Prestress force-release tests at elevated temperatures - Gradient anchorage stability for prestressed EB CFRP strips

During the last decades, traffic loads by heavy vehicles have increased due to requirements of the current society and this fact has to be considered when the state of existing bridges is evaluated. Many concrete structures, for instance lateral cantilevers of highway boxgirder bridges, can be strengthened on the upper side with Externally Bonded (EB) Carbon Fiber Reinforced Polymer (CFRP) strips in order to increase their load-bearing capacity in flexure. These additional reinforcements can be applied as an unstressed or prestressed system. The main objective of this paper is to analyze the influence of elevated temperatures for instance due to the application of warm mastic asphalt on the bond between the concrete substrate, the epoxy resin layer, and the externally prestressed bonded CFRP strip. Background of the study is a non-mechanical anchorage system for prestressed composite strips known as the gradient anchorage. A literature review collecting the most important experimental works performed in this field together with an own experimental investigation performed at Empa is presented in this paper. Besides, an analytical linear elastic model has been proposed to study the behavior of such prestressed systems after the releasing stage.