Structural performance of permanent post-tensioned CFRP ground anchors with strap ends

Permanent post-tensioned ground anchors are widely used for the stabilization of different structures in civil engineering. Carbon fiber-reinforced polymer (CFRP) tendons have been proposed to replace steel tendons in ground anchors to solve the durability problem. The objective of this research is to develop permanent post-tensioned CFRP ground anchors with strap ends to achieve a load-bearing capacity of at least 2500 kN. The CFRP tendon comprises a multi-strap anchor head on the ground side, in which the CFRP strap end is prefabricated in a lightweight high-strength grout cylinder confined with CFRP rings. The confinement rings deviate the spreading forces occurring in the strap region into the cylinder’s axial direction. The grout cylinder is stepwise axially loaded in compression. The uniaxial compressive stress vs strain curves, including the softening branch, of four cement- and resin-based grout materials were obtained in compression experiments. The concrete compression model developed by Sargin was successfully applied to also describe the compressive grout behavior and subsequently used for finite element (FE) analyses of the CFRP ground anchors. In a first stage, CFRP ground anchor heads with one-strap ends on the ground side were developed and investigated in pull-out experiments simulating anchor applications in rock and soil. In the rock application, the anchor can be used without additional confinement, while in the case of soil, an additional CFRP confinement ring is needed to prevent premature grout failure in the strap region. In a second stage, CFRP ground anchors with two-strap ends on the ground side were developed and investigated in pull-out experiments simulating anchor applications in different rock types. The anchors reached an average load-bearing capacity of 1384 kN with final failure occurring in the CFRP straps. In a third stage, based on the pull-out experiments on the one- and two-strap anchors and the newly conducted FE analyses on the one-strap anchors, the load-transfer mechanism in multi-strap anchors was investigated. An empirical model was developed for deriving the load-transfer diagram along the embedded straps for multi-strap anchors and subsequently applied to a new three-strap anchor with a targeted capacity of 2500 kN. Furthermore, since strap rupture occurred in the one- and two-strap anchors, the tensile behavior of non-laminated and laminated straps, both applicable for strap anchors, was investigated using experimental, numerical and analytical methods. An empirical model for the non-laminated straps and an analytical model for the laminated straps were developed to predict the strap capacity; in the latter, the strap anisotropy and friction at the strap-pin interfaces were also taken into account. The findings of this research therefore demonstrate that conventional steel anchors can be replaced by high-capacity CFRP strap anchors. This will lead to cost saving since the anchor can be easily handled and will no longer need to be monitored and replaced.