Freeze-thaw cycling of nano-SiO2-modified recycled aggregate in concretes: Hydro-thermal-mechanical modeling

Abstract

The freeze-thaw cycling in the cold region causes severe damage to the concrete, which seriously affects the durability of concrete. In this paper, nano-SiO₂ (NS) modified RAC (NS-RAC) specimens were prepared by impregnating RCA with a 3 % NS dispersion for 48 h with following frost resistance assessed experimentally. To further fully understand the performance of NS-RAC under freeze-thaw (F-T) cycles, a comprehensive hydro-thermal-mechanical coupling model was established to simulate the damage behavior of NS-RAC under F-T conditions. A random polygon microstructure model of RAC was developed, and the characteristics of RCA and three types of ITZs were discussed. The damage control equation for RAC during the F-T process was defined based on the coupling effects of percolation, temperature, and stress in a porous medium. The F-T cycle tests show that the compressive strength of NS-RAC samples after 100 F-T cycles is 9.04 MPa higher than that of RAC. Numerical simulation results indicate that, compared to RAC without NS, the maximum pore pressure, crystallization pressure, and maximum principal stress of samples with 3 % NS after the first F-T cycle are reduced by 3.84 %, 4.36 %, and 2.26 %, respectively. During the F-T cycle, with increases in ITZ width, aggregate gradation range, RCA replacement rate, and temperature difference, the internal stress of NS-RAC specimens increased to varying degrees. This study provides insights into the F-T damage of RAC in severe cold regions and promotes the practical engineering application of RAC in such environments.