Improving the salt frost resistance of recycled aggregate concrete modified by air-entraining agents and nano-silica under sustained compressive loading

This paper systematically investigated the combined effects of sustained compressive loading and the salt frost resistance for recycled aggregate concrete (RAC) incorporating air-entraining agents (AEA) and nano silica (NS). The effects of different RCA replacement ratios, freeze-thaw cycles, AEA dosage and compressive stress level on the freeze-thaw damage properties including the apparent morphology, mass loss rate and relative dynamic elastic modulus (RDEM) were discussed. The microstructure evolution of RAC samples before and after freeze-thaw cycles were characterized and analyzed through the SEM observation, microhardness measurement and X-CT tests. The synergistic modification mechanism of NS and AEA on RAC under salt frost action were investigated, and the link between the macroscopic and microscopic properties of air-entrained NS-modified RAC was further explored. The results showed that the RAC mixed with a certain amount of AEA and NS dosages exhibited the stronger salt frost resistance compared to that of control group. The effect of AEA dosage and compressive stress level exhibited similar changing trends on the salt frost resistance of RAC. When adding a moderate amount of AEA and applying a certain sustained compressive loading, the RDEM loss of RAC was reduced so that the salt frost resistance was partly improved. Similarly, the moderate AEA dosages can reduce the deterioration at ITZs of RAC. The compound use of NS and AEA proved to be as an efficient method to strengthen the microstructure of RAC, further improving the frost resistance under the coupled actions of sustained compressive loading and chloride salt attack. This study provides a feasible approach to improve the salt frost resistance of RAC under the coupled actions of sustained compressive loading, which is expected to promote the application and extension of RAC in complex environmental regions.