Bond behavior and durability of CFRP-to-steel bonded joints under cyclic loading and freeze-thaw and salt fog

There are many structural members of metallic bridges, namely girders and trusses, requiring rehabilitation and strengthening in a large number of countries. The utilization of polymers reinforced with carbon fibers (CFRP) has been considered of great potential for that purpose and many laboratorial tests have been made in the past 30 years to study the behavior of structural members strengthened with CFRP, based on which structural solutions have been designed and built. Despite that, more research is necessary to obtain data on the mechanical response and durability of those composite materials and structures, as well as on the CFRP/steel joints, to improve design procedures and define more suitable strengthening methodologies. One of the topics of the thesis is the study of the bond behavior of CFRP/steel joints under mechanical or thermal loading. In a first phase, tests of double-strap joints, with several different bonded lengths were subjected to monotonic loading and a simplified method was developed to estimate the responses and compare with the experimental results. Following that phase, pseudo-cyclic and cyclic tests were made to examine the capacity of the joints for those types of loading. The damage developed in the interface was analyzed in a way that allowed the calculation of the dissipated energy and related it to the slip between the CFRP strips and the steel plates. The durability of the bonded CFRP/steel joints was also studied. The joints as well as adhesive coupons, separately, were exposed to accelerated aging imposed by (i) temperature cycles between -20ºC and +20ºC with testing of effects at selected stages (1,000h, 2,500h, 3,500h, 5,000h and 10,000h), and (ii) salt fog cycles lasting for 5,000h. Both monotonic and cyclic tests were made to evaluate the deterioration caused by aging and different mechanisms were identified. Stresses due to resistance to free thermal expansion and due to galvanic corrosion were also addressed. Analytical solutions based on local bond laws, either bilinear or tri-linear, were used to estimate the debonding of the CFRP laminates from steel. It was also confirmed that the capacity of the joint is associated with the initiation of damage in the adhesive caused by steel corrosion. A “loading threshold” was detected approximately equal to 50% of the ultimate monotonic loading capacity of the joints, below which cyclic loading does not damage the CFRP/steel integrity. Experimental results showed that the normalized slip at the interface (S/Sult) and the normalized dissipated energy, local and global (wd/Gf and Ed/Et, respectively), may be inter-related both for reference, unaged samples, and aged samples. Moisture is a critical factor in temperature cycles around 0ºC, called freeze-thaw for simplicity, while temperature effects alone for cycles between -20ºC and +20ºC were negligible. Galvanic corrosion between CFRP and steel is generated by the accumulation of corrosion products in the joints, creating electric bridges that lead to deterioration of the CFRP laminates. Local corrosion of steel causes stress concentrations that may lead to brittle failure under monotonic or cyclic loading