Durability Of Concrete Strengthened With FRP Under Environmental And Loading Conditions

Fiber reinforced polymer (FRP) is increasingly being used for retrofit and repair of concrete structures, due to its light weight, high strength-to-weight ratio, and easy to install. However, there is insufficient information on the environmental durability of concrete strengthened with FRP, especially under coupled action of aggravating environment and sustained load. Based on the research background and sponsored by the Natural Science Foundation of China "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 carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP), interface bond between CFRP and concrete, and CFRP strengthened concrete beam subjected to sustained load and aggressive environment, which can be summarized as follows:1. Based on the classical elasticity theory and elastic-plastic property of adhesive, a simplified formula to calculate the effective bond length of FRP single lap bond (SLB) is presented. Twelve groups of FRP SLB specimens were made by hand. Tests of the coupons were conducted under static loading. The shear stress distribution in the overlap zone was measured, which leads to the derivation of the effective bond length of FRP SLB. The test results indicate that maximum shear stress occurs at the ends of the overlap zone for any length of the bond; and the effective bond length is estimated to be between 75 mm and 100 mm in Carbon specimens and between 100 mm and 125 mm in Glass specimens. The computed results using the presented formula are in agreement with experimental ones. Then the study evaluates the durability of CFRP SLB under aggressive environmental condition such as wet dry cycles, freeze thaw cycles and hygrothermal aging either in an unstressed state or loaded in about 30% or 60% of the initial ultimate load. The results confirm that the effective bond length of CFRP SLB using the presented formula meets durability requirement of specific application.2. Durability of CFRP and GFRP under aggressive environmental conditions such as wet dry cycles, freeze thaw cycles and hygrothermal aging either in an unstressed state or loaded in about 30% or 60% of the initial ultimate load are investigated. The results indicate that the coupled action of sustained load and environmental conditions further reduce the ultimate strength and strain of CFRP and GFRP. However, no significant change in the elastic modulus of CFRP and GFRP is observed. Based on the "strength loss" phenomenological reflects the damage in the CFRP, a simple residual strength damage model of CFRP under sustained load and freeze thaw cycle is presented. Based the tested results, a degradation model is developed to predict the long term durability performance of GFRP under aggressive environmental conditions such as wet dry cycles, freeze thaw cycles and hygrothermal aging.3. CFRP and GFRP were exposed to alkaline solution at 30°C, 40°C, 50°C and 60°C, then the tensile tests of FRP coupons were conducted. The results indicate that the tensile strength, elastic modulus and elongation of CFRP and GFRP specimens gradually decrease with the increase of exposure time, and the decrease is more pronounced under higher temperatures. Based on the results, a modified Arrhenius analysis is developed to predict the long term durability performance of CFRP and GFRP.4. Durability of CFRP-concrete bonded interfaces under aggressive environmental conditions such as wet dry cycles; freeze thaw cycles and hygrothermal aging either in an unstressed state or loaded in about 30% or 60% of the initial ultimate load are investigated. The results indicate that the single action of environmental conditions reduce the interfacial fracture energy G_f, local slip at debonding initiationδ_f and the peak shear stress; the coupledaction of sustained load and the environmental conditions further reduce the interfacial fracture energy G_f, local slip at debonding initiationδ_f and the peak shear stressτ_f, and thereduction are more pronounced under higher level of sustained load.5. The eight concrete beams reinforced with CFRP subjected to an aggressive environment under sustained load are investigated. The beams were pre-cracked with a four point flexural load, bonded carbon fiber reinforced-polymer sheets, and placed into wet dry saline water (Nacl) either in an unstressed state or loaded in about 30% or 60% of the initial ultimate load. The test results show that the coupled action of wet dry saline water and sustained bending stresses appeares to significantly affect the load capacity and failure mode of beam strengthened with CFRP, mainly due to degradation of bond between CFRP and concrete. However, the stiffness is not affected by coupled action of wet dry cycles and sustained load.