Axial compression performance of recycled concrete short columns confined with basalt fiber-reinforced polymer tubes

To promote the harmless treatment and resource utilization of waste concrete, it can be processed into recycled aggregate and used to partially or fully replace natural aggregates in the production of recycled concrete. However, recycled concrete generally exhibits inferior mechanical properties compared with normal concrete, particularly lower compressive strength, which restricts its engineering applications. To address this limitation, this study investigates the axial compressive behavior of circular short recycled concrete columns confined with basalt fiber-reinforced polymer (BFRP) tubes through experimental testing and numerical simulation. The primary variables include the number of circumferential basalt fiber sheet layers (0, 2, and 4) and the recycled coarse aggregate replacement ratio (0%, 50%, and 100%). Failure modes, ultimate load-carrying capacity, and axial compressive stress–strain responses of the core concrete were analyzed to evaluate the effects of the recycled coarse aggregate replacement ratio and the number of basalt fiber sheet layers. The test results indicate that unconfined specimens failed due to concrete crushing, whereas all BFRP tube-confined specimens failed by circumferential rupture of the BFRP tube, followed by crushing of the core concrete. The axial stress–strain responses of BFRP tube-confined recycled concrete and normal concrete are similar and can be divided into two stages. In the initial stage, the stress–strain response of confined concrete is nearly identical to that of unconfined concrete, indicating that the confinement effect is negligible. With further loading, progressive internal damage in the core concrete leads to increased lateral expansion, thereby activating the confinement provided by the BFRP tube. Consequently, the stress–strain curve continues to rise with pronounced strain hardening, and the slope of the ascending branch, axial compressive strength, and ultimate axial strain all increase with the number of basalt fiber sheet layers. Both the recycled coarse aggregate replacement ratio and the number of basalt fiber sheet layers significantly influence the load-carrying capacity of the short columns. Under the same confinement condition, the ultimate load-carrying capacity decreases with increasing recycled coarse aggregate replacement ratio. For unconfined specimens and those confined with two and four layers of basalt fiber sheets, the ultimate load-carrying capacity of recycled concrete short columns with a 100% replacement ratio was reduced by 21.5%, 13.6%, and 16.0%, respectively, compared with the corresponding normal concrete short columns. Nevertheless, the confinement efficiency of BFRP tubes increases with both the number of basalt fiber sheet layers and the recycled coarse aggregate replacement ratio. Compared with unconfined concrete columns, the ultimate load-carrying capacity of normal concrete columns (0% replacement) increased by 17.0% and 94.4% with two and four layers of basalt fiber sheets, respectively; for recycled concrete columns with a 50% replacement ratio, the corresponding increases were 23.8% and 86.0%; and for recycled concrete columns with a 100% replacement ratio, the corresponding increases further reached 28.8% and 108.1%. Under the test conditions, recycled concrete short columns with a 100% replacement ratio confined by two layers of basalt fiber sheets exhibited a higher ultimate load-carrying capacity than unconfined normal concrete columns, demonstrating the effectiveness of BFRP tube confinement in enhancing the compressive performance of recycled concrete. Furthermore, three-dimensional nonlinear finite element models of BFRP tube-confined concrete short columns were developed using the ABAQUS software. Comparisons between the numerical predictions and experimental results show good agreement in terms of failure modes, ultimate load-carrying capacity, and axial stress–strain responses. Parametric analyses reveal that the enhancement effect of BFRP tube confinement is primarily manifested in the strain-hardening stage. Both the axial compressive strength and ultimate axial strain increase progressively with the number of basalt fiber sheet layers, accompanied by an increased slope of the strain-hardening segment. Compared with specimens confined by a single basalt fiber sheet layer, the axial compressive strength of recycled concrete confined by two to five layers increased by 12.5%, 38.0%, 69.2%, and 109.1%, respectively.