Axial load-bearing capacity in composite columns with sleeve anchor stiffeners and recycled concrete: experimental and numerical study

This study examines the behaviour of strengthened Concrete-Filled Steel Tube (CFST) columns using sleeve anchor stiffeners. It evaluates the effects of different numbers (three and six) and positions (40 mm and 75 mm from the edges) of sleeve anchors on structural strength, comparing these configurations to non-strengthened CFST columns. Both recycled and conventional concrete were used as infill materials to assess performance. The main objectives are to analyze the strengthened CFST columns’ behaviour and develop theoretical models for their ultimate axial capacity, a topic not covered in existing codes. Twenty CFST specimens were tested under axial load and the results validated finite element (FE) models. Additionally, FE models were developed using ABAQUS to explore parameters not experimentally tested, such as sleeve anchor numbers, positions, tube-section thickness, and concrete types. Findings indicate that six sleeve anchors significantly enhance axial load capacity and minimize lateral deformations. Optimal placement of sleeve anchors (40 mm from edges) can achieve substantial material savings. High-strength concrete CFST columns showed improved ultimate axial capacity and reduced local buckling. Discrepancies between experimental results and FE model predictions ranged from 5 to 11%. The study also incorporates constitutive models for recycled concrete to optimize load-carrying capacity and sustainability.