Glass fiber-reinforced polymers (GFRPs) have received increasing attention in recent years due to their overall performance of light weight, low cost and corrosion resistance, and they are increasingly used as reinforcement in concrete structures. However, GFRP material has low elastic modulus and linear elastic properties compared with steel bars, which introduces different bonding characteristics between bars and concrete. Therefore, a reliable monitoring method is urgently needed to detect the bond slip in GFRP-reinforced concrete structures. In this paper, a piezoceramic-based active sensing approach is proposed and developed to find the debonding between a GFRP bar and the concrete structure. In the proposed method, we utilize PZT (lead zirconate titanate) as two transducers. One acts as an actuator which is buried in the concrete structure, and the other acts as a sensor which is attached to the GFRP bar by taking advantage of machinability of the GRRP material. Both transducers are strategically placed to face each other across from the interface between the GFRP bar and the concrete. The actuator provokes a stress wave that travels through the interface. Meanwhile, the PZT patch that is attached to the GFRP bar is used to detect the propagating stress wave. The bonding condition determines how difficult it is for the stress wave traveling through the interface. The occurrence of a bond slip leads to cracks between the bar and the concrete, which dramatically reduces the energy carried by the stress wave through the interface. In this research, two specimens equipped with the PZT transducers are fabricated, and pull-out tests are conducted. To analyze the active sensing data, we use wavelet packet analysis to compute the energy transferred to the sensing PZT patch throughout the process of debonding. Experimental results illustrate that the proposed method can accurately capture the bond slip between the GFRP bar and the concrete.

Mechanical properties of glass fiber reinforced polymer (GFRP) composites degrade under the combined effects of mechanical load and alkaline solution, affecting the service ability and safety of GFRP reinforced structures. In this study, GFRP bars were loaded with cyclic tension at different stress levels and immersed in alkaline solution for days to investigate the tensile properties and degradation law of GFRP bars. The degradation mechanisms were studied at micro-, meso- and macro-scales with scanning electron microscopy (SEM) and three-dimensional X-ray microscopy, respectively. The results show that tensile strength and degradation rate of GFRP bars are mainly dependent on the different stress levels and alkaline solution. When stress level is higher, the tensile strength degrades more quickly, especially in the early stages of soaking. With the loading and immersion time, the elastic modulus and Poisson’s ratio increase at first and then decrease. The ultimate tensile strain is relatively stable, whereas the ultimate elongation is significantly reduced. A strength-degradation model was proposed and fit well with experimental data, demonstrating that the model can be applied to predict tensile strength degradation under combined effects of the load and alkaline solution.

Concrete columns reinforced with GFRP bars, partially or fully replacing steel, can effectively meet the durability requirements of structures in corrosive environments. Axial compression tests were conducted on four groups of concrete columns with hybrid (mixed) reinforcement and single-type reinforcement. The results show that all specimens failed by concrete splitting, with several longitudinal bars buckling and extensive the concrete cover spalling at failure. In terms of ductility, specimens with hybrid longitudinal reinforcement and GFRP stirrups exhibited the highest value. Under conventional steel stirrup conditions, variations in longitudinal reinforcement configuration minimally affected ductility, whereas specimens reinforced entirely with GFRP bars exhibited the lowest ductility. The ultimate load capacity formula, which comprehensively accounts for the contributions of both longitudinal and transverse reinforcement, yielded theoretical values in close agreement with the experimental results. The test results indicate that the utilization rate of stirrups at the mid-height of hybrid reinforcement columns is greater than that in columns with only ordinary steel or only GFRP reinforcement. The reinforcement configuration showed little influence on the mechanical performance of the concrete itself in the columns. In terms of longitudinal reinforcement utilization, the hybrid steel-GFRP-reinforced columns performed better than those reinforced with GFRP bars alone.