Mechanics and subcritical cracking of FRP-concrete interface
The need for safe, effective, and efficient methods to strengthen and upgrade our nation's infrastructures is clear. Strengthening Reinforced Concrete (RC) members using Fiber Reinforced Polymer (FRP) composites through external bonding has emerged as a viable technique to retrofit/repair deteriorated infrastructures. The interface between the FRP and concrete plays a critical role in this technique. This study proposes a life-cycle analytical framework on the integrity and long-term durability of the FRP-concrete interface through a combined analytical, numerical, and experimental approach. A novel three-parameter elastic foundation model (3PEF) is first established to provide a general tool to analyze and evaluate the design of the FRP strengthening system. This model correctly predicts the location where debonding can occur. To simulate the interface stress redistribution and creep deformations accumulated during service life due to the strong time dependent features of the adhesive layer, linear viscoelastic analytical solutions are then developed for the FRP-strengthened RC beams. Small cracks usually exist within the FRP-concrete interface, making fracture mechanics a more appropriate tool to evaluate the integrity of the FRP-concrete interface. Analytical solutions of energy release rate (ERR) and its phase angle at the tip of a crack along the FRP-concrete interface are obtained. Under the synergistic effects of the service loads and environments species, these small cracks can grow slowly even if the ERR at the crack tip is lower than the critical value. This slow-crack growth process is known as environment-assisted subcritical cracking. A series of subcritical cracking testing are conducted using a wedge-driven testing to gain the ability to accurately predict the long-term durability of the FRP-concrete interface. It has been found that water, deicing salt and alkaline solutions can substantially reduce the ERR at the crack tip needed to drive the subcritical crack growth along the epoxy-concrete interface. Once the small cracks grow to the critical length, critical debonding will occur, leading to the premature failure of the structure. A nonlinear fracture mechanics model using a Cohesive Zone Model (CZM) is finally developed to simulate this final failure phase of the FRP-concrete interface.