Factors influencing bond of CFRP to steel

Many infrastructures require structural strengthening, e.g. (Miller et al., 2001; Libby and Mullins, 2001; Bank, 2006; Kirk and Mallet, 2007), and the use of fiber reinforced polymers on that endeavor has been growing in spite of uncertainties related to their lifetime behavior. Premature debonding of external strengthening under adverse environmental conditions is a major source of concern, including in the case of carbon fiber reinforced polymers (CFRP) used in the strengthening of steel structures, namely bridges. Failure of those CFRP-steel structural members may occur at the joints and this study examines factors that alter or explain reduction of the load capacity of the latter, namely surface treatment prior to bonding, the glass transition temperature (Tg) of the adhesive, the exposure to freeze-thaw (FT) cycles, that artificially accelerate aging of the components, and the effects of temperature nearing Tg on the mechanical strength of the joints. An experimental program was undertaken to analyze the influence of these factors on the performance of the joints. The steel surface was subjected to sand blasting (6.3bar), abrasive grinding (6.9bar) Sa 2, to compare with no treatment, and the case of rust induced by 10 days exposure to salt fog was also considered. Tests made at different temperatures allowed comparison of ultimate capacities of strains and failure modes, Table 1, confirming rapid deterioration. The quantification of the losses of mechanical resistance was made essentially by shear tests of double strap specimens. Changes of bond-slip responses with increasing operational temperature up to and above adhesive Tg were significant. Preparation of surface by sand blasting was found more effective than by grit steel blasting, but further tests are needed to ascertain that degrees of treatment neared optimum roughness in each case. Adhesion failures after FT were analyzed and compared with evolution of Tg showing negligible changes, in general. Scanning electronic microscope images were used to help interpret results. However, elemental component changes obtained by EDS at selected spots were found inconclusive.