Impact of Test Setup on Tensile Behavior of Adhesive Anchors

Abstract

This report describes a study of adhesive anchors in two types of tension tests. Confined tension (CT) tests are relatively easy to implement; however, the closely placed reaction may create undesirable and unrealistic confining condition to test anchor. Unconfined tension (UCT) tests are then needed to verify the tensile capacities, which usually correspond to lower bond strength than those from CT tests. Therefore, bond strengths obtained from CT tests must be reduced, and a reduction factor of 0.75 has been stipulated in ACI 355.4 document. This reduction factor (alpha_setup) is currently being challenged. In this study, we attempt to explore the impact of test setup on behavior of adhesive anchors using laboratory tests and finite element (FE) analyses. A total of 28 CT tests and 44 UCT tests were conducted. The measured capacities were found proportional to the embedment depths, indicating that the current uniform bond stress model in ACI 318-19 is reasonable. The tests indicated that the current reduction factor is reasonable when the bond strength is determined as the 5%-fractile from CT tests. The reduction factor (0.75) is unconservative if the bond strength is determined as the average strength from CT tests and a reduction factor of 0.5 is reasonable. The FE analyses indicate the the impact of lateral confining pressure from closely placed reaction in CT tests is reasonably negligible. Instead, the concrete surrounding adhesive anchors in UCT tests may develop dilation/splitting cracks near the top portion of concrete, leading to partial disengagement of adhesive from concrete and thus reduced bond resistance. Finally, concrete breakout may occur at top simultaneously with bond failure. This behavior, widely observed only in UCT tests, may also be responsible for the calculated capacity reduction. The above conclusion is limited by the fact that the vinylester adhesive used in this study had a low-medium bond strength as the average bond strength with ½-in. diameter anchors was about 2800 psi.