Delamination effects in reinforced concrete slabs strengthened with circular composite patches

The behavior of reinforced concrete slabs strengthened with fully or partially bonded (delaminated) circular patches is analytically investigated. The model derived follows the concepts of the high-order theory, and uses variational principles, equilibrium, and compatibility requirements, the constitutive equations of reinforced concrete (RC) members and composite laminates, and the fracture-mechanics concept of energy release rate. A substructuring approach, in which the localized response of the strengthened area is modeled assuming circular axis-symmetric behavior, is adopted. The investigated substructure consists of fully bonded and delaminated regions, where the delaminated faces can slip horizontally one with respect to another. A distinction is made between delaminations with contact, in which the delaminated faces accommodate vertical normal compressive stresses, and delaminations without vertical contact, in which the cracked interface is free of stresses on any kind. The field and governing equations of the fully bonded, delaminated (with or without contact), and unstrengthened regions, as well as the boundary/continuity conditions that combine these regions together, are derived. The influence of the existence of a delaminated area at the center of the slab and the effect of its size on the localized and overall behavior are investigated numerically. The elastic energy release rates associated with the growth of the delaminated area and their influence on the failure mode of the strengthened structure are also studied. The investigation reveals that the formation of a delaminated region reduces the composite action of the RC slab and the bonded patch, is involved with stress concentrations near the edge of the region, and may trigger an unstable delamination failure of the strengthened slab.