Gradient anchorage method for prestressed CFRP strips: Principle and application

The 'gradient anchorage method' for highly prestressed Carbon Fiber Reinforced Polymers (CFRP), introduced by Professor Urs Meier at Empa in the beginning of the 2000s, foresees a purely CFRP-epoxy-concrete connection at the strips ends without any remaining anchor plates and bolts. In order to avoid a debonding failure due to the high shear stresses, the anchorage force is gradually decreased by intermediate sector-wise accelerated epoxy curing at elevated temperatures. Within the framework of an R&D project in collaboration with the industrial partner S&P Clever Reinforcement from Switzerland, the 'gradient anchorage method' was considerably improved for commercial application. In the project, several key parameters of the anchorage were investigated and optimal values have been determined. For example the temporarily anchorage resistance of partly cured epoxy adhesive under elevated temperatures were determined with experimental pull-off and releasing tests. This paper focuses on the development of the complete prestressing and heating device allowing a practical application on site, will be presented. The functionality of the newly developed control of the heating elements is described. Finally, the complete working procedure to apply a highly prestressed CFRP strip is presented and explained in detail. Compared to conventional prestress methods and anchorages for CFRP strips, the total duration necessary to establish a sufficient anchorage is considerably shortened.