Full-scale testing of high-strength RACFST columns subjected to axial compression

The axial compression performance of concrete filled steel tubular (CFST) columns using high-strength recycled aggregate concrete (RAC) instead of normal concrete (NC) was studied in an attempt to use demolition debris for effective recycling in construction works. Five specimens (three RAC-filled steel tubular (RACFST) columns and two reference CFST columns) were tested to investigate the influence of tube shape (circular or square), concrete type (NC or RAC) and internal structure (installation of a steel reinforcement cage or not). The test results indicate that damage development and failure mode of RACFST columns are similar to those of CFST columns. For the same cross-sectional area, steel ratio and material strength, circular section specimens were found to have a higher load-bearing capacity and better deformability than square section specimens. The replacement of NC with RAC has relatively less influence on the axial compressive behaviour of square section specimens than on circular specimens. A steel reinforcement cage inside the square section specimen was found to strengthen confinement to the core concrete, which significantly improved the bearing capacity and deformability. The results obtained are compared with the ultimate strengths of RACFST columns predicted using existing design codes. Considering the size effect, a formula for calculation of the bearing capacity of square CFST columns is suggested; comparisons of the predicted results show good agreement with the experimental data.