The experimental results of the bond behavior between glass fiber reinforced polymer (GFRP) bars and concrete under several aggressive environments were studied. To give a comprehensive and comparative evaluation, a total 60 GFRP bar-concrete pullout specimens were divided into a control group and a treated group being immersed in tap water, alkaline and saline solutions for up to 270 days. The influences from aggressive environment, resin type and exposure period on the bond strength, together with the degradation mechanism and failure mode were explored. The time-dependent test results showed a clear decrease in the bond strength of treated group, with notably various degradation trends during different exposure periods. It is found that the saline solution induced the average bond strength loss of all corresponding specimens as 51.5% at 270 days, followed by alkaline solution (39.5%) and tap water (29.0%).
Considering the low-permeability of reactive powder concrete (RPC) and the influence of pressurized liquid on crack development, this paper studies the mechanical properties of sealed and unsealed prismatic RPC samples under different confining pressures. It is determined that the stress-strain relations of sealed and unsealed RPC samples are almost consistent before the stress reaches its proportional limit, but the pressurized oil greatly accelerates the failure of the unsealed sample after this point. The strength loss of unsealed samples increases with an increase in the confining pressure. The failure patterns of the sealed RPC samples change with the confining pressure, but those of the unsealed samples are almost same. The unsealed samples are split by a vertical crack under every confining pressure. Additionally, it is determined that the crack development leads to a difference in the lateral strains in the perpendicular directions for sealed prismatic samples, and the difference between the lateral strains at the peak stress in different directions increases linearly as the confining pressure increases. In contrast, for unsealed samples the lateral strains at the peak stress in the perpendicular directions are close.
Inspection data of actual concrete structures should be analyzed to elucidate the deterioration mechanism and construct a regression model. Although machine learning can be applied to this problem, inspection data are not suitable because machine learning targets big data with a uniform density and a balanced distribution. This study applies machine learning to a regression model of the crack damage grade in concrete bridges, using imbalanced inspection data. The model performance is improved by analyzing the influence of undersampling. Undersampling is conducted step-wise, and the models are constructed by learning all the undersampled data. The cross-validation of these models yielded the regression errors on each crack damage grade to evaluate the model performance considering the bias of data imbalance. Based on the results, the effect of undersampling on the model performance is analyzed, and the appropriate model is selected. Additionally, the influence of the model difference on the evaluation is investigated via historical change or factor analysis to confirm the effect of undersampling. This article not only presents a case study of a regression task for crack damage grades in concrete bridges, but also describes a strategy to maximize the use of imbalanced data for regression problems.
Shear failure experiments of concrete beams containing a weak layer were conducted with a focus on the bifurcation of shear localization appearing at the boundary between structure and soil foundation. Low-strength concrete, which is analogous to artificial soft rocks and strengthened foundation, was used to create a weak layer that caused dispersal and bifurcation of the shear localization area, resulting in ductile fracturing of members. Pulverization of hardened cement paste and gravelization (the loss of aggregate particle’s cementation) were observed in shear planes appearing in the weak layer. This confirmed the difficulty of simulating bifurcating shear localization solely by the constitutive law of concrete, which assumes firm cementation by hardened cement paste. In reference to the simulation of the disintegrated concrete slabs for bridge decks under fatigue loads, the transient model from hardened concrete to gravelized assembly was proposed, and it was successfully applied to the bifurcating shear localization of weak layers of low-strength concrete.
Fiber Reinforced Polymer (FRP) composites in the form of laminates and fabrics are installed on reinforced concrete members, using externally bonded (EB) techniques for strengthening. However, the utilisation of capacity of FRP composites is limited due to debonding observed in the strengthening system, due to FRP and concrete interfacial shear stress. Carbon fiber reinforced polymer (CFRP), in a new form of strand sheets were recently developed and have not been adequately investigated upon. These strands are less than 1.0 mm in diameter is woven with thread to obtain the desired width of the sheet. Such strand sheets are likely to have enhanced bond strengths due to the gaps between the strands that increase the surface available for bonding with the adhesive compared to laminates. This paper presents details of an experimental investigation of the influence of CFRP strand sheets on the flexural performance of CFRP strengthened reinforced concrete (RC) beams. In the experimental study, RC beams strengthened with CFRP strand sheets and lami-nates, were tested under four-point bending and compared with an un-strengthened specimen. The main parameter varied was the elastic modulus of the CFRP strand sheet and width ratio (wf /b) of high strength CFRP strand sheets. The relative contribution of the CFRP strand sheets to the bending moment capacity was observed to be significantly higher than that in the case of the CFRP laminate. The failure mode was observed to change from debonding to concrete cover separation with an increase in width ratio. The rupture was observed as the failure mode in the specimens strengthened with high modulus CFRP strand sheets. As a result, an increase in the bending-moment capacity and ductility of the beams was also observed. Based on the observed failure modes, models for predicting the mechanical behaviour of various systems were assessed.
Recent progress in finite element analysis aids the simulation of seismic vibration of an entire reinforced concrete (RC) building structure and indicates that drying shrinkage cracks affect seismic resistance performance. Polypropylene fiber-reinforced concrete (PFRC) is a promising material since the fibers will reduce the cracks and strains under drying shrinkage. This paper attempts to quantify the vibration characteristics of PFRC walls by means of a drop-weight test and finite element analyses. Four wall specimens having the same geometry and bar arrangement are prepared. After a one-year drying shrinkage period, the walls are subjected to impact loading of a constant collision velocity of 5 m/s, using a steel drop weight of 398.8 kg. Shear cracks are observed in the restrained wall made of plain concrete, while cracks are insignificant in the PFRC wall. Three-dimensional (3D) nonlinear finite element analyses are conducted to simulate all behaviors from drying shrinkage cracking up to the time of impact loading, and to estimate the vibration characteristics. The analysis results indicate that the polypropylene fiber content reduces the elongation of the natural period by an average of 13.7%.