Predictive multiscale model of delayed debonding for concrete members with adhesively bonded external reinforcement

The research described in this paper relates to flexural concrete members strengthened by means
of external reinforcement bonded adhesively onto the surface, in particular, by fiber-reinforced
polymeric strips, sheets, or laminates. The research is specifically devoted to external reinforce-
ment being already in tension under a dead load or a low fraction of live load. The concrete
cover exhibits initial flaws, which were unimportant when a member was not externally rein-
forced, but that may give rise to slow crack propagation up to delayed debonding when the
member is externally reinforced. This paper presents a model for predicting the delayed de-
bonding. The common debonding models, including code provisions, focus mainly on the struc-
tural and material scales, and thus ignore delayed failure. On the contrary, this new model
focusses on the mesoscale, which considers the velocity of crack growth that leads to debonding.
While on the nanoscale and microscale the crack velocity depends on the microstructure, on the
mesoscale the crack velocity depends on the interfacial bond shear stresses and crack length. This
dependence can be synthetized by the ratio between the mode II stress intensity factor, KII, and
its critical value KIIC. The model describes the delayed debonding in terms of interaction between
bond shear stress and time (ultimate domain maximum bond shear stress versus delayed time).