Experimental Investigation of the Shear Resistance Mechanism on Hybrid NSC-UHPC Predamaged and Undamaged Unidirectional Bridge Slabs

This paper investigates the shear behavior of unidirectional hybrid slabs made of a normal-strength concrete (NSC) substrate and an ultrahigh-performance concrete (UHPC) overlay on the tensile side. The impacts of two important aspects on the shear behavior were studied. First, various strengthening configurations (thickness, with or without NSC substitution, with existing and/or new rebars) were investigated, specifically two configurations scarcely studied in the scientific literature. Second, the state of damage of the NSC slab prior to the overlay application was examined for the first time. The hybrid slabs tested behaved monolithically until the establishment of a composite mechanical action occurring at up to 1.66 times the reference shear resistance. The composite mechanical action offered structural hardening with significant increase of shear resistance up to 2.50 times the reference shear resistance. Strain distribution and reorganization was monitored on the slabs using digital image correlation technology, and showed creation of a strut and tie system in four successive steps. Both the UHPC thickness and the total area of longitudinal rebar had a significant impact on the ultimate shear resistance. The effect of the load history on the ultimate shear resistance was limited, and partly masked by the combined effect of the type of NSC-UHPC interface and the overlay configuration influencing the crack pattern.