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•Complete shear and tensile laws for repair interface of NSC-UHPC beams are proposed and validated.•CF and IS models were developed respectively for good and poor surface preparation of repair interface.•Replication of NSC-UHPC beams behavior is significantly improved by using CF and IS models.•CF model most significant parameters are initial and peak cohesion stresses, shear stiffness and tensile stiffness.•IS model most significant parameter is the shear stiffness.
Normal-strength concrete (NSC) structural components strengthened with ultra-high-performance concrete (UHPC) overlays are widely studied and have demonstrated their efficiency in improving structural capacity and durability. The behavior of UHPC-strengthened beams or slabs is highly influenced by the properties of the NSC-UHPC interface. This project aimed to develop NSC-UHPC interface models by determining their detailed shear and tensile laws in UHPC-strengthened beams or slabs, which are missing presently in the literature. Determination of the interface models was realized through test data analysis of 700 interface characterization specimens found in the literature and inverse analysis with nonlinear finite element (FE) calculations of 14 UHPC-strengthened beams and slabs test results. Two interface models were proposed for two types of interfaces with good or poor surface preparations, respectively: a concrete fracture (CF) model and an interface sliding (IS) model. Results showed that FE calculations with CF and IS models provide accurate replication of the mechanical behavior of strengthened beams and slabs in terms of stiffness (K), ultimate shear capacity (P), and deflection at ultimate shear capacity (Δ), while the calculations with perfect bond model were not satisfactory. Parametric studies on the nine interface parameters indicated that peak cohesion stress, shear stiffness, and tensile stiffness greatly impact the shear behavior of strengthened beams in the CF model, while only shear stiffness had a significant effect on the shear behavior of strengthened slabs in the IS model. Suitable choices for interface model parameters, as in the proposed CF and IS models, provided accurate results for modeling the bending and shear behaviors of UHPC-strengthened concrete beams or slabs.