Study on Mechanical Properties of Steel Fiber-Reinforced Geopolymer Concrete under Multi-Axial Loads

Steel fiber-reinforced geopolymer concrete (SFRGPC) is an inorganic cementitious material with environmentally friendly features. As compared to conventional concrete, SFRGPC has greater strength and durability, but it is brittle, making it similar to ordinary concrete. To date, the triaxial mechanical properties that regulate SFRGPC’s structural performance at serviceable and ultimate-limit conditions remain poorly understood. In this study, we conducted experimental and theoretical analyses of these properties. Conventional triaxial testing is used to investigate the effects of varying steel fiber contents and ratio of length to diameter under different confinement pressures on SFRGPC’s mechanical properties. The failure mode, maximal strength, stress–strain curve, maximum strain, and compressive toughness were analyzed and discussed. Under uniaxial compression, the failure mode of the SFRGPC specimens was a longitudinal split failure. The brittleness of the SFRGPC can be eliminated, and its resistance to breaking can be greatly improved by increasing the volume of steel fibers and the confining pressure in the mixture. The steel fiber content and ratio of length to diameter have obvious influence on the compressive strength of SFRGPC. As the steel fiber content increased, the compressive strength increased by 1.15–1.44 times; as the ratio of length to diameter increased, the compressive strength increased by 1.21–1.70 times. The increase in confining pressure can improve the compressive strength of concrete. With the increase in confining pressure, the increase trend of compressive strength becomes smooth. The confining pressure, steel fiber content, and steel fiber length have substantial influences on the compressive toughness index ${\eta }_{c3}$. Under increasing confining pressure, ${\eta }_{c3}$ increases linearly; however, after confining pressure is higher than 5 MPa, ${\eta }_{c3}$ tends toward a steady state when the confining pressure increases. Using numerical simulation, we also investigated the size effect of SFRGPC under triaxial load. The concrete cylinder’s strength does not significantly decrease as its size increases.