An experimental investigation of the bond performance of BFRP bars with recycled-aggregate concrete of different particle sizes

The pull-out test was performed in this study to investigate the bond performance between basalt fiber reinforced polymer (BFRP) bars and recycled concrete. Three factors were considered: aggregate size (5-10 mm, 11-15 mm, and 16-20 mm), bond anchorage length of bars (3D, 5D, and 7D), and the recycled aggregate replacement rate (0%, 30%, 60%, and 100%). The bond-slip mechanism, failure mode, bond stress distribution, and bond-slip curves were all analysed. Based on the results, it was found that situations with a high recycled aggregate replacement rate, large aggregate particle sizes, or long bond lengths have an increased likelihood of experiencing splitting failure. As the aggregate size increases, the bond strength between the BFRP bars and the concrete would also increase, in ordinary concrete, a 22.54% increase was observed, whereas in recycled concrete, the maximum increase reached 48.20%. The effect would be more pronounced on recycled concrete compared to ordinary concrete, and it would also be more significant in specimens with BFRP bars compared to steel bars. Additionally, the optimal bond anchorage length was determined to be 5D. The bond strength exhibited a trend of decrease followed by an increase as the recycled aggregate replacement rate increased. The maximum reduction of 59.7% in bond strength was observed when the replacement rate of recycled aggregate reached 60%. Moreover, when the replacement rate of recycled aggregate increased to 100%, there was a decrease of 19.05% in bond strength compared to the specimen without recycled aggregates. Based on the experimental test data and existing models, an improved two-stage bond-slip constitutive model was proposed.