Experimental Study On Durability Of FRP Strengthening Civil Engineering Structures Under Load And Aggressive Environments

At present, Fiber Reinforced Polymer (FRP) has been extensively used in strengthening structures in civil engineering, and the durability of FRP strengthening Structure has deserved great attention by engineers and researchers. Based on the research background and sponsored by prophase research special item of national great basic research: "durability research of FRP strengthened concrete members in severe environments"(2004CCA04100) and NSFC "durability research of concrete structures strengthened by FRP under loading and aggressive environments" (50608013), experimental researches have been carried out in the dissertation on durability of bond interface between FRP and composite. The aggressive environments included water immersion, salt water immersion, acid solution immersion, alkali solution immersion, wet-dry cycling and freeze-thaw cycling action and etc. The loading effect was included in researching durability Of concrete and steel structures strengthened by FRP under wet-dry cycling. The detail works listed as follow:(1) Based on experiments of the interface bond between FRP and concrete under water immersion, salt water immersion, alkali solution immersion, wet-dry cycling and freeze-thaw cycling action, the durability of bond was studied. In this study, the fracture toughness was introduced in analyzing the influence of durability by harmful environments. The experiments results showed that the interface failure characteristic of FRP and concrete under aggressive environments was different form those specimens kept at room temperature. After environments exposure, the bond tip displacement load curve is still linear in beginning stage and the crack initiation load and initiation fracture toughness decreased. Salt water environment showed the more severe effect on FRP bonds than water immersion, especially for the longer exposure time. At the same exposure time, the alkali environment had more sever effect on FRP bonds than water immersion. For the wet-dry cycling environment, the short stage effect on FRP bonds was similar with water immersion, but for longer exposure time, the effect on FRP bonds by wet-dry cycling was more sever than water immersion. Freeze-thaw cycling environment showed a deterioration effect the interface bond between FRP and concrete. With the increase of freeze-thaw cycling, the initiation fracture toughness of specimens exhibited the larger decreased trend.(2) Loading effect was introduced in the durability research of bond interface between FRP and concrete. 30% or 50% of ultimate capacity was preloaded before the beam specimens with rotated hinge were put into the test case. The durability of FRP strengthened concrete under loading and wet-dry cycling was studied. The results showed that the wet-dry cycling had adverse effect on bond behavior of FRP and concrete. For the small preload (30% of ultimate capacity), the preload had the less effect on the durability of FRP and concrete. For the larger preload (50% of ultimate capacity), the preload had adverse effect on the durability of FRP and concrete. Comparing with the specimens of 0 and 30% preload, the capacity of the specimens with 50% ultimate capacity had the more decrease.(3) Based on the bond test of FRP and I typed steel with rotated hanger under dry-wet cycling and freeze-thaw cycling, durability of FRP and concrete/steel was studied. The test results showed that the wet-dry cycling had severe effect on bond between FRP and steel. The ultimate capacity of FRP strengthened steel structures only left 43% after 15 cycling comparing with those specimens kept at room temperature. With the increase of wet-dry cycling, the ultimate capacity of FRP strengthened steel beams had a more decrease trend. For the longer exposure time, the decrease trend was gentler than before. The freeze-thaw cycling also had severe effect on bond between FRP and steel. The ultimate capacity of FRP strengthened steel structures only left 40% after 25 cycling comparing with those specimens kept at room temperature. For the more cycling times, the capacity of specimens showed no decrease trend and no correlation existed in freeze-thaw cycling times and ultimate capacity.(4) Loading effect was introduced in the durability research of bond interface between FRP and steel. 15%, 25% or 30% of ultimate capacity was preloaded before the beam specimens with rotated hinge were put into the test case. The durability of FRP strengthened steel beams under preloading and wet-dry cycling was studied. The results showed that the couple action of preloading and the wet-dry cycling environments had adverse effect on bond behavior of FRP and steel. The ultimate capacity of FRP strengthened steel structures with 30% ultimate preload only left 22% after 120 cycling comparing with those specimens kept at room temperature. For the different preload level (0, 15%, 25% or 30% of ultimate capacity), the capacity of specimens had no evident correlation with exposure time. For the high level load and longer time serve, the bond behavior between FRP and steel is not stable and suddenly failure would be happened under load and aggressive environments.