Shear failure in conventional reinforced concrete (R/C) beams is characterized by pronounced brittleness, limited energy dissipation capacity, and a high propensity for catastrophic collapse due to the absence of discernible warning signs. This study investigates the use of high-ductility fiber-reinforced cementitious composites (Engineered Cementitious Composites, ECC) to enhance the shear performance of concrete beams. An experimental program was conducted on both R/C and reinforced ECC (R/ECC) beams subjected to shear, complemented by a novel numerical approach to quantify the contributions of arch action (V_a[jls-end-space/]) and beam action (V_b[jls-end-space/]) to shear resistance. The findings reveal a strong positive correlation between the efficiency of arch action and overall shear performance, including shear carrying capacity, deformation capacity, and ductility. Building on these insights, an optimized design strategy incorporating partial ECC replacement and pre-defined voids is proposed, illustrating the potential of a mechanism-driven approach to achieve superior shear behavior in structural elements.

The increasing demand for sustainable development in engineering practice has triggered researchers to explore solutions to reduce the CO₂ footprint caused by Portland cement (PC) production. Alkali-activated concrete (AAC), made by alkali activation of industrial by-products, poses to be a sustainable alternative to traditional Portland cement concrete. Despite vast studies on its material properties, there is still insufficient knowledge on the structural performance of reinforced AAC members, which impedes its widespread application. Bond between concrete and embedded reinforcement is critical for the structural behaviour of reinforced members, including crack propagation (crack opening and crack spacing), load carrying capacity, deformational capacity and seismic resistance. In this chapter, a critical review on the bond behaviour between AAC and reinforcement is given. A vast variety of tests have been used for studying the AAC-reinforcement bond. Similar to traditional PC concrete, different setup configurations and specimen geometries affect not only the measured bond strength, but also the nature of the bond response. Therefore, in this review different bond tests are discussed, focusing on application of both conventional steel and fibre reinforced polymer (FRP) as reinforcement. Given that AAC is a wide class of materials with largely varying mechanical properties, this study systematically classifies bond results based on different types of precursors used. Furthermore, numerical methods to predict AAC flexural response as well as the applicability of existing PC concrete design codes are summarized. It is concluded that AAC beams show comparable short-term bond behaviour with traditional PC concrete of the same strength class. Although the design codes for traditional concrete turn out to be usually applicable for the AAC-reinforcement bond, opposite trends were also reported. Finally, whereas the short-term behaviour has been widely investigated for AAC bond, systematic studies dealing with its long-term behaviour are lacking. Time dependent effects must be considered when developing reliable guidelines and recommendations for future structural design of AAC.

The introduction of alkali-activated concrete (AAC) technology to the construction industry represents a significant step toward a sustainable development and a cleaner environment by reducing environmental pollution. Currently, its application is relatively limited compared to traditional Portland cement based concrete (PCC). However, AAC has a remarkable potential for future growth and innovation despite several associated challenges and limitations. The current Chapter highlights recent progress in AAC mix design and its mechanical properties, paving the way for a broader application. The universally recognized international standards and codes for AAC, its mix design and evaluation of its long-term performance are still emerging. Unlike conventional PCC, AAC encompasses a wide class of materials with wide varying chemical composition and reaction mechanisms, depending on the choice of constituent materials (precursors and alkali activators). The mechanical properties, while diverse, reflect the flexibility of the material in response to different compositions and curing conditions. Though, non-uniformity makes consistent AAC usage challenging on the scale of PCC. Nevertheless, ongoing research and development efforts by RILEM TC 294-MPA are dedicated to tackling these challenges and enhancing the efficacy and widespread adoption of AAC technology.

Alkali-Activated Concrete (AAC) is considered as a promising alternative to conventional Portland Cement Concrete (PCC) due to its potential to reduce environmental impacts. However, its application in practical engineering is limited by, among others, insufficient understanding of the long-term structural behaviour of reinforced and prestressed AAC elements. To address this, a series of experiments were conducted on composite girders to investigate the long-term flexural behaviour. The composite girder is formed by a prefabricated prestressed AAC inverted-T girder with cast-in-situ ACC topping concrete. The midspan deflection of two composite girders, subjected to self-weight and additional sustained loading, were measured over a 9-month period. Subsequently, flexural tests under four-point bending configuration were performed at the age of 9 months and the reference age of 28 days. The results showed that the specimens tested at 9 months exhibited reduced initial stiffness, decreased cracking load and larger crack widths in the precast prestressed girder compared to those tested at 28 days. The reduction in stiffness likely stems from decreased elastic modulus and structural cracking. Meanwhile, the lower cracking load arises from prestress losses caused by ongoing (restrained) shrinkage and creep, consistent with AAC material test observations. Larger crack widths observed in the precast girder may result from a degradation of bond between AAC and prestressing strands over time. The distinct failure patterns of the 9-month specimens (anchorage failure for sample subjected to self-weight only and flexural failure for sample exposed to additional sustained load), highlighted the role of creep on bond behaviour between prestressing strands and AAC, particularly as a function of varying stress levels at the level of strands. Finally, analytical models were applied to evaluate the prestress loss and flexural behaviour of the specimens. The effective prestressing force and cracking loads at both testing ages were overestimated when the effects of (partially) restrained deformations between precast and cast-in-situ AAC were neglected. More accurate analytical predictions were achieved when these long-term effects and the level of restraint in the composite girder were considered.

Engineered cementitious composite (ECC) has been effectively applied in shear-critical structures due to its high ductility under tension and fiber bridging effect to resist crack opening and sliding. This study employed a novel monitoring system incorporating distributed strain gauges to investigate the shear resistance mechanism in reinforced ECC beams. The system enabled the measurement of full-length strain distribution along the stirrups and longitudinal reinforcement. By capturing stirrup strains precisely along the critical shear cracking path, the shear contributions from transverse reinforcement (V_s) and ECC matrix (V_c) could be accurately quantified. A total of 20 reinforced ECC beams were tested under shear, and the role of governing parameters (e.g., shear span-to-depth ratio, stirrup and longitudinal reinforcement ratio) was analysed. Based on the observed shear failure mechanism, a modified truss-strut model and a simplified equation for predicting shear strength are proposed for the shear design of reinforced ECC beams.

Understanding the bond behavior between reinforcement and concrete under varying confinement conditions is essential for the design and performance assessment of reinforced concrete structures. This study employs a discrete lattice model to investigate the reinforcement-concrete bond mechanism, focusing on crack propagation, fracture processes, and stress distribution. Experimental data involving lap-spliced reinforcement bond test under different confinement conditions serve as benchmarks. In the model, concrete, reinforcement, and their interface are discretized into beam elements, while the interface properties remain constant and independent of confinement conditions. A key finding is that generating the lattice mesh through the Delaunay triangulation scheme enables the model to reproduce realistic strut-cracking patterns and conical stress transfer phenomena, thereby capturing stirrup-induced passive confinement effects without modifying interface properties. The results clarify the role of stirrup confinement in restricting concrete dilatancy and bond splitting, while bond failure is shown to depend on concrete fracture under weak confinement and on interface failure only under strong confinement. Overall, this study not only validates the discrete lattice approach for reinforced concrete bond modeling but also provides deeper insights into lap-splice failure mechanisms, offering a robust framework for structural assessment and design.

To enable wide structural application of alkali activated concretes (AACs) as a sustainable alternative to Portland cement concretes (PCCs), it is vital to understand their interaction with steel reinforcement. Even before applying mechanical loading, significant internal loads (strains), originating from cement hydration, changes in temperature and (internal) relative humidity, can be imposed and will affect reinforcement-concrete interaction. Herein, early-age and long-term induced steel strains have been measured on embedded reinforcement bars instrumented with distributed fiber optical sensors (DFOS) in reinforced concrete beams. A GGBFS-based alkali activated concrete (S-AAC) was compared to a reinforced CEM III/B-based concrete (denoted as S-PCC). Both concretes have been fog-cured for 28 days and successively exposed to 20 °C and 55 % relative humidity for a duration of 1 year. By combining bonded and unbonded DFOS strain measurements, mechanical steel strains induced by autogenous and drying shrinkage could be decoupled from temperature effects. DFOS allowed to detect local strain peaks and microcracks and determine microcrack widths along the length of reinforcement bars. Autogenous shrinkage developed rapidly in S-AAC due to the fast polymerization reaction of GGBFS and negligible early-age expansion, leading to 3 times larger shrinkage at 28 days compared to S-PCC. Yet, unlike S-PCC, S-AAC showed a slower development of shrinkage induced deformations than plain S-AAC, indicating that a significant portion of shrinkage in S-AAC is creep. In addition, autogenous shrinkage induced deformations led to microcracking in S-AAC, contributing to a lowered development of steel deformation. Under drying, S-AAC showed a rapid increase in microcrack width without further significant increases in the mean steel deformation. Microscopic images confirmed the presence of more pronounced microcracking in S-AAC compared to S-PCC. These findings are relevant to gain understanding of the long-term behavior of reinforced AACs.

This study aims to investigate the effect of incorporating different quantities of steel fibres recovered during concrete recycling on the mechanical properties of new steel fibre reinforced concrete (SFRC). Mixes contained 20 kg/m³ and 25 kg/m³ of steel fibres, with recovered steel fibres at replacement levels of 0%, 10%, 30%, and 100%. The recovered fibres were tested and categorized to determine the effect of recycling on fibre properties. The compressive strength, elastic modulus, stress–strain behaviour in compression, residual flexural strength of SFRC and inductive test were tested. The results demonstrate that incorporating a small proportion of recycled fibre alongside virgin fibre is a feasible approach, with a 10% recycled fibre replacement yielding superior performance compared to using 100% virgin fibre alone.

Self-healing concrete, with its ability to autonomously repair damages, holds promise in enhancing its structural durability and resilience. Research on self-healing concrete in the past decade has advanced in understanding the mechanisms behind healing, exploring various healing agents, and assessing their effectiveness in concrete structures. However, the full potential of self-healing concrete remains untapped unless its effects are effectively integrated into the design practices of reinforced concrete structures. Realizing this challenge, this paper synthesizes the current research progress and discusses the possibilities to consider self-healing into design codes. The focus was placed on two specific benefits of applying self-healing concrete: one centered on durability and the other on mechanical performance. Specifically, the effect of self-healing on impeding chloride penetration into cracked reinforced concrete was discussed first. Modifications of parameters in existing predictive models based on different types of healing approaches were recommended. Furthermore, the possible impact of the self-healing capacity in mitigating the stiffness reduction of concrete was also discussed. Equations that can describe the stiffness regained due to healing action are presented. In each part of the case study, limitations and challenges still hindering standardization and wider application in the construction field are discussed.

The use of 3D printed polymers in the form of lattice reinforcement can enhance the mechanical properties of cementitious composites. Methods like Fused Deposition Modelling (FDM) 3D printing enable their creation, but this process has a large (negative) effect on their mechanical properties, with a large dependency on the printing direction. Continuing on our previous study concerned with modelling the anisotropic behaviour of 3D printed polymeric reinforcement, this work focuses on the reinforcement-matrix bond. Because of the layer-by-layer filament extrusion process of the 3D printing technique, the edges of FDM 3D printed polymers are typically composed of ellipses. Based on this, it is hypothesized that morphological effects as a result of the 3D printing technique enhance the bond between 3D printed reinforcement and cementitious matrix: The elliptic geometry potentially facilitates interlocking with the cementitious mortar, thereby possibly enhancing the bond behaviour in certain directions. To investigate the geometrical directional-dependent features at the edges of 3D printed polymers in more detail, micro-scale models are developed. Geometrical effects induced by different printing configurations are studied. The simulation results are verified through meso-scale pull-out experiments. The interlocking effects as a result of the 3D printing technique show to be significant seeing a bond strength increase of up to 56 % in one of the print configurations compared to the direction without any geometrical effects.

Concrete-to-concrete interfaces are brittle and reinforced with steel to ensure force transfer and provide ductility. Recent research in ceramics and polymers shows that by implementing intricate interlocking geometries, named bistable interlocks, toughness can be added to inherently brittle materials and their connections. In this research, the “bistable interlock” concept is applied to cementitious materials (strain-hardening cementitious composites, SHCC) offering a novel approach to increase the toughness of concrete interfaces. A bistable interlock mechanism is achieved by geometrically designing double-radii surface morphologies that can lock into two hardening positions under tensile loads and is combined with material hardening interlock of SHCC. The investigation focused on the effects of interface shape (straight vs. curved) and geometric characteristics (key length and diameter of interface keys), and interface treatments (as-cast, lubricated, and prefabricated). The findings highlight the critical role of interface treatment. Specimens with an untreated, strong interface were unable to activate bistable behavior, primarily failing due to key rupture. Lubricated interfaces facilitated key pullout, demonstrating in curved specimens up to 80% higher energy absorption compared to untreated specimens. The tensile strength of the architectured interface reached about 30% of the SHCC strength, whereas its deformation capacity was doubled. These results underscore the potential for customized, tough connections and their application in the design of precast concrete components.

With the development of waste recovery techniques, previous research has revealed that coarse fractions of municipal solid waste incineration (MSWI) bottom ash (BA) after proper treatment could be applied in the construction sector, while the fines are seldom recovered in practice and normally landfilled. This study explores the potential application of fine MSWI BA (0–2 mm) as a supplementary cementitious material (SCM) in Portland cement (PC) mixtures. Mechanical and chemical pre-treatment approaches have been designed with various conditions to optimize the treating process. The chemical and mineralogical compositions, as well as the metallic Al content in BA were characterized before and after the pre-treatment. It was found that both methods are effective in removing the metallic Al content in BA, Moreover, BA derived from mechanical treatment exhibited more contribution to the hydration reaction in PC mixtures, as revealed by the amount of reaction products and mineral phases formed in hardened trial mixtures. BA obtained was further partially blended in PC mortars to evaluate the performance as compared to SCMs and inert fillers. It was found that treated BA resulted in a slight retarding effect on the reaction kinetics. Treated BA behaved better than the coal fly ash to contribute to the strength development, while the inclusion of BA did not lead to significant influences on the workability.

This study investigates the structural behaviour and self-healing performance of hybrid reinforced concrete (RC) beams, enhanced with a 1.5-cm-thick self-healing cover composed of bacteria-embedded strain hardening cementitious composite (SHCC), for its potential in crack width control and crack healing. The research focuses on the performance under both flexural and shear loading, examining aspects such as load-bearing capacity, surface crack pattern, crack propagation between layers, and healing effectiveness. Results demonstrate the successful activation of the healing function, alongside improvements in structural performance. Under flexural loading, hybrid beams exhibited greater load-bearing capacity and significantly improved crack control ability. The maximum crack width of the hybrid beams exceeded 0.3 mm at 124.7 kN load, whereas in the control beam the largest crack exceeded 0.3 mm at only 59.8 kN load. Under shear loading, while the influence of the cover on structural capacity was minimal, it notably improved post-peak ductility and energy dissipation. Interface delamination was not observed in both cases. The results of the current study demonstrate the potential of delivering the self-healing mechanism precisely where it is most needed, which presents a scalable and economically viable strategy for integrating self-healing technology into standard construction practices.

The fiber's bridging effect across the shear cracks is considered to play an important role of resisting shear in engineered cementitious composite (ECC), and fiber reinforced material in general. To quantify the shear crack kinematics (i.e., shear crack opening and sliding displacements) in reinforced ECC (R/ECC) beams, a crack measuring algorithm based on the full-field displacement spectrum is developed by using the Digital Image Correlation (DIC) technology. In addition, a novel distributed strain-measuring methodology was used to detect the strain distribution along the transverse and longitudinal reinforcement. Reinforced beams made of traditional concrete (R/C) and mortar (R/M) were used as reference. Through aforementioned monitoring schemes, the role of matrix (V_c) and stirrups (V_s) in shear resistance mechanism could be independently understood and evaluated. The R/ECC beams exhibited much higher V_c than the reference reinforced concrete (R/C) beams (by 68%∼104%). Nevertheless, the shear crack measuring results revealed that the higher shear strength in R/ECC did not always result from the fiber's bridging effect across the critical shear crack (CSC) but of high shear-resisting contribution from ECC in shear-compression zone. For a better understanding of the shear failure mechanisms, phenomenological models of shear crack kinematics in R/C and R/ECC beams are proposed.

Betonnen bruggen kunnen worden versterkt door het storten of plakken van een laag ultra-hogesterkte vezelversterkt beton (UHPFRC) aan één of beide kanten van de liggers. Hiermee kan de dwarskrachtcapaciteit worden vergroot. Bij deze toepassing is de hechting tussen het oude en het nieuwe beton van groot belang. In een bachelor eindproject aan de TU Delft is gekeken of de delaminatie tussen normaal beton en UHPFRC met infrarood thermografie kan worden beoordeeld.

In the current study, experiments were carried out to investigate the cracking behaviour of reinforced concrete beams consisting of 1-cm-thick layer of Strain Hardening Cementitious Composite (SHCC) in the concrete cover zone. The hybrid SHCC/concrete beams with different types of interfaces were tested and compared with control reinforced concrete beams without a SHCC layer. A new SHCC/concrete interface that features a weakened chemical adhesion but an enhanced mechanical bonding was also developed to facilitate the activation of SHCC. The beams were tested in four-point bending configuration, while Digital Image Correlation (DIC) was used to evaluate crack pattern development and crack widths. Results show that hybrid beams possessed similar load bearing capacity but exhibited an improved cracking behaviour as compared to the control beam. The maximum crack width of the best performing hybrid beams exceeded 0.3 mm at approximately 53.3 kN load, whereas in the control beam it exceeded 0.3 mm at only 32.5 kN load. It is thus expected that the hybrid beams developed in the current study will possess an improved durability and enhanced self-healing potential as a result of having smaller cracks, leading to an extended service life at the expense of minimal additional cost.

Although alkali activated concretes (AACs) are promising for reducing the carbon emissions of concrete, in order to enable their wide application it is vital to understand their long-term behaviour. Herein, we report the development of mechanical properties of a ground granulated blast furnace slag (GGBFS)-based AAC and a binary fly ash (FA) /GGBFS-based AAC exposed to 55% relative humidity and 20 °C up to the age of 5 years. For comparison, two ordinary Portland cement (OPC) concretes were monitored for 3.5 years. For the GGBFS-based AAC, after an initial decrease within the first 6 months the elastic compressive modulus stabilized, while its tensile splitting strength continued to decrease for the tested period of 5 years. The binary AAC showed a continuous decrease in its tensile splitting strength for 5 years and a reduction in its compressive strength after 2 years. No decreases in mechanical properties were observed in OPC-based concretes. To reveal underlying mechanisms, additional analyses were performed. Permanent degradation was observed in both AACs; the binary AAC mainly suffered from carbonation, and the GGBFS-based AAC showed microcracking. These cracks were probably caused by drying shrinkage and drying-induced chemical changes. Based on the measured mechanical properties of AAC, crack widths and stiffness of reinforced AAC beams under bending were analytically evaluated and compared to experiments. Decreases in bending stiffness and increases in crack width were observed for reinforced AAC beams tested at later ages. A bimodular approach is proposed to predict the reduction of bending stiffness in the studied AACs over time. These findings are relevant to understand serviceability limit states of reinforced AACs.

The number of hybrid concrete structures is increasing due to the need for repairing/strengthening existing structures and the development of new hybrid concrete systems. The structural response of these hybrid structures might be governed by the strength of the interface between the two concretes, making it essential to characterize the mechanical response of the interface. In this research, a notch beam tests is proposed to investigate the structural behavior of the interface. Hybrid beams consisting of Strain Hardening Cementitious Composites (SHCC) and conventional concrete are designed with a notch at mid-span and are tested under a four-point bending configuration. The effect of interface treatment (i.e. surface roughness) and the curing condition is tested using two sets of hybrid beams. The first set has three beams which are cured in sealed conditions until the day of testing and the interface is varied between smooth, profiled and roughened. The second set has two beams with smooth interface where one beam is seal cured and the other one is exposed to drying in the laboratory. The opening of the interface is visualized using Digital Image Correlation (DIC) and quantified using Linear Variable Differential Transformers (LVTDs) during testing of the hybrid beams. It is observed that increasing the roughness of the interface leads to higher load-bearing capacity and controlled opening of the interface. The beam exposed to drying showed somewhat reduced capacity, possibly due to the pre-damage caused by differential shrinkage of the two concretes.

Strain Hardening Cementitious Composite (SHCC) is an innovative type of fibre-reinforced cement-based composite that has superior tensile properties. Because of this, it holds the potential to enhance the shear capacity of reinforced concrete (RC) beams, if applied properly. This paper presents the general and distinctive properties of SHCC as well as a literature review of topics related to the contribution of SHCC layers to the shear resistance of RC beams with and without shear reinforcement. Based on the analysed results, it is concluded that the main characteristics of SHCC are its microcracking behaviour, high ductility, and increased tensile strength (between 2 and 8 MPa) at large deformations. When used in structural elements, SHCC develops multiple parallel cracks compared to concentrated cracks in conventionally reinforced concrete. The biggest disadvantage of SHCC is its significant drying shrinkage. Although showing high variability, using SHCC as laminates with a thickness of 10 mm improves the shear capacity of hybrid RC beams, but debonding of interfaces in a hybrid system occurs in some cases.

Concrete is characterized in terms of its engineering properties, mainly strength and stiffness, which are subsequently used in structural design. However, the apparent (i.e., measured) concrete properties are not intrinsic but dependent on the conditions under which the measurement is performed. Herein a combined experimental and numerical study is performed to clarify the effects of hygral gradients and resulting eigen-stresses in self-restrained concrete on its apparent strength. Compressive strength, splitting tensile strength and Young's modulus are tested. Three mixes of varying strength grades are studied: normal strength, high strength, and ultra-high performance fibre reinforced concrete. Samples are subjected to two different curing conditions. To investigate the effect of size on the apparent properties, 50 mm, 100 mm and 150 mm cubes are tested. Depending on the size of the specimen there can be an underestimation or overestimation of up to 25% of the real concrete (e.g., splitting, direct tensile) strength.