Torsional behavior of reinforced slag-based geopolymer concrete columns under compression

Although extensive research has been conducted on geopolymer concrete (GPC) at material level, its structural behavior under complex stress states remains inadequately understood, especially under combined compression and torsion. In this study, monotonic torsion tests under combined compressive loading were performed on nine GPC columns and three ordinary Portland cement concrete (PCC) columns to examine the influence of concrete type and strength, axial compression ratio, and reinforcement strength ratio on the structural behavior. The investigation included analyses of failure modes, torque-twist relationships, load-carrying capacity, post-cracking stiffness, ductility, and softening response. Experimental results revealed that GPC columns achieved higher cracking torque and torsional load resistance than their PCC counterparts, accompanied by distinct crack propagation patterns. Compression force enhanced the cracking torque of GPC and PCC by 37.0% and 13.8%, respectively. Increasing compression force decreased the diagonal crack angle and longitudinal bar stress. With the higher strength grades of concrete, PCC specimens displayed typical brittle characteristics, including the reduced post-cracking stiffness, greater softening rates, and lower ductility. In contrast, GPC specimens exhibited concurrent increases in ductility and softening rate with concrete strength enhancement, along with denser and more numerous cracks. Current design codes provide satisfactory capacity predictions, but modifications to control parameters are recommended for specimens with high longitudinal reinforcement ratios (>1.5%) to ensure accurate and safe design.