Long-term behavior of bond between FRP and concrete exposed to a humid subtropical environment : experimental study and predictive modeling

Abstract

Fiber reinforced polymer (FRP) composites have been widely applied in many fields such as the aerospace industry for many years. Compared with steel which is a commonly used modern construction material, the FRP composites have the advantages of excellent corrosion resistance and high strength-to-weight ratio. The corrosion resistance of FRP can benefit the long-term performance of reinforced structures, while the high strength-to-weight ratio leads to great ease in site handling, which reduces labor cost and interruptions to existing services. While a large number of studies have been conducted for the short-term performance of FRP-strengthened reinforced concrete (RC) structures, uncertainty is still remaining in their long-term performance. Indeed, lack of proper understanding of the durability of the FRP-strengthened RC structures has in return impeded a wider adoption of this technique in practice.In this present study, the long-term performance of FRP-to-concrete interfaces and FRP-strengthened RC beams subjected to accelerated wet-dry cycles, which simulate a sub-tropical environment, is investigated. The thesis consists of four parts of work: (1) Evaluation of the degradation of materials used in the FRP-strengthened RC structures to provide a reference for the following durability study on FRP-to-concrete interfaces and FRP-strengthened RC beams. The concrete, FRP composites and adhesive materials were tested after 8 months of accelerated dry-wet cycle exposure. (2) Examination of the long-term performance on FRP-to-concrete interfaces subjected to accelerated wet-dry cycle exposure, with the aim to establish a series of exposure-based interfacial bond-slip models;(3) Assessment of the durability of FRP-strengthened RC beams, which experienced 8 months of accelerated dry-wet cycle exposure. Finite Element (FE) analysis was conducted to simulate their mechanical performance with due consideration of the degradation of materials and FRP-to-concrete interfaces; (4) The long-term performance of FRP-strengthened RC beams under sustained load. Four beams were loaded under two different load levels for 9 months and the beam deflections were continuously measured using the fiber optic sensing technique. FE simulations were also conducted to reproduce the time-dependent beam deflections in comparison with the test results. Static loading tests were also conducted on the beams at the end of the sustained loading. The following findings have been obtained from the above studies: Degradations were observed in FRP-to-concrete interface after 8-month exposure, while their degree varied in different types of FRP systems. The proposed exposure-dependent bond-slip model gave a good prediction of the degraded bond behavior; (b) The load capacity of FRP-strengthened RC beams decreased by 1.4% to 10.8% after 8-month exposure in wet-dry cycles, while increase in stiffness of beams was also observed; (c) Carbon FRP (CFRP)-strengthened RC beams were subjected to 300 days sustained loading. The time dependent deflections were 1.76~2.60 times the instantaneous deflections. However, no significant change was observed on the load-carrying capacity of the strengthened beams after the sustained loading regardless of the sustained load levels. (d) The FE models developed in this study with the implementation of appropriate bond-slip models of the FRP-to-concrete interface, can effectively predict the long-term behavior of FRP-strengthened RC beams with due consideration of the effects of weathering and sustained loading.