An innovative and precise confinement level definition of concrete filled steel tubular columns with geopolymer recycled aggregate concrete

In recent years, the utilization of structural members made of concrete-filled steel tube (CFST) sections has increased due to their positive effect on the system's performance. In addition, there is a need to find sustainable and environmentally friendly materials that make a successful step toward saving environmental resources and reducing pollution. In this study, the two approaches were combined by the utilization of recycled aggregates forming what is known as geopolymer recycled aggregate (GRA) CFST columns. A novel definition of the confinement level was proposed for CFST square columns with a new relationship between the residual load coefficient and the confinement level. The nonlinear finite element analysis (NLFEA) method was used to examine the behavior of GRACFST columns. The investigated parameters include the following: (i) recycled aggregate replacement ratio (R%) as 0, 50, and 100, (ii) infill concrete strength (30, 45, and 65 MPa), and (iii) width-to-thickness ratio (B/t) as (52.0, 32.0, 23.4, and 18.7). It was found that increasing the (R%) maximized the benefit of increasing the steel tube thickness for high-strength concrete compared to reversed behavior for normal-strength geopolymer concrete. In addition, the proposed levels definition and the introduced relationship between the confinement level and the ratio between the residual and the ultimate load capacities were real and accurate as evidenced by the validation against literature with +-10% error.