Title: Impact of Retarder-Induced Roughness on Shear Friction Capacity using Conventional and High-Strength Reinforcement
Author(s): Stephan Ahn;Paolo M. Calvi;Dawn E. Lehman;Marc O. Eberhard
Appears on pages(s):
Keywords: shear friction, retarder-induced, high-strength reinforcement, concrete surface condition, concrete strength, steel reinforcement strength, reinforcement ratio, cold-joint,
Shear friction is used to transfer shear forces between two reinforced concrete members or two members with dissimilar materials. Shear transfer across a plane represents a complex phenomenon that depends on the interactions between several variables, such as concrete surface condition and cohesion, concrete strength, and steel reinforcement strength and reinforcement ratio. Prior research has focused on surface condition as well as properties of the reinforcing steel. However, these data have not provided conclusive results. Although it is clear that roughening the interface of a cold-joint contributes to increase its the shear-transfer capacity, the roughening methods currently used in practice are labor intensive. In addition, the ACI code prohibits the use of high strength steel reinforcement. To investigate these parameters methodically, an experimental research program was undertaken. The test matrix included 24 cold-joint specimens. Of those, half had untreated (“or smooth”) interface, while half were intentionally roughened using a surface retarder. Different types of retarder were investigated separately to ensure minimum roughness depth. The test series also investigated the impact of parameters such as steel reinforcement strength and reinforcement ratio. More specifically, for different specimens, varying amount of grade 60 and grade 80 steel reinforcing bar were used across the cold-joint interface. The experimental results indicated the following: (1) roughening the interface increases the shear-transfer capacity of the joint, (2) using the recommended retarder is an economical and structurally reliable method to achieve a roughened interface, (3) the reinforcement does not yield at the peak load and therefore, increasing the strength of the reinforcement does not provide a corresponding increase in shear-transfer capacity, and (4) increasing the reinforcement ratio has a more significant impact on the smooth-surface specimens. The results were compared with the shear friction design equations currently reported in the ACI code and in the American Association of State Highway Transportation Officials (AASHTO). The comparison indicated that both codes underestimate the shear friction capacity of cold-joint specimens with low reinforcement ratios, and that AASHTO code overestimates the shear friction capacity of roughened cold-joint specimens with high reinforcement ratios. Incorporating this data into a previously compiled shear-friction database, a new shear friction equation was proposed. The new equation has a separate cohesion term that separates the contributions of the steel reinforcement and cohesion and includes has higher strength upper limits for smooth-surface specimens and lower strength upper limits for roughened specimens.