Compressive behavior of geopolymer recycled brick aggregate concrete confined by steel tubes

Geopolymer recycled brick aggregate concrete (GRBAC) is an environmental-friendly material and its strength and ductility can be improved by employing steel tube for confinement. However, there is little research on the mechanical performance and constitutive model of GRBAC under the confinement of steel tube in the past. In this paper, the axial compressive behaviors on 25 stub columns of GRBAC confined by steel tubes were experimentally studied, and the test variables were reference compressive strength (45 and 65 MPa), recycled brick aggregate (RBA) replacement ratio (0, 30%, 50%, 70% and 100%) and thickness of steel tube (2.5, 3.7 and 5 mm). The characteristics such as failure mode, load-deformation curve, ultimate strength, compressive stiffness, ductility and peak strain of the stub columns were presented and analyzed. The results demonstrate that the adverse effect of RBA content on the ultimate strength is weakened due to the confinement effect of the external steel tube, and the compressive strength of confined GRBAC is 1.73-3.45 times that of unconfined specimen. With the increase of RBA replacement ratio, the axial compressive ultimate strength and stiffness of the steel tube confined GRBAC decrease, and the ductility increases. Compared with the specimen without RBA, the axial compressive ultimate strength and stiffness of the specimens with 100% RBA replacement ratio decrease by 21.8%- 33.3% and 31%-40%, respectively. Based on the existing conventional core concrete model, considering the characteristics of GRBAC, a finite element constitutive model and a fiber constitutive model suitable for GRBAC confined by steel tubes were proposed.