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.

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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.

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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.@en

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.

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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.

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In the current study, experiments and numerical simulations were carried out to investigate the cracking behavior of reinforced concrete beams consisting of a very thin layer (i.e., 1 cm in thickness) of SHCC in the concrete cover, tension zone. A novel type of SHCC/concrete interface that features a weakened chemical adhesion but an enhanced mechanical interlock bonding was developed to facilitate the activation of SHCC. The study involved testing hybrid SHCC/concrete beams that have various types of interfaces. The results were compared to the control reinforced concrete beams that do not have SHCC in the cover. Four-point bending tests were performed with the beams and Digital Image Correlation (DIC) was utilized to track the development of crack pattern and crack width. Results show that hybrid beams possessed similar load bearing capacity but exhibited a significantly improved cracking behavior as compared to the control beam. With a 1-cm-thick layer of SHCC, the maximum crack width of the best performing hybrid beam exceeded 0.3 mm at 53.3 kN load, whereas in the control beam the largest crack exceeded 0.3 mm at 32.5 kN load. The hybrid beam with the proposed new interface formed 10 times more cracks in SHCC than the hybrid beam with a simple smooth interface and had an average crack width less than 0.1 mm throughout the loading. The lattice model has successfully showcased its ability to predict and offer valuable insights into the fracture behavior of hybrid systems. The simulation results indicate that the presence of a weak interface bond, coupled with mechanical interlocking, can effectively facilitate the activation of SHCC, resulting in the formation of more cracks and a delayed progression towards the maximum crack width. As the volume ratio of SHCC used in the hybrid beams is only 6%, the current study highlights the strategic use of minimum amount of SHCC in the critical region to efficiently enhance the performance of hybrid structures.

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A promising solution for reducing the carbon footprint of concrete is the use of alkali-activated concretes (AAC). Before this material can be widely applied, its long-term behaviour needs to be understood, especially since some studies reported a decrease of mechanical properties over time. Similarly, Prinsse et al. reported decreasing mechanical properties, especially elastic modulus and flexural and splitting tensile strength for the studied slag-based AAC (S100) and the blended slag- and fly-ash-based AAC (S50) up to the tested age of 2 years. They hypothesized that these decreases could be only temporarily. To test that hypothesis, this study continued to monitor the mechanical properties of both AACs up to the age of 5 years. As a reference, two OPC-based concretes (OPCC), with different strength classes, are monitored up to the age of 3.5 years. In addition, the internal structures of the concretes are assessed for carbonation and internal micro cracking. S100 shows stabilization of the elastic modulus and the compressive strength, whereas the tensile splitting strength continued to decrease up to 5 years. This is attributed to a combination of carbonation and drying, since the microscopic analysis showed increased porosity around the ITZ and in the carbonated region. In addition, S50 shows an ongoing decrease of all tested mechanical properties, which is attributed to carbonation. No decreases in mechanical properties are found for OPCC.

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Ultra-high performance fiber-reinforced concrete (UHPFRC) exhibits high compressive and tensile strength together with outstanding durability. Due to these superior properties, UHPFRC is promising for retrofitting existing reinforced concrete (RC) bridges. While research and on-site applications show the significant improvement of RC structures strengthened with UHPFRC in flexure, information regarding the shear behavior of such UHPFRC composite structures is limited. Therefore, the primary objective of the present study is to investigate the efficiency of UHPFRC in enhancing the shear strength of RC beams. The material properties including the compressive and tensile strength, and shrinkage of UHPFRC are experimentally measured. The shear deficient reference beam (RB) is designed, and UHPFRC is applied on the lateral sides of the RB. Two different bonding techniques to apply UHPFRC are employed: (1) casting fresh UHPFRC in-situ; and (2) gluing precast UHPFRC plates with epoxy resin. The interface properties under each technique are examined. Results demonstrate that compared to RB, strengthened beam (ST) with bonded prefabricated UHPFRC using epoxy resin shows an around 110% and 60% enhancement in strength and ductility, respectively. However, with in-situ casting of UHPFRC, due to restrained shrinkage, the delamination between UHPFRC and concrete beam occurs and a negligible strengthening effect is observed. The findings indicate that the ability of UHPFRC can be fully utilized only provided that the interface strength is sufficient to prevent premature debonding for the hybrid UHPFRC-concrete structure.@en

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.@en

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.@en

Hoewel geopolymeerbeton op laboratoriumschaal uitgebreid is onderzocht, zijn praktische toepassingen en ervaringen nog maar beperkt beschikbaar. Bovendien bestaan er voor een brede en grootschalige constructieve toepassing een aantal uitdagingen. Er is een onderzoeksproject gestart waarin een zelfverdichtend geopolymeerbeton (ZGB) is ontwikkeld voor toepassing in een brug. In de tweede fase van het onderzoek zijn de mogelijkheden voor productie op industriële schaal onderzocht.@en

Ultra-high performance fiber reinforced concrete (UHPFRC) is an advanced cementitious composite with high compressive strength and low permeability. Due to its excellent mechanical properties and superior durability, UHPFRC is considered promising for strengthening of the existing concrete bridges. In order to examine its strengthening efficiency for shear capacity, an experimental study is carried out on shear-deficient beams without stirrups. Strengthening method comprising precast UHPFRC laminates being glued with epoxy resin on two lateral sides of the reinforced concrete beams, is examined. To investigate the robustness of the system under severe exposure conditions, some beams are subjected to freeze-thaw (FT) cycles. Beams are tested to failure under three-point bending configuration. Test results show that for epoxy resin bonding, UHPFRC shear strengthening is a promising method to increase the load and deformational capacity, and to limit the crack openings. The load capacity is doubled, and the deformational capacity is increased by around 60%. After exposure to 30 FT cycles, the strengthening efficiency and fracture behaviour of UHPFRC composite beams seem not to be affected. It seems that the interfacial bond strength is sufficient to prevent premature debonding between UHPFRC and NC, which under combined action of environmental exposure (e.g. FT) and mechanical loading might become a challenge. Finally, a finite element model is developed to predict and understand the shear behaviour of the reference and strengthened beams. In general numerical results show good agreement with the experimental results in terms of failure pattern and peak load prediction once the perfect bond model is used for the interface between UHPFRC and NC. In order to better understand the role of governing parameters on the shear capacity of the composite member, parametric studies are conducted focusing on the role of varying UHPFRC softening behaviour and UHPFRC-concrete interface properties.

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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.

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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.

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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.

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In the current study, experiments were carried out to investigate the structural performance of reinforced concrete (RC) beams with a self-healing cover zone. The cover zone consists of a 1.5-cm-thick layer of bacteria-embedded strain hardening cementitious composite (SHCC) for a combination of crack width control and crack healing. The aim is to bring together two emerging technologies (i.e., self-healing and strain-hardening) that show great potential for realizing highly efficient concrete structures. RC beam without the self-healing cover was also prepared as the control specimen for comparison purposes. The experimental program includes loading the beams to failure in four-point bending configuration and sawing the beams to segments for crack pattern analysis and crack healing. Results show that the beams with self-healing cover exhibited a 45-60% improvement in structural capacity. The crack patterns of the hybrid beams were also largely modified. While the reference beam formed only a few major cracks, the hybrid beams formed around 40 fine cracks in the constant bending moment region with an average crack width smaller than 0.2 mm even at maximum load. By having an improved cracking behavior and an enhanced self-healing capacity, it is expected that the beams with a self-healing cover will possess an extended service life at the expense of minimal additional cost.@en

Ultra-high performance fiber-reinforced concrete (UHPFRC) has emerged as one of the promising materials for strengthening of concrete structures. For the strengthening application of UHPFRC, one of the primary concerns is to evaluate the degradation of bond behavior and structural re-sponse of strengthened elements under harsh environmental conditions. Therefore, an experi-mental program has been carried out to investigate the interfacial behavior between UHPFRC and normal concrete, as well as the shear performance of UHPFRC-concrete hybrid beams subjected to combined freeze-thaw cycles and mechanical load. In this study, two groups of shear-deficient reinforced concrete beams were first strengthened by UHPFRC precast panels using epoxy resin. Then, the specimens subjected to 0 and 30 freeze-thaw cycles were loaded to failure under three-point bending. The results indicate that the utilization of epoxy resin is an effective bonding tech-nique to ensure the integral performance of the composite beams and the shear capacity is greatly enhanced with the application of UHPFRC. In addition, it is observed that the effect of applied freeze-thaw regime on the UHPFRC-concrete interfacial bond strength and shear resistance of unstrengthened and strengthened beams is negligible.@en

Net toen we dachten dat we het coronaverhaal achter de rug hadden en dat alles weer naar ‘normaal’ zou gaan, kregen we een, misschien nog wel serieuzere, nieuwe schok te verwerken. De afgelopen weken waren we getuige van vele zeer alarmerende beelden. Zo veel natuurrampen: overstromingen, hittegolven, hoogst gemeten temperaturen ooit, ‘orkanen van vuur’, zo veel slachtoffers!... En eigenlijk gebeurde het allemaal in een oogwenk. En het is geen toeval of iets wat we eens in de 100 jaar mogen verwachten. Het is veel erger. Volgens het rapport [1] van IPCC (Intergovernmental Panel for Climate Change) komen deze gebeurtenissen overeen met de prognoses van specialisten en zullen dergelijke extreme situaties in de toekomst steeds vaker voorkomen. En wie is verantwoordelijk? Jij, ik…? Wij allemaal!@en

The construction industry is experiencing a significant increase in hybrid concrete structures due to the need for repairing/strengthening of existing structures and the development of novel hybrid structures. The crack development and the ultimate capacity of hybrid concrete structures may significantly be governed by the properties of interface between the two concretes, making the quantification of interface properties essential. A large number of bond tests have been reported in literature but most of them do not result in a failure directly/entirely at the interface (unless the interface is very weak), resulting in only a lower bound estimate of the interfacial strength. Furthermore, the reported interfacial properties are only determined from small-scale bond tests where structural effects (like shrinkage) are limitedly taken into account. In the current study, the most commonly used bond tests are critically assessed in terms of the stress distribution caused by their inherent boundary conditions. Furthermore a testing procedure is then discussed which can allow for the quantification of the interfacial properties. A possible structural test is also designed which forces the failure to localize at the interface and allows to determine interface properties considering structural effects.@en

The bond between concrete and reinforcement is one of the critical parameters influencing the structural behavior of reinforced concrete (RC). This research proposes a mathematical methodology to scale the reinforcement-concrete bond-slip relationship in a beam lattice modeling framework. A simplified, generalized approach based on stochastic analysis is proposed to model the interaction between the reinforcing bar and surrounding concrete at the macroscale. The approach considers the randomness of the lattice mesh and the mesh size and adopts an analytical model for the interface assuming the pull-out failure of reinforcement as input, thereby including also the mesoscale geometric effect of ribs. By using the geometric configuration of Delaunay triangulation in the random lattice mesh, the interface elements can reproduce the basic conical stress transfer mechanism in concrete. Consequently, depending on boundary conditions, and without changing the interface properties, a splitting failure and bond-slip relation for splitting failure can be predicted. The model is systematically validated in different types of pull-out tests, through flexural and finally shear tests. With limited input (properties of the concrete and analytical equation for pull-out failure), having a (strong) physical background, the model was shown to capture the fundamental fracture mechanisms in RC under different loading and confinement conditions.

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