4 environment employing different percentages of fine RCA (FRCA) and coarse RCA (CRCA) classified as type A. Two experimental phases were conducted in which 300 kg of cement/m3 and an effective water:cement ratio of 0.48 (Phase 1) and 0.52 (Phase 2) were employed with different percentages of the CRCA and FRCA aggregates. The types of properties determined were physical (density, absorption, accessible porous), mechanical (compressive, splitting tensile strength, modulus of elasticity), shrinkage, and durability (sorptivity, water penetration). After exhausting analyses of the experimental values and structural code requirements, the successful use of a simultaneous combination of up to 60 % CRCA and 30 % of FRCA was defined in Phase 1. In addition, the use of 50 % CRCA and 20 % of FRCA were defined in Phase 2. Keywords: Coarse and fine recycled aggregates; structural recycled aggregate concrete, physical and mechanical properties, drying shrinkage, sorptivity. Chapter 4 31 4.1. Objective In the present research work, the physical, mechanical, drying shrinkage and durability (sorptivity and water penetration) properties of concretes made with CRCA and FRCA were determined. The main goal was to define the maximum percentages of CRCA and FRCA usable for structural concrete production with a characteristic compressive strength of 30 MPa (C30/37) to be exposed to XC1 to XC4 environments, corrosion induced by carbonation can happen due to humidity presence. Where exposure conditions are: XC1 is dry or permanently wet; XC2 is wet, rarely dry; XC3 is moderate humidity; and XC4 is cyclic wet and dry. Two experimental phases were conducted. In Phase 1, all of the concrete mixtures (NAC, concretes produced using only CRCA and concretes made with CRCA and FRCA) were produced with 300 kg of cement and an effective water: cement (w/c) ratio of 0.48. The percentages of 20 %, 30 %, 50 % and 100 % CRCA were used for concrete production. In addition, different percentages of FRCA were included (concretes with low CRCA were produced with a low % of FRCA), up to 100 % FRCA in concretes made with 100 % CRCA. Phase 2 concretes were produced using 300 kg of cement and an effective w/c ratio of 0.52. Due to the higher w/c ratio, the recycled concretes were produced using up to 60 % CRCA (including 20 %, 30 % and 50 %). In addition, up to 50 % FRCA was used as replacement for natural sand, together with CRCA. The obtained values for the measured properties formed the basis for the maximum percentages of CRCA and FRCA were defined for structural concrete production in concretes produced with different w/c ratios. 4.2. Concrete production All the concrete mixtures were designed to be exposed to XC1–XC4 environments [20], which require a minimum characteristic design strength (fck) of 30 MPa (C30/37). Two concrete production phases (Phase 1 and Phase 2) were conducted using the effective w/c ratios of 0.48 and 0.52, respectively. In both phases, 300 kg of cement/m3 and different percentages of CRCA and FRCA fractions were employed. Table 4. 1 shows the concrete mixtures produced in Phase 1. NAC was designated as NAC1. In addition, the concretes produced using only CRCA were designated as RAC-CX1, where X was 20 %, 30 %, 50 % or 100 % of the replacement ratio (in volume) of NAs coarse aggregate. Furthermore, the concretes produced with FRCA and CRCA were designated RAC-CX-FY1, where Y was the percentage of FRCA employed in the replacement of NAs sand. The FRCA used was from 0 % to 100 % as replacement for natural sand. As shown in Table 4. 1, the conventional concrete (NAC1) was designed with a total w/c ratio of 0.525, and its effective w/c ratio was calculated and maintained constant in all the produced concretes. All concretes were produced using an effective w/c ratio of 0.48 and 300 kg of cement /m3. The effective water (free water) is the water amount reacted with cement. In order to calculate the effective water value, if the aggregates were oversaturated (the FRCA was used in that condition), the water released from aggregates had to be considered free water. However, when the aggregates were humid but not saturated, their effective water absorption capacity had to be considered. The effective absorption capacity was considered the water amount absorbed by the aggregates in 30 min. While the effective absorption capacities of fine and coarse NAs were 70 % and Chapter 4 32 20 %, respectively, of their total absorption capacities, the FRCA and CRCA aggregates’ effective absorption capacities were 100 % and 70 %, respectively, of their total absorption capacity. However, the FRCA was used in an oversaturated condition. Consequently, the total water in concrete was calculated by the sum the effective water and the water inside the aggregates (humidity plus effectively absorbed water). As shown in Table 4. 1, the total w/c ratio was higher with increasing volumes of RCA used in concrete production. The total water values were consistent with other studies that employed up to 100 % of CRCA and FRCA in concrete production [46,68]. In Phase 1, the RCA replacement percentages were determined based on European regulations and the results of other researchers [15,26,46,48]. The concretes RAC-C201, RAC-C301, RAC-C501 and RAC-C1001 were produced according to the Spanish Structural Concrete code (SC-BOE) [20], UNE EN 206, NF EN 1260 2003 and RILEM. For the simultaneous use of both CRCA and FRCA fractions, the RECYBETON [177] proposal was followed in creating the mixtures C60-F201, C60F301 and C50-F201. In addition, the C50-F501, C100-F201 and C100-F1001 concrete mixtures were produced based on Danish legislation, which allows for the use of aggregates provided that their density is not less than 2220 kg/m3[48]. Table 4. 1 Mix proportions of Phase 1 concrete. All the concretes were produced with an effective water/cement ratio of 0.48. Cement Total Water FNA FRCA CNA 1 CRCA 1 CNA 2 CRCA 2 S P Slump Mix (Kg) (Kg) (Kg) (Kg) (Kg) (Kg) (Kg) (Kg) (%) (%) (mm) NAC1 300 157.5 994 - 247 - 831 - 1.5 - 100 RAC-C201 300 167.7 994 - 198 41 665 143 1.9 - 140 RAC-C301 300 172.8 994 - 173 62 582 215 1.9 - 150 RAC-C501 300 182.4 952 - 144 121 416 358 1.2 0.4 200 RAC-C1001 300 213.5 849 - - 414 - 626 1.2 0.4 100 RAC-C50-F201 300 190.0 762 163 144 121 416 358 1.2 1.5 180 RAC-C50-F501 300 203.4 476 407 144 121 416 358 1.2 1.5 200 RAC-C60-F201 300 194.8 762 163 115 145 333 429 1.2 1.5 200 RAC-C60-F301 300 199.3 667 244 115 145 333 429 1.2 1.5 160 RAC-C100-F201 300 222.6 679 145. - 626 - 626 1.2 1.7 200 RAC-C100-F1001 300 254.0 - 721 - 414 - 626 1.2 1.7 150 The NAC1, RAC-C201 and RAC-C301 concretes were produced using only the S admixture. However, the concretes produced with a higher percentage of RCA more S admixture were needed to achieve similar workability. Consequently, in order to maintain the workability with a lower risk of segregation, the S admixture amount was reduced, and P (multifunctional) admixture was employed in all of the other concretes. Chapter 4 33 The consistencies of all concretes were determined following the EN 12350-2 specification. All the concretes achieved a slump value of 150–200 mm except NAC1 and RAC-C1001. The NAC1 concrete was produced with the lowest S admixture amount and achieved a 100 mm slump. In addition, the RAC-C1001 needed a higher amount of P admixture. However, it was verified that all of the concretes had adequate fluid-liquid consistency with the 1.20 % S and 1.5 % P admixture. Table 4. 2 shows the concrete mixtures produced in Phase 2. In this case, the NAC2 was produced using a total w/c ratio of 0.55. In addition, all the concrete mixtures were produced with an effective w/c ratio of 0.52 and 300 kg of cement/m3. Due to the use of a higher w/c ratio than that employed in Phase 1, and in order to control the total water amount in concrete, a maximum of 60 % of CRCA (RAC-C602) and 50 % of FRCA (RAC-CX-F502) were employed for concrete production. Furthermore, the C20-F52 and C30-F102 concrete mixtures, made with low percentages of CRCA and FRCA, were fabricated to analyse the influence of FRCA on concrete properties. Table 4. 2 Mix proportions of Phase 2 concrete. All the concretes were produced with an effective water/cement ratio of 0.52 Cement Total, Water FNA FRCA CNA 1 CRCA 1 CNA 2 CRCA 2 S P Slump Mix (Kg) (Kg) (Kg) (Kg) (Kg) (Kg) (Kg) (Kg) (%) (%) (mm) NAC2 300 165 981.2 - 242.9 - 815.4 0.00 1.0 1.2 160 RAC-C202 300 173.9 981.2 - 194.3 43.1 652.3 147.4 0.7 1.0 180 RAC-C302 300 179.1 981.2 - 170.0 64.4 570.8 219.1 0.7 1.0 160 RAC-C502 300 190.8 940.3 - 141.7 125.2 407.7 370.1 1.0 1.2 160 RAC-C602 300 195.9 940.3 - 113.3 150.3 326.2 444.5 1.0 1.2 170 RAC-C20-F52 300 176.9 932.2 41.6 194.3 43.1 652.3 147.4 0.7 1.0 200 RAC-C30-F102 300 183.7 883.1 84.4 170.0 64.4 570.8 219.1 0.7 1.0 90 RAC-C50-F202 300 199.6 752.3 159.5 141.7 125.2 407.7 370.1 1.0 1.2 160 RAC-C50-F502 300 212.8 470.2 398.8 141.7 125.2 407.7 370.1 1.0 1.2 160 RAC-C60-F202 300 204.7 752.3 159.5 113.3 150.3 326.2 444.5 1.0 1.2 180 RAC-C60-F302 300 209.1 658.2 239.3 113.3 150.3 326.2 444.5 1.0 1.2 170 In order to maintain workability with a lower risk of segregation, all the concrete mixtures, were produced using S admixture and P (multifunctional). As a result, all the concretes, except for RACC30-F102, had adequate fluid-liquid consistency with the 0.7–1.0 % S and 1.0–1.2 % P admixture. It was determined that the RAC-C30-F102 would require more amount of admixtures due to the high humidity of CRCA and oversaturated FRCA. In addition, no relevant loss of workability was shown in concretes produced using FRCA. Chapter 4 40 fctm=0.3·fck2/3 Eq.4 (4) Figure 4. 3 shows the ratio of the experimental value with respect to the numerical (theoretical) value obtained by each concrete produced. In Phase 1 (Figure 4. 3-a), it was observed that the numerical method (Eq.4 [4]) accurately estimated the experimentally obtained tensile strength values of NAC1, RAC-C201 and RAC-C301 concretes. In addition, the numerical value was similar, with a difference of around 5 %, with respect to the experimental values in all the RAC concretes except RAC-C1001 and RAC-C100-F1001. In Phase 2 (Figure 4. 3-b), all the obtained results were above the dotted line (i.e. the theoretical method underestimated the experimentally obtained values) except for RAC-C502, RAC-C60-F202 and RAC-C60-F302. The use of recycled aggregates in concrete, regardless of the fraction used, did not cause significant decreases in tensile strength. Therefore, the formula specified in the structural concrete code was validated as helpful for estimating the tensile strength of concrete containing RCA. 0,6 0,7 0,8 0,9 1,0 1,1 1,2 1,3 1,4 Ratio Experimental value/numerical (SC-BOE) value Splitting Tensile Strength (Experimental/Theorical BOE) Elastic Modulus(Experimental/Theorical BOE) 0,6 0,7 0,8 0,9 1,0 1,1 1,2 1,3 1,4 Ratio Experiemntal value/numerical (SC-BOE) value Splitting Tensile Strength (Experimental/Theorical BOE) Elastic Modulus(Experimental/Theorical BOE) a) b) Figure 4. 3 Ratio of the experimental value to respect to numeral value of splitting tensile strength and modulus of elasticity a) phase 1 and b) phase 2. 4.3.2.3 Elastic modulus The elastic modulus values of the produced concretes are provided in Table 4. 5. All of the RACs achieved a lower modulus elasticity value than the NACs. In addition, the value was lower when higher percentages of RCA were employed. However, the RAC produced in Phase 1 (with a lower w/c ratio) achieved a higher decrease with respect to NAC than did the RAC produced in Phase 2. In Phase 1, concrete produced with 20 %, 30 %, 50 % and 100 % CRCA obtained 11.5 %,12.3 %, 15.2 % and 24.9 % lower modulus values, respectively, than NAC1. Other researchers [51] found Chapter 4 41 similar values of 14 % and 21 % reductions when concretes were produced with 50 % and 100 % CRCA replacement ratios. Dimitriou et al. [191] also reported a 12–33 % reduction in concrete made with 100 % CRCA. In addition, when the RAC was produced with up to 30 % FRCA, the modulus of elasticity was reduced to less than 5 % with respect to the corresponding concrete produced using only CRCA. However, in the concretes produced with 50 % and 100 % FRCA, the modulus of elasticity decreased 7.4 % and 14.9 %, respectively, with respect to the corresponding CRCA concrete. Other researchers also found that replacing 30 % CRCA and FRCA decreased the elastic modulus of concrete [192]. Moreover, several researchers corroborated the negative influence of using high percentages of FRCA in the value of modulus of elasticity [51,193]. Dimitriou et al. [191] reported a reduction of up to 27 % of modulus elasticity when concrete was produced with 100 % CRCA and 25 % FRCA. The greater porosity, paste volume and deformability of RAC in comparison to NAC contributed to lower modulus of elasticity values [57,106,194,195]. In Phase 2, all of the RAC produced using only CRCA achieved similar values. Moreover, there was no variation in the modulus of elasticity value when the concrete was made using 5 % and 10 % FRCA, compared to the corresponding concrete produced with CRCA. However, the mixes produced with a greater than 20 % FRCA replacement ratio showed considerable decreases in modulus of elasticity. The RAC-C50-F502 and RAC-C60-F302 concretes had 17.8 % and 22.4 % lower modulus of elasticity than NAC2. In comparing the Phase 1 and Phase 2 values, while the NAC1 concrete achieved a higher modulus than the NAC2, the concretes produced using the same percentages of CRCA and FCRA in phase 2 achieved similar modulus elasticity values to those in phase 1. According to the structural concrete code [20], the modulus elasticity of NAC concrete can be estimated following Eq.4 (5). Considering that the NAC (30/37) concrete should have a minimum fcm of 38 MPa, its modulus elasticity should be 33 GPa. Considering this value, the RAC-C1001, RACC100-F201, RAC-C100-F1001 and RAC-A-C60-F302 concretes achieved values that were too low and, consequently, had too high strain. Ecm=22∗(fcm 10)0.3 Eq.4 (5) where Ecm is the elastic modulus of the concrete and fcm is the mean compressive strength. Furthermore, Figure 4. 3 shows the ratios of the experimental and numerical (Eq.4 [5]) modulus elasticity values for each produced concrete. In Phase 1 (Figure 4. 3-a), it was observed that numerical estimation (using Eq.4 [5]) of modulus elasticity was similar to the values obtained experimentally when concrete was made with up to 50 % CRCA and natural sand. In contrast, with concretes produced using fine aggregates, the numerical method overestimated the experimental values (the obtained ratios were below the dropped line), with the experimental value 8 % lower than the numerical value. Chapter 4 42 However, the difference between experimental and numerical values was much greater when 100 % CRCA was employed with natural or FRCA aggregates. In contrast, in Phase 2, the numerical method (Eq.4 [5]) adequately estimated the modulus elasticity of RAC, except for RAC-C60-302, which experimentally achieved a low value. Consequently, it can be concluded that although the SC-BOE defines the Eq.4 (5) for a concrete produced with a maximum 20 % CRCA, it adequately estimates the values of concretes produced with up to 60 % CRCA and 20 % FRCA. According to Figure 4. 3, the influence of the recycled aggregates on the reduction of the modulus of elasticity value with respect to theoretical value was greater in Phase 1 than in Phase 2 (due to a lower w/c ratio and higher strength). Consequently, the estimation formula approximates better with lower strength concretes (Phase 2). However, as mentioned, the numerical estimation was good in all concretes produced using up to 50–60 % CRCA and up to 20 % FRCA, taking into account both phases. 4.3.3. Drying shrinkage Figure 4. 4 illustrates the drying shrinkage (µƐ) and mass loss (%) values up to 91 days for all concrete mixtures produced in Phase 1. The obtained test results revealed that concrete with a higher percentage of RCA suffered more shrinkage. In addition, the use of FRCA significantly increased shrinkage. These increases occur mainly in the early ages, tending to stabilize over time [51]. Figure 4. 4-a shows that all the concretes produced using only CRCA achieved similar shrinkage values as NAC1 except RAC-C1001, which achieved considerably more shrinkage (59.9 %) at 91 days. The RAC-C201, RAC-C301 and RAC-C501 concretes suffered 7.7 % more shrinkage than NAC1. Figure 4. 4-b shows that the use of FRCA in concrete production increased the drying shrinkage value. The RAC-C50-F201 and RAC-C60-F201 increased in 13.7% and 16.5%, respectively, with respect to NAC1. However, using 30% of FRCA, the RAC-C60-F301 achieved a 39.1% higher shrinkage value than NAC1. In addition, the use of 50% of FRCA considerably increased the shrinkage value. The RAC-C50-F501 reached a 60.2% greater value than NAC1. Furthermore, the RAC-C100-F1001 concretes achieved 99.3 % more shrinkage than NAC1 at 91 days. Based on these results, it can be concluded that the shrinkage increased when the replacement value of FRCA was greater than 30%, together with 60% of CRCA. The obtained results were in accordance with the results obtained by other authors [186]. Figure 4. 4-c shows that while NAC1 lost 2.2 % of mass, concrete mixtures up to 30 % CRCA and with 50 % CRCA achieved a mass loss of 2.7 % and 3.5 %, respectively, after 91 days of drying test exposure. However, similar to shrinkage values, in mixtures produced using up to 60 % CRCA and 30 % FRCA, the weight losses increased up to 3.6 % (see Figure 4. 4-d). In addition, the RACC100-201 and RAC-C100-1001 concretes suffered a mass loss of 4.4 % and 5.4 %, respectively. Other researchers obtained similar results and observed that replacing 15% and 30 % CRCA caused a mass loss of 2,0% and 2.5 % with drying shrinkage values of -550 and -790 µm/m respectively [196]. In addition, the concretes produced with 20 %, 50 % and 100 % CRCA had maximum loss values of 1.3 %, 1.9 % and 2.1 %, respectively, with drying shrinkage ranging from -311 to -717 µm/m [123]. Chapter 4 43 According to Etxeberria et al. [183], total replacement of both fractions caused a mass loss of 4.2 % with all drying shrinkage values were −1000 µm/m. A clear trend was observed showing that mass loss increased as the percentage of recycled aggregates increased. a) b) c) d) Figure 4. 4 Drying shrinkage development in phase 1 a) drying shrinkage CRCA1; b) drying shrinkage CRCA1&FRCA1; c) mass loss CRCA1; d) mass loss CRCA1&FRCA1 Figure 4. 5 shows the drying shrinkage and mass loss (%) obtained by concretes produced in Phase 2. The RAC in Phase 2 obtained a greater shrinkage value than did NAC2. Figure 4. 5-a and Figure 4. 5-b illustrate that all the concrete produced in Phase 2 achieved shrinkage values lower than 420 µm/m, which is a good value according to ACI [122]. This determines that conventional concrete’s typical drying shrinkage values are −200 to −800 when a high w/c ratio is used. Chapter 4 44 Although Phase 2 concrete mixtures were produced using a higher w/c ratio than Phase 1, the shrinkage values obtained in Phase 2 concretes were lower. This was because the climatic room condition varied a little with exterior environmental conditions. The Phase 1 concretes were manufactured in the spring and summer months, and the average climatic room conditions were 22 ℃ and 51 % relative humidity. In contrast, the Phase 2 concretes were manufactured in the autumn and winter months, when the average climatic room conditions were 19 ℃ and 53 % relative humidity, so a lower temperature and higher humidity than in Phase 1. a) b) c) d) Figure 4. 5 Drying shrinkage development in phase 2 a) drying shrinkage CRCA2; b) drying shrinkage CRCA1&FRCA2; c) mass loss CRCA2; d) mass loss CRCA1&FRCA2 Figure 4. 5-c and Figure 4. 5-d show the mass loss of concrete specimens submitted to the drying process. While the NAC2 concrete achieved a mass loss of 3.0 %, all the RACs produced with CRCA achieved a mass loss of 3.5 % after 91 days. Similar to Phase 1, concrete with up to 20 % FRCA maintained the mass loss achieved by concrete produced with only CRCA. However, concrete produced with 30 % FRCA reached a 5 % mass loss. These values are similar to those found by other researchers [123,183]. Chapter 4 45 The mass loss percentages on Phase 2 concrete were higher than Phase 1 concrete, while the shrinkage values achieved were lower. However, as Medjigbodo et al. [197] described, the mass loss depended on the free water amount (it was higher in phase 2), which did not directly influence in shrinkage values. In addition, due to a higher humidity value in the chamber for phase 2 concretes, their decreasing internal relative humidity was lower, suffering lower capillary stress and lower shrinkage than that in phase 1 concrete. Figure 4. 6 shows the ratio between the shrinkage value of each of the concretes produced with respect to NAC in Phase 1 (Figure 4. 6-a) and Phase 2 (Figure 4. 6-b) experimental phases. It was clearly observed that the shrinkage RAC/NAC ratio value of Phase 1 concretes was lower than that of Phase 2 concretes when they were produced with the same amount of recycled aggregates, probably due to a higher total water amount in phase 2. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Ratio shrinkage all concrete/NAC1 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Ratio shrinkage all concrete/NAC₂ a) b) Figure 4. 6 Relative ratio of shrinkage at 91 days a) Phase 1 (effective w/c ratio of 0.48) b) Phase 2 (effective w/c ratio of 0.52) In Phase 2, unlike in phase 1, the use of up to 60% of CRCA increased the shrinkage value with respect to conventional concrete, NAC2 (see Figure 4. 6-b). While the RAC-C202 concrete obtained a 23 % higher shrinkage value than that of NAC2, the RAC-C302, RAC-C502, and RAC-C602 concretes reached an increase of up to 39.7 %. Moreover, the use of up to 20 % of FRCA achieved a low rise in shrinkage value with respect to those obtained by concretes using only CRCA. However, the shrinkage increased up to 71.9 % with respect to NAC2 when 30 % or more FRCA was employed in concrete production together with CRCA. However, as previously mentioned, the total drying shrinkage values obtained by all concretes produced in Phase 2 were low. As previously demonstrated, the use of recycled aggregate increases the shrinkage value of concrete due to the presence of old cement paste in CRCA and FRCA and, consequently, requires a higher quantity of water [47]. Chapter 4 46 In order to verify the shrinkage estimation value numerically in comparison to the experimentally obtained data, the Structural Concrete Code (SC-BOE) [20] and Eurocode 2: EN 1992-1-1 (EC-02) [198] were considered. According to SC-BOE, the drying shrinkage is estimated numerically using Eq.4 (6): εcd(t)=(t−ts) (t−ts)+0.04√ho3.kh.εcd,0 Eq.4 (6) where: t is the concrete age in days (91 days); ts s is the concrete age when it was exposed to drying conditions (1 day); h0 is the notion size equal to 2Ac/u, where Ac (5625 mm2) is the concrete cross-sectional area and u (300 mm) is the perimeter of the section exposed to drying; kh is the coefficient (taking the value of 1.00), which depends on the average thickness h0; and εcd,0 is the initial drying shrinkage of concrete in mm/m (Eq.4 [7]). εcd,0=0.85[(220+210.αds1)exp(−αds2 fcm fcmo)].10−6.1.55[1−(HR HRo)3] Eq.4 (7) where fcm is the average compressive strength (N/mm2) in cylindrical specimens; fcm0 = 10 N/mm2; αds1 is a coefficient that depends on the type of cement, which is 6 for R cement class; αds2 is a coefficient that depends on the type of cement, which is 0.11 for R cement class; HR is the relative humidity of the environment; and HR0 = 100 %. According to Eurocode 2: EN 1992-1-1 (EC-02) [198], the drying shrinkage is numerically estimated with Eq.4 (8): εcds(t−ts)=[(220+210.αds)exp(−0.012𝑓𝑐𝑚,28)].10−6.1.55 [ 1− ( RH 99.( 35 𝑓𝑐𝑚,28)3≤99 ) 3 ] .[ (𝑡−𝑡𝑠) 0.035∗ℎ𝑛2+(𝑡−𝑡𝑠)]0.5𝑛𝑠ℎ𝑅𝐴 Eq.4 (8) where αds is a coefficient that depends on the type of cement, which is 6 for R class cement; RH is the relative humidity of the environment; ts is the age of the concrete at the beginning of drying (1 day); t is the time elapsed since concrete casting (i.e. concrete age, which was 91 days); hn: is the notion size = 2 Ac/u, where Ac is the concrete cross-sectional area and u is the perimeter exposed to Chapter 4 47 drying; shRA is a factor when RCA is employed in concrete production between 0.20 < αRA  0.40 (shRA = 1+0.8 αRA, where αRA is a ratio between the quantity of FRCA and CRCA and the total quantity of aggregates, fine and coarse aggregates, employed). According to EC-02, recycled aggregates may be used in normal concrete production without any particular consent if done in accordance with the provisions of type A recycled aggregate as defined in EN 206. Figure 4. 7 shows the ratios between experimentally obtained shrinkage data with respect to the numerically obtained values (SC-BOE and EC-02) for all of the concretes produced in Phase 1. The values obtained in Phase 2 were not analysed because, due to the low temperature and relative humidity of the climate room, the obtained values were lower than the expected ones. Figure 4. 7 shows that the EC-02 [198] estimation method, which considered the RCA influence using the factor shRA, adequately estimated the values for all of the concretes. However, SC-BOE [20] methods underestimate the RAC shrinkage value because it is only regarded as applicable for NAC and RACC20. However, it adequately estimated the behaviour of concrete produced using up to 60 % CRCA and 20 % FRCA because they have limited influence on drying shrinkage values. 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6 1,8 2,0 Ratio Experimental value/ numerical estimation SC-BOE EC-02 Figure 4. 7 Analysis of shrinkage estimation, the ratio of experimental results/numerical estimation following SC-BOE and EC-02 for the concretes produced in Phase 1. 4.3.4. Durability properties. 4.3.4.1 Sorptivity Figure 4. 8 displays the capillary absorption values obtained over time for the concretes produced in Phase 1 and Phase 2. In addition, Table 4. 3 shows the sorptivity values for each of the produced concretes. All of the RACs achieved higher values than the NACs. Chapter 4 48 In Phase 1, the concretes produced using 20 % to 100 % of the CRCA in substitution for coarse aggregates obtained values of 30.2 % to 39.6 %, a higher sorptivity value than that of NAC1. In Phase 2, due to the concrete production have higher w/c ratios than Phase 1, the CRCA influence on sorptivity was lower. Consequently, the concretes made using 20 % to 30 % CRCA had sorptivity values that only increased by 10 % as compared to NAC2. In addition, the concretes produced with 50 % and 60 % CRCA achieved sorptivity values up to 28.3 % greater than NAC2. Similarly, Pedro and Brito [51] obtained sorptivity values for RAC concretes produced with 100 % CRCA and natural sand and achieved compressive strengths that were 46.9–59.4 % higher than the NAC. In addition, other researchers [59,180] have described sorptivity increases with RAC that were lower when the concretes were produced with a higher w/c ratio, reaching an increase of 22 % and 37 % when concretes were produced with a 100 % CRCA replacement ratio. The concrete also employing FRCA achieved a higher sorptivity value than that produced only with CRCA (see Figure 4. 8-a and Figure 4. 8-b). In Phase 1, the concrete made with 50 % CRCA and 20 % or 50 % FRCA increased by 62.5 % and 103.7 %, respectively, as compared to NAC1. In addition, they achieved sorptivity values of 0.0466 mm/min0.5 and 0.0493 mm/min0.5, respectively. In Phase 2, although the RAC-C50-F202 and RAC-C50-F502 concretes had lower increases with respect to NAC2 (43.5 % and 74 %), they achieved sorptivity values of 0.0557 mm/min0.5 and 0.0676 mm/min0.5, respectively. Other researchers [51] also found that concrete produced with 50 % FRCA and CRCA suffered increases of up to 72 % with respect to NAC. However, it should be noted that RAC-C60-F202 and RAC-C60-F302 achieved adequate performance with sorptivity values of 0.0429 mm/min0.5 and 0.0438 mm/min0.5, respectively. In terms of the comparison of Phase 1 and Phase 2 values, all of the concretes produced using CRCA in Phase 1 achieved lower sorptivity values than NAC2. The use of FRCA increased the sorptivity value. The Phase 1 concretes achieved lower sorptivity values than the corresponding concretes in Phase 2. 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 0 2 4 6 8 10 12 Capilary Absoption (mm) Time (min0.5) NAC₁RAC-C20₁ RAC-C30₁RAC-C50₁ RAC-C100₁RAC-C50-F20₁ RAC-C50-F50₁RAC-C60-F20₁ RAC-C60-F30₁RAC-C100-F20₁ RAC-C100-F100₁ 0,0 0,2 0,4 0,6 0,8 1,0 1,2 1,4 0 2 4 6 8 10 12 Capilary Absoption (mm) Time (min0.5) NAC₂RAC-C20₂ RAC-C30₂RAC-C50₂ RAC-C60₂RAC-C20-F5₂ RAC-C30-F10₂RAC-C50-F20₂ RAC-C50-F50₂RAC-C60-F20₂ RAC-C60-F30₂ a) b) Figure 4. 8 Sorptivity values at 28 days in a) Phase 1 and b) Phase 2 Chapter 4 49 According to several researchers [199], concrete with a sorptivity value lower than 0.10 mm/min0.5 is considered durable. However, other researchers [200] proposed lowering that value to 0.05 mm/min0.5 for reasons of safety. Pedro and Brito [51] expressed that the quality of concrete is low if the sorptivity value is higher than 0.2 mm/min0.5, medium if the coefficient is between 0.2 mm/min0.5 and 0.1 mm/min0.5, and good if the coefficient is lower than 0.1 mm/min0.5. According to those criteria, it can be concluded that all of the concretes in Phase 1 and Phase 2 were in the high quality and durable range. However, considering a more restrictive condition of 0.05 mm/min0.5, in Phase 1, the RCA-C60-F301 would define the limit, and the RCA-C50-F20 would be the limit, in Phase 2. 4.3.4.2 Water penetration under pressure Figure 4. 9 displays the water penetration values obtained for concretes produced in Phase 1 and Phase 2. According to the Structural Concrete Code (SC-BOE) [20], the average (Da) and maximum (Dmax) water penetration under pressure should be lower than 30 mm and 50 mm, respectively, for aggressive exposure classes of concrete. Consequently, the obtained results show that all of the mixes, regardless of the percentage and type of RCA used in concrete production, met the requirements established by the Structural Concrete Code (SC-BOE). In Phase 1, the concretes produced using only CRCA as replacement for NAs achieved an average water penetration value of about 8.0 mm. In addition, the maximum water penetration values were 13 mm. When the fine fraction was used in concrete production, the average (Da) water penetration depth values of all the concretes remained similar to that of the NAC1. However, the Dmax increased to 22 %, 16 % and 26 % in RAC-C50-F501, in RAC-C100-F201 and RAC-C100-F1001, respectively, as compared to NAC1. The results obtained in this research were lower than those found by Kapoor and Singh [201], who observed that recycled concretes manufactured with 100 % CRCA increased water penetration by 30 %. In Phase 2, the average and maximum water penetrations ranged from 8.03 mm to 13.70 mm and 12.25 mm to 22 mm, respectively. The higher water penetration was observed with RAC-C50-F502, RAC-C60-F202 and RAC-C60-F302, with variations of 29.4 %, 31.4 % and 72.5 %, respectively, compared to NAC2. As a general rule, the greater the AR substitution in a mixture, the greater the water penetration [144]. However, the water penetration under pressure property seems to be more influenced by the effective w/c ratio [183,202] and not by the RCA used, causing doubt regarding an effective durability property. In terms of comparing Phase 1 and Phase 2 values, the NACs (NAC1 and NAC2) achieved similar values. In addition, the recycled concretes produced in Phase 1 (using an effective w/c ratio of 0.48) that used up to 60 % CRCA together with up to 30 % FRCA achieved values similar to NAC. Chapter 5 56 Table 5. 2 shows that the RAC achieved 18% lower compressive strength than that of NAC-0.47 (all concretes were made using the same effective water-cement ratio of 0.47). However, the RAC achieved similar strength to NAC-0.51 concrete, which was made with an effective water-cement ratio of 0.51 (see Figure 5. 1). These findings highlight the influence of recycled aggregates on the mechanical properties of concrete and emphasise the importance of adjusting the water-to-cement ratio to achieve comparable compressive strength to that of NAC. Although all the RACs had the same effective water-cement ratio, RACs manufactured with type CEM IIAL cement exhibited slightly lower compressive strength than those obtained with type CEM IIAS and type CEM IIIB cement. This difference could be attributed to environmental temperature: the RACs made with CEM IIAL were produced in spring/summer, while the others were produced in autumn/winter [204]. These factors could influence the setting and curing process of the concrete, directly affecting its final strength. Figure 5. 1 a-c describes the ratio of compressive strength value obtained by each concrete with respect to that of NAC-0.51 at 7, 28 and 56 days. In general, NAC-0.47 achieved between 3% and 17% higher strength than NAC-0.51 concrete at different ages. (a) CEM-IIAL (b) CEM-IIAS 0 0,2 0,4 0,6 0,8 1 1,2 NAC₀₄₇ RAC-C50 RAC-C50-F10 RAC-C50-F20 Ratio comprresivestrenght all concretes/NAC0,51 7 days 28 days 56 days NAC₀₅₁ 0 0,2 0,4 0,6 0,8 1 1,2 NAC₀₄₇ RAC-C50 RAC-C50-F10 RAC-C50-F20 Ratio comprresivestrenght all concretes/NAC0,51 7 days 28 days 56 days NAC₀₅₁ Chapter 5 57 (c) ) CEM-IIIB Figure 5. 1 Relative compressive strength at all concrete ages produced with cements: a) type CEM IIAL; b) type CEM IIAS; c) type CEM III-B Figure 5. 1-a describes the ratio when the concretes were produced using the cement CEM IIAL. While the RAC-C50 and RAC-C50-F10 achieved similar values to NAC-0.51 at any age, the RACC50-F20 obtained a decrease of up to 8.5% at 56 days. Figure 5. 1-b shows the results obtained for concrete produced with CEM IIAS. All the RAC, including the RAC-C50-F20, achieved a similar strength to the NAC-0.51 concrete. Similarly, Figure 5. 1-c shows that RAC made with CEM IIIB cement achieved a slightly higher strength than NAC-0.51. The results (Figure 5. 1) show that the different substitution levels of RCA (50% CRCA and up to 20% FRCA) did not have a significant detrimental impact on compressive strength. In addition, as discovered in previous work [126], it was verified that the concrete produced with 50% CRCA and 20% FRCA (RAC-C50-F20) achieved similar compressive strength to concrete containing only CRCA (RAC-C50) and NAC when the RAC and NAC were made with an effective water-cement ratio of 0.47 and 0.52, respectively. Gao and Wang [62] reported that concrete produced with a higher percentage of FRCA caused a reduction of the compressive strength value. However, several researchers suggested that incorporating 30% FRCA as a replacement for natural sand could still achieve satisfactory properties [63,64]. Evangelista and Brito [63] and Pedro and Brito [51] also confirmed the use of up to 30% FRCA for structural concrete production. According to the results, the NAC and RAC concrete (produced with 50% CRCA and up to 20% FRCA) achieved similar compressive strength when an effective water-cement ratio of 0.51 and 0.47 was used in concrete production, respectively. They all achieved a suitably designed compressive strength of C30/37 for structural applications. 0,0 0,2 0,4 0,6 0,8 1,0 1,2 NAC₀₄₇ RAC-C50 RAC-C50-F10 RAC-C50-F20 Ratio comprresivestrenght all concretes/NAC 051 7 days 28 days 56 days NAC₀₅₁ Chapter 5 58 5.3.2. Drying Shrinkage Figure 5. 2 a–c and Figure 5. 3 a–c illustrates the drying shrinkage (µƐ) and mass loss (%) values, respectively, over 91 days for the NAC-0.51 and RAC concretes produced using cement CEM IIAL (a), CEM IIAS (b) and CEM IIIB (c). (a) (b) (c) Figure 5. 2 Drying shrinkage development at 91 days: a) CEM II/AL, b) CEM II/AS, c) CEM III/B Chapter 5 59 (a) (b) (c) Figure 5. 3 Mass loss development at 91 days: a) CEM II/AL, b) CEM II/AS, CEM III/B. According to shrinkage values, the concretes produced with CEM IIAL achieved a drying shrinkage value between -496.8 and -562.7 µm/m at 91 days (see Figure 5. 2-a). The RAC-C50 concrete achieved a 7.3% higher drying shrinkage value than that of the NAC. Furthermore, the RACC50-F20 concrete achieved a 13.3% higher drying shrinkage than NAC concrete (the RAC-C50-F10 concrete was not tested). According to the results obtained from the concretes produced using CEM IIAS (see Figure 5. 2-b), the obtained shrinkage values were between -318.82 and -496.52 µm/m. Figure 5. 2-b shows that RAC-C50 and RAC-C50-F10 had similar drying shrinkage values which were 18% and 16%, Chapter 5 60 respectively, higher than NAC-0.51. Moreover, the RAC-C50-F20 concrete had 56% higher drying shrinkage than NAC-0.51. In accordance with the results obtained from the concretes produced with CEM IIIB (see Figure 5. 2-c), all the concretes achieved similar drying shrinkage values, between -437.3 and -489.6 µm/m. Figure 5. 2-c shows that RAC-50 only exceeds 3% of the value obtained by NAC-0.51 concrete, while RAC-C50-10 and RAC-C50-20 were 11% and 12% higher than NAC-0.51, respectively. The use of RAC reduced stiffness caused by the amount of adhered mortar in the recycled aggregate [126,205,206]. This property is closely associated with the modulus of elasticity, which is the principal mechanical indicator of material stiffness [205]. Vintimilla and Etxeberria [126] determined that RAC concrete produced using CEM IIAL with 50% CRCA and 20% FRCA achieved a 19% lower modulus elasticity and a 14% higher drying shrinkage value than those of NAC. Bendimerad et al. [31] confirmed that the increase in drying shrinkage was associated with a decrease in modulus. According to the achieved results, FRCA strongly influenced and increased the shrinkage value, consequently, the concrete produced with 20% FRCA reached the highest shrinkage value regardless of the obtained compressive strength, as all the concretes exhibited similar compressive strength. Despite this increase in shrinkage, all the values were considered acceptable according to ACI [122], which states that the typical drying shrinkage values of NAC range from -200 to -800 when a high water-cement ratio is employed. In addition, in general, RAC presents deviations similar to those NAC, but between them showed a moderate disperse in most cases. During the first four days of curing within a drying chamber, the concretes produced with lower clinker (CEM IIIB cement, see Figure 5. 2-c) achieved the highest shrinkage value. Drying shrinkage mainly occurs during the early ages and tends to stabilise over time [51] . However, this behaviour is more apparent when SCM is employed as a concrete binder [118,119]. The NAC-0.51 produced with CEM IIIB, CEM IIAL and CEM IIAS cement reached -200 µm/m, -170 µm/m and -110 µm/m, respectively. In addition, the shrinkage value increased as the percentage of RCA used increased. The RAC-C50-F20 produced with CEM IIIB cement achieved a drying shrinkage value of -280 µm/m in the first four days of drying. However, as mentioned above, the shrinkage values stabilised over 28 days. Figure 5. 3 a-c show the mass loss (in %) of each concrete produced with CEM IIAL, CEM IIAS and CEM IIIB, respectively. The three NAC-0.51 mixes achieved a similar mass loss of 2.2%, 2.3%, and 1.9%, respectively. As expected, the concrete mass loss increased as the percentage of recycled aggregates rose. Similarly, for the drying shrinkage value, the RAC-C50-F20 produced with CEM IIAL achieved the highest mass loss with 3.5%, followed by the RAC-C50-F20 produced with CEM IIAS and lastly, CEM IIIB with a mass loss of 3.1% and 2.86%, respectively (see Figure 5. 3). These values are consistent with the results found by other researchers [126,207,208]. In all cases, higher drying shrinkage was closely associated with a higher mass loss when comparing concretes that employed the same type of cement. All control concretes exhibited an average mass loss of approximately 2%, while the incorporation of fine and coarse recycled aggregates resulted in an increment of approximately 3% to 3.5%. Chapter 5 61 The formulations provided by the Structural Concrete Code (SC-BOE) [20] and the Eurocode 2: EN 1992-1-1 (EC-02) [198] were used to predict drying shrinkage in RAC concretes. The calculation method used was described in a previous paper [126]. In order to determine the shrinkage value following the Structural Concrete Code (SC-BOE) [20], the following factors should be considered: the compressive strength at 28 days; concrete specimen size; ambient RH; and the type of cement (the CEM IIAL was considered high early strength (Class CR); the CEM II/AS; and CEM IIIB cements were considered ordinary early strength (Class CN)). However, it must be noted that the SC-BOE does not consider the use of RCA. However, the use of RCA to estimate the shrinkage value is considered in Eurocode 2: EN 19921-1 (EC-02) [198]. The influence of CRCA and FRCA was calculated by applying a specific factor (shRA) in the formula to determine the drying shrinkage of RAC. The shRA is described as 1+0.8 αRA, where αRA represents the ratio between the recycled aggregates quantity (CRCA and FRCA) and the total quantity of aggregates (coarse and fine aggregates) employed. This factor (shRA) is applied when the RCA is employed in replacement of 20-40% of NAs (0.20 < αRA  0.40) [126,198]. In this research work, the αRA factors was defined by 0.27, 0.31, and 0.36 for the RACC50, RAC-C50-F10, and RAC-C50-F20 concretes, respectively. To demonstrate the effectiveness of the codes in predicting drying shrinkage in concrete, Figure 5. 4 illustrates the ratio between the experimentally obtained drying shrinkage value of each concrete and the value determined using the following standards: a) Structural Concrete Code (SC-BOE) and b) Eurocode 2: EN 1992-1-1 (EC-02). 0 0.2 0.4 0.6 0.8 1 1.2 1.4 NAC₀₅₁ RAC-C50 RAC-C50-F10 RAC-C50-F20 Ratio Drying shrinkage stain all concrtes/SC-BOE CEM II/AL CEM II/AS CEM III/B SC-BOE (SC-BOE) 0 0.2 0.4 0.6 0.8 1 1.2 1.4 NAC₀₅₁ RAC-C50 RAC-C50-F10 RAC-C50-F20 Ratio Drying shrinkage strain all concretes/EC-02 CEM II/AL CEM II/AS CEM III/B EC-O2 (EC-02) (a) (b) Figure 5. 4 Shrinkage estimation analysis depicted through ratios of a) experimental results/ numerical estimation SCBOE and b) experimental results/numerical estimation EC-02 Chapter 5 62 According to Figure 5. 4-a, the Spanish Structural Concrete Code (SC-BOE) is not exact in the estimation of NAC-0.51 concrete's shrinkage value. It has been observed that the concrete produced with CEM IIAL and CEM IIAS cements achieved a 10-15% lower shrinkage value than the value estimated by SC-BOE. However, the concrete produced with CEM IIIB cement achieved a 15% higher shrinkage value than that of the value estimated for SC-BOE cement. Although the type of cement was considered in the drying shrinkage calculation for the SC-BOE, it was not considered in a higher early shrinkage caused by high BFS content cement (CEM IIIB), which can influence total drying shrinkage [118,119]. Moreover, the compressive strength at 28 days is the primary parameter considered in SCBOE estimation; this proved to be similar in all NAC-0.51 concretes. However, as Figure 5. 4-a indicates more parameters besides the compressive strength should be considered. Revilla-Cuesta [129] suggested that a partial correction coefficient should be used for every change in concrete composition, including aspects such as the type of concrete (vibrated, high-performance, or selfcompacting), the content of RA, the maturity of the RA, and the addition of an alternative binder. The SC-BOE adequately estimates the shrinkage values for RAC-C50 and RAC-C50-F20 concretes made with CEM IIAL as well as the RAC-C50 and RAC-C50-F10 concrete made with CEM IIAS as the use of RCA slightly increased the shrinkage value of concretes. However, the RCA-C50F20 made with CEM IIAS achieved a 38% higher shrinkage value than the value estimated by SCBOE. The SC-BOE estimated adequately the shrinkage value of RCA-C50-F20 made with CEM IIAL as it achieved a lower strength than any concrete produced with this cement. Consequently, it can be stated that the SC-BOE can adequately estimate the drying shrinkage of concrete produced with 50% CRCA and up to 10% FRCA. However, it estimates a lower shrinkage rate than the value obtained experimentally when 50% CRCA and 20% FRCA are employed in concrete production. In addition, all the concretes made using CEM IIIB reached a higher shrinkage rate than estimated by SC-BOE. Several researchers have reported that the code estimations could create a ± 30% dispersion in the results [54,129]. Moreover, this difference increased when recycled aggregates were used. As mentioned previously, concrete with a high BFS content exhibits higher early shrinkage, which can influence total drying shrinkage [118,119]. Furthermore, the specimens were placed in a climatic chamber after a short period of curing (after 1 day of casting) [118], which also influenced the increase in experimentally obtained shrinkage values. Figure 5. 4-b describes the ratio between the experimental results and the values determined by EC-02. The values estimated by EC-02 for concrete produced with CEM IIAL and CEM IIAS were higher than those obtained experimentally. However, similar to SC-BOE, the NAC-0.51 concrete produced using CEM IIIB, EC-02 estimated a lower shrinkage value than it achieved experimentally. Moreover, as mentioned above, EC-02 considers shrinkage increase as a factor due to the use of recycled aggregates. Consequently, the EC-02 prediction of RAC drying shrinkage is more accurate for the experimental results than the values obtained by the SC-BOE. Chapter 5 63 5.3.3. Chloride Ion Penetration Table 5. 3 describes the chloride ion penetrability values and their standard deviation (values given between brackets) of produced concrete mixtures measured at 28 and 56 days of curing. The ASTMC1202 test classified the chloride ion penetrability as low (1000-2000 Coulomb), moderate (2000-4000 Coulomb), and high (> 4000 Coulombs of total passed charge) [209]. This research found that chloride ion penetrability varied significantly according to the type of cement used, as several researchers have stated [113,210–212]. In addition, a direct correlation was observed between the percentage of recycled aggregate replacement ratio and chloride ion penetrability. This is a fact also defined in previous research works [9,40]. Table 5. 3 Chloride ion penetrability and the standard deviation (described in brackets) determined in Charge pass in coulombs. 3 IIAL IIAS IIIB (Coulombs) Δ (%) (Coulombs) Δ (%) (Coulombs) Δ (%) 28d 56d 28d 56d 28d 56d NAC-0.51 5314(2) 4096(271) 23 2897(111) 1976(129) 32 674(15) 501(12) 26 RAC-C50 4479(441) 4065(71) 9 2535(136) 1962(80) 23 610(9.0) 503(8) 18 RAC-C50-F10 6038(596) 4448(97) 26 3130(58) 2293(5) 27 626(16) 531(15) 15 RAC-C50-F20 6401(569) 4944(178) 23 4515(91) 2866(66) 37 740(40) 532(18) 28 () Standard deviation. Δ (increase of resistance) Figure 5. 5 shows the ratio between the charge passed from each concrete produced with respect to 4000 coulombs (the maximum value considered a moderate corrosion risk concrete). Figure 5. 5 a,b describe the data at 28 and 56 days, respectively. Figure 5. 5-a shows that all concretes manufactured with CEM IIAL exhibit high values of chloride ion penetration. The addition of BFS to cement reduced the ion penetrability of the concrete as Kopecký and Balázs et al. [212], stated. Chapter 5 64 (a) (b) Figure 5. 5 The ratio of chloride ion penetrability (determined in charge pass) for all concretes with respect to maximum value of 4000 Coulombs: a) 28 days and b) 56 days Based on the influence of RCA use, the RAC-C50 achieved lower chloride ion penetrability than that of NAC, independent of cement type. However, it must be mentioned that the RAC-C50 and NAC0.51 were produced with effective water-cement ratios of 0.47 and 0.51, respectively. In agreement with the study conducted by Kopeckó and Balázs et al. [212], it was shown that an increase in the w/c ratio leads to an increase in the depth of chloride penetration, while keeping the same cement content constant. Moreover, when FRCA was employed for concrete production, and more evidently with the use of 20% FRCA in the replacement of natural sand, the chloride ion penetrability increased. In addition, this was more evident when cement without BSF (CEMII AL) or low BSF (CEM IIAS) was used for concrete production. The high porosity and microcracks of the old mortar are present on the RCA surface, resulting in the increased permeability of chloride ions [209,213]. Researchers have demonstrated that RAC exhibits more capillary channels than NAC, these are primarily attributed to the introduction of interfacial transition zones (ITZs) between natural aggregates and old cement mortars, as well as the presence of microcracks in the RCA [40,87]. However, Etxeberria et al. [214] have demonstrated that the total charge passed value for all concretes mixed using CEM IIIB cement with different percentages of recycled mixed aggregates (volumes of 0%, 25%, 50% and 100%) ranged from 800 to 1400 coulombs. The authors have also demonstrated that an adequate cement type was necessary to increase chloride ion penetration resistance in concrete production. Sim and Park [91] concluded that the incorporation of FRCA had a minimal impact on chloride ion penetration. They observed that the type of cement used had a more significant influence on concretes performance than the quantity of recycled aggregates. In addition, Table 5. 3 shows that the standard deviation of concretes produced with CEM IIAL was higher than that produced with the CEM IIAS. In addition, the concretes produced with CEM IIIB achieved the lowest deviation standard. These findings highlight variability in concrete properties due to different cement types. Notably, CEM 0 0.25 0.5 0.75 1 1.25 1.5 1.75 NAC₀₅₁ RAC-C50 RAC-C50-F10 RAC-C50-F20 Ratio Chloride Ion Penetration all concretes/4000(ASTMC1202) CEM-IIAL CEM-IIAS CEM-IIIB ASTMC1202 (High) ASTMC1202(Low) ASTMC1202( Moderate) MODERATE HIGH LOW 0 0.25 0.5 0.75 1 1.25 1.5 1.75 NAC₀₅₁ RAC-C50 RAC-C50-F10 RAC-C50-F20 Ratio Chloride Ion Penetration all concretes/4000(ASTMC1202) CEM-IIAL CEM-IIAS CEM-IIIB ASTMC1202 (High) ASTMC1202(Low) ASTMC1202( Moderate) HIGH MODERATE LOW Chapter 5 65 IIAL and CEM IIAS exhibited relatively high standard deviations, however, to ensure accurate values, more than two should be used. After a curing period of 56 days (see Figure 5. 5-b), the chloride penetration resistance increased in all the concretes. However, all the concretes produced using CEM IIAL, including NAC, still had very high chloride ion penetrability values. This fact can be attributed to the limestone base of CEM IIAL concrete [215] which had higher chloride ion permeability than those mixes with a higher replacement of SCM [215,216]. As a consequence, it was concluded that CEM IIAL cement was unsuitable for defined application due to its limited ability to resist chloride ion penetration. The obtained results of chloride penetrability in this work were slightly lower than those determined by Etxeberria and Castillo [217], in which the concrete produced with 50% coarse RCA and the same type of cement with effective water-cement ratio of 0.50 and a cement content 350 kg/m3 obtained 8799 C at 28 days and 6377 C at 56 days. In concrete produced using CEM IIAS, an improvement in chloride ion penetration resistance was observed from 28 to 56 days, with a range between 23% and 37% in all samples. This fact demonstrates that all concrete mixtures achieved a moderate level of resistance in terms of chloride ion penetration. In addition, all the concretes produced using type CEM IIIB cement had low chloride permeability at 28 and 56 days, independently of the percentage of RCA employed. As mentioned above, BFS cement enhances chloride penetration resistance in concrete due to its ability to immobilize chloride ions [93,94]. 5.3.4. Carbonation Resistance Table 5. 4 summarises the carbonation depth (in mm) and its standard deviation (between brackets), which was determined by testing each produced concrete after 91 days of exposure to 3% CO2, 57% RH and 20ºC. Although the NAC-0.51 concretes achieved the lowest carbonation depths (in each type of cement concretes), the RAC-C50 and RAC-C50-F10 concretes reached similar values to that of NAC-0.51 concrete, with the exception of the RAC-C50-F10 concrete produced with CEM IIAL, which had a 12.9% higher carbonation depth than the corresponding NAC-0.51. According to Guo et al. [40], the RAC and NAC achieved similar resistance and carbonation depth when the RAC was produced with a lower w/c ratio. Table 5. 4 Carbonation depth, their standard deviation, and the accelerated and theorical natural carbonation coefficient of all concretes Concrete types Carbonation depth(mm) at 90 days Carbonation coefficient kacc (mm/day0.5) knatTHEO (mm/year0.5) II AL II AS III B II AL II AS III B II AL II AS III B NAC-0.51 7.7(0.1) 6.0(0.1) 12.0(0.4) 0.81 0.65 1.22 1.84 1.48 2.79 RAC-C50 8.0(0.4) 6.1(0.2) 12.1(0.2) 0.84 0.68 1.27 1.9 1.55 2.89 RAC-C50-F10 8.7(0.2) 6.3(0) 12.1(0.1) 0.97 0.68 1.28 2.19 1.55 2.92 RAC-C50-F20 9.8(0.0 7.2(0.2) 12.9(0.1) 1.04 0.78 1.39 2.37 1.77 3.16 Chapter 6 72 the concrete was determined by combining effective water and the water within the aggregates (humidity plus the amount of effectively absorbed water). Table 6. 1 shows the mix proportions of the produced concretes; combinations of 50% and 60% CRCA with 0%, 10% and 20% FRCA were used. The described mixtures were produced using three types of cement (CEM II/AL, CEM II/AS and CEM III/B). As mentioned above, these substitution levels were validated in a previous study by Vintimilla and Etxeberria et al. [126] for structural concrete use. Table 6. 1 Mix proportions of concretes were produced with CEM II/AL, CEM II/AS and CEM III/B Materials Concrete types (kg) NAC0.51 NAC0.47 RACC50 RAC-C50F10 RACC50-F20 RCAC60 RCAC60-F10 RCAC60-F20 Cement 300 300 300 300 300 300 300 300 Total water 165 150 175.8 179.5 182.3 180.8 184.34 187.48 CNA 1 354.5 360.1 180.5 180.5 180.5 144.4 144.4 144.4 CNA 2 723.68 737.2 369.3 369.3 369.3 295.5 295.5 295.5 FNA 954.1 971.9 1014.2 875.7 778.4 973 875.7 778.4 CRCA 1 - - 165.8 165.2 165.6 198.9 198.6 199.7 CRCA 2 - - 338.8 339.1 337.2 406.0 406.1 404.2 FRCA - - - 87.1 174.1 - 87.1 174.1 P (%) 1/0.7 1 1 1/0.6 1 1/0.5 1 1/0.3 1 1/0.3 1 1/0.3 1 1/0.3 1 S (%) 1 1 1/1.5 1 1/1.5 1 1/1.5 1 1/1.5 1 1/1.5 1 1/1.5 1 effective w/c 0.51 0.47 0.47 0.47 0.47 0.47 0.47 0.47 Slump-IIAS (mm) 175 145 150 155 150 175 155 160 Slump-IIIB (mm) 175 160 135 150 150 140 150 125 Slump-IIAL (mm) 175 150 190 200 195 180 210 195 1 Plasticizer content utilized in CEM II/AL Table 6. 1 shows the slump values (determined according to the EN 12350-2 [203] specification) of the produced concretes. The concretes made with CEM II/AL exhibited a liquid consistency, and a greater amount of superplasticizer (P) was used than in the corresponding concrete produced with IIAS and IIIB cement. In order to control the consistency, the concretes produced with IIAS and IIIB were produced using a lower superplasticizer content and a slightly higher plasticizer content (1% S and 1% P), achieving a fluid consistency, as defined by the Structural Concrete Code (SC-BOE) [20]. The concrete produced with type IIIB cement achieved a lower slump value than the corresponding concrete produced using IIAS, with the same amount of admixtures. In addition, the concrete produced using FRCA achieved a similar or higher slump value than that of the concrete produced using only CRCA. Chapter 6 73 The concrete mixtures were prepared in a vertical axis mixer, and the aggregates were added first in order of size. They were mixed for 1 min, after which cement, followed by water and chemical admixtures, was added while the mixing process continued. After mixing for an additional minute, the concrete specimens were manually compacted and covered with plastic for 24-h of curing. Subsequently, the specimens were demolded and stored at 20 ± 2 °C with a relative humidity of ≥ 95% until 1 h before testing. All of the test elements were kept in the same conditions. 6.3. Results and Discussion 6.3.1. Compressive strength The compressive strength values (fcm, cub100) and their standard deviations for cubic concrete specimens (100 × 100 × 100 mm) are presented in Table 6. 2. The specimens, designed for exposure classes XC1–XC4 and XS1 with strength class C30/37, must meet the minimum characteristic and average strength values of 30 MPa and 38 MPa, respectively, for cylindrical specimens (150 × 300 mm), according to the Structural Concrete Code (SC-BOE) [20]. According to the differences in geometries and stress distributions between cubes and cylinders, Vintimilla and Etxeberria et al. [126] determined, in compliance with SC-BOE [20],[184], that the minimum average compressive strength for 100 mm cubes (fcm,cub100) should be 46 MPa at 28 days. In addition, the standard deviations were acceptable, with a higher dispersion at 7 days that decreased by 28 and 56 days, indicating reliable measurements. Table 6. 2 Compressive strength and its standard deviation (values in brackets) in all of the produced concrete. Concrete Reference IIAL IIAS IIIB 7d 28d 56d 7d 28d 56d 7d 28d 56d NAC-0.47 52.5(1.3) 62.9(1.3) 65.5(1.0) 54.5(1.3) 69.8(1) 71.3(1.9) 53.1(0.8) 67.2(0.4) 69.9(0.9) NAC-0.51 45.2(2.0) 56.2(1.6) 58.8(1.0) 54.1(2.0) 59.2(0.5) 64.7(0.2) 51.5(0.1) 57.2(1.2) 59.9(0.9) RAC-C50 48.6(2.5) 57.3(1.0) 59.9(1.7) 53.9(2.5) 59.2(2.3) 62.8(1.2) 53.7(0.3) 61.4(3.0) 62.0(1.0) RAC-C50-F10 46.9(1.8) 56.3(1.5) 57.5(0.3) 52.4(1.8) 59.7(1.3) 59.9(0.2) 53.4(2.8) 60.6(1.7) 61.6(0.7) RAC-C50-F20 44.9(2.4) 52.7(0.3) 53.8(0.4) 50.2(2.4) 60.7(0) 63.7(1.2) 53.6(1.5) 62.8(1.4) 62.9(1.3) RAC-C60 46,3(0.1) 54.8(0.8) 58.8(0.3) 51.6(1.2) 60.2(0.9) 60.9(0.2) 50.7(0.9) 58.8(0.6) 59(0.1) RAC-C60-F10 43.5(0.6) 54.7(1.0) 56.1(0.0) 49.8(0.5) 59.2(0.4) 62.2(0.3) 48.2(1.6) 58.2(2.0) 64.3(1.9) RAC-C60-F20 44.1(0.6) 54.6(0.8) 54.8(0.1) 45.1(0.9) 58.7(0.8) 61.7(2.0) 47.5(1.2) 61.4(0.5) 64.1(0.9) Compressive strength values for 100 mm cube specimens. () Standard deviation The compressive strength of RACs was 18% lower than that of NAC-0.47, even though both were produced with the same effective water–cement ratio of 0.47, as shown in Table 6. 2. However, RAC’s compressive strength was comparable to that of NAC-0.51, which had a higher effective Chapter 6 74 water–cement ratio of 0.51 (see Figure 6. 1). These results illustrate the need to adjust the water– cement ratio to attain compressive strength similar to that of NAC-0.51 due to the significant impact of recycled aggregates on concrete’s mechanical properties. As previously demonstrated [41,105], due to the higher porosity and Los Angeles coefficient (weaker particles) of the RCA than those of NA, the RAC produced with 50% CRCA could achieve similar mechanical properties to those of NAC when produced with a lower water−cement ratio. The compressive strength of RACs is influenced by the proportion of CRCA and FRCA employed in concrete production [221–223]. For instance, mixes with 50% CRCA (RAC-C50) generally exhibit better performance than those with 60% CRCA (RAC-C60), due to the inferior mechanical quality of the CRCA and potential weaknesses in the old interfacial transition zone (ITZ) or adhered mortar [221,224]. The RAC produced using 10% FRCA achieved acceptable strength, likely owing to the enhanced compaction and improved bonding of the fine fraction with the cement paste [126,190]. This effect was particularly pronounced when RACs were produced using CEM III/B and CEM II/AS cements, which enhance matrix densification and chemical bonding due to their SCMs [112] and the presence of Portland cement mortar in FRCA. Table 6. 2 demonstrates that the RACs made with CEM II/AL cement obtained a slightly lower compressive strength than those made with CEM II/AS and CEM III/B cements despite having the same water−cement ratio. This difference is attributed to the seasonal production periods; RACs with CEM II/AL were produced in spring/summer, whereas the others were manufactured in autumn/winter, impacting the setting and curing processes of the concrete [204]. Figure 6. 1 a–c illustrate the compressive strength ratios of each RAC relative to conventional concrete (NAC-0.51) at 7, 28, and 56 days. In addition, NAC-0.47 achieved a 3% to 17% higher strength than NAC-0.51 at various ages. (a) CEM II/AL 42.5 R (b) CEM II/AS 42.5 N/SRC Chapter 6 75 (c) CEM III/B 42.5 N-LH/SR Figure 6. 1 Relative compressive strength at all concrete ages with cements: a) Type CEM II/AL, b) Type CEM II/AS, c) Type CEM III/B Figure 6. 1a (concrete produced using IIAL cement) shows that RAC-C50, RAC-C50-F10, and RAC-C60 achieved similar values to NAC-0.51 at all ages, while RAC-C50-F20 showed a decrease of up to 8.5% at 56 days. Additionally, RAC-C60-F10 and RAC-C60-F20 exhibited reduced early compressive strength (7 days), but this difference was lower at 56 days, achieving a 7% lower strength than NAC-0.51. Figure 6. 1b shows the results for concretes produced with CEM II/AS cement. The concretes produced using 10–20% FRCA obtained a slightly lower strength at 7 days; however, at 28 days, all RACs, including RAC-C60-F20, achieved similar strength to NAC-0.51. At 56 days, the compressive strength of all RACs was between 3% and 7.5% lower. All of the RAC produced using IIIB cement achieved a similar or higher strength than that of NAC-0.51 at 28 and 56 days (see Figure 6. 1c). At 7 days, the concretes produced using 60% CRCA and fine aggregates (10%, 20%) showed lower compressive strength than NAC-0.51. Furthermore, as discovered in previous studies [105,126], the RAC-C50-F20 concrete (produced with 50% CRCA and 20% FRCA) achieved similar strength to RAC-C50 (using natural sand) and NAC0.51. However, several researchers [51,62–64] have suggested that incorporating up to 30% FRCA as a replacement for natural sand could still achieve satisfactory properties. As mentioned, all of the RACs produced, including RAC-C60-F20, with an effective w/c ratio of 0.47, achieved a similar strength to NAC-0.51 at 28 days. Although the inclusion of FRCA slightly reduced the early strength, which was probably due to the higher water amount caused by the higher WA capacity (old mortar was the main component [225,226]) and weaker bonding of the fine recycled aggregates [51,88,221], this effect was mitigated as the concrete matured, reaching compressive strengths comparable to that of conventional concrete (NAC-0.51) at 56 days. All mixtures achieved a compressive strength of C30/37, which is suitable for structural applications. Chapter 6 76 6.3.2. Drying Shrinkage Figure 6. 2 a–c,d–f illustrates the values of drying shrinkage (µƐ) and mass loss (%) over 91 days for NAC-0.51 and the RACs produced using CEM II/AL, II/AS, and III/B. In addition, the standard deviation of the shrinkage values are described by the vertical lines. −4.0 −3.0 −2.0 −1.0 0.0 0 7 14 21 28 35 42 49 56 63 70 77 84 91 Mass Loss (%) Time (days) NAC-0.51 RAC-C50-F20 RAC-C50 RAC-C60-F20 a) d) CEM II /AL 42.5 R −4.0 −3.0 −2.0 −1.0 0.0 0 7 14 21 28 35 42 49 56 63 70 77 84 91 Mass Loss (%) Time (days) NAC-0.51 RAC-C50 RAC-C50-F10 RAC-C50-F20 RAC-C60 RAC-C60-F10 RAC-C60-F20 b) e) CEM II /AS-42.5 Chapter 6 77 −4.0 −3.0 −2.0 −1.0 0.0 0 7 14 21 28 35 42 49 56 63 70 77 84 91 Mass Loss (%) Time (days) NAC-0.51 RAC-C50 RAC-C50-F10 RAC-C50-F20 RAC-C60 RAC-C60-F10 RAC-C60-F20 c) f) CEM III/B-42.5 Figure 6. 2 Drying shrinkage value development and mass loss at 91 days and their standard deviation: (a) and (d) CEM II/AL, (b) and (e) CEM II/AS, (c) and (f) CEM III/B. All concretes produced using CEM II/AL (see Figure 6. 2a) achieved similar shrinkage values (from −496.8 to −576.7 µm/m) at 91 days. The use of FRCA slightly increased the shrinkage value; the RAC-C50-F20 and RAC-C60-F20 concretes exhibited 13.3% and 16.1% higher drying shrinkage, respectively, than that of NAC-0,51, while the RAC-C50 concrete showed a 7.3% higher drying shrinkage than that of NAC-051 (the RAC-C50-F10, RAC-C60, and RAC-C60-F10 concretes were not tested). The concretes produced using CEM II/AS (Figure 6. 2b) achieved shrinkage values between −318.8 and −501.2 µm/m. The RACs with up to 60% CRCA and 10% FRCA obtained between 15.8% and 19.7% higher drying shrinkage values than NAC-0.51. In addition, the RAC-C50-F20 and RACC60-F20 concretes had similar behaviors, with shrinkage values up to 57.2% higher than that of NAC0.51. However, the values obtained in the RACs were lower than those obtained in concretes produced using IIAL. All of the concretes produced with CEM III/B (Figure 6. 2c) obtained similar drying shrinkage values ranging from −437.3 to −534 µm/m. RAC-C50 and RAC-C60 achieved 3% and 5% higher shrinkage values, respectively, than NAC-0.51. The use of FRCA increased the shrinkage value of concrete, while RAC-C50-F10, RAC-C50-F20, and RAC-C60-F10 showed 11%, 12%, and 13.4% higher shrinkage values than that of NAC-0.51, and RAC-C60-F20 obtained a 22.1% higher shrinkage value than that of NAC-0.51. All shrinkage values measured for the RACs were within the acceptable limits according to the guidelines of the American Concrete Institute (ACI) [122], which stipulate an ultimate shrinkage strain Chapter 6 78 of −780 to −800 μm/m, establishing this range as the threshold for drying shrinkage in conventional concrete when a high w/c ratio is used. During the first four days of curing, concretes with lower clinker content (CEM III/B) exhibited the highest shrinkage values (Figure 6. 2c). NAC-0.51 with CEM III/B, CEM II/AL, and CEM II/AS reached −200 µm/m, −170 µm/m, and −110 µm/m, respectively, at 4 days, and it was stabilized over 28 days. Early age drying shrinkage tends to stabilize over time, especially with an SCM as a binder [51,105,118,119]. The shrinkage value increased with higher RCA content, with similar deviations to those of NAC, though with moderate variations in most cases. Figure 6. 2d–f illustrates the mass loss percentages for the produced concrete mixes. The NAC0.51 mixes showed mass losses of 2.2% with CEM II/AL, 2.3% with CEM II/AS, and 1.9% with CEM III/B. As expected, the mass loss was greater in mixes with a higher content of RCA (as a consequence of a higher total water amount). The RACs produced using CEM II/AL achieved a mass loss of up to 3.5%, the CEM II/AS mixes obtained values between 2.8% and 3.8%, and the CEM III/B mixes obtained mass losses from 2.86% to 3.4%. These results align with those of prior studies [105,126,159,208,227], confirming that greater drying shrinkage correlates with increased mass loss across similar cement types. In order to validate the drying shrinkage value prediction according to codes, the Structural Concrete Code (SC-BOE) [20] and Eurocode 2: EN 1992-1-1 (EC-02) [50] were used to predict drying shrinkage in RACs, as described in a previous study [126]. SC-BOE [20] does not consider the use of RCA. However, factors such as the compressive strength at 28 days, concrete specimen size, ambient RH, and cement type are considered. CEM IIAL was classified as having a high early strength (Class CR), while CEM II/AS and CEM III/B were classified as having an ordinary early strength (Class CN). Eurocode 2 (EC-02) [50] considers RCA to estimate shrinkage. The influence of CRCA and FRCA was calculated using a factor (ηshRA) in the formula for the drying shrinkage of RAC, defined as 1 + 0.8 αRA, where αRA is the ratio of recycled aggregates (CRCA and FRCA) to total aggregates. This factor is applied when the RCA replaces 20–40% of the NAs (0.20 < αRA ≤ 0.40) [50,126]. In this study, the αRA values were 0.27, 0.31, and 0.36 for the RAC-C50, RAC-C50-F10, and RAC-C50-F20 concretes, respectively, while for the RAC-C60, RAC-C60-F10, and RAC-C60-F20 concretes, the values were 0.32, 0.37 and 0.42. Figure 6. 3 illustrates the ratio between the experimentally obtained drying shrinkage value of each concrete and the value determined using the formulations provided by a) the Structural Concrete Code (SC-BOE) [20] and b) Eurocode 2: EN 1992-1-1 (EC-02) [50]. Chapter 6 79 a) SC-BOE b) EC-02 Figure 6. 3 Shrinkage estimation (a) experimental results/numerical estimation (SC-BOE); (b) experimental results/numerical estimation (EC-02). According to Figure 6a, SC-BOE [20] does not accurately estimate the shrinkage value of NAC0.51 concrete. In order to estimate the shrinkage value, the parameters of the compressive strength at 28 days (as the primary parameter) and the type of cement were considered. However, it was observed that, although the type of cement is considered, SC-BOE does not adequately assess the shrinkage of CEM III/B concrete due to a high early shrinkage caused by the high BFS content in the cement, significantly influencing the total drying shrinkage [118,119]. NAC-0.51 produced with CEM III/B cement had a 15% higher shrinkage value than that estimated using SC-BOE [20]. In addition, NAC0.51 produced with the CEM II/AL and CEM II/AS cements obtained 9% and 15% lower shrinkage values, respectively, than those calculated using the SC-BOE [20]. SC-BOE accurately estimates the shrinkage of the RAC produced using up to 60% CRCA and 10% FRCA with CEM II/AL and CEM II/AS. However, the RAC-C60-F20 concrete made with CEM II/AL and CEM II/AS achieved 6% and 35% higher shrinkage values, respectively, than the value estimated using the code. In addition, all concrete mixtures produced with CEM III/B achieved higher shrinkage values than those estimated with SC-BOE. Researchers have reported that SC-BOE estimations can have a ±30% dispersion, and they are increased in RACs [117,228]. The higher early shrinkage of concrete produced using a high BFS content influences the total shrinkage [118,119]. This highlights the need to consider other factors for shrinkage estimation, such as environmental conditions, cement types and properties, types of aggregates, and mineral additions [117]. Figure 6. 3b shows the ratio between the experimental results and the EC-02 estimates. EC-02 overestimated the shrinkage values of the NAC-0.51 and RAC concretes produced using CEM II/AL and CEM II/AS. Similarly to SC-BOE, EC-02 underestimated the shrinkage value of NAC-0.51 made with CEM III/B. However, EC-02 accurately predicted shrinkage for the RAC with CEM III/B. EC02 considers an increase in shrinkage due to recycled aggregates, making its predictions more accurate than those of SC-BOE for drying shrinkage in RACs. Chapter 6 80 6.3.3. Chloride Ion Penetration The ASTMC1202 test categorizes chloride ion penetrability into low (1000–2000 Coulombs), moderate (2000–4000 Coulombs), and high (>4000 Coulombs) levels [229]. Table 6. 3 presents the average chloride ion penetrability for the concrete mixtures at 28 and 56 days of curing. This study confirms that chloride ion penetrability significantly varies with the type of cement used, which is consistent with previous findings [113,210–212], where a concrete produced with IIAL achieved higher permeability than IIAS concretes and IIIB concretes, which achieved the lowest permeability. In addition, the data in Table 6. 3 reveal that concretes made with CEM II/AL displayed higher variability, as measured with the standard deviation, compared with those with CEM II/AS, while CEM III/B mixtures showed the lowest variability. Moreover, a direct correlation was found between the RCA replacement ratio and chloride ion penetrability, as reported in earlier studies [9,40,51]. Concrete produced with higher percentages of RCA achieved a higher chloride permeability when the same type of cement was used. However, due to the lower effective water−cement ratio employed in RAC compared with NAC-0.51 (all with similar compressive strength), RAC-C50 and even the concrete produced with a higher percentage of RCA achieved higher chloride ion penetration resistance than the corresponding NAC-0.51 concrete. Furthermore, the effect of FRCA on chloride penetration was more evident than that of CRCA due to the higher amount of adhered mortar in FRCA [40]. In addition, NAC-0.47, which was produced with a total water−cement ratio of 0.50 as required for the XS1 environment, exhibited the highest chloride ion permeability resistance. Table 6. 3 Chloride ion penetrability and standard deviation as determined in charge passed in coulombs. IIAL IIAS IIIB Concrete Types (Coulombs) ∆ (%) (Coulombs) ∆ (%) (Coulombs) ∆ (%) 28d 56d 28d 56d 28d 56d NAC-0.47 4451(194) 3971(94) 11 2145(281) 1766(44) 18 530(2) 408(5) 23 NAC-0.51 5314(2) 4096(271) 23 2897(111) 1976(129) 32 674(15) 501(12) 26 RAC-C50 4479(441) 4065(71) 9 2535(136) 1962(80) 23 610(9) 503(8) 18 RAC-C50-F10 6038(596) 4448(97) 26 3130(58) 2293(5) 27 626(16) 531(15) 15 RAC-C50-F20 6401(569) 4944(178) 23 4515(91) 2866(66) 37 740(40) 532(18) 28 RAC-C60 5726(250) 5140(636) 10 2648(71) 2329(28) 12 651(4) 570(4) 12 RAC-C60-F10 6009(197) 5454(33) 9 3425(78) 2736(64) 20 707(44) 668(14) 6 RAC-C60-F20 6549(567) 6048(43) 8 4696(17) 3239(119) 31 843(69) 767(3) 9 Standard deviation (values are given in brackets) Table 6. 4 a,b depicts the ratio between the charge passed by each concrete mix with respect to 4000 Coulombs (the threshold for moderate corrosion risk) at 28 and 56 days, respectively. Figure 6. Chapter 6 81 4a indicates that all concretes made with CEM II/AL exhibit high chloride ion penetration. The incorporation of BFS into the cement reduced the concrete’s ion penetrability [212]. Although the influence of RCA usage was evident, RAC-C50 (produced with an effective w/c ratio of 0.47) demonstrated lower chloride ion penetrability than NAC (produced with an effective w/c ratio of 0.51), regardless of the cement type used, as reported by Kopecký and Balázs et al. [212]. Furthermore, when FRCA was used, particularly with a 20% replacement of natural sand, the chloride ion penetrability increased, mainly when cement without BFS (CEM II/AL) or with low BFS content (CEM II/AS) was employed. Evangelista and de Brito et al. [9] observed that concrete produced using FRCA exhibited lower resistance to chloride ions than NAC. This is attributed to the high porosity and high volume of adhered mortar in the FRCA, which enhance the permeability to chloride ions [213,229]. a) 28 Days b) 56 Days Figure 6. 4 The ratio of chloride ion penetrability (determined in charge passed) for all concretes concerning maximum value of 4000 Coulombs: a) 28 days and b) 56 days Figure 6. 4 also shows that the RAC produced using cement with a greater amount of GGBS achieved the highest chloride ion penetration resistance. Etxeberria et al. [214] reported that concretes mixed with CEM III/B cement and varying recycled aggregate contents (0%, 25%, 50%, 100%) passed charges between 800 and 1400 Coulombs, which was categorized as low chloride ion penetrability. Similarly, Sim and Park [91] noted a minimal impact of FRCA on chloride ion penetration. At 56 days of curing (Figure 6. 4b), the chloride ion penetration resistance of the produced concretes increased. However, the concretes made using CEM II/AL maintained elevated levels of chloride ion penetrability. This heightened permeability is primarily attributed to the limestone content in the CEM II/AL concrete, which facilitates greater chloride ion migration compared with that in mixes utilizing higher supplementary cementitious material (SCM) substitution [215,216]. The chloride penetrability values observed in this study were marginally lower than those reported by Etxeberria and Castillo [217], where concrete composed of 50% coarse RCA and identical cement— maintaining an effective water−cement ratio of 0.50 and a cement content of 350 kg/m³—demonstrated chloride values of 8799 Coulombs at 28 days and 6377 Coulombs at 56 days. In contrast, 4 shows that Chapter 6 88 The RAC-C50 concretes achieved less than a 3% increase in Kacc compared with NAC-0.51 in every cement type. The RAC-C60 concretes also showed a similar or slightly superior carbonation rate to that of NAC-0.51, except for the concrete produced with IIAS, which achieved 20% higher Kacc than that of NAC-0.51. These trends are consistent with findings from prior research [101,114,217,237]. In addition, the use of 10% FRCA with 50% CRCA in RAC-C50-F10 resulted in a slight 5% increase in Kacc, except for the one produced with CEM II/AL, which achieved a 17% increase, compared with the NAC-0.51 concrete. In contrast, the RAC-C60-10 concretes produced with the CEM II/AL, CEM II/AS, and CEM III/B cements achieved 21%, 24%, and 5% higher Kacc values than that of the corresponding NAC0.51. Moreover, the use of 20% FRCA in the substitution of natural sand led to substantial increases in Kacc in comparison with the values obtained for NAC, especially in concretes produced with IIAL and IIAS cement, which achieved up to 25% higher Kacc values than those of NAC-0.51 concretes. The lower performance of RAC than that of NAC can be attributed to the higher porosity of recycled aggregates, a factor that was especially pronounced in the case of FRCA [51]. The Kacc value can be employed to estimate the theoretical natural carbonation coefficient (knatTHEO) (Table 8) [218,219] of each type of concrete produced using Eq.6 (3). Kacc KnatTHEO=(∅acc)0.5 (∅natTHEO)0.5, Eq.6 (3) Here, Kacc and knatTHEO are the CO2 concentrations in the accelerated carbonation (3%) and natural carbonation processes (430 ppm, in Barcelona), respectively. The obtained values of KnatTHEO were compared with the natural carbonation rate that was determined experimentally. 6.3.7. Natural Carbonation Resistance After four days of curing, the produced concrete specimens were exposed for one year under natural environmental conditions in Barcelona with an average of 431 ppm CO2, 56.8% RH, and 20.1 °C, as illustrated in Figure 6. 8. The average carbonation depth data after one year of exposure and the calculated natural carbonation rate (knat) for the NAC and RAC concretes are described in Table 6. 6. Chapter 6 89 0 100 200 300 400 500 600 0 10 20 30 40 50 60 70 80 RH (%)& T (ºC) RH (%) Temp (ºC) CO2 (ppm) CO2 Concentration (ppm) Figure 6. 8 Environmental conditions for natural carbonation. The NAC-0.51 concretes achieved a similar or slightly higher carbonation depth than that of NAC0.47 and RAC in each corresponding cement type. The influence of the cement type was apparent; concretes made with the CEM III/B cement showed significantly higher carbonation depth, followed by those with CEM II/AL. Moreover, concretes with CEM II/AS achieved the lowest carbonation depth, which was similar to the accelerated carbonation process. In concrete produced using the IIIB cement, with a large amount of SCMs, due to the reduction of portlandite (available CaO) and the lower pH buffering capacity, the susceptibility to carbonation was increased [109]. While CEM II/AS and CEM II/AL have similar clinker amounts (approximately 88%), IIAS has more hydraulic CaO available, which gives it a higher carbonation resistance than IIAL. The IIAL contains 10% limestone filler [109,238–240]. Chapter 6 90 Table 6. 6 Carbonation depth and natural carbonation coefficient of all concrete Mixtures Carbonation depth(mm) at 1 year Carbonation coefficient knat (mm/year0.5) II AL II AS III B II AL II AS III B NAC-0.47 4.2(0) 3.5(0.04) 5.1(0.09) 3.82 3.16 5.08 NAC-0.51 4.2(0.02) 3.7(0.04) 5.7(0.11) 4.24 3.50 5.55 RAC-C50 4.2(0.02) 3.5(0.42) 5.2(0.15) 3.89 3.32 5.19 RAC-C50-F10 4.2(0.15) 3.7(0.04) 5.3(0.1) 3.97 3.50 5.33 RAC-C50-F20 4.2(0.09) 3.6(0.09) 5.4(0.31) 3.93 3.49 5.45 RAC-C60 4(0.11) 3.5(0.04) 5.1(0.04) 3.68 3.18 5.12 RAC-C60-F10 4.1(0.02) 3.5(0.02) 5.1(0.04) 3.63 3.21 5.15 RAC-C60-F20 4.2(0.15) 3.7(0.15) 5.2(0.04) 3.90 3.40 5.05 Figure 6. 9 describes the ratio between the Knat coefficient of RAC with respect to that of NAC0.51 (concretes with similar compressive strength) for each corresponding cement type. It shows that the RAC achieved a lower Knat value than that of NAC-0.51. This behaviour differed from the accelerated carbonation process in which the Kacc value of RAC was higher than that of NAC-0.51. The concrete specimens were exposed to natural carbonation after 4 days of curing, which was probably due to water inside the RCA, which helped with internal curing and reduced the carbonation rate [241–243]. In addition, the higher calcium availability in RAC could influence the improvement of the carbonation resistance of RAC with respect to NAC-0.51. Figure 6. 9 Ratio of the RAC Knat to the NAC-0.51 Knat. Chapter 6 91 6.3.8. Knat vs KnatTHEO Table 6. 7shows the ratio between KnatTHEO (see Table 6. 5) and Knat (see Table 6. 6) of the concretes obtained through accelerated and natural carbonation processes, respectively. The results indicate that the Knat coefficient was between 2.0 and 2.8 times higher in NAC and between 1.6 and 2.4 times higher in RAC compared with the KnatTHEO coefficient value. These findings align with those of other studies [115], which report that the experimentally determined natural carbonation coefficient (Knat) is 1.6 to 1.8 times higher than the carbonation rate estimated using an accelerated test. The concretes were exposed to natural carbonation between May and June (at 20–26 °C and 55% RH) after only 4 days of curing, which increased the carbonation rate of the concretes and, more importantly, of NAC (RAC may achieve internal curing). Moreover, Knat was determined using only data from 1 year of exposure, and it may be reduced after a longer period of exposure [244]. Table 6. 7 Relationship between Natural Carbonation and Accelerated Carbonation in Concretes with NA and RCA knat and knatTHEO Relationship MIX CEM II/AL CEM II/AS CEM III/B NAC-0.47 2.5 2.8 2.5 NAC-0.51 2.4 2.5 2.0 RAC-C50 2.2 2.3 2.0 RAC-C50-F10 1.9 2.4 2.0 RAC-C50-F20 1.8 2.1 1.9 RAC-C60 2.1 1.9 1.9 RAC-C60-F10 1.7 1.9 1.7 RAC-C60-F20 1.7 1.9 1.6 Tests have demonstrated that the carbonation depths in both RAC and NAC increase when samples are cured shortly in a humid room and undergo a long curing process in drier environments, highlighting the importance of curing conditions [79,245]. The natural carbonation, influenced by both the duration of exposure and interaction with realistic environmental cycles, results in deeper penetration of CO2 into concrete compared with that in the controlled environments of accelerated tests [79,246]. Figure 6. 10 visually compares the carbonation depth obtained in the NAC-0.47, NAC-0.51, RACC60 and RAC-C60-F20 (with RAC having the highest percentages) concretes produced using different types of cement under natural conditions over one year versus under accelerated conditions over 91 days. Chapter 6 92 Figure 6. 10 Visual comparison of the carbonation depth in natural (1 year) and accelerated conditions (91 days) for concretes with different cement types. 6.3.9. Carbonation Analysis at 50 and 100 years Table 6. 8 details the predicted carbonation depth for each type of concrete calculated using the Knat value over 50 and 100 years of expected service lives. The Spanish Structural Concrete Code (SC-BOE) [20] specifies the minimum cover depth for concrete structures exposed to XC3 and XC4 environments, corresponding to a design life of 50 and 100 years. For XC3 exposure conditions, a Chapter 6 93 minimum cover of 20 mm and 30 mm is required for a 50 and 100 year lifespan, respectively. For XC4 exposure conditions, the required minimum cover increases to 25 mm and 35 mm, respectively. In addition, EC-02, as shown in Table 11 [50], describes that the minimum cover −given the data from SCBOE [20]− should be increased by +5 mm when recycled aggregates are used. Table 6. 8 Carbonation depth after lifespan of 50 and 100 years. Concrete types knat (50 years) knat (100 years) II AL II AS III B II AL II AS III B NAC-0.47 27.0 22.3 35.9 38.2 31.6 50.8 NAC-0.51 30.0 24.8 39.2 42.4 35.0 55.5 RAC-C50 27.5 23.5 36.7 38.9 33.2 51.9 RAC-C50-F10 28.1 24.8 37.7 39.7 35.0 53.3 RAC-C50-F20 27.8 24.6 38.5 39.3 34.9 54.5 RAC-C60 26.0 22.5 36.2 36.8 31.8 51.2 RAC-C60-F10 25.6 22.7 36.4 36.3 32.1 51.5 RAC-C60-F20 27.6 24.0 35.7 39.0 34.0 50.5 Min. Cover (mm) XC3 20+5* 25+5* XC4 30+5* 35+5* * EC-0.2 recommends increasing the minimum cover by 5 mm when recycled aggregates are used According to the results (Table 6. 8 and Figure 6. 11), all of the concretes produced using CEM II/AL and CEM II/AS achieved 50 years of service life with a minimum cover of 30 mm defined for an XC4 environment (SC-BOE [20] regulations). Although a minimum cover of 20 mm was generally insufficient for XC3 conditions, CEM II/AS adequately met the 50-year service life requirements according to the EC-02 recommendation. In addition, in the concretes produced with CEM II/AS and CEM II/AL, the minimum cover of 35 + 5 mm was adequate for 100 years of service life in an XC4 environment (SC-BOE [20] and EC-02 [50] regulations). In all of the concretes mentioned, the RAC achieved similar or even higher carbonation resistance than the NAC. Figure 6. 11 shows the ratio of carbonation depths after a lifespan of 50 and 100 years (values described in Table 6. 8) with respect to the minimum cover defined by SC-BOE for XC3 (20 mm and 25 mm). In addition, the ratios of the minimum cover requirements of XC3 (EC-02), XC4 (SC-BOE) and X4 (EC-02) with respect to that of XC3 (SC-BOE) are described. Chapter 6 94 0.0 0.5 1.0 1.5 2.0 2.5 Ratio Carbonation depth (50 years)/ 20mm(XC3) CEM II-AL CEM II-AS CEM III-B XC3 (SC-BOE) XC3 (EC-02) XC4(SC-BOE) XC4( EC-02) 0.0 0.5 1.0 1.5 2.0 2.5 Ratio Carbonation depth (100 years)/ 25mm(XC3) CEM II-AL CEM II-AS CEM III-B XC3 (SC-BOE) XC3 (EC-02) XC4(SC-BOE) XC4( EC-02) (a) (b) Figure 6. 11 Comparison of the carbonation depth relative to exposure class XC3 for all concretes (a) 50 years, (b) 100 years. 6.4. Conclusions The following conclusions can be drawn from the results of this study: • This study demonstrates that incorporating up to 60% CRCA and 20% FRCA can result in compressive strength levels comparable to those of NAC by lowering the RAC’s water–cement ratio by approximately 0.04. This underscores the potential of RCA as a sustainable alternative while maintaining structural integrity. Regarding the drying shrinkage: • All RAC produced, with a maximum of 60% CRCA and 20% FRCA and independently of the cement type employed (IIAL, IIAS and IIIB), achieved admissible values at 91 days (maximum of -580 μm/m); • The concrete produced using up to 60% CRCA achieved a similar shrinkage value to that of NAC-0.51 concrete produced with the same cement. However, the concrete using 20% FRCA (with 60% of CRCA) made with CEM II/AL, CEM II/AS, and CEM IIIB achieved 16.1%, 57.2%, and 22.1%, respectively, higher shrinkage than that of the NAC. However, all achieved lower drying shrinkage values than the acceptable values defined by the ACI (up to −800 μm/m); • EC-02 is more accurate than SC-BOE in predicting the drying shrinkage of RAC regardless of the cement type used. Both standards fail to accurately estimate NAC-0.51 produced with CEM III/B, as they prioritize 28-day compressive strength over the initial shrinkage values. This Chapter 6 95 underscores the need for more comprehensive models incorporating environmental factors and different cement and aggregate types. Regarding the durability properties: • RACs with 50% and 60% CRCA (produced with an effective w/c ratio of 0.47) exhibit similar or higher resistance to chloride ion penetration and carbonation in comparison with NAC-0.51 (effective w/c ratio of 0.51) when they have similar compressive strengths; • The concretes (including the RACs) produced with CEM III/B obtained a low chloride concentration at the concrete surface (Cs) and low non-steady state diffusion coefficient (Dnss) values. However, the use of 20% FRCA increased chloride ion penetration; • RAC-C60-F20 achieves high durability in chloride-aggressive environments when it is produced using cements with high BFS content, such as CEM III/B. In addition, it achieves higher chloride resistance than that of NAC produced using CEM II/AL and CEM II/AS; • RAC-C60-F10 produced with CEM II/AS achieves moderate chloride ion penetrability resistance, while any concrete, including NAC, produced with IIAL presents no resistance to chloride ions; • The concretes made with CEM II/AS, followed by CEM II/AL and CEM III/B, achieved the highest carbonation resistance, independent of the type of aggregates used; • The use of 20% FRCA increased the carbonation rate (up to 25%) compared with NAC-0.51 when an accelerated carbonation test was carried out. However, under natural conditions, these RAC concretes exhibited a lower carbonation rate than that of NAC-0.51; • The obtained natural carbonation rate (Knat) values were between 2.0 and 2.8 times higher for NAC and between 1.6 and 2.4 times higher for RAC than the theoretical natural carbonation rate (knatTHEO) obtained from the accelerated carbonation test. Recycled concrete with up to 60% CRCA and 20% FRCA achieved an adequate drying shrinkage value and showed satisfactory durability performance in XC1 to XC4 and XS1 environments, depending on the type of cement used: • RAC-C60-F20 produced with CEM II/AS cement achieved adequate carbonation resistance in XC3 and XC4 environments, ensuring a service life of 50 years. However, for chloride resistance, only concrete with up to 60% CRCA and 10% FRCA exhibited moderate chloride ion penetration values and lower chloride diffusion coefficients (Dnss) than those of CEM II/AL cement; • RAC-C60-F20 produced with CEM II/AL cement, while providing adequate carbonation resistance in XC4 environments, presented high chloride ion penetration and a high Dnss value, with values slightly higher than those of NAC-0.51; Chapter 6 96 • RAC-C60-F20 produced with CEM III/B exhibited low carbonation resistance and very high chloride penetration resistance, as indicated by lower surface chloride concentrations (Cs) and reduced Dnss values, similarly to NAC-0.51. For future research, it is recommended that the long-term durability and resistance to environmental factors such as corrosion and carbonation in structural concretes made using RCA are explored. This investigation should focus on RAC with a similar compressive strength to that of NAC, which can be achieved by adjusting the effective water−cement ratio in RAC mixes. Furthermore, it is crucial to delve deeper into the analysis of concrete produced using 60% CRCA and 20% FRCA, evaluating its behaviour under various environmental and exposure conditions, including real-scale tests. Chapter 7 97 1. 7. Chapter 7. Fine and coarse recycled aggregates-Type B in concrete production Limiting the maximum fine and coarse recycled aggregates-Type B used in structural concrete Authors: Carla Vintimilla, Miren Etxeberria Journal: Construction and Building Materials Publication Date: Volume 459, 17 January 2025, 139791 DOI: https://doi.org/10.1016/j.conbuildmat.2024.139791 ABSTRACT The use of mixed recycled aggregate (MRA) is increasingly being adopted as a sustainable solution to the environmental impact of concrete production. This study investigates the suitability of using varying proportions of fine MRA (FMRA) and coarse MRA (CMRA) classified as type B MRA (MRA with a maximum of 30% masonry waste) to produce structural concrete suitable for exposure to XC1 to XC4 environments. Two experimental phases were conducted. In Phase 1, an effective water-to-cement ratio of 0.48 was employed, along with up to 100% CMRA and up to 25% FMRA, to produce concrete. In Phase 2, the effective water-to-cement ratio was 0.52, with up to 50% CMRA and up to 15% FMRA used in concrete production. In the two phases, 300 kg of CEM IIAL 42.5 R cement was used, and the physical properties (density, absorption and accessible porosity), mechanical properties (compressive strength, splitting tensile strength and modulus of elasticity), shrinkage properties and durability properties (sorptivity and water penetration) were assessed. In addition, the criteria stipulated in Eurocode 2 and in the Spanish structural concrete code (SC-BOE) were used to validate the structural MRA concrete produced. The results indicate that the simultaneous combination of up to 40% CMRA and 15% FMRA in Phase 1 achieves compressive strength and durability properties similar to those of conventional concrete produced with an Chapter 7 104 compressive strength similar to that of NAC. In addition, the strength increased progressively over the curing period (Table 7. 4, data in parentheses). The compressive strength in MRACs exhibited an evolution similar to that observed in NAC1. Employing FMRA in combination with CMRA yielded low compressive strength. The MRACC20-F51 and MRAC-C30-F101 concretes achieved a 15% lower compressive strength than that of NAC1 at day 28 (Table 7. 4& Figure 7. 2-a). However, employing 5% FMRA in producing concrete did not yield a negative impact, as MRAC-C20-F51 achieved an only 5% lower strength than MRACC201, while MRAC-C30-F101 achieved an 8.2% lower strength than MRAC-C301. In addition, the MRAC-C40-F151 and MRAC-C50-F251 concretes achieved a 20% lower strength than the NAC1 concrete. The MRAC-C50-F251 concrete achieved a 7.8% lower strength than MRAC-C501. Table 7. 4 Compressive strength of all concretes produced (increase in compressive strength in %) Phase 1 Mixtures Compressive strength Phase 2 Mixtures Compressive strength 7 d 28d 56d 7 d 28d 56d NAC₁ 55.7 64.3 (15.4) 64.7 (0.6) NAC₂ 44.7 53.9 (20.5) 57.8 (7.4) MRAC-C20₁ 53.5 61.6 (15.1) 61.7 (0.2) MRAC-C20₂ 43.9 48.8 (10.9) 53.1 (8.9) MRAC-C30₁ 53.7 62.5 (16.4) 63.4 (1.4) MRAC-C30₂ 43.9 48.0 (9.3) 53.3 (11.0) MRAC-C50₁ 47.2 57.4 (21.7) 58.5 (1.9) MRAC-C40₂ 40.3 47.0 (16.6) 51.5 (9.6) MRAC-C100₁ 38.1 45.7 (20.0) 46.3 (1.3) MRAC-C50₂ 39.7 46.8 (17.8) 51.2 (9.3) MRAC-C20-F5₁ 48.5 58.3 (20.2) 58.5 (0.3) MRAC-C20-F5₂ 42.4 48.1 (13.6) 54.4 (13.0) MRAC-C30-F10₁ 46.9 57.2 (21.9) 57.8 (1.0) MRAC-C30-F10₂ 40.3 47.6 (18.1) 50.9 (6.9) MRAC-C40-F15₁ 46.6 55.1 (18.2) 55.4 (0.5) MRAC-C40-F15₂ 39.4 45.7 (16.1) 49.6 (8.4) MRAC-C50-F25₁ 44.6 52.3 (17.3) 53.2 (1.7) MRAC-C50-F15₂ 36.5 43.4 (18.7) 46.2 (6.6) * Compressive strength values for 100-mm cubic specimens. *(in brackets) Increase in compressive strength: from day 7 to day 28 and from day 28 to day 56 of curing Chapter 7 105 a) b) Figure 7. 2 Relative compressive strength at day 28: (a) Phase 1 (effective w/c = 0.48) and (b) Phase 2 (effective w/c = 0.52) In Phase 2, the MRACs achieved a less than 11% lower compressive strength vis-à-vis that of NAC2 at day 7 of curing except for MRAC-C50-F152, which achieved an 18.3% lower strength than NAC2. At 28 and 56 days of curing, the compressive strengths of the MRAC-C202, MRAC-C302, MRACC402 and MRAC-C502 concretes were lower than that of NAC2 by 9.5%, 10%, 12.8% and 13.1%, respectively (Figure 7. 2-b). MRAC produced with a CMRA percentage composition of up to 50% achieved similar properties to those of NAC [21,89,148,169,247,254]. Based on 28 days values, MRAC-C20-F52 and MRAC-C30-F102 achieved a 10% lower compressive strength than NAC2 (Table 7. 4& Figure 7. 2-b). Although MRAC-C40-F152 and MRACC50-F152, respectively, achieved 15% and 19.5% lower strengths than that of NAC2, MRAC-C40F152 achieved a 2.4% lower strength than that of MRAC-C402. In addition, MRAC-C50-F152 achieved a 6.4% lower strength than that of MRAC-C502. The small differences in compressive strength can be attributed to an enhanced interfacial zone resulting from the use of rough ceramic, along with the inherent strength of the ceramic material [57,196]. Pedro et al. [51] and Díaz et al. [169] observed that using the 25/25 and 50/50 replacement percentage proportions for MRA (fine/coarse) yielded 3–10% and 10–17% lower strengths, respectively than the NAC. Mas et al. [253] found that the use of 15% and 30% of MRA (0/8) reduced compressive strength by 20.6% and 24.1%, respectively, indicating sensitivity to high levels of MRA replacement. The total water/cement ratio of concrete increased as a higher percentage of MRA was used in concrete production. In addition, as shown in Table 7. 1 and Table 7. 2, the concretes produced in Phase 2 (with an effective w/c ratio of 0.52) had a higher total water-cement ratio than those of Phase 1 (with an effective w/c ratio of 0.48). Moreover, Figure 7. 3 shows that by analysing each phase independently, the concretes produced with a higher percentage of MRAC and, consequently, with a 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Ratio compressive strenght all concrete/NAC 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Ratio compressive strenght all concrete/NAC Chapter 7 106 higher total water-cement ratio, achieved lower compressive strength. Due to the higher water absorption of MRA and weaker bond to the cementitious matrix, MRA deteriorates the concrete microstructure, reducing its ability to reach optimal strength [150,168]. Figure 7. 3 also shows that the MRAC concrete produced with a similar total water-cement ratio but a lower effective water-cement ratio (phase 1) achieved higher strength than the concrete produced in phase 2. NAC-300₁ MRAC-C20₁MRAC-C30₁ MRACC50₁ MRAC-C100₁ MRAC-C20-F5₁ MRAC-C30-F10₁MRAC-C40-F15₁ MRAC-C50-F25₁ NAC-300₂ MRAC-C20₂MRAC-C30₂ MRAC-G40₂MRAC-C50₂ MRAC-C20-F5₂MRAC-C30-F10₂ MRAC-C40-F15₂ MRAC-C50-F15₂R² = 0.9072 R² = 0.8875 40.0 45.0 50.0 55.0 60.0 65.0 70.0 0.50 0.55 0.60 0.65 0.70 0.75 0.80 Compressive strength (MPa) Total water/ cement PHASE 1 PHASE 2 Figure 7. 3 Compressive strength ratio vs Total w/c ratio The analysis presented in Table 7. 4 and Figure 7. 3 reveals that all the MRACs produced in Phase 1 (effective w/c ratio = 0.48) exhibited a greater or similar compressive strength to that of NAC2 (effective w/c ratio = 0.52) at 28 and 56 days of curing, respectively, except for MRAC-C1001. Reducing the w/c ratio of the MRAC to 0.48 significantly enhanced its strength, allowing some mixtures to achieve values comparable to NAC2 (describe via a grey square in Figure 7. 3) and perform similarly in structural applications. Optimising the w/c is critical for MRAC to achieve compressive strengths comparable to those of NAC; however, the effectiveness of this approach is critically dependent on the quality of the RA used. The concretes were designed for a C30/37 strength class rating. Based on the SC-BOE [20], the minimum fck and the average compressive strength (fcm) should be 30 MPa and 38 MPa Eq.7 (1), respectively, for a concrete cylinder of Ø150 mm and length 300 mm. In addition, the strength in cubic specimens measuring 150 × 150 × 150 mm (fcm,cub150) is calculated relative to cylindrical specimens (fcm,cyl) using Eq.7 (2) [20]. Furthermore, the relationship between the compressive strengths of cubic specimens with different dimensions is expressed as Eq.7 (3) [126]. Based on these calculations, a minimum average strength of 46 MPa in cubic specimens (fcm,cub100) was required in the MRAC concretes produced to meet the validation requirements of the code. Although the strength of the Chapter 7 107 MRACs (Table 7. 4) was lower than that of the NAC, all the concrete mixtures in Phase 1 produced with an effective w/c ratio of 0.48 met the strength requirement, except MRAC-C100₁. In addition, all the concrete mixtures in Phase 2 met the strength criteria, except MRAC-C40-F152 and MRAC-C50F15₂. fcm = fck + 8MPa Eq.7 (1) fcm,cub150 = fcm,cyl  0.9 Eq.7 (2) fcm,cub100= 1.09· fcm,cub150 Eq.7 (3) 7.3.2.2 Splitting tensile strength All MRAC concretes obtained lower splitting tensile strength than the NAC concrete. In addition, the concretes produced with a higher percentage of MRA suffered a higher decrease in splitting tensile strength (see Table 7. 5). In Phase 1, MRAC-C201 and MRAC-C301 exhibited splitting tensile strengths that were slightly lower than that of NAC1 by 0.9% and 5.3%, respectively. However, MRAC-C501 and MRAC-1001 achieved 13% to 22.3% lower splitting tensile strength than NAC1. Several researchers [149,196,253] have also observed similar behaviour in MRAC concretes. Bravo et al. [251] and Cantero et al. [252] report that concrete produced with 50% CMRA achieved a 9–26% lower splitting tensile strength than NAC. Furthermore, according to several research studies [247,255], concretes produced with 100% CMRA can achieve up to 30% lower strength than NAC. According to Etxeberria&Gonzales et al. [254], the lower tensile strength is contingent upon the large nominal size of MRA (20 mm), and the flat shape of MRA gains is instrumental in the achieving of the lower splitting tensile strength. Incorporating FMRA into the concrete resulted in a much lower splitting tensile strength (Table 7. 4). The MRAC-C20-F51, MRAC-C30-F101 and MRAC-C50-F251 concretes achieved 4.5%, 7.7% and 19.8% lower strengths than the corresponding MRAC-C201, MRAC-C301 and MRAC-C501 concretes. MRAC produced using CMRA and FMRA achieved between 12.7% and 33% lower strength than NAC1. Similarly, Pedro et al. [51] determined that concrete produced using fine and coarse RA (FMRA/CMRA) in replacement ratios of 25/25, 50/50, 100/0, 0/100 and 100/100 yielded 13.8% to 38% lower splitting tensile strength than NAC. According to Nedeljković [57], concrete produced using 10%, 30% and 50% FMRA (in replacement of NA sand) and 100% coarse NA yielded 10.2%, 10.8% and 17.8%, respectively, lower strength than NAC. However, several studies [145,196,256] have determined that the use of FMRA and ceramic fine aggregates yields a higher splitting tensile strength than that of NAC concrete a similar trend as that observed for compressive strength. Chapter 7 108 Table 7. 5 Splitting tensile strength and modulus of elasticity (MRAC/NAC ratio) Phase 1 Mixtures Splitting tensile Strength (MPa) Modulus of elasticity (GPa) Phase 2 Mixtures Splitting tensile Strength (MPa) Modulus of elasticity (GPa) NAC₁ 3,79 40,45 NAC₂ 3.77 38.79 MRAC-C20₁ 3.76 (-0.91) 35.83 (-11.44) MRAC-C20₂ 3.7 (-1.86) 35.57 (-8.29) MRAC-C30₁ 3.59 (-5.25) 35.26 (-12.84) MRAC-C30₂ 3.62 (-3.85) 35.14 (-9.41) MRAC-C50₁ 3.29 (-13.14) 34.54 (-14.61) MRAC-C40₂ 3.55 (-5.84) 32.79 (-15.47) MRAC-C100₁ 2.95 (-22.3) 25.75 (-36.34) MRAC-C50₂ 3.28 (-13.05) 32.34 (-16.63) MRAC-C20-F5₁ 3.59 (-5.38) 34.97 (-13.55) MRAC-C20-F5₂ 3.54 (-6.1) 34.57 (-10.88) MRAC-C30-F10₁ 3.30 (-12.7) 33.66 (-16.79) MRAC-C30-F10₂ 3.33 (-11.72) 32.38 (-16.52) MRAC-C40-F15₁ 3.28 (-13.5) 31.94 (-21.04) MRAC-C40-F15₂ 3.30 (-12.38) 31.8 (-18.02) MRAC-C50-F25₁ 2.54 (-33.0) 29.53 (-27.01) MRAC-C50-F15₂ 3.11 (-17.50) 29.34 (-24.36) In Phase 2, much like in Phase 1, it was observed that the MRAC-C302 and MRAC-C402 concretes achieved 3.9% and 5.8% lower strength than NAC2. However, the strength difference was as high as 13.1% when 50% CMRA (MRAC-C502) was employed. Several researchers [257,258] have determined that concrete produced with 25% and 50% CMRA achieves up to 4% and 9%, respectively, lower flexural tensile strength than NAC. In contrast to Phase 1, concrete produced with up to 15% FMRA and CMRA in Phase 2 (Table 7. 4) achieved similar strength to the corresponding concrete produced using only CMRA and NA sand. MRAC-C20-52, MRAC-C30-F102, MRAC-C40-F152 and MRAC-C50-F152 achieved 4.2%, 7.8%, 6.54% and 4.5% lower strength than their corresponding concrete produced using only CMRA and NA sand. Several researchers indicate that concrete made using a high percentage of MRA in replacement of NA achieves slightly lower or similar, but comparable splitting tensile strength to NAC [148,205,259]. According to several researchers [103,185,260], the splitting tensile strength reduction in recycled concrete can be assumed to be up to 15-20% vis−à−vis that of NAC, without compromising safety and without a critical risk of cracking. According to SC-BOE [20], required 2.9 MPa for C30/37 concrete, all the MRAC achieved adequate parameter values, except MRAC-C100₁ and MRAC-C50-F25₁. The strength values achieved by the specimens in Phase 1 and Phase 2 indicate that NACs and concretes produced with the same percentage compositions of CMRA and FMRA achieve comparable properties notwithstanding the higher w/c ratio employed in Phase 2. This behaviour is attributable to the beneficial effects of water in the aggregates, which enhance the properties of the concrete through the hydration of the cement paste [196]. Based on the SC-BOE [20], the splitting tensile strength (fctm) of NAC can be determined as a function of its fck (Eq.7 [4]). For each type of concrete produced, the fck was evaluated by taking into account the fck of each sample (fcm,cub), as outlined in Table 7. 3, using Eqs.7 (3), 7 (2) and 7 (1), Chapter 7 109 then the fctm was determined. The relationship between the experimental and theoretical values for the different types of concrete are presented in Figure 7. 4. fctm= 0.3fck/2/3 Eq.7 (4) a) b) Figure 7. 4 Ratio of the experimental value to the theoretical value of the splitting tensile strength: (a) Phase 1 (effective w/c = 0.48) and (b) Phase 2 (effective w/c = 0.52) During Phase 1 (Figure 7. 4-a), the theoretical method (based on Eq.7 [4]) exhibited precision, with differences of below 5% for NAC1 and MRAC specimens, except for MRAC-C50-F251 concrete in which the theoretical method overestimates its splitting tensile by 21%. In Phase 2 (Figure 7. 4-b), it was observed that the theoretical method underestimated the experimental values. This suggests that although the specific formula in the SC-BOE [20], which is based on compressive strength, performs well for low w/c concretes, it may not fully capture the splitting tensile strength value when concrete was produced with an effective w/c ratio of 0.52. 7.3.2.3 Elastic modulus Based on the modulus of elasticity at 28 days Table 7. 5), the MRAC concretes achieved a lower modulus of elasticity than the NAC concretes due to the higher porosity of MRA, [64,171,261]. The value of the modulus of elasticity decreased even further when higher percentages of MRA were utilized [126,185,247]. The modulus of elasticity of concrete produced in phase 1 and phase 2 was similar, as the modulus of elasticity is influenced by the coarse aggregate used in the concrete production [262]. In addition, 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Ratio Splitting tensile Strenght (Experimental/theoricalSC-BOE) 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Ratio Splitting tensile Strenght (Experimental/theoricalSC-BOE) Chapter 7 110 due to the low stiffness of the adhered mortar, employing more than 5% of FMRA increases the reduction of modulus of the MRAC with respect to that of NAC concrete [263,264]. Based on the results of the tests (Phase 1 and Phase 2), the concretes produced with 20%, 30%, 40%, 50% and 100% CMRA and NA sand achieved approximately 9%, 11%, 13%, 15% and 36% lower modulus of elasticity than NAC. According to several researchers [171,214,252], MRAC produced with up to 25%, 50% and 100% CMRA achieved 5–10%, 15–25% and 25–30%, respectively, lower modulus of elasticity than NAC. In addition, several researchers [265] have determined that concrete produced with 100% coarse brick aggregates has up to 68% lower modulus of elasticity than NAC. When FMRA was employed in addition to CMRA, the modulus of elasticity of MRAC was lower than that of concrete produced only using CMRA. The use of 10% and 15% of FMRA with up to 40% of CMRA yielded a less than 10% lower modulus of elasticity than the corresponding concrete produced using only CMRA. However, the MRAC-C40-F15 concrete achieved a 20% lower modulus of elasticity than NAC. In addition, mixes incorporating a 25% FMRA (MRAC-C50-251) exhibited a significantly low modulus of elasticity, achieving a 12.14% lower modulus of elasticity than the MRAC-C501 concrete and a 27.01% lower modulus than NAC1. Pedro et al. [51] found that concrete produced with coarse and fine replacements (CMRA/FMRA) of 25/25 and 50/50 achieved 15% and 20% lower modulus, respectively, than NAC. The modulus decrease obtained by MRAC with respect to NAC in phases 1 and 2, except MRAC-C100₁ and MRAC-C50-F25₁, were lower than the values published by the Building Contractors Society of Japan [266], which establishes that RAC (produced using fine and coarse RA) achieve 25–40% lower value than NAC. The MRAC-C100₁ and MRACC50-F25₁ achieved a decrease of 36.34% and 27.01%, respectively, values established by the Building Contractors Society of Japan. To validate the values achieved by MRACs, the experimentally achieved values were compared to the theoretically determined values, taking into account both the SC-BOE [20] and Eurocode 2: EN 1992–1-1 (EC-02)[50]. The modulus of elasticity for NAC concrete can be estimated using Eq.7 (5) as per SC-BOE [20]. Therefore, a NAC (30/37) concrete with a minimum fcm of 38 MPa (cylindrical specimen) should have a minimum modulus of elasticity of 33 GPa. In light of this value, the MRACC1001, MRAC-C50-F251, and MRAC-C50-F152 concrete mixtures exhibited lower values, indicating higher deformability (Table 7. 5). Ecm = 22.(fcm 10)0.3 Eq.7 (5) The ratio between the experimental value vis-à-vis the theoretical value achieved by each concrete, per the SC-BOE, are presented in Figure 7. 5-a. When MRAC was produced with an effective w/c ratio of 0.48 (Phase 1), the modulus of elasticity of MRAC produced with up to 50% CMRA and natural sand was higher or similar than the theoretically estimated value. However, the experimental value for CMRA-C1001 was 21% lower than the theoretically estimated value. Furthermore, when Chapter 7 111 FMRA was employed in producing concrete, the theoretical method overestimated the experimental values (experimental values were lower than the theoretically estimated values), with experimental values being up to 14% lower than the theoretically estimated values, except for MRAC-C20-F5, for which the values were similar. In contrast, when MRAC was produced with an effective w/c of 0.52, Eq.7 (5) adequately estimated the modulus of elasticity value of all the MRACs, except for MRACC50-F152, for which the experimental value was lower than the theoretically estimated value. According to EC-02, the modulus of elasticity is theoretically estimated using Eq.7 (6): Ecm= KE.(1−0.25.αRA).fcm1/3 Eq.7 (6) Where the coefficient kE represents the type of aggregate and is 9,500 [21], and αRA is the ratio of the utilized quantity of FMRA and CMRA to the total quantity of aggregates employed, taking fine and coarse aggregates into account [50]. Figure 7. 5-b shows that Eq.7 (6) underestimates the modulus of elasticity of MRAC produced using CMRA for all specimens produced in Phase 2 (employing an effective w/c of 0.52) and Phase 1 (employing an effective w/c of 0.48), except for MRAC-C1001, which achieved a 6.0% lower experimentally determined value than the theoretically estimated value of its modulus of elasticity. Based on the information presented in Figure 7. 5, it is noteworthy that the impact of MRA on the difference between the experimentally determined modulus of elasticity vis-à-vis the theoretical value was greater in Phase 1 than in Phase 2. This phenomenon is attributable to the lower w/c ratio and higher strength in the former (Phase 1) than in the latter (Phase 2). 0.7 0.8 0.9 1.0 1.1 1.2 Ratio Modulus of elacticity (Experimental value/ Theoretical SC-BOE) Phase 1 effective w/c=0.48 Phase 2 effective w/c=0.52 0.7 0.8 0.9 1.0 1.1 1.2 Ratio Modulus of elacticity (Experimental value/ Theoretical Ec-02) Phase 1 effective w/c=0.48 Phase 2 effective w/c=0.52 a) SC-BOE b) EC-02 Figure 7. 5 Analysis of modulus of elasticity estimation: (a) Ratio of experimental value/theoretical SC-BOE (b) Ratio of experimental/theoretical EC-02 Chapter 7 112 The MRAC40-F15 produced in the two phases − with a reduction ranging from 11.44% to 21.04% in Phase 1 and 8.29% to 16.63% in Phase 2− meet the ranges accepted by the SC-BOE [14] with a lower reduction than the 20% reduction proffered by several authors [51,185,222,255] who propose that concrete mixtures with recycled aggregates within these reduction margins can be suitable for structural applications. The modulus of elasticity is crucial for controlling deformations under load, and despite the observed reductions, structures built with MRAC still maintain their deformability within safe limits. 7.3.3. Drying shrinkage The drying shrinkage (µƐ) and percentage mass loss (%) values achieved by all the concrete mixtures produced in Phase 1 are outlined in Figure 7. 6-a. CMRA-C201 and CMRA-C301 achieved lower or similar shrinkage values than NAC1, and employing FMRA in the production of CMRAC20-F51 and CMRA-C30-F101 yielded a slightly higher shrinkage value than NAC1. These results are also presented in Figure 7. 7-a. in which the ratio of the MRAC shrinkage values vis-à-vis those of NAC1 are outlined. However, when MRAC was produced with a high percentage composition of MRA (coarse and fine), it achieved high shrinkage values. This behaviour is attributable to the high porosity of fine MRA and the consequent low density resulting from the presence of hardened mortar within its structure. This phenomenon results in low material stiffness and the material’s ability to restrain deformation [263]. Based on these results, it can be concluded that significant changes in concrete shrinkage occurs when concrete is produced using 50% CMRA (with natural sand) and when concrete is produced with 40% CMRA and 15% FMRA. MRAC-C501 and MRAC-C1001, respectively, achieved 20% and 72.8% higher drying shrinkage values than NAC1 (Figure 7. 7-a). MRAC-C1001 achieved a drying shrinkage value of −855.4 μ/m. Vintimilla and Etxeberria [126] determined that concrete produced using 100% CRCA and NA sand achieves a 60% higher shrinkage value than NAC. In addition, Pedro et al. [51] observed that concrete produced with 100% coarse MRA and NA sand achieves a 72% higher shrinkage value than NAC. Furthermore, the MRACC40-F151 and MRAC-C50-F251 concretes in this study exhibited 16.2% and 25.3% higher drying shrinkage values than NAC1 at 91 days (Figure 7. 7-a), achieving values of −574.9 μ/m and −620.2 μ/m, respectively. Figure 7. 6-b describes the mass loss of all the concretes produced in Phase 1. Similar to the shrinkage values, concrete made with a high MRA achieved a high mass loss. NAC1 achieved a mass loss of 2.2%, MRAC-C201 and MRAC-C20-F51 achieved a mass loss value of 3.2%, MRAC-C301, MRAC-C30-F101 and MRAC-C40-F151 achieved an approximate mass loss value of 3.6%, MRACC501 and MRAC-C50-F251 achieved mass loss values of 3.8% and 4.1%, respectively, and MRACC1001 achieved a mass loss of 5.5% the highest mass loss value recorded. These values are slightly higher than those observed by Vintimilla and Etxeberria [126], who determined that concretes produced with up to 100% CRCA and natural sand achieved up to 3.5% mass loss. Similarly, concrete produced with coarse and fine RCA (CRCA/FRCA) in ratios from 50/20 to 100/100 suffered mass losses ranging from 3.4% to 5.4%. In a previous study [105], concrete produced employing a 50/20 ratio of different cement types resulted in a mass loss of 2.5% to 3.5%. Chapter 7 113 -6.0 -5.0 -4.0 -3.0 -2.0 -1.0 0.0 0 7 14 21 28 35 42 49 56 63 70 77 84 91 Mass Loss (%) Time (Days) NAC₁MRAC-C20₁ MRAC-C30₁MRAC-C50₁ MRAC-C100₁MRAC-C20-F5₁ MRAC-C30-F10₁MRAC-C40-F15₁ MRAC-C50-F25₁ a) b) Figure 7. 6 Drying shrinkage values in Phase 1: (a) Drying shrinkage MRCA1&FMRA1, (b) Mass loss MRCA1&FMRA1 a) b) Figure 7. 7 Relative shrinkage ratio at day 91: (a) Phase 1 (effective w/c = 0.48) and (b) Phase 2 (effective w/c = 0.52) Concrete produced with a high water–cement ratio suffers high shrinkage [121,267]. Although the concrete mixtures in Phase 2 had a higher water content than the Phase 1 concretes, the shrinkage values in Phase 2 were lower. This can be attributed to slight variations in the atmospheric conditions of the climatic room during the test procedure. Phase 1 was conducted during spring and summer, with stable atmospheric conditions of the climatic room, featuring an average temperature of 22℃ and a relative humidity (RH) of 51%. On the other hand, in Phase 2, concretes were assessed in autumn and winter while the maintenance works were set in the climatic room. Consequently, although the average atmospheric conditions in the climatic room were 19 ℃ and 53% RH, it caused -900 -800 -700 -600 -500 -400 -300 -200 -100 0 0 7 14 21 28 35 42 49 56 63 70 77 84 91 Drying shrinkage strain (µm/m) Drying time (days) NAC₁MRAC-C20₁ MRAC-C30₁MRAC-C50₁ MRAC-C100₁MRAC-C20-F5₁ MRAC-C30-F10₁MRAC-C40-F15₁ MRAC-C50-F25₁ 0.0 0.5 1.0 1.5 2.0 Ratio shrinkage all concrete/NAC₁ 0.0 0.5 1.0 1.5 2.0 Ratio shrinkage all concrete/NAC₂ Chapter 7 120 a) b) Figure 7. 12 Water penetration under pressure: (a) Phase 1 (effective w/c = 0.48) and (b) Phase 2 (effective w/c = 0.52) 7.4. Conclusions The results of the tests performed in this study support the following conclusions: • The MRACs produced −using up to 50% coarse MRA with natural sand and up to 40% coarse MRA combined with 15% fine MRA, with an effective w/c ratio of 0.48 (Phase11) and an effective w/c ratio of 0.52 (Phase 22) achieved densities 2.2 to 2.33 kg/dm³, approximately 5% lower than the density of NACs. Furthermore, accessible pores remain below 15%, ensuring acceptable quality. • MRAC concretes meet the compressive strength requirements for structural concrete intended for use in XC1 to XC4 environments (46 MPa cubic specimens), except for MRAC-C1001, MRAC-C40-F152 and MRAC-C50-F152. • MRAC specimens achieve a lower strength than NAC when produced with an identical effective w/c ratio. However, the use of a 0.04 lower effective w/c ratio in MRAC-C30-F10, MRACC40-F15 (fine and coarse fractions), and MRAC-C50 (only coarse fraction) yields similar strengths to that of NAC. • The splitting tensile strength and modulus of elasticity of MRAC decrease with an increase in the MRA percentage proportion. The effective w/c ratio does not significantly impact those properties for MRACs with the same MRA replacement percentage. • The splitting tensile strength of all MRACs, except for MRAC-C1001 and MRAC-C50-F251, meets the required 2.9 MPa for C30/37 concrete, which is suitable for use in XC1 to XC4 environments. 0 10 20 30 40 50 60 Water Penetration (mm) Average Penetration Da ≤30mm Maximun Penetration Dmax ≤ 50mm 0 10 20 30 40 50 60 Water Penetration (mm) Average Penetration Da ≤30mm Maximun Penetration Dmax ≤ 50mm Chapter 7 121 • Concrete with maximum replacements of 40% CMRA and 15% FMRA achieves modulus of elasticity values comparable to those of NACs with modulus of elasticity values engineered using the SC-BOE method for C30/37 concrete. The EC-02 method, which takes into account the impact of MRA components, provides satisfactory estimates for all MRACs. • Concrete incorporating up to 50% CMRA (without FMRA) in Phase 1 and 30% in Phase 2 exhibited a drying shrinkage value similar to that of NACs. The use of up to 15% FMRA with 40% of CMRA in Phase 1 slightly increased (16%) the shrinkage value of concrete whereas in Phase 2, the use of 10% FMRA resulted in a 44% increase in shrinkage. • The theoretical method of the SC-BOE, structural code criteria, accurately estimates shrinkage values for concrete with up to 30% CMRA and 10% FMRA. Although EC-02, which employs the ηshRA factor, limits the use of MRA to up to 40% CRMA or 30% CMRA together with 10% FMRA, it provides accurate estimates for all concrete types. • MRACs exhibit higher sorptivity values than NACs due to their higher porosity. However, the concrete produced with up to 40% CMRA and 15% FMRA achieves sorptivity values below 0.05 mm/min0.5, which indicates good durability. • All concretes produced in this study meet the water penetration threshold for XC1 to XC4 environments stipulated by the SC-BOE. In general, an increase in the FMRA content results in an increased water penetration threshold. Based on the findings of this study, concrete produced with 40% CMRA and 15% FMRA and an effective w/c ratio of 0.48 achieves compressive strength and durability properties similar to those of conventional concrete produced with an effective w/c ratio of 0.52. However, to guarantee mechanical performance of concrete produced with 30% CMRA and 10% FMRA (which represents a 20% replacement of the total volume of aggregates) satisfies all the specification requirements. Further investigation is required to determine the long-term durability effect of MRACs especially concrete produced with 40% CMRA and 15% FMRA (which represents a 28% replacement of the total volume of aggregates). Utilizing MRA (both fine and coarse) is a viable option for producing concrete, notwithstanding not being specified in SC-BOE. In addition, small quantities of FMRA can be incorporated successfully. Chapter 8 122 1. 8. Chapter 8. General conclusions and future research lines 8.1 General conclusions This study analyzed the feasibility of using recycled concrete aggregates (RCA) and mixed recycled aggregates (MRA) in Optimize use of Recycled Aggregate in High Durability Structural Concrete: An Experimental Study. Different replacement ratios, the maximum viable substitution limit for structural concrete production, of these recycled aggregates were established for RAC applications in XC1-XC4, XS1 environments, with a compressive strength of 30/37 MPa. In all experimental phases, a cement dosage of 300 kg per cubic meter of concrete was used. The developed mixtures were thoroughly evaluated in terms of their physical, mechanical, and durability properties, ensuring they meet the requirements for structural applications in sustainable and long-lasting environments. The general conclusions of the investigations are listed below: Properties of recycled aggregates • RCA-Type A, consists of over 90% concrete and natural stone (RCU95 classification). The research indicates a density range of 2.32-2.36 kg/dm3 and water absorption between 5.16% and 5.73%. These values meet the EN 206 standard for Type A aggregates (> 2.1 kg/dm3 density and <7% water absorption, according to SC-BOE). • RCA-Type A can comply with key performance indicators for structural concrete. The aggregates also meet the requirements for sand equivalent, Los Angeles coefficient, flakiness index, and alkali-aggregate reactivity (expansion < 0.1% after 14 days). • MRA-Type B, with concrete and natural stone exceeding 50% and ceramic content below 30% (meeting EN 206 Type B classification: RC50, RCU70, Rb30-, Ra5-, FL2and XRg2). Density ranges from 2.08 to 2.28 kg/dm3, exceeding the UNE EN 206 requirement of >1.7 kg/dm3. The water absorption of coarse MRA (CMRA) fractions (8.75-9%) exceeds the 7% limit set by SC-BOE. However, the study highlights that the average water absorption of the combined NA and CMRA remains below the threshold when used in concrete production. • MRA exhibits inferior performance compared to NA and RCA-Type A (lower density, higher porosity), but it can still be suitable for concrete if mix designs account for the higher water absorption and potential impact on workability. Using a lower percentage of MRA can also help mitigate these effects Chapter 8 123 The Adjustments in Mix Design To achieve properties comparable to natural aggregate concrete (NAC), certain adjustments in the mix design with recycled aggregate concrete (RAC) and mixed recycled aggregate concrete (MRAC) are required. In the RAC mixtures, the natural coarse aggregate was initially replaced by coarse RCA (CRCA) at levels ranging from 20% to 100%. Subsequently, fine recycled concrete aggregate (FRCA) was incorporated in adjusted proportions, reaching up to 100% in the mixes with the highest CRCA content. For the MRAC mixtures, the natural coarse aggregate was substituted with coarse mixed recycled aggregate (CMRA) in proportions ranging from 20% to 100%. Additionally, fine mixed recycled aggregate (FMRA) was introduced, replacing up to 25% of the natural sand. • Throughout the experimental campaign, the amount of chemical admixtures, including plasticizers and superplasticizers, must be adjusted when concrete is produced using recycled aggregates to achieve adequate workability. The required dosage was higher in RACs and MRCs compared to NAC, which consistently remained within a range between fluid and liquid. • Adjustments in mix design showed that reducing the water-cement ratio in RAC and MRAC had a positive impact on its durability. The NAC and RAC were designed with effective watercement ratios of 0.47 and 0.51, respectively. This adjustment, reducing the effective watercement ratio by approximately 0.04 in RAC and MRACs with respect to that of NAC, allowed for a more equitable durability assessment, compensating for the higher porosity and water absorption of RCA. As a result, the reduction in the water-cement ratio improved the RAC’s resistance to carbonation and chloride ions, bringing its performance closer to that of NAC. • The water absorption of RAs is significantly higher in RCA-Type A and particularly in MRAType B than that of NA. This characteristic directly influences the design and performance of concrete, as excessive effective absorption capacity can affect workability and effective w/c ratio, consequently, the cement hydration. Therefore, the use of aggregates with high moisture percentages is recommended, preferably in a range of 70-80% of their absorption capacity. This practice allows for optimization of the effective water-cement ratio and improvement of mix consistency, as well as ensures better mechanical performance and durability in structures made with recycled concrete. Physical and Mechanical Properties • The concretes, RAC (up to 60 % CRCA with 30 % FRCA) and MRAC (up to 50% CMRA with natural sand and up to 40% CMRA with 15% FMRA) exhibit slightly lower densities (approximately 5%) compared to natural aggregate concrete (NAC), ranging from 2.2 to 2.33 kg/dm³, but remain within acceptable limits for structural applications. The accessible pore values are maintained below 10% for RAC and 15% for MRAC, ensuring adequate quality. • This research has demonstrated the feasibility of utilizing high percentages, up to 100%, of both coarse and fine RCA and MRA in concrete production. Although both types of recycled concrete exhibit a decrease in mechanical properties compared to NAC, they remain suitable for a wide range of non-structural applications. Chapter 8 124 • The research has demonstrated that concretes incorporating up to 60% CRCA and 20% FRCA, as well as up to 40% CMRA and 15% FMRA, exhibited a reduction in splitting tensile strength compared to NAC. In RAC, the incorporation of up to 20% FRCA showed no significant variation compared to concretes made with only CRCA. Similarly, MRAC with up to 15% FMRA achieved strengths similar to those of concretes made with only CMRA, and in some cases, even improved the mechanical properties. Notably, different water-tocement ratios had no significant impact on the splitting tensile strength of NAC, RAC, and MRAC. • This study demonstrates that reducing the effective water-cement ratio by approximately 0.04 in RAC and MRACs (effect. w/c ratio of 0.47) ensures compressive strength equivalent to that of NAC (effect. w/c ratio 0.51) with a minimum compressive strength of 30/37 MPa. Concretes with 100% CRCA and those incorporating 60% CRCA and 30% FRCA simultaneously achieve compressive strengths comparable to NAC. Similarly, concretes with 50% CMRA and those with 50% CMRA combined with 25% FMRA also reach equivalent compressive strengths. However, exceeding these replacement levels leads to significant reductions in compressive strength. • The use of recycled aggregates in concrete (RAC and MRAC) reduces the modulus of elasticity as the percentage of NA replacement increases; however, mixes with up to 60% CRCA and 30% FRCA, and 40% CMRA with 15% FMCA achieve acceptable values according to structural regulations. It was observed that recycled sand has a more significant impact on this reduction compared to the coarse fraction. Moreover, while the modulus of elasticity in NAC varies with the water/cement ratio, in RAC, it depends directly on the percentage of recycled aggregate replacement. Shrinkage and Predictive Models: Existing models for predicting concrete behaviour require adjustments for RAC: • The study demonstrates that concrete mixtures incorporating up to 100% CRCA, even in combination with 20% FRCA, consistently exhibited drying shrinkage values below the -800 με/m threshold recommended by the American Concrete Institute (ACI). Similarly, mixtures containing MRA, up to 50% CMRA and 25% FMRA showed satisfactory performance, maintaining acceptable shrinkage levels. These results remained consistent across different water-to-cement ratios (0.47 and 0.51). However, it decreased for higher percentages of recycled aggregates. • All RAC mixtures, with up to 60% CRCA and 20% FRCA—regardless of the type of cement used (CEM II/AL, CEM II/AS, and CEM III/B)— and with effective water-cement ratio of 0.47, achieved acceptable shrinkage values at 91 days, remain below the -600 με/m. • Shrinkage prediction models for RAC exhibit varying levels of accuracy. The SC-BOE model provides precise predictions for concretes with up to 60% CRCA and 20% FRCA, as well as for mixes with up to 30% CMRA and 10% FMRA. However, its accuracy decreases for higher percentages of recycled aggregates. • The EC-02 model, which incorporates the ηshRA factor (0.4 for RCA and 0.2 for MRA), demonstrates greater precision compared to the SC-BOE model. This model provides Chapter 8 125 accurate predictions for concretes with up to 60% CRCA and 20% FRCA, as well as for mixes with up to 40% CMRA or 30% CMRA together with 10% FMRA. The inclusion of these specific factors enhances the model's ability to account for the unique properties of recycled aggregate concrete. • Both models have limitations, especially when predicting shrinkage in concretes with high blast furnace slag content cements (such as CEM III/B). These findings highlight the need for more comprehensive models that account for environmental factors, cement types, and the specific properties of recycled aggregates to enhance shrinkage predictions, especially in the early stages of concrete curing. Durability • RAC incorporating up to 60% CRCA and 20% FRCA, as well as 40% CMRA and 15% FMRA, demonstrated satisfactory performance in terms of sorptivity, with values below 0.05 mm/min⁰·⁵, indicating good durability. Furthermore, all concretes, including RAC and MRAC, complied with the water penetration under pressure requirements established by the Structural Concrete Code. • RACs exhibited comparable or superior chloride resistance to NAC when both had similar compressive strengths, with up to 60% CRCA replacement, regardless of the cement type. However, increasing FRCA content up to 20% FRCA tends to increase chloride penetration. The use of CEM III/B cement significantly improved chloride penetration in all concretes, including RACs with up to 60% CRCA and 20% FRCA. The selection of cement type is crucial: CEM III/B cement proved to be the most effective, followed by CEM II/AS, which showed acceptable values with up to 60% CRCA and 10% FRCA. On the other hand, the concrete produced using CEM II/AL exhibited the poorest performance in terms of chloride penetration, rendering it unsuitable for these types of aggressive environments. • The study highlights the complexity of carbonation resistance across concretes produced using different types of cement. It confirms the superior performance of concretes made with CEM II/AS cement, followed by CEM II/AL, and finally, CEM III/B in terms of carbonation resistance, regardless of the aggregates used. Notably, RACs with up to 60% CRCA and 20% FRCA exhibited similar carbonation resistance to NAC with similar compressive strength, indicating that RCA can be effectively utilized without significantly compromising performance. This finding underscores that carbonation resistance is primarily governed by the type of cement rather than the aggregate replacement, supporting the viability of incorporating high levels of recycled aggregates in concrete production. • The inclusion of up to 60% CRCA was found to be suitable for carbonation resistance, yielding performance comparable to that of NAC with similar compressive strength. Additionally, the incorporation of 20% FRCA produced notable results: while it increased the carbonation rate in accelerated tests, it exhibited better performance than NAC under natural exposure conditions. A key observation was that the natural carbonation rate was significantly higher than that predicted by accelerated tests. This discrepancy underscores the urgent need for the development of more accurate models for assessing the long-term durability of concrete in service. Chapter 8 126 • The obtained natural carbonation rate (Knat) was 2.0 to 2.8 times higher for NAC and 1.6 to 2.4 times higher for RAC compared to the theoretical natural carbonation rate (KnatTHEO) derived from the accelerated carbonation test. These results highlight the disparity between natural and accelerated carbonation rates, emphasizing the need for careful consideration when extrapolating long-term carbonation performance from accelerated testing. Influence of Cement Type in durability of RAC. The type of cement plays a crucial role in the performance of concretes with recycled aggregates: • CEM II/A-S provides a well-balanced performance in terms of carbonation resistance and chloride penetration. CEM II/AS cement demonstrates excellent performance in concrete mixes, particularly for carbonation resistance. In RAC-C60-F20 concrete, it ensures adequate durability in XC3 and XC4 environments, achieving a 50-year service life. This cement type also exhibits resistance to chloride penetration. Concrete incorporating up to 60% CRCA and 10% FRCA exhibits moderate chloride ion penetration values and lower chloride diffusion coefficients. • CEM III/B offers superior chloride resistance but lower carbonation resistance. RAC-C60-F20 produced with CEM III/B, is suitable for environments exposed to chlorides due to its high chloride penetration resistance, as indicated by the low surface chloride concentrations (Cs) and reduced Dnss values. However, it exhibits the lowest carbonation resistance compared to the other mixes analyzed. • CEM II/A-L is less effective in enhancing the durability of RACs. RAC-C60-F20, produced with CEM II/AL cement, demonstrated adequate carbonation resistance in XC4 environments. However, its performance in terms of chloride ion resistance was significantly lower, making it less suitable for aggressive exposure conditions. Feasibility and Potential of RCA and MRA in Concrete for Structural Applications • This research has demonstrated the feasibility of incorporating both CRCA and FRCA in concrete mixtures. Specifically, the simultaneous incorporation of FRCA is feasible, allowing for a mixture containing 60% CRCA and up to 20% FRCA while maintaining the required physical, mechanical, and durability properties, provided that the appropriate cement type is selected according to the exposure conditions. • Concrete containing MRA is suitable for structural applications, allowing up to 40% CMRA with 15% FMRA. • These findings significantly exceed the 20% limit for coarse recycled concrete aggregates stipulated in the Spanish Structural Concrete Code (SC-BOE), demonstrating the feasibility of higher substitution rates. The successful application of fine recycled aggregates further expands the potential of recycled materials in concrete production. • This research lays the foundation for an increased utilization of recycled aggregates in the construction industry, promoting sustainability and fostering a more efficient circular economy within the sector. Chapter 8 127 8.2 Future Research Lines • An improved future research line would investigate the behaviour of concretes with 60% CRCA and 20% FRCA under diverse environmental conditions, including marine, industrial, urban, and extreme temperature settings. The study should encompass long-term durability trials in real structures, performance optimization using various cement types and admixtures, microstructural analysis of the interfacial transition zone, and assessment of specific durability aspects such as chloride penetration, carbonation, and sulphate resistance. • It is recommended to expand research on the durability of concrete with MRA through a comprehensive approach encompassing: exposure to various environmental conditions (including marine, industrial, and urban areas), freeze-thaw cycles, and high temperatures; optimization of mixtures using different types of cement, additives, and mineral admixtures; study of MRA pre-treatment and improvement methods; and conducting long-term tests on actual structures. This research should include analysis of carbonation, chloride penetration, and resistance to various aggressive agents, as well as the development of predictive models to estimate the service life of MRA concrete in different scenarios. • Perform structural tests such as compression, flexural, and shear tests on both conventional and recycled aggregate concrete elements to compare their load-bearing capacity, stiffness, and deformation behaviour. • Develop predictive models and simulation tools to evaluate the performance of structural elements made with recycled aggregates, considering the effects of various cement types and environmental exposure conditions. • Based on the findings, propose new guidelines and recommendations for incorporating recycled aggregates into structural concrete, with a focus on optimizing both performance and sustainability. • Future studies should aim to conduct a comprehensive Life Cycle Assessment (LCA) of concrete incorporating Type A and Type B recycled aggregates. 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