Prediction of Interface Shear Stresses in Elastoplastic Moment-Varying Zones of Advanced Composite Material-Strengthened Concrete Members

Adhesive bonding of plates of advanced composite materials (ACMs) to reinforced concrete structural members is increasingly used to strengthen such members in flexure. The integrity of the strengthened member under load depends on the level of hybrid structural action developed between the concrete member and the plate through the adhesive. High shear stresses at the adhesive-to-concrete interface trigger fracture of the concrete, and so induce failure by loss of this hybrid action. In zones of varying moment along the member, these high interface shear stresses can arise from the requirement, after the steel yields, for high axial stress gradients to develop in the ACM plate. In these moment-varying zones of elastoplastic activity, the steel and concrete are into their nonlinear regimes of material behavior, and the height of the neutral axis changes nonlinearly along the member. These nonlinear effects, which generate complex behavior in elastoplastic zones, are not represented in existing approaches for predicting interface shear stresses. This paper fills that gap by presenting analytically exact expressions for interface shear stress which allow for vertical shift of the neutral axis and for material nonlinearities. The expressions reveal that distinct nonlinear disparities exist between the interface shear stress variations and the shear force variations in moment-varying elastoplastic zones. The model is verified against test data, and is then used to establish magnitudes of elastoplastic interface shear stress in a plated member under different types of load. The stresses are shown to reach critical values, and their variations along the plated member are seen to be load sensitive. It is concluded that this model for elastoplastic interface shear stresses is suitable for incorporation into procedures for design of ACM-strengthening schemes.