Evaluation indexes of the frost resistance of recycled aggregate concrete and improvement mechanisms: A review

The recycling and utilization of waste concrete can reduce the extraction of natural building materials and alleviate environmental pollution, which is an inevitable choice to promote sustainable development. The objective of this paper is to enhance the frost resistance of recycled aggregate concrete (RAC) in order to prolong its structural service life. Based on the existing literature, this study first reviews and analyzes the current evaluation indicators for the frost resistance performance of RAC. It compares the effectiveness and advantages and disadvantages of evaluation indicators such as mass loss rate, relative dynamic elastic modulus, and compressive strength loss rate. A research approach is proposed for a comprehensive evaluation method of RAC frost resistance based on multiple indicators. Additionally, the damage patterns of the new and old paste and the interfacial transition zone (ITZ) in RAC under freeze-thaw cycles are explained on a microscopic level, revealing the deterioration mechanism under freeze-thaw action. Furthermore, the influence of coarse aggregate modification, mineral admixtures, and different fibers on the frost resistance of RAC under freeze-thaw cycles is reviewed from macro- micro multi-scales, and a strategy for enhancing RAC frost resistance is constructed. The research indicates that using physical or chemical strengthening methods such as particle reshaping, thermal grinding, mechanical crushing, chemical reagent soaking, chemical grouting, and nano- silica solution treatment to improve RCA quality, and completely replacing natural coarse aggregates, can produce RAC that meets the F300 frost resistance grade. Additionally, incorporating 5 % rubber particles and reducing their size can increase the air content of RAC, effectively buffering freeze-thaw stress. Compared to RAC without rubber particles, the compressive strength loss rate can be reduced by 15 %-35 %. Future research should focus on improving the freeze resistance of RAC at the fine- and nano-scales.