Bond behavior of fiber-reinforced plastic (FRP) reinforcements with concrete

The bond behavior between Fiber Reinforced Plastic (FRP) rods and concrete is one of the most important aspects to predict the short- and long-term performance of FRP-reinforced concrete structures. The objectives of this study are to understand the bond characteristics and to identify the important parameters that control the bond behavior and load transfer at the interface. Direct pull-out tests performed on cubic specimens were used to study the FRP/concrete bond behavior. This method allows the measurement of the loaded- and free-end slips of the FRP rod and placement of a strain probe inside the rod to measure internal strain distribution in both axial and radial directions along the bonded length without affecting the FRP/concrete interface. The internal probe technique was verified and found to be effective in measuring the strain values during pull-out tests.; The direct pull-out tests were conducted on glass/vinylester, carbon/vinylester, and carbon/epoxy FRP rods with smooth, machined and wrapped FRP rods making axisymmetric configurations. In addition, commercially available glass/vinylester rods and carbon/epoxy cable and rods with different surface deformations were tested. The typical results are given as nominal bond or shear stress vs. free-and loaded-end slip. Experimental results obtained from strain probes used during the pull-out test are also presented as nominal bond or shear stress vs. strain. For the smooth, machined and wrapped FRP rods instrumented with an internal probe, the local bond and shear stress vs. global and local slip were calculated for the segments between the gage locations. For smooth rods the bond strength is low and tends to be controlled by the finish and composition of the resin-rich surface layer. In the case of the machined rods, the bond is developed through the interlock between the lugs and concrete and is controlled by the shear strength of the lugs, which is resin dependent. For the commercially available rods with spiral deformation and sand coating, the maximum bond was reached after a considerable amount of free-end slip. Bond behavior in these rods is controlled by friction due to sand particles and interlock with the rod deformations. The bond failure is FRP controlled and the strength of concrete appears to have no effect on the bond strength and failure mechanism of the machined FRP rods when appropriate concrete cover is provided.