Shear Strength of Bonded Concrete

The goal of repairs to existing concrete structures is for the repaired concrete structure to return, as closely as possible, to its original design strength and stiffness. At the repair surface, the bond between the original and new concrete layers is of primary importance and must provide a continuous transfer of the design loads at a minimum and preferably approach the original concrete strength. Although studies since the mid-1800s to the present day have not resulted in an accepted standard test to verify the bonded concrete shear strength, many studies suggest that actual shear strength at the bonded concrete interface is on the order of 5 to 10 times, and sometimes more than 10 times, the values currently adopted in present standards. This paper summarizes the results of selected historical studies, and recent research at the University of Colorado Denver, revealing shear strengths much higher than the current ACI 562 for repairs and rehabilitation might mandate. Structural engineers would benefit from an improved understanding of shear strength at a bonded concrete interface and, should the existing standards be more realistically revised, be able to design more efficient and cost-effective concrete repairs.