Seismic strengthening of reinforced concrete columns with straight carbon fibre reinforced polymer (CFRP) anchors

Abstract

The use of Externally Bonded Fibre Reinforced Polymer (EBR-FRP) systems is an established technique for the structural improvement of existing buildings but the technique features disadvantages. Premature FRP-to-concrete debonding has been commonly highlighted as one of the main problems, together with the difficulty of fully wrapping the structural element when the structure presents complex geometries. FRP straight anchors are used to transfer the forces from the FRP sheet into the structural element, ameliorating these 2 problems, but a comprehensive design method for FRP anchors has not yet been established despite the increased use and research attention given to FRP anchors. A research project was undertaken with the ultimate aim of developing design equations suitable for wide implementation of FRP anchors with EBR-FRP systems. The first stage of the project involved monotonically testing single-anchors in tension to investigate the behaviour and capacity of isolated FRP anchors. However, a number of factors that may have a significant influence on the capacity of the anchors could not be investigated with direct tensile single-anchor testing. Many of these factors are related to seismic response, such as the behaviour of the anchors when subjected to tension-compression cycles and the effect of dynamic loads, which are highly relevant for the New Zealand context. To address some of the aspects not covered within the single-anchor tests, 6 full-scale reinforced concrete columns replicating the ground-level columns of an existing understrength building in Wellington were tested. In addition to the control column, 2 confinement schemes and 2 anchorage configurations were tested, all using pseudo-static loading, with the aim being to verify that the peak moment developed during testing was forecasted accurately. Column test 6 was a proof-of-concept test, with the objective of the test being to verify whether the experimentally established column moment and displacement ductility capacities could be predicted using a standardised design procedure. For column test 6 the FRP sheet was installed without bonding the bottom part of the sheet to the column, and instead anchors were used to hold the sheet in place, which allowed an increase in tensile strain of the FRP sheets above the value that would cause debonding. This elevated strain enabled a larger column drift to occur when compared to the previous column tests, indicating that the displacement ductility capacity of the column could be designed by changing the height of the debonded area.