Distinguished bond behaviour of CFRP sheets in unbonded post-tensioned reinforced concrete beams versus single-lap shear tests

•The first study experimentally compares FRP-to-concrete bond behaviour in large-scale UPC beams vs shear tests.•The interfacial bond behaviour in the beams was very different compared to the single-lap shear tests.•The CFRP debonding strain in UPC beams was higher than prediction by existing models.•A proposed model is suggested for use in UPC beams. The interfacial bond relationship between carbon fiber reinforced polymer (CFRP) sheets and concrete beams was developed based on single-lap shear tests on concrete blocks, which has been popularly used in the literature. However, the interfacial bond behaviour between CFRP sheets and concrete beams is different from that in single-lap shear tests. This study experimentally investigates the difference in the interfacial bond behaviour between CFRP sheets and concrete blocks in single-lap shear tests vs CFRP sheets and concrete beams, i.e. unbonded post-tensioned concrete beams (UPC). The experimental investigation comprises of ten full-scale UPC T-beams, including eight uncracked/pre-cracked UPC T-beams strengthened with CFRP sheets and two unstrengthened beams. These strengthened beams had a different design, such as various CFRP ratios and with/without U-wraps CFRP anchors. Meanwhile, seven corresponding single-lap shear tests were also carried out to compare to those from the beams. The experimental results showed that the interfacial bond behaviour in the beams was very different from that in the single-lap shear tests in terms of the debonding strain and strain distribution of CFRP sheets. Strain of CFRP sheets in the UPC beams exhibited stress concentration at cracks and substantial non-uniformity, particularly at high load levels. The debonding strain of CFRP sheets in the strengthened UPC beams was significantly higher than that in single-lap shear tests by 250% for the beams without U-wraps anchors and up to 490% for the beams with U-wraps anchors. The use of CFRP U-wraps anchors considerably enhanced the debonding strain up to 127% regarding the beams without anchors. Also, the accuracy of some existing codes and models such as fib Bulletin 14 and CNR-DT 200/2004 in estimating the bond strength of CFRP-to-concrete joints in UPC beams was evaluated in this study.