Digital image correlation for shear testing of masonry

Final Thesis developed by: Bermudez Sequerra, Edna Paulina Directed by: Pelà, Luca Garcia-Ramonda, Larisa Bachelor in: Civil Engineering Barcelona, October 2024 Department of Civil & Environmental Engineering Digital image correlation for shear testing of masonry Digital image correlation for shear testing of masonry by EDNA PAULINA BERMUDEZ SEQUERRA A thesis submitted to the Department of Civil and Environmental Engineering in partial fulfillment of the requirements for the degree of Bachelor of Science in Civil Engineering at the Polytechnical University of Catalonia October 2024 Authored by: Edna Paulina Bermudez Sequerra Department of Civil and Environmental Engineering Supervised by: Larisa Garcia-Ramonda Post-doctoral Researcher at UPC-BarcelonaTech, Thesis Supervisor Supervised by: Luca Pel` a Full Professor of Structural and Construction Engineering at UPC-BarcelonaTech, Thesis Supervisor Abstract Masonry is one of the oldest construction methods still used worldwide. This construction technique involves the combination of two distinct components: masonry units and a binder, with the units either laid dry or bound together. The extensive use of masonry throughout history is driven by the abundant availability of natural materials and the simplicity of its construction methods. This versatility has made masonry timeless and adaptable, suitable for both ordinary and innovative designs. A significant amount of the world’s existing buildings consist of load-bearing masonry structures. Many of these buildings possess cultural heritage or architectural value and may be located in seismically active areas. Therefore, understanding the shear behavior of masonry is essential for assessing their structural integrity and preservation. Among the various available masonry units, this thesis focuses on the shear characterization of masonry constructed with perforated bricks. These are commonly used in contemporary construction due to advantages such as reduced weight, enhanced fire resistance, improved thermal and acoustic insulation, and lower embodied carbon. However, the experimental performance of exposed brick masonry remains under-researched. This thesis aims to research on the behavior of masonry constructed with exposed perforated bricks, analyzing their behavior under shear stress. The results will identify failure modes using Digital Image Correlation (DIC), and the shear strength will be characterized according to European standards. The findings will determine the cohesion and friction coefficient of this type of masonry, to better understand the masonry’s performance under in-plane actions. Keywords: Masonry ·Triplets ·Brickwork ·Preforated bricks ·Shear characterization ·Laboratory testing ·Digital Image Correlation (DIC) ·Asymmetry 2 Resumen La mamposter´ ıa es uno de los m´ etodos de construcci´ on m´ as antiguos que se siguen utilizando en todo el mundo. Esta t´ ecnica de construcci´ on implica la combinaci´ on de dos componentes distintos: unidades de mamposter´ ıa y un aglutinante, y las unidades se colocan en seco o se unen entre s´ ı. El amplio uso de la mamposter´ ıa a lo largo de la historia se debe a la abundante disponibilidad de materiales naturales y a la sencillez de sus m´ etodos de construcci´ on. Esta versatilidad ha hecho que la mamposter´ ıa sea atemporal y adaptable, adecuada tanto para dise˜ nos ordinarios como innovadores. Una parte importante de los edificios existentes en el mundo est´ a formada por estructuras portantes de mamposter´ ıa. Muchos de estos edificios son patrimonio cultural o tienen valor arquitect´ onico y pueden estar situados en zonas s´ ısmicamente activas. Por lo tanto, comprender el comportamiento a cortante de la mamposter´ ıa es esencial para evaluar su integridad estructural y su conservaci´ on. Entre las diversas unidades de mamposter´ ıa disponibles, esta tesis se centra en la caracterizaci´ on a cortante de la mamposter´ ıa construida con ladrillos perforados. Estos ladrillos se utilizan com´unmente en la construcci´ on contempor´ anea debido a ventajas tales como la reducci´ on de peso, la mejora de la resistencia al fuego, la mejora del aislamiento t´ ermico y ac´ustico, y la reducci´ on del carbono incorporado. Sin embargo, el comportamiento experimental de la mamposter´ ıa de ladrillo visto sigue estando poco investigado. Esta tesis tiene como objetivo investigar el comportamiento de la mamposter´ ıa construida con ladrillos perforados vistos, analizando su comportamiento bajo esfuerzo cortante. Los resultados identificar´ an los modos de fallo mediante Correlaci´ on Digital de Im´ agenes (DIC), y se caracterizar´ a la resistencia a cortante seg´un los est´ andares europeos. Los resultados determinar´ an la cohesi´ on y el coeficiente de fricci´ on de este tipo de mamposter´ ıa, para comprender mejor el comportamiento de la mamposter´ ıa bajo acciones en el plano. Palabras clave: Mamposter´ ıa ·Tripletas ·Ladrillos preforados ·Comportamiento a cortante · Ensayos de laboratorio ·Correlaci´ on Digital de Im´ agenes (DIC) ·Asimetria 3 Resum La mamposteria ´ es un dels m` etodes de construcci´ o m´ es antics que encara s’utilitzen arreu del m´ on. Aquesta t` ecnica de construcci´ o implica la combinaci´ o de dos components diferents: unitats de mamposteria i un aglutinant, i les unitats es col·loquen en sec o s’uneixen entre si. L’´us extensiu de la mamposteria al llarg de la hist` oria es deu a la disponibilitat abundant de materials naturals i a la senzillesa dels seus m` etodes de construcci´ o. Aquesta versatilitat ha fet que la mamposteria sigui intemporal i adaptable, adequada tant per a dissenys ordinaris com innovadors. Una part important dels edificis existents al m´ on est` a formada per estructures portants de mamposteria. Molts d’aquests edificis s´ on patrimoni cultural o tenen valor arquitect` onic i poden estar situats en zones s´ ısmiques actives. Per tant, comprendre el comportament a tallant de la mamposteria ´ es essencial per avaluar-ne la integritat estructural i la conservaci´ o. Entre les diverses unitats de mamposteria disponibles, aquesta tesi es centra en la caracteritzaci´ o al tall de la mamposteria constru¨ ıda amb maons perforats. Aquests maons s’utilitzen comunament en la construcci´ o contempor` ania a causa de beneficis com la reducci´ o de pes, la millora de la resist` encia al foc, la millora de l’a¨ ıllament t` ermic i ac´ustic, i la reducci´ o del carboni incorporat. Tanmateix, el comportament experimental de la mamposteria de ma´ o vist segueix estant poc investigat. Aquesta tesi t´ e com a objectiu investigar el comportament de la mamposteria constru¨ ıda amb maons perforats vistos, analitzant el seu comportament sota esforc¸ tallant. Els resultats identificaran els modes de fallada mitjanc¸ant Correlaci´ o Digital d’Imatges (DIC), i es caracteritzar` a la resist` encia al tall segons els est` andards europeus. Els resultats determinaran la cohesi´ o i el coeficient de fricci´ o d’aquest tipus de mamposteria, per comprendre millor el comportament de la mamposteria sota accions en el pla. Paraules clau: Mac¸oneria ·Tripletes ·Maons preforats ·Comportament a tallant ·Assajos de laboratori ·Correlaci´ o Digital d’Imatges (DIC) ·Asimetria 4 Acknowledgments I would like to extend my deepest gratitude to my thesis supervisors, Dr. Larisa GarciaRamonda, Professor Luca Pel` a, and PhD Candidate Emerson Cuadros-Rojas, for their invaluable guidance, insightful advice, and support throughout the course of my research. Their expertise and generosity in sharing their knowledge have been fundamental in shaping this work. My heartfelt thanks also go to the laboratory technicians, Tom´ as Garcia, Carlos Hurtado, Ernest Craus, and Robert Mc-Aloon, whose assistance during the experimental phase was appreciated. I am also grateful to Edwin Cabrera for his construction of the specimens, which played a vital role in this project. I would also like to express my appreciation to Guillem and Elisabet, not only for their support but for making my university experience enjoyable and unforgettable. Their support and companionship during this time have been invaluable. Lastly, I want to express my deepest love and gratitude to my family. Their constant support, encouragement, and unconditional love throughout in my life have been my greatest source of strength. Thank you for always being there for me. 5 Contents Abstract 2 Resumen 3 Resum 4 Acknowledgments 5 1 Introduction 13 1.1 Motivation for the present research ........................ 13 1.1.1 Masonry ................................. 13 1.1.2 Objective and focus of the thesis ..................... 14 1.2 Outline of the thesis ................................ 14 2 State of the art 15 2.1 Masonry ...................................... 15 2.1.1 Composite material ............................ 15 2.2 Mechanical behaviour of masonry ........................ 19 2.2.1 Uniaxial compressive behaviour ..................... 19 2.2.2 Uniaxial tensile behaviour ........................ 19 2.2.3 Biaxial behaviour ............................. 20 2.2.4 In-plane behaviour ............................ 21 2.3 Digital image correlation (DIC) .......................... 23 6 CONTENTS 3 Experimental programme 27 3.1 Materials ..................................... 27 3.1.1 Bricks ................................... 27 3.1.2 Mortar .................................. 30 3.2 Specimens preparation .............................. 33 3.2.1 Test setup ................................. 36 4 Results 41 4.1 Characteristic initial shear strength 𝝉and internal friction angle 𝝓....... 41 4.2 Precompression stress level equal to 𝝈=0.2MPa ................ 43 4.3 Precompression stress level equal to 𝝈=0.6MPa ................ 46 4.4 Precompression stress level equal to 𝝈=1.0MPa ................ 48 5 Conclusions 53 5.1 Future works ................................... 53 6 Sustainability Analysis and Ethical Implications 55 6.1 Environmental impact ............................... 55 6.2 Economic impact ................................. 55 6.3 Social impact ................................... 56 6.4 Ethical implications ................................ 56 6.5 Impact in relation to the Sustainable Development Goals (SDG’s) ....... 56 A Appendix for Chapter 3 59 B Appendix for Chapter 4 63 B.1 Precompression stress level equal to 𝜎= 0.2 MPa ................ 64 B.2 Precompression stress level equal to 𝜎= 0.6 MPa ................ 66 B.3 Precompression stress level equal to 𝜎= 1.0 MPa ................ 67 References 69 7 List of Figures 2.1 Different masonry brick bonds. (a) Stretcher bond; (b) Header bond; (c) English bond; (d) English cross bond. Illustration adapted from [4]. ........... 16 2.2 Perforated bricks. ................................. 17 2.3 Bucket with mortar mixed with water. ...................... 18 2.4 Second type of failure under uniaxial tensile stress. Illustration adapted from [7]. 20 2.5 Modes of failure of solid clay units under biaxial loading. Illustration adapted from [7]. ...................................... 21 2.6 Typical failure modes of masonry walls subjected to combined in-plane vertical and horizontal loads. Illustration adapted from [8]. ............... 22 2.7 Schematic experimental set up for the 2D DIC method. Illustration adapted from [12]. ..................................... 24 2.8 Random speckle pattern. ............................. 24 2.9 Sketch of a reference square subset before and a target subset after deformation [12]. ........................................ 25 3.1 Dimensions of the perforated brick used in the experimental campaign. ..... 27 3.2 Preparation of the bricks: (a) Observed rough surface of the brick; (b) Flat surface of the brick once polished. ........................ 29 3.3 Characterisation of the normalized compressive strength 𝑓𝑏,𝑐 of the brick according to EN 772–1:2011+A1:2016 [13] : (a) Ibertest 3000 machine; (b) Setup of the test. .................................... 29 3.4 Bricks tested for their compressive strength. ................... 29 3.5 Tampering of a layer to ensure proper compaction of the specimen. ....... 30 8 Chapter 2 State of the art 2.1 Masonry Masonry is one of the oldest building materials that remains widely used in modern construction. Its construction process involves the stacking of units with or without mortar for cohesion. Therefore, its main characteristic is its simplicity making it a straightforward yet effective technique that has been successfully employed since ancient times. The choice of materials for masonry units is largely influenced by the availability of local resources, with stones, bricks, or blocks being the most common options depending on the surrounding environment. Masonry is often classified as a composite material since the quality of the final product relies on both the materials used and the craftsmanship involved. It is primarily utilized in the construction of walls, serving as either load-bearing or partitioning shear walls, as cladding for protection or aesthetic purposes, or as infill in RC frames [4]. To ensure high-quality masonry with robust load-bearing capacity using bricks and mortar, adherence to specific craftsmanship guidelines, known as bond rules, is essential. These rules specify four primary bond patterns (shown in Figure 2.1), that dictate how brick courses are arranged and offset to achieve optimal structural integrity. Additionally offset of bricks plays a vital role in the wall’s load-bearing capacity. A larger offset, which means a deeper racking back of the bricks, significantly enhances the wall’s resistance to longitudinal cracking. 2.1.1 Composite material As previously stated, masonry is a construction technique that involves the use of two distinct components. Therefore it is classified as a composite material. This fact, adds complexity to 15 CHAPTER 2. STATE OF THE ART (a) (b) (c) (d) Figure 2.1: Different masonry brick bonds. (a) Stretcher bond; (b) Header bond; (c) English bond; (d) English cross bond. Illustration adapted from [4]. accurately characterize its behavior, particularly when subjected to shear stress. The overall properties of masonry are heavily influenced by the attributes of its individual components [5]. 16 CHAPTER 2. STATE OF THE ART Brick In the early days of masonry, walls were constructed using large, rough-hewn stones placed directly on top of each other without mortar, a method known as Cyclopean masonry. During the Classical Age, builders began using uniformly shaped stone blocks with smooth faces to construct walls and piers, though mortar was still not employed. This technique was famously applied in the construction of many temples on the Acropolis in Athens and later in the Roman Colosseum [1]. In contrast, the ancient societies of Mesopotamia faced a challenge due to the limited availability of hard, durable rocks. To address this, they developed methods to produce artificial building materials. Initially, they used sun-baked bricks, which were porous and prone to deterioration over time. To improve the durability and strength of these bricks, the use of kiln firing was introduced. This process involved heating the clay bricks in a kiln to significantly increase their hardness and resistance. Fired bricks, produced through this technique, offered much greater longevity and have continued to be a fundamental construction material throughout history and into modern times. Nowadays, a wide variety of artificially manufactured bricks and blocks are used, such as clay masonry units, calcium silicate masonry units, aggregate concrete masonry units or autoclaved aerated concrete masonry units. Brick types include solid, hollow, perforated (Figure 2.2) and multiperforated variations. Perforated bricks, in particular, are extensively used in contemporary construction for both structural and non-structural purposes. Their global popularity stems from several key advantages: they are lighter, which reduces the overall weight of the structure; they offer enhanced fire resistance; they improve both thermal and acoustic insulation; and they have a lower embodied carbon footprint. These benefits make perforated bricks a highly advantageous choice in modern building practices. Figure 2.2: Perforated bricks. 17 CHAPTER 2. STATE OF THE ART Mortar The use of binders—substances that set and harden—in masonry construction is an ancient technique that has evolved significantly over time. Mortar, a workable paste used to bind masonry blocks together and fill the gaps between them, becomes hard when it sets, forming a rigid aggregate structure. Early mortars were created by mixing mud and clay. The ancient Egyptians later introduced gypsum as a binder, while the Persians used bitumen. A great advancement in the evolution of masonry came with the Etruscans’ discovery of lime. They found that when limestone was burned and mixed with water, it produced lime, which hardened over time. This discovery was crucial as it significantly improved the quality and durability of mortar. The innovation continued with the addition of pozzolana, a volcanic ash that reacts with calcium hydroxide in the presence of water. This mixture not only enhanced the mortar’s strength but also allowed it to set underwater. Figure 2.3: Bucket with mortar mixed with water. Today, modern mortars are typically composed of a mixture of sand, a binder such as cement or lime, and water. This formulation ensures that mortar remains effective in binding masonry blocks and filling gaps, while also setting into a durable and rigid aggregate structure. Masonry as a composite material Masonry presents a complex mechanical behavior due to its composition made up of blocks, which are typically made from quasi-brittle materials bonded together with mortar. It is also due to its structure, such as the pattern in which the blocks are arranged (bonds). The general behavior of masonry is influenced by the mechanical properties of its individual components (blocks and mortar) and the bond between them. This fact contributes to variability behavior in terms of their mechanical properties across the different sections, due to its heterogeneity and creation of multiple interfaces between them. 18 CHAPTER 2. STATE OF THE ART Masonry components generally exhibit quasi-brittle behavior in both tension and compression, with compressive strength and fracture energy being significantly higher compared to tensile properties. The bond between blocks and mortar is often weak, characterized by a normal stress-dependent cohesive-frictional behavior in shear and a cohesive behavior in tension (with minimal cohesion in dry stone masonry), including softening of cohesion. This results in a highly nonlinear overall response of masonry, making it difficult to predict the shear response [6]. Masonry is also an anisotropic material, this means that its properties vary depending on the direction of the applied load and the orientation of the blocks and the mortar joints. Anisotropy can be seen in various aspects: elastic behavior (elastic anisotropy), strength properties (strength anisotropy), and post-peak response (brittleness anisotropy). 2.2 Mechanical behaviour of masonry 2.2.1 Uniaxial compressive behaviour The uniaxial compressive behavior of masonry, particularly in the direction normal (perpendicular) to the bed joints, has traditionally been considered the most significant structural property of the material. This behavior is typically assessed using a test called the stacked bond prism test. The fundamental issue with masonry under uniaxial compression is the difference in elastic properties between the masonry units (such as bricks or blocks) and the mortar. When masonry is subjected to uniaxial compression, the mortar experiences triaxial compression, while the units are subjected to a combination of compression and biaxial tension. This difference in stress distribution leads to the development of vertical cracks within the units, which initially form along the middle line of the specimen and propagate through the vertical joints. As the load increases, additional vertical cracks develop, particularly on the smaller sides of the specimen, eventually causing the masonry to fail by splitting . Moreover, it has been observed that higher compressive strength in masonry leads to a more brittle failure mode [7]. 2.2.2 Uniaxial tensile behaviour The uniaxial tensile behavior of masonry, particularly for tensile loading perpendicular to the bed joints, is generally characterized by failure due to the relatively low tensile bond strength between the bed joint and the masonry unit. In such cases, the tensile strength of the masonry can be roughly approximated by the tensile bond strength between the joint and the unit. In masonry constructed with low-strength units but with a higher tensile bond strength between the bed joint and the unit, such as when high-strength mortar is used or when the units have numerous small perforations that produce a dowel effect, failure may instead occur when the 19 CHAPTER 2. STATE OF THE ART stresses exceed the tensile strength of the unit. Here, the masonry tensile strength can be approximately equated to the tensile strength of the unit. Two different types of failure can occur depending on the relative strength of the joints and units. In the first type, stair-stepped cracks through both head and bed mortar joints. In the second type of failure, cracks run almost vertically through the units and head mortar joints (Figure 2.4). Figure 2.4: Second type of failure under uniaxial tensile stress. Illustration adapted from [7]. 2.2.3 Biaxial behaviour Page [7] conducted the most comprehensive experimental investigations on masonry subjected to proportional biaxial loading. For uniaxial tension, failure typically occurred through cracking and sliding of the head and bed joints. However, the effect of lateral tensile stress on tensile strength is unknown due to a lack of experimental data. Conversely, lateral compressive stress reduces tensile strength, attributed to damage in the composite material, including micro-slip of joints and micro-cracking of units. In tension-compression loading scenarios, failure may occur solely through cracking and sliding of the joints or through a combined mechanism involving both units and mortar joints. In uniaxial compression, a smooth transition to different failure modes is observed under biaxial compression. Failure in biaxial compression typically manifests as splitting of the specimen at mid-thickness in a plane parallel to its free surface, regardless of the principal stress orientation. For principal stress ratios significantly less than or greater than one, the orientation of stresses plays a crucial role, leading to failure through a combined mechanism involving joint failure and lateral splitting. The increase in compressive strength under biaxial compression can be attributed to friction in the joints and internal friction within the units and mortar joints. 20 CHAPTER 2. STATE OF THE ART Figure 2.5: Modes of failure of solid clay units under biaxial loading. Illustration adapted from [7]. 2.2.4 In-plane behaviour The in-plane performance of unreinforced masonry walls is influenced by multiple factors such as their geometry, bond type, vertical loads, boundary conditions, and the mechanical properties of the materials used . Different failure modes can develop depending on the vertical loading, the quality of the bond between the masonry unit and the mortar, and the strength of that bond [8]. The sliding shear failure occurs along the mortar joints, producing horizontal or stair-stepped diagonal cracks caused by low bond strength at the mortar-masonry interface or when the compressive stresses acting on the wall are reduced. The diagonal shear failure occurs when the 21 CHAPTER 2. STATE OF THE ART Figure 2.6: Typical failure modes of masonry walls subjected to combined in-plane vertical and horizontal loads. Illustration adapted from [8]. tensile strength of the masonry is surpassed along the principal direction, leading to diagonal cracks along the wall. In regular masonry, this failure happens with strong mortar and weak units, and a good bond behavior at the mortar-masonry interfaces. Finally, the flexural failure, also known as rocking or toe crushing, is a type of failure that occurs when the tensile or compressive strength at the ends of the wall’s cross sections is exceeded, resulting in nearly horizontal or vertical cracks [9]. In regular masonry - a regular arrangement of blocks or bricks, either with or without mortar joints - there are two primary in-plane shear failure modes. The first one is the sliding shear failure and the second one is the diagonal shear failure with cracking of units. These failure modes are primarily influenced by the cohesion of the joints and the tensile strength of the masonry units, respectively. In masonry involving weak mortar, the low cohesion can lead to sliding failure. This sliding mechanism is often analyzed using the Mohr–Coulomb criterion: 𝜏=𝜇·𝜎+𝑐=𝑡𝑎𝑛𝜙 ·𝜎+𝑐(2.1) This criterion determines the shear strength, 𝜏, the friction angle, 𝜙obtained from the coefficient of friction, 𝜇-, the level of normal compression, 𝜎and the cohesion, 𝑐. To characterise the mechanical parameters 𝜏and 𝜙, couplet and triplet tests are carried out, which are bond tests that involve two or three masonry units joined with mortar. There are other two primary testing methods to investigate the in-plane behavior of masonry walls under lateral loads: the diagonal compression test and the shear-compression test. The diagonal compression test is primarily used to determine the tensile strength of masonry. It involves applying a compression force along a diagonal, thereby eliminating lateral compression and creating a pure shear stress condition in the wall’s midsection. The shear-compression test offers a more comprehensive analysis of the effective in-plane 22 CHAPTER 2. STATE OF THE ART response of masonry walls. It simulates a biaxial stress state by simultaneously applying vertical compressive and horizontal shear forces. This loading condition more accurately reflects the behavior of walls in masonry structures subjected to both vertical and horizontal loads. Given the mechanical behavior of masonry and its limited performance under shear stress, it is crucial to thoroughly characterize these properties to understand the stability and overall performance of masonry structures. Shear tests are used to assess the properties of materials, providing empirical data that can help predict how the materials will behave under various loading conditions, which is crucial for ensuring safety and effective design. The importance of incorporating results from shear tests into structural models help improve the design methods to ensure that they meet the building code requirements, and enhance the overall safety and performance of structures. In this context, Eurocode 6 (EN 2010) [10] serves as an important standard for the design of buildings and civil engineering works involving unreinforced, reinforced, prestressed, and confined masonry. It addresses essential requirements for resistance, serviceability, and durability of structures. Additionally, Eurocode 6 outlines execution standards necessary to ensure the quality of materials and workmanship in accordance with the design rules. However, it is important to note that seismic design requirements are addressed separately in Eurocode 8. Eurocode 8 (EN 2004) [11] is crucial for the design and construction of buildings and civil engineering works in seismic regions. Its primary objectives include the protection of human lives, ensuring occupant safety during seismic events, limiting damage to minimize structural harm, and maintaining the operational integrity of critical structures post-earthquake. Both Eurocode 6 and Eurocode 8 are vital for the effective design of masonry structures. Eurocode 6 ensures that the basic mechanical properties and quality of masonry are adequately addressed, while Eurocode 8 specifically enhances the safety and performance of these structures in seismic conditions. 2.3 Digital image correlation (DIC) Digital image correlation is a widely used contactless technique that processes images to accurately measure deformations in specimens. This technique works by capturing deformation across the entire surface of the specimen, which enables the determination of the developed strain at each point on the specimen’s surface. Therefore, it is possible to reconstruct the displacement and strain fields during the deformation process [12]. The implementation of the 2D DIC method involves three sequential steps: the specimens and the set up of the test are prepared, then the capturing of images of the specimen’s planar surface before and after loading and finally the processing of the captured images using a software to obtain the displacement and strain data. 23 CHAPTER 2. STATE OF THE ART The typical set up of the 2D DIC method is shown in Figure 2.7. The specimen’s surface need a random gray instensity distribution, knows as speckle pattern, which deforms along with the specimen to provide deformation data. This speckle pattern can be achieved either using a spray with black paint with a white background (Figure 2.8) or it can be the natural texture of the specimen’s surface. Figure 2.7: Schematic experimental set up for the 2D DIC method. Illustration adapted from [12]. The placement of the camera is normal to the specimen’s surface in order to capture its planar surface during the different loading states, this is due to a change in magnification of the captured images when there is an out-of-plane motion, which causes additional in-plane displacements. To ensure accurate results, this should be avoided. Additionaly, geometric distortion within the imaging system must not occur, however they often occur, causing impairment of the ideal alignment between the physical and the imaged points, producing additional displacements. Once the digital images of the specimen have been captured before and after deformation, the DIC computes the detected movement of each image point by comparing the images of the specimen’s surface in their different states. Figure 2.8: Random speckle pattern. 24 CHAPTER 3. EXPERIMENTAL PROGRAMME A total of 12 prisms were prepared, in order to be tested at four different ages (3 prisms/age), which were 7, 14, 21 and 28 days to ensure the accuracy of the testing procedure and to evaluate the evolution of the mortar’s mechanical properties over time. The specimens were stored at room temperature and covered with a plastic to ensure the humidity. Characterization of mortar – Flexural Strength As it was previously mentioned, three prisms for each age of the specimens were tested to determine the mortar’s flexural strength. According to the European standard EN 1015-11:1999 [14] , the load was applied at a uniform rate in the range 10 N/s to 50 N/s, so that failure occurs withing a period of 30 s to 90 s. In this experimental programme, the applied load was kept constant at a rate of 10 N/s. Therefore this was done using the Ibertest 10, which is hydraulic press machine with a capacity of 10 kN. (a) (b) Figure 3.6: Flexural test setup according to EN 1015-11:1999 [14]: (a) Set up of the test; (b) Typical failure mode. The calculation of the flexural strength of the mortar is obtained using the following equation provided in the standard: 𝑓𝑚, 𝑓 =1.5𝐹𝑙 𝑏𝑑2(3.1) Where •F is the maximum load applied to the specimen [N] •l is the distance between the axes of the support rollers [mm] •b is the width os specimen [mm] •d is the depth of the specimen [mm] 31 CHAPTER 3. EXPERIMENTAL PROGRAMME The results of the flexural strength of the hardened mortar are shown in Table 3.2. As expected, the binding mortar increases its flexural strength with curing period, given that the mortar continues to hydrate and gain strength over time. The coefficient of variation is within the range that indicates a good consistency and uniformity of the material’s properties. Table 3.2: Flexural Strength 𝑓𝑚, 𝑓 of the binding mortar used for the construction of the specimens. Days Prism Maximum Force [kN] Flexural Strength [MPa] 𝑓𝑚, 𝑓 [MPa] CoV 7 1.1 * * 0.94 5.1%1.2 0.39 0.90 1.3 0.41 0.97 14 2.1 0.51 1.19 1.16 10%2.2 0.44 1.03 2.3 0.54 1.26 21 3.1 0.68 1.60 1.53 9.0%3.2 0.58 1.37 3.3 0.69 1.62 28 4.1 0.72 1.68 1.78 5.0%4.2 0.77 1.81 4.3 0.79 1.85 * No data available, due to the prism’s breakage during the demoulding process. 0 5 10 15 20 25 30 Age [Days] 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 fm,f [MPa] Mortar flexural strength evolution Figure 3.7: Evolution in time of the flexural strength of the binding mortar. 32 CHAPTER 3. EXPERIMENTAL PROGRAMME Characterization of mortar – Compressive Strength Following the completion of the flexural strength test, the two halves of each prism were subsequently utilized to characterize the compressive strength of the mortar in accordance to the EN 105-11:1999 [14]. The load was applied without shock and increased until failure occurred, at a rate of 75 N/s using the Ibertest 10 for the specimens of age 7 and 14 days, whereas for the specimens of 21 and 28 days the Ibertest 200 was used, in case the compressive strength exceeded the machine capacity of 10 kN. (a) (b) (c) Figure 3.8: Compressive test according to EN 1015-11:1999 [14] : (a) Set up of the test; (b) Halves used from the flexural test; (c) Typical hourglass failure mode. In Table 3.3, the results of the compressive strength of the binding mortar, 𝑓𝑚,𝑐 , over time are represented. It can be observed that there is a compressive strength development that increases over time, due to the cure and hardening of the mortar. The smallest variation in compressive strength occurs at 28 days, indicating that the mortar has reached a stable and consistent strength level by this time. The consistent low variability in the test results across all age groups suggests the testing method is accurate and produces reliable data. 3.2 Specimens preparation The standard EN 1052-3:2002 [3] specifies a method for determining the in plane initial shear strength of horizontal bed joints in masonry. A total of 12 specimens of standard triplets were constructed as seen in Figure 3.10. The triplets consisted of three bonded bricks with two mortar joints with an average thickness equals to 11mm. For the construction of the specimens, the bricks were carefully selected with no flaws, such as cracks. The bearing surfaces of the masonry units were cleaned of any adherent dust, and 33 CHAPTER 3. EXPERIMENTAL PROGRAMME Table 3.3: Compressive Strength 𝑓𝑚,𝑐 of the binding mortar used for the construction of the specimens. Days Prism Maximum Force [kN] Compressive Strength [MPa] 𝑓𝑚,𝑐 [MPa] CoV 7 1.1A 4.30 2.69 2.86 4.9% 1.1B 4.47 2.79 1.2A 4.59 2.87 1.2B 4.72 2.95 1.3A 4.48 2.80 1.3B 4.93 3.08 14 2.1A 5.89 3.68 3.78 6.2% 2.1B 6.15 3.84 2.2A 6.04 3.77 2.2B 6.03 3.77 2.3A 6.64 4.15 2.3B 5.50 3.44 21 3.1A 8.27 5.17 4.94 3.9% 3.1B 7.88 4.92 3.2A 7.56 4.73 3.2B 7.86 4.92 3.3A 8.25 5.16 3.3B 7.60 4.75 28 4.1A 9.13 5.71 5.80 2.8% 4.1B 9.55 5.97 4.2A 9.15 5.72 4.2B 9.14 5.72 4.3A 9.65 6.03 4.3B 9.01 5.63 34 CHAPTER 3. EXPERIMENTAL PROGRAMME 0 5 10 15 20 25 30 Age [Days] 0 1 2 3 4 5 6 7 fm,c [MPa] Mortar compressive strength evolution Figure 3.9: Evolution in time of the compressive strength of the binding mortar. Figure 3.10: Constructed triplet specimens. then they were submerged in water in order to reduce their absorption ratio from the mortar. The mortar was then mixed with the corresponding amount of water, as previously mentioned in section 3.1.2. 35 CHAPTER 3. EXPERIMENTAL PROGRAMME (a) (b) (c) Figure 3.11: Construction process of the specimens: (a) Sumberging the bricks; (b) Mixing of the mortar; (c) Alignment and leveling of the masonry units. The lower unit was placed horizontally on a clean leveled surface, and was filled with mortar. The next unit was positioned to achieve an approximate mortar joint thickness of 11 mm. The masonry unit was checked for linear alignment and level using a measuring tape and a spirit level, as seen in Figure 3.11c. Excess mortar was then removed with a trowel, and the procedure was repeated for the top unit. In Figure 3.12, it is displayed the average geometric characteristics of the triplet specimens, which show a total width of 163 mm and a height of 235 mm. The specimen consists of three bricks of 47 mm separated by two mortar joints, each 11 mm wide. After the construction of the specimens, the triplets were covered with a damp cloth and then covered with plastic (Figure 3.13). The specimens were stored under laboratory conditions for 28 days. The triplets were then painted white for the base of the speckle pattern for the DIC. 3.2.1 Test setup According to the European Standard EN 1015-3:2002 [3] at least three specimens at three different precompression levels must be tested. All of the 12 constructed specimens were tested. This was done in order to achieve more representative results, since there were differences in the maximum obtained forces. 36 CHAPTER 3. EXPERIMENTAL PROGRAMME Figure 3.12: Geometric characteristics of the specimens in mm. Figure 3.13: Constructed specimens covered with a damp cloth and a plastic sheet. Figure 3.14 shows the setup of the test. The machine Instron 8505 was used to apply the shear force through a rolling loader, which has a load capacity of 200 kN. Given that the normalised compressive strength of the brick had a value of 18.96 MPa ≥10 MPa, the EN 1052-3:2002 [3] standard states that the precompression levels to be used should give values of around 0.2 MPa, 0.6 MPa and 1 MPa. These precompressions stresses were kept constant during the entire test using a loading jack with a capacity of 98.07 kN. Strawboards were placed both on the distribution plate and retaining plate in order to ensure an 37 CHAPTER 3. EXPERIMENTAL PROGRAMME equally stress precompression stresses to avoid local normal stress concentrations. The end of the units of each specimen were supported using pieces of steel to ensure good contact in order to reduce the possible bending effect. Once the specimen was placed, cameras were set and adjusted both on the frontal and posterior face in order to monitorise the displacement field using Digital Image Correlation. Load displayers were positioned within the camera’s range to later synchronsize the beginning of the test with the application of the load of the DIC obtained information. For the implementation of the DIC, two different cameras were used: a NIKON to capture pictures on the posterior part of the setup and an integrated camera in the software for the front part. The loading protocol comprised two stages. The first stage involved the application of the precompression load, perpendicular to the mortar joint, at a rate of 10 kN/min. Once the precompression was reached for each level it was kept constant. In the second stage the imposed displacement was applied at a rate of 0.5 mm/min through a ball hinge placed in the centre of the top central steel plate, in compliance with the standard. In parallel the cameras for the DIC acquisition were also started. Table 3.4: Correspondence of cameras used according to the analyzed faces. Precompression [MPa] Specimen Face Shell Camera 0.2 6Front Thin GOM Posterior Thick Nikon 9Front Thick GOM Posterior Thin Nikon 11 Front Thick GOM Posterior Thin Nikon 0.6 7Front Thick GOM Posterior Thin Nikon 10 Front Thick GOM Posterior Thin Nikon 12 Front Thick GOM Posterior Thin Nikon 1.0 1Front Thin GOM Posterior Thick Nikon 3Front Thin Nikon Posterior Thick GOM 8Front Thick GOM Posterior Thin Nikon The 2D DIC technique was used to monitor the deformation and the failure modes for the frontal and back faces of the specimens. The software used was Zeiss Inspect. Five virtual points were 38 CHAPTER 3. EXPERIMENTAL PROGRAMME Figure 3.14: Set up of the test. positioned within the middle brick where the shear force was applied in order to accurately obtain its the displacement, Figure 3.15 shows their positioning. It must be noticed that the picture is upside down since the camera was positioned in that way, however this did not affect the results. Figure 3.15: Distribution of the virtual points. 39 CHAPTER 3. EXPERIMENTAL PROGRAMME 40 CHAPTER 4. RESULTS Table 4.2: Experimental results of shear test on the triplet specimens corresponding to a precompression stress level equal to 𝜎𝑐=0.60 MPa. ID Front Back 𝐹𝑚𝑎𝑥 [kN] 𝜎[MPa] 𝜏[MPa] 744.5 0.60 0.85 10 45.23 0.60 0.86 12 46.77 0.60 0.90 Average 45.50 0.60 0.87 CoV 3% — 3% 01234 Displacement [mm] 0 0.2 0.4 0.6 0.8 1 Shear Stress [MPa] 0.6 MPa NIKON Thin shell 0.6MPa 7 NIKON Thin Shell 0.6MPa 10 NIKON Thin shell 0.6MPa 12 GOM Thick shell 0.6MPa 7 GOM Thick shell 0.6MPa 12 Figure 4.6: Comparison of the experimental shear 𝜏displacement 𝛿curve for precompression stress level 0.6 MPa obtained by DIC for the thin and thick shell. 47 CHAPTER 4. RESULTS (a) (b) Figure 4.7: Final condition of specimen 12. (a) Sliding failure observed on both sides of the specimen; (b) Buckling failure observed in the middle brick of the specimen. Figure 4.8: Development of cracking on the thick shell of specimen 12 throughout the displacement - load curve for precompression stress level 0.6 MPa obtained by DIC. 4.4 Precompression stress level equal to 𝝈=1.0 MPa The following table, presents the experimental results for precompression stress level equal to 𝜎=1.0 MPa. The average shear strength is 1.10 MPa (CoV = 13%), which increases with higher normal stresses. Due to the composite nature of masonry, this scattering of results is considered acceptable. The curves in Figure 4.9 show greater variability. In this case, the displacement is more pronounced in the thin shells before reaching the peak compared to the thicker ones. This 48 CHAPTER 4. RESULTS increased displacement may be attributed to the asymmetry of the brick, where buckling could have occurred due to its difference in width. All of the specimens did not separate, nevertheless the thinner shells failed by buckling on specimens 3 and 8, and on both shells in specimen 1. (Figure 4.10). For this precompression stress level, the crack evolution of specimen 1 is represented in Figure 4.11. The same behaviour is observed as in the previous cases, where vertical cracks start to develop on the supports region. However, in this case, before reaching the peak cracks on the upper region are generated. Once the peak is reached, more cracks start to grow followed by horizontal cracks due to buckling. Table 4.3: Experimental results of shear test on the triplet specimens corresponding to a precompression stress level equal to 𝜎𝑐=1.00 MPa. ID Front Back 𝐹𝑚𝑎𝑥 [kN] 𝜎[MPa] 𝜏[MPa] 1 60.6 1.00 1.15 3 49.58 1.00 0.94 8 63.2 1.00 1.21 Average 57.79 1.00 1.10 CoV 13% — 13% A comparison of the obtained parameters, 𝑐and 𝜙, as well as the displacement behavior of the triplets with data from previous studies reveals both similarities and differences. In the study by Segura et al. (2021) [16], for masonry constructed with solid extruded bricks and Portland cement mortar, the cohesion was found to be 0.165 MPa with an internal friction angle 49 CHAPTER 4. RESULTS 0 1 2 3 4 5 Displacement [mm] 0 0.2 0.4 0.6 0.8 1 1.2 1.4 Shear Stress [MPa] 1.0 MPa GOM Thin shell 1.0MPa 1 GOM Thin shell 1.0MPa 3 NIKON Thin shell 1.0MPa 8 NIKON Thick shell 1.0MPa 1 NIKON Thick shell 1.0MPa 3 GOM Thick shell 1.0MPa 8 Figure 4.9: Comparison of the experimental shear 𝜏displacement 𝛿curve for precompression stress level 2.0 MPa obtained by DIC for the thin and thick shell. Figure 4.10: Final condition of specimen 1, where buckling failure is observed. of 36.27°. These values reflect a lower cohesion and a slight difference in the internal friction angle when compared to this present research. The higher cohesion observed in this study may be attributed to the presence of perforations within the bricks, which likely enhanced an interlocking mechanical effect and, subsequently, increased cohesion. Furthermore, Calder´ on et al. (2022) [17] conducted a study for multiperforated clay bricks. The study reported a higher value for the cohesion of 1.1 MPa compared to the on this research of 0.39, however their angle of internal friction angle has a similar value of 36.8°. However their results had more scatter compared to this research. 50 CHAPTER 4. RESULTS Figure 4.11: Development of cracking on the thick shell of specimen 1 throughout the displacement - load curve for precompression stress level 1.0 MPa obtained by DIC. 51 CHAPTER 4. RESULTS 52 Chapter 5 Conclusions This study focused on investigating the shear strength and mechanical behavior of masonry triplets subjected to different precompression levels. The main objective was to evaluate the cohesion (𝑐) and internal friction angle (𝜙), which define the shear strength and displacement behavior of these triplets. A total of 12 specimens were constructed and tested, though only the most consistent results were selected to ensure accurate analysis. All of the computed parameters were calculated based on the equations from EN 1052-3 [3]. The maximum shear stresses for each precompression values were plotted, and a linear regression was found, resulting in a failure envelope consistent with the Mohr-Coulomb failure criterion. From this, the mean initial shear strength ( 𝑓𝑣𝑜) and internal friction angle (𝜙) were determined, with 𝑓𝑣𝑜 =0.312 MPa and 𝜙=29.45°. Experimental results demonstrated that shear strength increased with higher precompression, and that the average shear strength was 0.52 MPa at 0.20 MPa precompression, 0.87 MPa at 0.60 MPa precompression and 1.10 MPa at 1.00 MPa precompression. The variation in the results ranged between 3-13%, which is considered acceptable in masonry testing. The predominant failure mode observed was sliding shear failure along the mortar-brick interface, although buckling was also noted both in thick and thin shells in most of the specimens. The analysis of the shear stress - displacement curves showed an almost linear behavior prior to reaching peak shear stress, followed by a sudden drop. 5.1 Future works What this study has characterized was a specific type of shear failure occurring at the joint. To gain a more comprehensive understanding of the shear behavior across the entire masonry unit, additional tests would need to be performed, such as: •Diagonal compression test: Diagonal cracking, a common failure mode of masonry walls 53 CHAPTER 5. CONCLUSIONS subjected to shear, often occurs following seismic events. To replicate this type of loading, diagonal compression tests are conducted either in the laboratory or on-site. •Shear compression test: A vertical load is continuously applied to introduce precompression into the wall, which is securely fixed to the ground. Subsequently, the top of the wall is either pushed or pulled, subjecting the wall to shear forces. •A study of this type of brickwork using Finite Element Methods (FEM). 54 Chapter 6 Sustainability Analysis and Ethical Implications In this final chapter, the environmental, economic and social impacts due to the development and execution of this final degree project is discussed, assessing the ethical considerations related to these impacts. Furthermore, it the project contributes directly and indirectly to several Sustainable Development Goals (SDGs) outlined in the 2030 Agenda, reinforcing its alignment with global sustainability objectives. 6.1 Environmental impact During the development of the this final thesis, the environmental footprint includes the consumption of resources such as energy and materials, as well as waste generation. The use of computers and laboratory equipment, i.e. machines, contributes to energy consumption, while the testing activities generate waste from the triplet specimens, i.e. the bricks and mortar. The construction of masonry built with perforated bricks, which are lighter and more environmentally friendly than traditional solid bricks, contributes to reducing the 𝐶𝑂2𝑒in construction. 6.2 Economic impact The economic impact during the development phase is reflected in the resources required for the completion of the final thesis. These include material costs for laboratory testing, such as bricks and mortar, and the time invested by the student, the supervisors, the laboratory technicians and the bricklayer. In terms of project execution, the economic viability of the proposed solution lies in the potential savings it offers through the use of perforated bricks, sincee they are lighter and easier 55 CHAPTER 6. SUSTAINABILITY ANALYSIS AND ETHICAL IMPLICATIONS to transport and also require less material. This reduction in material consumption may lower construction costs, making the solution economically attractive. 6.3 Social impact The social impact during the development of the thesis is primarily experienced by those involved in the project. The student gains knowledge and practical skills in the structural analysis of masonry and manipulation of the DIC technology, while the supervisors provide guidance, contributing to the development of new expertise in the field. Furthermore, the execution of the project has the potential to positively impact society by improving the safety and sustainability of masonry buildings constructed with perforated bricks. The project’s results could enhance building resilience, particularly in areas prone to seismic activity. This would have a direct impact on public safety and benefiting communities that rely on masonry structures for housing and public infrastructure. 6.4 Ethical implications Several ethical considerations related to environmental, social, and economic impacts have been taken into account. The project responds to the need for increased knowledge in the field of the structural behavior of masonry with perforated bricks under shear stress, particularly by using modern techniques such as Digital Image Correlation (DIC). This need was defined by the interest in improving the structural strength of masonry constructions, thereby contributing to the safety of structures in seismic zones. 6.5 Impact in relation to the Sustainable Development Goals (SDG’s) This final thesis, contributes to various Sustainable Development Goals (SDGs) of the 2030 Agenda, particularly SDG 9, SDG 11 and SDG 12. SDG 9: Industry, Innovation, and Infrastructure This project encourages research and development of new techniques to characterize the behavior of masonry under shear stress, utilizing modern technologies such as Digital Image Correlation (DIC). This contributes to innovation in the construction sector and promotes the use of more efficient and sustainable materials, such as perforated bricks. 56 Appendix B Appendix for Chapter 4 Table B.1: Recorded paramaters during testing for all precompressions stress levels. Precompression stress level [N/mm2] Specimen 𝐹𝑖,𝑚𝑎𝑥 [N] 𝐹𝑝𝑖 [N] 𝐴𝑖[mm2]𝑓𝑣𝑜𝑖 [N/mm2]𝑓𝑝𝑖 [N/mm2] 0.2 6 26500 5100 26290 0.50 0.19 9 27861 5100 26070 0.53 0.20 11 26874 5100 26070 0.52 0.20 0.6 7 44490 15400 26180 0.85 0.59 10 45226 15400 26180 0.86 0.59 12 46767 15400 26070 0.90 0.59 1 1 60590 25600 26290 1.15 0.97 3 49584 25600 26290 0.94 0.97 8 63199 25600 26180 1.21 0.98 63 APPENDIX B. APPENDIX FOR CHAPTER 4 B.1 Precompression stress level equal to 𝜎= 0.2 MPa Figure B.1: Final condition of specimen 6, where buckling is observed on the thin shell. (a) Sliding failure on the left mortar joint. (b) Sliding failure on the right mortar joint. Figure B.2: Final condition of specimen 9. 64 APPENDIX B. APPENDIX FOR CHAPTER 4 (a) Buckling failure observed on the thin shell of the specimen. (b) Buckling failure observed on the thick shell of the specimen. (c) Sliding failure observed. Figure B.3: Final condition of specimen 11. 65 APPENDIX B. APPENDIX FOR CHAPTER 4 B.2 Precompression stress level equal to 𝜎= 0.6 MPa (a) Buckling failure observed on the thick shell of the specimen. (b) Sliding failure observed on the right mortar joint. Figure B.4: Final condition of specimen 7. Figure B.5: Final condition of specimen 10, where the mortar joints did not separate from the brick. 66 APPENDIX B. APPENDIX FOR CHAPTER 4 B.3 Precompression stress level equal to 𝜎= 1.0 MPa Figure B.6: Final condition of specimen 3, where buckling failure in the middle brick is observed. Figure B.7: Final condition of specimen 8, where buckling failure in the middle brick is observed. 67 APPENDIX B. APPENDIX FOR CHAPTER 4 68 References [1] Mario Como. Statics of Historic Masonry Constructions, volume 9 of Springer Series in Solid and Structural Mechanics. Springer International Publishing, Cham, 2017. [2] Emerson Cuadros-Rojas, Larisa Garcia-Ramonda, Pere Roca, and Luca Pel` a. Experimental analysis of the compressive behaviour of perforated brick masonry using digital image correlation. Construction and Building Materials, 431:136471, June 2024. [3] European Committee for Standardization (CEN), EN 1052-3 Methods of test for masonry - Part 3: Determination of initial shear strength, 2002. [4] Nils Kummer and Bert Bielefeld. Masonry Construction. Birkh¨ auser Verlag, Boston, basics edition, 2022. 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