International Symposium on Nanotechnology in Construction Materials NICOM8 Corrosion Sensing Properties of Carbon Black-Based Cementitious Smart Coatings

Available online at www.sciencedirect.com ScienceDirect Structural Integrity Procedia 00 (2024) 000–000 www.elsevier.com/locate/procedia 2452-3216 © 2024 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of NICOM8 Chairpersons International Symposium on Nanotechnology in Construction Materials NICOM8 Corrosion Sensing Properties of Carbon Black-Based Cementitious Smart Coatings Gabriele Milonea,b * , Maria Cruz Alonsob, Christos Vlachakisa, Jean-Marc Tullianic and Abir Al-Tabbaaa aDepartment of Engineering, University of Cambridge, Trumpington Street, Cambridge CB2 1PZ, UK bEduardo Torroja Institute for Construction Sciences (IETCC), Spanish National Research Council, C. de Serrano Galvache, 4, Madrid 28033, Spain cDepartment of Applied Science and Technology, INSTM R.U. Lince Laboratory, Politecnico di Torino, Corso Duca degli Abruzzi 24, Turin 10129, Italy Abstract This research explores the sensing capabilities of carbon black (CB)-based cementitious coatings for detecting deformations arising from reinforcement corrosion. The investigation focused on a chlorides-contaminated reinforced mortar element subjected to controlled accelerated corrosion. The objective is to utilize smart coatings' sensing properties to establish a link between electrochemical attacks and the mechanical effects induced by corrosion. Differently from their more frequent application in the literature, this type of study focused on a chemical attack rather than physical. The sensors were employed to quantify the increase in internal stress and strain due to oxide formation and propagation within the matrix. The sensors exhibit good sensitivity to corrosion progression identifying attack penetration on the reinforcement and crack formation on the surface. The research was initiated with the implementation of a protocol designed to efficiently accelerate corrosion in reinforced mortar beams, considering varying rebar exposure. Subsequently, a correlation was established between the electromechanical response of the smart coatings and the ongoing corrosion in the substrate, culminating in surface fracture development. Positioned transversally to the beam's longitudinal direction, all sensors consistently provided accurate crack propagation measurements up to an average width of 116 ± 45 μm. Additionally, the sensors demonstrated the ability to provide crack development measurements also when positioned at varying distances from the directly affected rebar sections. This study expands the use of smart carbon-based coatings, positioning them as multifunctional systems beyond traditional structural applications. * Corresponding author. E-mail address: [email protected] 2 G. Milone et al./ Structural Integrity Procedia 00 (2024) 000–000 © 2024 The Authors. Published by ELSEVIER B.V. This is an open access article under the CC BY-NC-ND license (https://creativecommons.org/licenses/by-nc-nd/4.0) Peer-review under responsibility of NICOM8 Chairpersons Keywords: corrosion sensing; smart materials; carbon-based cementitious sensors; structural health monitoring 1. Introduction Reinforced concrete (RC) is the most widely used construction material in the world (Gagg, 2014). By combining concrete's ability to withstand compression with steel's ductility to tension, this material is an ideal composite for all types of structures (Fehling, Leutbecher, and Roeder, 2011; Wight, 2016). However, as concrete is prone to cracking (Safiuddin et al., 2018), one of the main challenges for RC structures revolves around reinforcement corrosion (Popov, 2023). The corrosion of steel rebars within concrete is initiated through the penetration of chlorides or carbonation, reducing the alkalinity of concrete and, consequently, breaking the natural passivation of steel (Zhu et al., 2016). This leads to the progressive weakening of the rebar’s cross-sectional area and adversely affecting the bond between steel and concrete (Syll and Kanakubo, 2022), which is crucial for the structural performance of reinforced concrete elements (Coccia, Imperatore, and Rinaldi, 2016). The repair and maintenance of corroded structures lead to environmental issues and high costs, equivalent to half of the yearly construction budget spent on renovation of existing structures to extend their service life (Cailleux and Pollet, 2009). Hence, the development of effective prevention and protection techniques aims at limiting corrosion for maintaining safety and serviceability levels of existing structures (Angst, 2018; Abbas and Shafiee, 2020). Different strategies include corrosion inhibitors, alternative reinforcement materials, coatings, and electrochemical methods (Goyal et al., 2018). Despite the variety of techniques available, each corrosion monitoring method is characterized by different efficiencies and limitations, such as invasiveness, cost, and the inability to provide early warning signs (Komary et al., 2023). For instance, while electrochemical methods provide precise corrosion rates, they require direct access to the steel (Popova and Prošek, 2022). Similarly, fiber Bragg grating (FBG) based acoustic emission, though capable of real-time monitoring, struggles with background noise and interpreting data accurately (Jinachandran and Rajan, 2021). Infrared thermography offers a non-destructive method for corrosion assessment. Its accuracy, however, can be affected by external thermal variations and the placement of the reinforcement within concrete (Kobayashi and Banthia, 2011). The emergence of self-sensing construction materials offers a novel solution to overcome the limitations of traditional corrosion detection in reinforced concrete, by integrating sensing capabilities directly into construction materials (Han, Ding, and Yu, 2015). These smart materials are cement systems, such as pastes, mortar or concrete, that have been doped with a single or a combination of electrically conductive fillers to enhance their sensing properties (Ding et al., 2019; Tian et al., 2019). Monitoring is made possible through the analysis of electrical changes in electrical resistance of these conductive based material under external stimuli such as strain, damage, temperature and moisture (Chung, 2023). Self-sensing materials also detect and monitor corrosion, addressing some of the issues associated with more traditional methods, as a consequence of the deformation and cracking induced in the concrete cover due to the generation of iron oxides. Jin et al. (2017) explored how the incorporation of various chloride ion contents affects the electrical response of carbon-based cementitious composites. They found that the presence of ions supported graphene fillers in creating additional conductive networks within the matrix. Alternatively, carbon-based cementitious binders can be used as a pseudo reference electrode which, embedded in concrete, allows to monitor the different corrosion states of steel (Jin et al., 2019). Nonetheless, the methodology for these applications – generally referred to as “bulk” – present significant limitations, i.e., invasiveness, due to the substitution of concrete with the conductive material, and cost, associated with the large filler amount needed to enhance the conductivity of an entire structural element. Moreover, this methodology does not represent general corrosion observations since carbon-based particles may act as protective layers on steel which suppress both metal oxidation and oxygen reduction, limiting the overall rebar corrosion (Coating, 2012). Alternatively, cement-based systems can be applied as external sensors – “coatings” – which have the potential to monitor corrosion development on a reinforced substrate. Hence, this research aims at exploring the potential of carbon black (CB)-based cementitious coatings as smart, multifunctional systems for early corrosion detection and monitoring G. Milone et al./ Structural Integrity Procedia 00 (2024) 000–000 3 in reinforced concrete structures. Coatings provide greater flexibility in terms of deployment for new and existing infrastructure, and also in fabrication due to their smaller size and thus lower costs (Chung, 2023; Ding et al., 2019). The corrosion monitoring capability of these sensing coatings, similar to conventional gauges, was achieved by measuring the substrate’s inner state of strain and relating it with the movements of corrosion products between the rebar and the cementitious matrix (Grattan et al., 2009; Routoulas and Batis, 1999). Differently from the traditional techniques mentioned above, this corrosion sensing method has the potential to monitor broader areas while preserving the substrate structure. The research methodology involves the sensing capabilities of smart coatings in chloride-induced corrosion scenarios. To emulate such corroding conditions in a brief amount of time, an accelerated corrosion test was pursued on all reinforced mortar specimens (Miró et al., 2021). This test spanned 24 days and permitted the development of corrosion approximately 53 times faster than natural settings. By employing CB-based smart coatings, a link between electrochemical attacks and the mechanical effects induced by corrosion was established. Sensors were utilized as high-precision gauges to quantify the increase in internal stress and strain due to oxide formation and propagation within the matrix (Andrade, Alonso, and Molina, 1993). Visual analysis proceeded in parallel to qu alitatively assess the oxides’ propagation and their influence on the mortar’s inner state of stress. This sensing capability was measured in both free and partially confined rebar scenarios, showing the versatility of the coatings under different structural conditions. This work serves as a starting point for cementitious coatings in corrosive environments, setting the stage for further exploration of multifunctional sensors. 2. Materials and methods 2.1. Materials The smart coatings used in this study were formulated by combining Portland cement (CEM I – 52.5N, supplied by Hanson Cement, UK), conforming to BS EN 197-1, with conductive carbon black powder, as outlined in Table 1 (sourced from Alfa Aesar, US). To ensure uniformity and optimal workability across different carbon black concentrations, MasterGlenium C315 (BASF, UK) was utilized as a superplasticizer by weight of carbon black. Table 1. Carbon black properties as per the manufacturer. Appearance (colour) Black Form Powder Ash (%) ≤0.50 Electrical resistivity (Ω∙cm) ≤0.25 pH 7.6 Moisture (%) 0.12 Average particle size (nm) 42 Surface area (m2/g) 75 Bulk density (g/L) 170-230 These sensors had dimensions equal to 7.5 mm × 3 mm × 30 mm and embedded two copper wires (20 mm in length, 1 mm in thickness), procured from RS Components, UK. Their application was aided by the use of small pincers that controlled the position of the electrodes and prevented movement during casting and curing. To ensure that the thickness of the sensors was in line with their nominal value, five thickness measurements were obtained for all coatings along their longitudinal direction by means of a caliper. The resistivity was calculated on this basis to prevent geometrical variability in the samples. Table 2 displays the mix design used for casting the sensor coatings for electromechanical testing. Table 2. Mix design of the coating composition tested in this study (kg/m3). Name Cement Water Carbon black Dispersant CB dosage [wt%] CB dosage [vol%] 4 G. Milone et al./ Structural Integrity Procedia 00 (2024) 000–000 CB3 2906.8 1327.5 85.9 8.6 3.0 15.7 For the substrate preparation, the reinforced mortar was designed in view of the corrosion test. The mortar mix comprised cement CEM I – 42.5R (supplied by Ciments molins, Spain), standardized sand (produced at IETcc) as per UNE EN 196-1, and water, with a water-cement ratio maintained at 0.5 for all samples. To simulate chloride-induced corrosion environments, 2% of NaCl by weight of cement was added directly into the mixing water until its dissolution. The reinforced bars, type D500SD (provided by Iturrino suministros industrials, Spain), were 6 mm in diameter. Some steel rebars were coated with a protective two-phase epoxy resin to limit the corrosion spread within predetermined sections of the bar. Finally, the sensors were installed on the mortar’s surface using a different twophase rapid-hardening epoxy resin, from Huntsman Advanced Materials (US) and supplied by RS components (UK). 2.2. Sample preparation The mortar beams subjected to accelerated corrosion had standard dimensions of 40 mm × 40 mm × 160 mm. They incorporated one steel rebar, ensuring a cover depth of 4.8 mm from the surface to facilitate targeted crack development. This cover-to-diameter ratio (c/ϕ = 0.8) was critical in directing crack propagation along the mortar beam's surface, aiming to simulate real-world structural degradation patterns under accelerated corrosion conditions. The rebars were either fully exposed to corrosion or partially limited through epoxy coating. Hence, the corrosion test included specimens with both confined and unconfined rebars subjected to accelerated corrosion, as specified in Table 3. Table 3. Types of rebar setting subjected to accelerated corrosion test and related corrosion monitoring sensors applied. Steel exposure Sensor no. Epoxy confinement of steel [cm] Sensor distance from exposed rebar [mm] Applied sensors Unconfined #1, #2, #3 0 4.8 X, Y, Z Confined #4 4 36.3 Y #5 8 76.2 #6 12 116.1 In agreement with UNE EN 196-1:2005, the mortar prisms were cured under controlled conditions (at 22 ± 2°C, RH = 98%) for 7 days. Such a short curing period was selected to give sufficient time for the cementitious matrix to harden and to limit its natural corrosion development. Afterwards, fully cured cement-based sensors were applied on top of 7-days old mortar prism. Figure 1 shows the sensors' encapsulation and subsequent application on the top surface of the substrate prism. These coatings were positioned transversally to the longitudinal axis of the rebar, each spaced 40 mm from the other (Figure 2), ensuring stable adherence and operational integrity throughout the corrosion testing phase. In unconfined rebar tests, the three sensors were named X, Y, and Z according to their distance from the rebar side directly connected to the corrosion equipment (Figure 2b). G. Milone et al./ Structural Integrity Procedia 00 (2024) 000–000 5 Figure 1. Visual representation of sensing coating preparation and installation: (a) epoxy resin mixing; (b) sensor encapsulation; and (c) sen sor application on mortar surface. Figure 2. (a) Schematic and (b) photographic representation of sensing coatings onto mortar substrate. Final configuration of three sensors X, Y and Z along the transversal direction of the prism across the rebar axis. Identical configuration for samples #1, #2 and #3 . Selected rebars were coated with epoxy resin, varying the length of encapsulation across specimens to study the effect of confinement on corrosion spread and sensor response. The rebar sections were encapsulated symmetrically in the middle, with encapsulation extending over 4, 8, and 12 cm, as shown in Figure 3a. For partially confined rebars, a single sensing coating (sensor Y) was applied to the middle section of the beam (Figure 3b). The primary objective of this study was to assess the corrosion sensing capabilities of a smart carbon black (CB)-based coating installed at varying distances from the corrosion-affected section; specifically, at 36.3, 76.2, and 116.1 mm from the unconfined steel. 6 G. Milone et al./ Structural Integrity Procedia 00 (2024) 000–000 Figure 3. Visual representation of (a,c,e) mould configuration and (b,d,f) mortar prisms with partially confined rebars for samples #4, #5 and #6: (a,b) encapsulation extending 4 cm and sensor distanced 36.3 mm from unconfined steel; (c,d) encapsulation extending 8 cm and sensor distanced 76.2 mm from unconfined steel; (e,f) encapsulation extending 12 cm and sensor distanced 116.1 mm from unconfined steel. 2.3. Experimental program 2.3.1. Accelerated corrosion test The corrosion test required a unique setup to induce accelerated corrosion within the mortar specimens, focusing on electrochemical attack kinetics. The sensing property capability of smart coatings was related to the mechanical effect induced by the corrosive electrochemical attack in the steel/cement interface. Prior to initiating the accelerated corrosion, the depasivation state of the rebar, as a consequence of the addition of NaCl to the mortar, was assessed through the measurement of electrochemical methods, offering insights into the initial corrosion status of the steel rebars. This was based on the linear polarization test (Rp) which relies on the linearity of polarization curves around the corrosion potential (i.e., Ecorr ± 20 mV) (Stern, 1958). By applying a small amplitude through an alternating signal to the working electrode (i.e., steel rebar), the corrosion current (Icorr) was determined using the Stern-Geary equation (Stern and L.G., 1957): Icorr=B/Rp (1) The constant B can be determined from Tafel slopes of the cathodic and anodic polarization curves (Song, 2000; Kouřil, Novák, and Bojko, 2006), and it was assumed equal to 26 mV in this investigation (Andrade and Alonso, 1996). The resultant value was adjusted to account for the ohmic drop between the working and reference electrodes. The obtained output (Icorr) was compared with the standards – UNE 112072:2011 – to obtain an understanding of the system’s corrosion level. Following the initial 7-day curing of the mortar prisms (before sensor application), such a G. Milone et al./ Structural Integrity Procedia 00 (2024) 000–000 7 value was expected not to be excessively high; nonetheless, due to the presence of NaCl in the matrix, the corrosion current was not negligible and, therefore, its measurement was required for the subsequent corrosion acceleration. The application of a constant corrosion current, facilitated by a TG 97 Galvanostat (Bank Elektronik, Germany), standardized the rate of oxide formation across specimens, ensuring a controlled environment for studying the corrosion's mechanical impact and the subsequent response of the smart coatings. Figure 4 displays the setup used in this study where a stainless-steel sheet worked as the counter electrode (cathode) connected to the rebar which became the working electrode (anode). The galvanostatic forced polarization ensured that the cathode behaved as the negative electron donor and the positive anode was the corroded rebar (Shi, Jia, and Atrens, 2012). Both the crack elongation and the oxide propagation within the matrix were visually assessed with a manual optical microscope (RS PRO USB digital microscope 5M, RS components, Spain). Figure 4. (a) Schematic and (b) graphical description of setup employed for applying constant current to an unconfined rebar embedded in a 40 mm × 40 mm × 160 mm mortar prism in order to accelerate its corrosion propagation. The mortar samples were all connected to the galvanostat – as specified in Figure 4b – which, in turn, was linked in series with a 500 Ω resistor. This setup provided a constant corrosion current of 3016 μA which, assuming a cylindrical geometry for the rebar, resulted in Icorr = 100 μA/cm² for all mortar beams. Such a value was chosen in 8 G. Milone et al./ Structural Integrity Procedia 00 (2024) 000–000 agreement with the literature to ensure rapid corrosion progress within the mortar sample (Andrade, Alonso, and Molina, 1993; Caré and Raharinaivo, 2007). This resulted in a constant mass loss for the steel rebar with an attack penetration (A.P.) of ~4.74 μm/day, according to equation 2 (Rodriguez and Andrade, 1990). ϕ(t)=ϕi-0.023·Icorr·t (2) This test was conducted for approximately 24 days until the extreme corrosion of all samples produced cracks with a width above ~500 μm. For partially confined rebars, the corroded section was smaller than its unconfined counterpart. Thus, a constant current of 2.3, 1.5, and 0.75 mA for samples with a rebar confinement of 4, 8, and 12 cm, respectively. These current values were determined by considering the exposed area of the rebar when applying the fixed corrosion current of 100 μA/cm². 2.3.2. Corrosion sensing measurement During the aforementioned accelerated corrosion test, the smart coatings were used as external sensors to assess the progress of the corrosion in the mortar substrates (Table 3). Via the 2-probe method (Miccoli et al., 2015), the application of alternate current was implemented daily using a potentiostat PGSTAT204 (Metrohm, Switzerland) to minimize the polarization effect in the system (20 Hz – 300 kHz; U = 0.5 V; 9 points per decade). The bulk resistance, obtained by deconvoluting the impedance spectrum, can be used to define the effective conductivity, defined by equation 3: 1 Rbulk (L A)=σbulk (3) where Rbulk is the resistance value corresponding to the ionic conduction of the interconnected pores in parallel with the electronic conduction through the conductive filler (Wang and Pang, 2019) [Ω], L is the distance between the pair of chosen electrodes or gauge length [m] and A is the cross-section of the coating sensors [m²]. The corrosion measurement of unconfined rebars was obtained from three identical sensing coatings applied perpendicularly to the rebar direction and, therefore, to the crack expansion (Figure 5). The sensors, distanced 40 mm from one another, worked as both strain and damage monitoring devices. The electrical output was given from the Nyquist plot obtained at different corrosion times and the damage was assessed via a portable microscope to measure crack width along the rebar on each surface. Once the corrosion test finished after 24 days of accelerated corrosion, the samples were transversally cut with an electric saw and subsequently manually split open to visualize the oxide production and propagation from the rebar to the surface under study. Figure 5. Graphical representation of fully corroded reinforced beam and sensor location perpendicular to fracture location. G. Milone et al./ Structural Integrity Procedia 00 (2024) 000–000 9 3. Results and discussion 3.1. Accelerated corrosion The accelerated corrosion tests, facilitated by the inclusion of 2% NaCl by weight of cement in the mortar mix, were employed to simulate a chloride-contaminated environment for steel rebar corrosion. Initial corrosion assessments via electrochemical measurements revealed that despite achieving moderate to high corrosion levels, as per UNE 112072:2011 standard, the actual rate of corrosion was insufficient to achieve elevated oxides production within a short time. Indeed, with a natural corrosion current of 0.630 ± 0.044 μA/cm², the samples followed an estimated attack penetration rate of ~0.09 μm/day. Therefore, the galvanostatic approach was employed, ensuring a consistent and predictable rate of oxide production (i.e., 4.74 μm/day in agreement with equation 2). To empirically assess the effectiveness of the theoretical attack penetration (equation 2), the diameter of fully corroded rebars was measured via stereoscope. Figure 6 depicts the difference between non-corroded and corroded rebar, treated with a solution of hydrochloric acid and hexamethylenetetramine for steel corrosion prevention, to remove the oxides on the rebars’ surface while limiting any further oxidation of the rebar in the acid. After applying Icorr = 100 μA/cm² for 24 days, the theoretical attack penetration (A.P.) reached ~114 μm. The rebar diameter after corrosion can be approximated to ~5.85 mm, resulting in an empirical A.P. of 150 ± 55 μm, slightly higher but still analogous to the theoretical value. Figure 6. Visual representation of diameter for (a) uncorroded and (b) corroded beam after applying I corr = 100 μA/cm2 for 24 days. A critical aspect in validating the accelerated corrosion protocol involves the relationship between fracture width on the mortar’s surface and oxides production and movement. For rebars positioned 4.8 mm below the surface, the first visible crack emerged for an attack penetration of 24 μm (within 5/6 days). Despite a slightly higher empirical A.P. (Figure 6b), the systems experienced the first crack occurrence on the surface at a slower rate than what was expected by theory (Alonso et al., 1996). Indeed, when currents drive chemical reactions, the efficiency of accelerated corrosion is typically less than 100%, mainly due to simultaneous heat generation (Alonso et al., 1996; Dzhioev, Kosov, and Von Oppen, 2013). Consequently, higher attack penetrations were necessary to achieve specific crack widths, when applying high corrosion rates (i.e., Icorr = 100 μA/cm²), and to compensate for the oxide diffusion through the pores of the cover (Alonso et al., 1996). Hence, both longitudinal and transversal cross-sections for the corroded specimens are presented in Figure 7 and Figure 8, proving the significant role of oxide propagation in influencing crack formation on the system’s surface. Moreover, Figure 7 reveals that oxides, originating from the rebar, exhibit a random movement toward any of the four surfaces parallel to the reinforcement. 16 G. Milone et al./ Structural Integrity Procedia 00 (2024) 000–000 By employing CB-based smart coatings, a link between electrochemical attacks and the mechanical effects induced by corrosion was established. Sensors were utilized to quantify the increase in internal stress and strain due to oxide formation and propagation within the matrix. This capability was evidenced in both free and partially confined rebar scenarios, showcasing the coatings' versatility across different structural conditions. For unconfined rebar, the coatings provided accurate tracking of crack development and progression up to an average width of 116 ± 45 μm, demonstrating their potential as early warning systems for structural integrity. Furthermore, in configurations involving confined rebar, the coatings were characterized by good sensitivity to the development of corrosion, even when positioned at large distances from the directly affected rebar sections. In conclusion, this research underscored the importance of smart construction materials in enhancing the durability and safety of civil infrastructure. The study extended the application of CB-based coatings beyond traditional uses, highlighting their potential as a multifunctional material. Their successful implementation of corrosion sensing represents an initial promising step towards the production of smarter and more resilient infrastructure. Future work will focus on better framing the production and propagation of the oxides within the corroded matrix, using varying acceleration rates of corrosion, and correlating them with the electrical response of these sensors. Indeed, corrosion development has been proven to be significant in the interpretation of structural health monitoring systems. 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