Shear friction strength of smooth construction joints and monolithic interfaces

This study examined the shear friction behavior of smooth construction joints and monolithic interfaces according to the equivalent compressive stresses generated from clamping forces of the transverse reinforcement crossing the joint and external forces applied normal to the joint. Twenty-one push-off specimens were tested to measure the relationship between direct shear stress and relative slip along the interfacial failure plane. Mechanical models for monolithic joints proposed by Hwang and Yang are extended to estimate the shear friction strength of the construction joints. From the mechanical model and test results, the cohesion and frictional angle of concrete along smooth construction joints are determined to be 0·11(f′_c)^0·65 and 32·8°, respectively, where f′_c is the concrete compressive strength. The construction joints have considerably lower shear capacities than the companion monolithic joints, by approximately 70% for shear cracking strength and 55–70% for shear friction strength, indicating that the reduction ratios decrease with the increase of the equivalent compressive stress. The reliability of the previous empirical equations for the shear friction strength is highly sensitive to the roughness of the interfacial failure plane, whereas the proposed model provides improved accuracy in predicting the shear friction strength of the monolithic and smooth construction joints.