In this paper, a fracture mechanics based micromechanical model is constructed to describe the retrofit effect on concrete structures with fiber reinforced plastics (FRP) sheets, the interaction between concrete crack and applied FRP sheets is treated by creating opposing bridging stress acting across the concrete crack flanks as a result of FRP stretching. The details of the bridging stress include all relevant FRP properties and the interfacial bonds between FRP and concrete. Fiber modulus, often believed to be the key element of a successful FRP wrap for strengthening concrete structures, is one of the critical parameters but not the only one. Such mechanical interaction dominates the concrete/FRP composite behavior from the first bent-over point to the peak load in a load/displacement curve. The detailed interaction depends on actual loading conditions such as compression, shear, or bending. The FRP retrofit effects under flexural and shear loads will be discussed and highlighted in this paper.
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