Study on axial compressive behavior of concrete-filled steel tube columns with mult-source mixed recycled coarse aggregate

Abstract

The quality of recycled coarse aggregate (RCA) is highly effected by the strength of original concrete. However, it is hard to confirm the source and strength of original concrete of mixed recycled coarse aggregate (MRCA) used in the actual engineering. In order to study the effects of multi-sources MRCA on the axial compressive behavior of concrete confined by steel tubes and use easily measured parameters to build a stress-strain model, 6 different types of MRCA were collected and the axial compressive tests and analysis of 63 plain mixed recycled concrete (PMRC) columns and 21 mixed recycled concrete-filled steel tube (MRCFST) columns were carried out. The results show that there are more cracks and holes in the microstructure of mixed recycled concrete (MRC), which results in a lower compressive capacity of PMRC than plain ordinary concrete (POC). The constraint of steel tube weakens the negative influence of MRCA on the compressive capacity of MRCFST, especially for the circular cross-section. The peak strain of MRCFST is smaller than that of ordinary CFST, and the reduction of peak strain of circular MRCFST is the smallest in different cross sections. Meanwhile, the peak strains of MRCFST columns in circular, square, and rectangular sections are all approximately linearly related to the apparent density (ρ), water absorption (ω) and crushing index (δ) of MRCA. The confinement coefficient has no obvious influence on the σ-ε curves of MRCFST with different sections, and the curves all exhibited the characteristic of “strain softening”. The predicted curves of the stress-strain model considering the effects of ρ, ω and δ parameters agree well with the experimental curves.