Effective bonding between adherents plays a key role in retrofitting concrete structures in civil engineering using fibre-reinforced polymers (FRPs). To ensure structural safety, it is critical to develop design codes, which account for uncertainties of materials, the environment, and load, to estimate bond behaviour under long-term exposure to harsh environments. Therefore, a reliability analysis was performed to study the bond behaviour of FRP–concrete interface under wet–dry cycles and sustained loading. Thirty double-lap, shear-bonded carbon FRP (CFRP)–concrete composite specimens were tested after wet–dry cycles and sustained loading exposure. The fracture energy G_f of the bond behavior between CFRP and concrete was directly obtained from the measured local bond-slip curves. Five widely used test methods were adopted to verify the possible distribution types of G_f. Based on the best fit distribution of G_f, a reliability index β was then calculated for the specimens. The effects of wet–dry exposure and sustained loading on β were analysed separately. The effects of the mean and standard deviation of the load on β were compared. It was found that the mean had a greater impact on reliability than the standard deviation, but neither changed the regulation of the exponential reduction of β with increasing wet–dry cycle time. Their impact was significant for a small number of wet–dry cycles but insignificant for more than 4000 wet–dry cycles.

Elevated temperature exposure has a negative effect on the performance of the matrix resin in Carbon Fiber Reinforced Plastics (CFRP) plates, whereas limited quantitative research focuses on the deteriorations. Therefore, 30 CFRP specimens were designed and tested under elevated temperatures (10, 30, 50, 70, and 90 °C) to explore the degradations in tensile properties. The effect of temperature on the failure mode, stress-strain curve, tensile strength, elastic modulus and elongation of CFRP plates were investigated. The results showed that elevated temperature exposure significantly changed the failure characteristics. When the exposed temperature increased from 10 °C to 90 °C, the failure mode changed from the global factures in the whole CFRP plate to the successive fractures in carbon fibers. Moreover, with temperatures increasing, tensile strength and elongation of CFRP plates decreases gradually while the elastic modulus shows negligible change. Finally, the results of One-Way Analysis of Variance (ANOVA) show that the degradation of the tensile strength of CFRP plates was due to the impact of elevated temperature exposure, rather than the test error.

Self-compacting Concrete Filled Square steel Tubes (SCFST) is used to strengthen square RC columns. In order to investigate the eccentric loading behavior of the strengthened columns, a generic fiber element model is proposed. The ultimate strengths and the complete load-deflection response at the mid-height of the strengthened columns are predicted by the model in the aid of computational procedure. To verify the accuracy of the computational results, related experiments have been performed. The obtained experimental results were then compared with the computational results. After finding close agreement between them, the effects of extensive experimental parameters on the behavior of strengthened columns are investigated, including tube wall thickness, concrete compressive strength and initial eccentricity. Furthermore, a simple design formula based on the verified model is proposed to calculate the ultimate strength of eccentrically compressed strengthened columns. The calculation results agree well with the experimental results.

Due to lack of research data on the rates of deterioration of FRP properties under a harsh environment exposure, it was pointed out in the design guidelines that the durability of FRP needs to be further developed. Therefore, in this study, 48 FRP samples were tested under wet-dry cycles exposure. The effect of wet-dry cycling times on the failure modes, tensile strength, and the probability distribution of different FRP (GFRP and CFRP) composite specimens were investigated. The experimental results showed that the wet-dry cycles have a significant adverse influence on the tensile strength, have a certain adverse effect on the elongation, and a very limited influence on the elastic modulus of FRP. According to the experimental results, a probability analysis was conducted on the degradation of tensile strength. Five widely used test methods were adopted to verify the possible distribution types of tensile strength, and a reliability index β was then calculated. Subsequently, the effects of the design tensile strengths of ACI-440, TR-55, GB 50608-2010, GB 50367-2013, European Fib Bulletin 14 and Italian CNR guidelines on the β were investigated. The investigation illustrates that only the design value of the TR-55 code can guarantee sufficient long-term safety of a CFRP composite, whereas all the six codes cannot guarantee the long-term safety of a GFRP composite and the partial safety factors in these codes are still not conservative. Therefore, a more conservative safety factor was suggested. Moreover, the design value of tensile strength needs to be further conservative when the standard deviation of the load is large.

This paper presents an experimental study of the seismic behavior of RC square columns strengthened with self-compacting concrete-filled carbon-fiber-reinforced polymer (CFRP)-steel tubes (CFCSTs). Eleven columns were tested under lateral cyclic load while simultaneously being subjected to constant axial load. The test parameters included the number of CFRP layers, the thickness of the steel tube, and the axial load level. The entire failure process, ultimate lateral load, and deformation capacity were analyzed. The experimental results indicated that the seismic bearing capacity of the RC columns could be improved significantly while the strengthening method was applied, because the ultimate lateral loads of the strengthened columns were 7.36–9.67 times that of the unstrengthened column, and the improvement ratio of ductility parameter reached 42.38–115.56%. The CFRP jacket could effectively delay or even prevent the local buckling of the steel tube, leading to better structural seismic performance. As the number of CFRP layers increased, the ultimate bearing capacity, ductility, and energy-dissipation capacity of the CFCST-strengthened columns increased slightly. The CFCST-strengthened columns under a higher axial load level exhibited better ultimate bearing capacity and energy-dissipation capacity but worse ductility. With the increase of the thickness of the steel tube, all seismic performance indexes significantly improved. The CFCST-strengthened columns were simulated by using the finite-element analysis software Abaqus 6.14–4. The comparison of the analytical results with the experimental results revealed that they are basically consistent with each other.