Experimental Study On Durability Of Precracked RC Beams Strengthened With CFRP Under Load And Dry-wet Conditions

At present, Carbon Fiber Reinforced Polymer (CFRP) is extensively used in strengthening structures in civil engineering reinforcement because of its advantages of high strength, light quality, good workability, good durability and so on. Except burdening load reinforced concrete structures strengthened with CFRP are still exposured in chemistry environment, dry-wet conditions, seawater environment, freeze and thaw cycles, and others aggressive environment. So the structures strengthened with CFRP sometimes can't normally work because of durability decrease. In practical project, reinforced concrete structures are usually damaged beforce strengthenment. So precracked reinforced concrete structures strengthened with CFRP under load and aggressive environment can simulate the structures strengthened with CFRP in practical project, which can be used for studying the strengthening structures durability decrease mechanism and providing information to the design of structures strengthened with CFRP.Based on the NSFC " Durability research of conerete structures strengthened by FRP under loading and aggressive environments" (50608013), this paper experimental study on durability of eight precracked RC beams strengthened with CFRP under load and dry-wet conditions, load level was designed to 30% and 60%, and dry-wet conditions was designed to 90 times and 180 times. Many conclusions have been obtained from experimental and theoretical study.(1) The bond interface of CFRP and concrete is soaked by seawater and weakened under dry-wet conditions. CFRP is easier debonding under load because seawater dry-wet conditions decrease the capability of the bond layer. Seawater acutely soak to the inside of specimen and decrease the capability of the bond layer, because the protection capability of the bond layer is decreased and the crack expand under load. The capability of specimen is acutely degenerated under load and dry-wet conditions.(2) The midspan cross section strain of RC beams under load and dry-wet conditions still fulfil the plane hypothesis. Before the steel bar yield , the strain of CFRP and utmost strain are declining with increase of load level and dry-wet conditions times, L-180-60%'s CFRP utmost strain declines 19.1% comparing with L-0-0's. The steel bar strain is declining with increase of load level. (3) The crack load is declining with increase of dry-wet conditions times, but is less affected by the load level. L-180-60%'s crack load raises 25.5% comparing with L-0-0's. The utmost load and the utmost deflection are declining with increase of load level and dry-wet conditions times, L-180-60%'s separately declines 6.6% and 10.82% comparing with L-0-O's. The crack space between and maximal crack width are declining with increase of the load level, but is less affected by dry-wet conditions times.(4) IC debonding of structures strengthened with CFRP relate to bonding stress caused by crack spreading and shearing force. The bonding slip caused by crack spreading is the main reason of IC debonding. Before the concrete is cracked, part debonding hardly affect the carrying capacity of specimen, contrariwise, part debonding near the crack decrease the carrying capacity of specimen after the concrete is cracked, which brings on the function of crack restraining declining and specimen stiffness increased by CFRP declining.