Pullout Simulation of Postinstalled Chemically Bonded Anchors

Chemically bonded postinstalled anchors have seen tremendous growth over the past few years for retrofits, as well as new construction. Currently, they are designed from proprietary tables provided by adhesive manufacturers based on laboratory pullout tests. Recently, Doerr et al. (1989), Cook (1993), and Eligehausen et al. (1984) have developed equations to predict pullout resistance of anchors. Since chemically bonded anchors result in the failure of both the concrete and adhesive-concrete interface, the equations attempt to predict the ultimate resistance of the anchor through the sum of the contributions from the concrete-failure cone and adhesive-concrete interface. However, this approach requires an estimate of both the average or maximum shear stress within the adhesive bond layer and the concrete-failure cone depths. To shed more light on the development of failure for these types of anchors, a state-of-the art elastoplastic finite-element analysis was performed and compared to experimental results. Besides being able to predict pullout resistance, concrete-failure cone depths, and orientations, the analysis revealed that failure initiates as a tension zone below the concrete surface at the anchor-adhesive interface and propagates with load toward the surface. In the process, both the concrete and adhesive material dilate increasing the confinement and shear resistance within the adhesive layer. Once the tension zone reaches the surface, the confinement is lost, resulting in a diminished shear resistance within the adhesive layer and anchor failure. After comparing a number of proposed methods to predict resistance to the experimental data, it was found that a simplistic, uniform bond stress applied over the whole anchor did an excellent job of predicting pullout capacity.