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

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.

Automated summary

This study investigates the difference in interfacial bond behavior between CFRP sheets and concrete blocks in single-lap shear tests compared to CFRP sheets and concrete beams. The experimental results show significant differences in debonding strain and strain distribution of CFRP sheets between the different test setups, with strengthened UPC beams exhibiting higher debonding strain than single-lap shear tests. The use of CFRP U-wraps anchors greatly enhances debonding strain in the beams. Additionally, existing codes and models for estimating bond strength of CFRP-to-concrete joints in UPC beams are evaluated.