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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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

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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Waste glass (WG), as a nonbiodegradable material, poses a threat to environmental protection. The reuse of WG as a raw material to replace cement or aggregate in concrete production is gaining attention for recycling purposes. However, the optimal proportion of WG in concrete mixtures and its particle size distribution are hard to determine. Large glass particles are prone to leading to the undesirable alkali–silica reaction (ASR) in concrete. Therefore, in this study, cement and aggregate in concrete mixtures are partially replaced by combinations of glass powder (<30 μm) and glass beads (0.2–1.7 mm), respectively. Glass concretes (GCs) containing waste glass at various replacement ratios (0, 10, 15, 20, and 30%) are prepared, and their flowability and compressive strength are evaluated and compared. Finally, steel tubes filled by ordinary concrete (OCFSTs) and steel tubes filled by glass concrete (GCFSTs) are fabricated and tested in axial compression. The test results show that the slump and slump flow increase when the replacement ratio is lower than 20%, and the maximum slump value (250 mm) is achieved for concrete with the use of 20% waste glass. With regard to compressive strength, as the glass replacement percentage is increased, the compressive strength of GC continues to reduce. The maximum decrease of compressive strength (merely 70% of compressive strength for original concrete) is observed in GC mixed with 20% glass, which might be attributed to the smooth surface of glass, consequently weakening the interfacial bond strength between the glass and matrix. In terms of the bearing capacity of GCFSTs, the axial compressive strength of GCFSTs decreases as more GC is used. However, no obvious reduction is observed compared to OCFSTs (less than 10% for GCFSTs containing 30% GP). Moreover, GCFSTs show greater (no less than 25% more) deformational ability at peak strength over OCFST columns, demonstrating that GC is a promising alternative for normal concrete. Finally, the feasibility of existing design codes (AISC, EC4, and GB50936-2014) to assess the bearing capacity of GCFSTs is evaluated by comparing the test and calculated results. The current codes, in general, give a conservative prediction and EC4 provides the closest value (predicted to experimental peak load ratio is 0.9).

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To enhance the structural and seismic resistance, as well as durability of concrete structures, an ultra ductile fiber reinforced cementitious composites called Engineered Cementitious Composite (ECC), also known as Strain Hardening Cementitious Composite (SHCC), was developed. ECC has a similar compressive and tensile strength to conventional concrete, but it exhibits a pseudo-strain-hardening behaviour under uniaxial tension with excellent crack control ability. The ultimate tensile strain of ECC can reach 3–12%, which is 300–1200 times higher than that of concrete. It is reported that ECC can also exhibit at least twice as high shear carrying capacity compared to traditional concrete, signifying a potential to use ECC material in shear-resistance elements. However, the shear resisting mechanism of reinforced ECC (R/ECC) members is still not clear. In most existing codes and models, the shear strength of reinforced structural members (Vu) is divided into two parts, i.e., shear resistance coming from the matrix (Vc) and from the transverse reinforcement (Vs). To quantify accurately Vc and Vs and also their development throughout the loading, a well-designed testing method consisting of continuous strain quantification along the stirrups, was used in this research. Six steel reinforced beams incorporating different matrix (ECC, concrete and mortar) were tested under four-point bending. The test results indicated that Vc changed continuously with the propagation of shear crack, whereas the stirrups that crossed the critical shear crack, did not always yield at the ultimate shear resistance.@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

Ultra-High Performance Fiber-Reinforced Concrete (UHPFRC) is, due to its superior mechanical properties and low permeability, a promising material for the restoration and improvement of the mechanical resistance and durability of existing Reinforced Concrete (RC) structures. This paper reviews the strengthening applications of UHPFRC in flexure, shear and punching shear, with a focus on shear performance of hybrid structures and the UHPFRC-concrete interface behavior which is governing the response of the hybrid beams. Holistic review approach is adopted considering not only structural behaviour of hybrid UHPFRC-concrete beams at the macro-scale, but also parameters governing the interface behaviour between concrete and UHPFRC at the meso- and micro-scale. Current analytical and numerical methods to predict the shear or punching shear capacity of RC structures strengthened with UHPFRC are reviewed and critically analyzed. Furthermore, the frequently overlooked role of interface, the effects of bonding technique, moisture exchange between the two materials, differential shrinkage and the role of coupled environmental and mechanical loads are discussed. It is observed that although extensive research work has been conducted to study the performance of hybrid UHPFRC-concrete structures, poor understanding of the behavior at the interface between concrete and UHPFRC, the role of thermal and hygral gradients and stress concentration for premature debonding, and the lack of reliable models and design codes impede the wide application of UHPFRC.@en