The purpose of this study was to evaluate the use of resin injection to repair cracks in ultra-high-strength concrete (UHSC) members. As a preliminary step, the applicability of the neutron diffraction method (NDM) to investigate the effect of repairs in UHSC specimens was examined. The experimental results showed that the NDM can measure stresses in rebars in UHSC and normal concrete specimens. Therefore, in this experiment, the NDM was used to measure the bond performance of repairs with epoxy resin around the slit in normal concrete and UHSC specimens and examine the effect of repair on the UHSC specimens. Displacement around the slit was measured using a PI-shape displacement transducer. The evaluation confirmed that the bond performance of the repaired area was recovered by resin injection regardless of the concrete strength. In addition, the displacement around the slit was smaller for the injected specimens than the non-injected specimens. These experimental results clarified that by injecting resin, the same bond repair effect could be obtained in UHSC and normal concrete specimens.
The tension stiffening behavior of reinforced concrete (RC) prisms, affected by the aggregate volume expansion induced by neutron irradiation, were numerically investigated using a rigid body spring network model. First, the model was validated by comparison with the uniaxial tension test results of wet- and dry-cured (with volume contraction of concrete) RC prisms. Subsequently, different degrees of expansion strain were applied to the aggregate elements in the RC prism model and the uniaxial tension loading was simulated again. Tension stiffening decreased under larger radiation-induced volume expansion of the aggregate owing to the corresponding decrease in the concrete tensile strength with increasing damage, this behavior changed considerably according to the restraint condition. Indeed, the Young’s modulus of the restrained concrete after aggregate expansion was larger than that of the unrestrained concrete after aggregate expansion. However, the compressive stress in the concrete after aggregate expansion was effectively transmitted to the rebar during uniaxial tension loading; this behavior indicated that RC could maintain its integrity under uniaxial tension even after 0.5% aggregate linear expansion.
Polymer modified mortar (PMM) is indispensable for repairing and reinforcing concrete structures owing to its excellent adhesion to concrete, compactness, and workability. However, PMM tends to spall when exposed to high temperatures because it contains organic polymers. In this study, a ring-restrained heating test was performed on normal cement mortar and PMM mixed with three types of polymers to investigate the factors that affect fire spalling, such as differences in fire spalling magnitude, restraint stress, and water vapor pressure. Furthermore, a tensile strain failure model based on thermal stress theory was used to evaluate temporal changes in fire spalling depth.
Carbon fiber reinforced polymer (CFRP) bars present advantages over conventional steel rebars such as non-corrosiveness and relatively high tensile strength. CFRP bars cannot directly substitute steel rebars as the main reinforcement for concrete structures due to a lack of understanding of the behavior of CFRP-reinforced concrete (RC) members. This research investigates the effect of surface characteristics of deformed FRP bars on the bond between FRP bars and concrete. Moreover, the research aims to understand how the bond property of FRP bars affects the behavior of CFRP RC beams under shear load. An analysis was conducted for a better understanding of this matter by producing different bond models with a reproductive pull-out test that takes into consideration the surface characteristics aspect of the FRP bar. The structural analysis for CFRP RC beams using the proposed bond models was conducted. It was observed that surface characteristics of CFRP rebars affect the bond-slip curves in a way that the ribbed type presents higher bond strength and greater shear stiffness modulus than the dented type. In addition, the analysis for CFRP RC beams using a ribbed bond model tends to generate steeper diagonal cracks leading to the shear failure in the CFRP RC beams.
Recently, the use of headed bars as mechanical anchorage in reinforced concrete beam-column joints has increased rapidly. This type of anchorage is used for T-shaped beam and exterior column joints at middle floors and for L- and T-shaped beam-column joints at the rooftops of buildings. However, few experimental data are available regarding the anchorage capacity of headed bars embedded in L-shaped beam-column joints. Nine pullout tests are conducted to investigate the anchorage capacity of headed bars embedded in L-shaped beam-column joints. Parameters affecting the anchorage capacity, such as the diameter, embedment length, location of the headed bars, and the number of supplementary bars are considered. The anchorage capacity of the headed bars is evaluated using empirical formulas proposed by Kubota and Murakami and a formula for concrete cone failure. The failure behavior of the specimens is concrete failure with cracks propagating in the diagonal direction at the maximum applied load. The anchorage capacity increases with the embedment length and diameter of the headed bars, number of supplementary bars, and concrete area. For most specimens, the Kubota and Murakami formulas overestimate the anchorage capacity, whereas formula for concrete cone failure yields lower anchorage capacities as compared with the test results.
Recycled tyre steel (RTS) fibre is favoured as a replacement for industrial steel fibre to reduce the environmental impact and material cost of fibre reinforced cementitious composites as well as a potential substitute for the commonly used polyvinyl alcohol (PVA) fibre to develop sustainable engineered geopolymer composites (EGC). This paper systematically examines the effect of hybrid PVA and RTS fibre dosage on the engineering properties of fly ash-slag based EGC, with special focus on uniaxial tensile behaviour and dynamic compressive and splitting tensile behaviour. Results indicate that combining RTS fibres with PVA fibres can effectively improve the drying shrinkage resistance of EGC. All studied EGC mixes exhibit expected strain-hardening and multiple cracking behaviour under uniaxial tension and about 5% enhancement in tensile strength is captured for EGC when 0.25% PVA fibre is replaced with RTS fibre. The incorporation of RTS fibres can improve the quasi-static compressive strength of EGC up to 31%, as compared to EGC with 2.0% PVA fibre. Replacing 0.25 to 0.5% PVA fibre with RTS fibre is beneficial to the dynamic mechanical properties of EGC, where up to 20% improvement in dynamic splitting tensile strength is found for EGC.
In this paper, the applicability of two deformation indices related to displacement of concrete members proposed by the authors has been verified for full-scale RC members with shear or bending failure under bilateral loads. These two indices, namely "thickness increment of RC member" assuming out-of-plane shear failure, and a newly proposed index, "relative displacement on compressive edge in RC member", which assumes in-plane shear failure and out-of-plane bending failure, are mainly discussed in this paper.
As a result, it was revealed that the deformation indices could estimate the failure mode of RC columns with shear failure or bending failure under horizontal bilateral loads. In addition, it was confirmed that one of the indices reached the limit value at about 90% or more of the maximum load, and that the element size dependence was small. It was also found that this index can reasonably evaluate the load carrying capacity as much as or better than the existing strain indices.