Tri-axial compressive behavior of coal reject concrete

Using coal reject concrete in steel tube or fiber-reinforced polymer (FRP) confined concrete columns for underground standing supports (i.e., underground water reservoir , tunnels and other infrastructures) is a plausible approach to mitigate the weaknesses of coal reject concrete, while the key issue of confinement mechanism of coal reject concrete under the tri-axial compression has never been investigated. To this end, tri-axial compression tests on 48 coal reject concrete cubes were carried out to investigate the effects of different coal reject aggregate volume replacement ratios (i.e., 0 %, 25 %, 50 % and 100 %) and active confining stresses ( i.e., either equal or unequal in the lateral directions) via the self-designed true tri-axial compression test instrument. The test results revealed that the typical axial stress-strain curve generally consists of an initial elastic ascending portion, a plastic ascending portion, and a post-peak softening portion. It also indicated that the plastic ascending segment could prolonged with the increased confinement, whereas the increase of the coal reject aggregate volume replacement ratio could significantly decrease the slopes of ascending segments. In addition, it shows that increasing either confining stress or coal reject aggregate content could increase the ductility. A new stress-strain model for actively confined coal reject concrete was then developed, which can accurately predict the maximum stress and strain of the confined coal reject concrete under different confined stresses, whereas current models for conventional concrete tended to be overestimated.