Repeated and Post-Repeated Flexural Behavior of Unbonded Post-Tensioned Concrete T-Beams Strengthened with CFRP Sheets

Studies on unbonded post-tensioned concrete (UPC) beams strengthened with fiber-reinforced polymer (FRP) sheets are limited and existing studies focus on monotonic loading; in particular, there are no studies on flexural behavioral characteristics of UPC beams strengthened with carbon FRP (CFRP) sheets subjected to repeated loads. This study deals with the flexural behavior of CFRP-strengthened UPC T-beams under repeated and post-repeated monotonic loading in which influences of varying load amplitudes, CFRP sheet ratios, and different CFRP U-wrap anchors were investigated. Experimental results showed that the high-amplitude repeated load considerably affected the response of the beams including creep strain in concrete, residual displacement, residual load-carrying capacity, crack width, CFRP sheet strain, and stress range in prestressing steel. During repeated loading, the CFRP sheets considerably decreased the residual displacement, energy dissipation, stress range in prestressing steel, maximum displacement, and crack width of the strengthened beams; these reductions were proportional to the number of CFRP layers and load cycles. The repeated loads tremendously affected the beams’ behavior under post-repeated monotonic loading particularly at the serviceability state but the influence became insignificant at the ultimate state. The U-wrap anchors considerably affected beam failure, increased the maximum strain of CFRP sheets and tendons, and enhanced the ductility of the beams under the post-repeated monotonic loads. The debonding strain of the CFRP sheets estimated from guidelines or codes was much smaller than the actual values for UPC beams.