Aimed at realizing the effective strengthening and durable repair of concrete structures, particularly in emergencies like traffic interruption triggered by broken roads and damaged bridges, nine groups of specimens were designed and tested in this paper to develop the high-early-strength Engineered Cementitious Composites (HES-ECC) featured as both high early-strength and superior long-term-deformability. The high-early-strength effect of sulphoaluminate cement, silica fume, and Portland cement on HES-ECC was compared, as well as their influence on the deformation ability of HES-ECC. Moreover, the interfacial behaviors between HES-ECC and existing concrete structure were clarified, considering the effects of interfacial agents, interfacial treatment methods, and interfacial roughness. The results indicate that HES-ECC with 6% silica fume mixed could obtain both the high early-strength and superior long-term-deformability. The flexural strength at 3 h could reach 66.67% of that at 28d. The compressive strength could reach up to 28.7 MPa at 3 h, and the ultimate tensile strain could remain 4.21% at 28d. Cement paste interfacial agent could enhance the chemical adhesive bonding between HES-ECC and existing concrete while polymer modified interfacial agent was incompatible. The increased roughness of chiseled interface was beneficial to both the bearing capacity and the deformation ability. Interfacial shear performance of the grooved interface was scarcely deteriorated even if the roughness decreased by 54.18% compared with the chiseled interface. The recommended interfacial treatment is chiseled interface combined with grooved interface, as well as a thickness of 1–2 mm cement paste interfacial agent. This study provides valuable and credible experimental data for promoting the application of HES-ECC in repairing existing concrete structures in practice.@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.@en

Application of fiber-reinforced polymer (FRP) reinforcement in concrete beams may cause large deflection and crack width, as well as low shear capacity and ductility due to relatively small stiffness of FRP materials. To avoid these unfavorable factors and evaluate the shear behavior of FRP-reinforced structural members, a high-performance strain-hardening cementitious composite (SHCC) is introduced to substitute conventional concrete in reinforced beams, and four-point bending test is conducted in this study. Six FRP-reinforced SHCC beams with different transverse reinforcement ratios and shear spans, as well as one concrete reference beam, were tested. According to the test results, the FRP-reinforced SHCC beam showed enhanced shear carrying capacity and superior ductility compared with the concrete beams. The shear span to effective depth ratio as well as the stirrup ratio has a great influence on the shear behavior of FRP-reinforced SHCC beams, including the failure mode, load-carrying capacity, crack propagation, and ductility. Finally, a simplified truss-strut model for predicting shear carrying capacity of steel or FRP-reinforced SHCC beams is proposed, and a good agreement is achieved with the experimental results.@en

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

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.@en