Experimental Investigation of Bond Fatigue Behavior of Concrete Beams Strengthened with NSM Prestressed CFRP Rods

Bond tests were conducted on 10 concrete beams strengthened with near-surface mounted (NSM) prestressed carbon fiber reinforced polymer (CFRP) rods under different fatigue load levels. In the NSM technique, grooves are cut on the tension side of the beams. The CFRP rods are then placed inside the grooves and prestressed. Then an epoxy adhesive is placed inside the groove to provide a bond between the concrete and the CFRP rod. The test variables included the type of CFRP rod (spirally wound or sand-coated) and the fatigue load level. The beams were tested in four-point bending. Unlike the bond failures for beams strengthened with nonprestressed CFRP rods, bond failure for beams strengthened with prestressed CFRP rods and tested under fatigue loading was by slipping between the CFRP rod and the epoxy that started at the support and propagated inward toward the loading point. The sand-coated rods showed a better bond fatigue performance than the spirally wound rods, where at a given load level the beams strengthened with sand-coated rods had longer fatigue lives than the beams strengthened with spirally wound rods. Also, for a given number of cycles, the beams strengthened with prestressed CFRP rods failed in bond at a lower applied load range than the beams strengthened with a nonprestressed CFRP rod. At onset of excessive slip (failure), the force distribution in the CFRP rod in the end region was the same for a given rod type. Thus, the shear stress value and distribution in the region close to the support was the same at onset of excessive slip (failure) for a given rod type regardless of the applied load level.