The effect of the aggressive environment on the bond strength of frp-concrete specimens

Bonding is a very important issue when FRP plates or sheets are used to strengthen R.C. beams. It depends on the mechanical and physical properties of concrete, composite and adhesive. Some studies can be found in the literature concerning the bond strength variation when the strengthening intervention is in close contact with an aggressive environment; in particular, the bond failure at high or low temperature and under high or low humidity content has been investigated. Nevertheless, few studies can be found on the effect of these degrading actions, when they occur as cyclic events and not as steady state actions. The exposition to deicing salts or freeze-thaw cycles may in fact deteriorate the mechanical properties of FRP, adhesive and concrete substrate. Due to the lack of information, the design rules against the effect of aggressive environment are currently very conservative, even in recent recommendations [3]. The present experimental study is a preliminary investigation of a quite wide research program aimed at the definition of the basic correlations between degradation phenomena and changes in mechanical parameters of the FRP-concrete bonding (fracture energy, peak shear stress, etc). A number of specimens have been preconditioned by thermal cycles and/or immersion in salt-spray fog environment. Then, pull-push debonding tests have been performed under standard conditions. Starting from experimental data, non-linear FRP-concrete interface laws after exposition on aggressive environment have been calibrated. The interface laws and force-plate elongation curves during the tests have been finally compared with the analogous results obtained from unconditioned FRP-concrete specimens.