Experimental Investigation on Shear-Torsion Behavior of Geopolymer Concrete Beams and Modified Capacity Model from Apparent Stiffness Variation

Abstract Geopolymer concrete (GPC) is a promising low-carbon alternative to ordinary portland cement concrete (OPC). Its mechanical behavior differs from that of OPC, and existing models developed primarily for OPC do not accurately predict the behavior of GPC beams under combined shear–torsion loading. This study experimentally investigates 17 GPC beams and three OPC beams and systematically analyzes the coupled effects of four key parameters—stirrup ratio, longitudinal reinforcement ratio, section height, and concrete strength—on shear–torsion behavior. The results show that, within the tested ranges, increases in concrete compressive strength, section height, stirrup ratio, and longitudinal reinforcement ratio lead to an increase in the ultimate load-bearing capacity of the beams. The shear and torsional capacities of geopolymer concrete beams are generally consistent with those of ordinary concrete beams, and both exhibit torsion-shear failure. However, GPC beams developed a larger number of more closely spaced cracks than OPC beams, with an average crack spacing of only about 60% of that in OPC beams. Under combined loading, the apparent (postcracking secant) stiffness was higher than that under single loading. Based on the Chinese design code, a modified model was developed to predict shear–torsion capacities by incorporating the observed stiffness variation through correction factors. The mean ratios of predicted-to-experimental shear and torsion capacities are 0.90 and 0.91, respectively, with coefficients of variation (COVs) below 0.10. Compared with direct calculations using Chinese, American, and European codes, the proposed model shows improved accuracy.