An experimental study was performed to evaluate the bond strength between two concrete layers of different ages, considering different mixtures of added concrete, with different strengths. The specimens first had the roughness of the substrate surface increased by sand blasting. Later, the new concrete was added. Afterwards, slant shear tests were performed to quantify the bond strength in shear. These tests indicated that increasing the compressive strength of the added concrete relative to the compressive strength of the substrate concrete improves the bond strength and changes the rupture mode from adhesive to monolithic. A finite element analysis showed that, increasing the difference between the compressive strengths of the added concrete and the substrate concrete, higher values of normal stress are present in the interface, for the same level of shear stress. The study presented in this paper reveals that the added concrete, normally with higher compressive strength than the substrate concrete, may possibly have an influence on concrete-to-concrete bond strength. Therefore, design of shear at the interface between concrete cast at different ages, according to Eurocode 2, could be improved, since here only the lowest strength concrete is considered.
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