Effect of pore water pressure on mechanical performance of recycled aggregate concrete under triaxial compression

The pore water pressure in concrete can significantly increase due to volume compression. Recycled aggregate concrete (RAC) possesses a more complex microstructure compared to natural aggregate concrete (NAC). Understanding the porosity and micromechanical properties of RAC is crucial for analysing its failure mechanism under the influence of coupled confining pressure and pore water pressure. This study compares the constituent proportions and micromechanical properties of interfacial transition zones (ITZs) and the adjacent paste matrix in NAC and RAC. Compressive stress-strain curves were obtained for concrete under coupled confining pressure and pore water pressure. The results indicate that the newly formed ITZ, which bonds to old mortar, outperformed the one bonded to natural aggregate when considering the same water-cement ratio. Compressive strength, ductility, and maximum volumetric strain gradually increased with increasing confining pressure. However, when pore water pressure was removed, compressive strength decreased while elastic modulus improved. Due to the inferior microstructures of RAC compared to NAC, the supportive effect of pore water becomes more pronounced. This is evident in the gradual increase in peak strain with increased pore water pressure for the stress-strain curves of RAC (100 % replacement ratio). Finally, a failure criterion and stress-strain theoretical model considering pore water pressure are proposed, and satisfactory fitting results are obtained.