Traction-separation law for CFRP-concrete interface fracture under mode II loading

The rehabilitation and retrofitting of concrete structural members using externally bonded Fiber Reinforced Polymer (FRP) strips has been steadily gaining use in recent years due to its many advantages, such as ease and speed of construction, low cost and maintenance, and high strength/weight ratio. An important design issue with significant performance and safety implications is the debonding of externally bonded FRP strips in flexural members, where the delamination is primarily due to Mode II facture. A significant amount of research has been conducted in this area, but there are concerns about interface durability. This study is based upon a fracture mechanics approach using Mode II single-shear tests to evaluate the durability of Carbon FRP (CFRP)-concrete interface subject to two combined environmental conditioning variables: (1) immersion in deionized water varying from 0 to 15 weeks; and simultaneously (2) controlled temperatures varying from 77°F to 140°F (25°C to 60°C) of the same samples immersed in water. A new method is proposed based on J-integral to obtain the fracture energy release rate and the traction-separation law, by measuring the load and slip at the debonding end only, which was verified by the traditional strain-based method. The durability of the interface is characterized by the energy release rate (ERR). By comparing the results with those from unconditioned companion specimens, it is found that considerable degradation of the interface integrity resulted with increased moisture duration and temperature. Representative delaminated FRP specimens were further studied using CMM (Coordinate Measuring Machine) and optical microscopy to obtain a qualitative understanding of the fractured surface. These results were in accord with the fracture testing results