Recycling of waste recycled aggregate concrete in freeze-thaw environment and emergy analysis of concrete recycling system

It is an inevitable trend to recycle and use construction waste. Recycled aggregate concrete (RAC) has been widely studied and put into use. However, it is an important issue to improve the utilization rate and sustainability of waste concrete that how to use RAC for generations and apply it in a durable environment (freezing and thawing, carbonation, chloride ion erosion, etc.). In order to explore the recycled coarse aggregate (RCA) recycling system suitable for use in freeze-thaw environment. This study research the multiple recycling (up to the third generation) of RCA and a sustainability analysis in freeze-thaw environment. Three recycling systems with different RCA substitution rates were studied. RAC and 2nd-RAC were prepared in the test. The RCA replacement rates were 0 %, 50 %, and 100 %. The compressive strength and the frost resistance of the 1st-RACs and 2nd-RACs were tested. RCA was obtained by crushing the RACs after freeze- thaw cycles. The properties of RCAs were tested. Finally, the emergy analysis theory was used to evaluate the sustainability of the recycling system. The results showed that all RACs could meet the compressive strength of 40 MPa. Among them, RAC-0 in 1st-RAC has the highest compressive strength of 51.1 MPa, while RAC-50-50 in 2nd-RAC has the highest compressive strength of 48.3 MPa. All RACs have good frost resistance and the frost resistance of the 2nd-RAC was better than that of the 1st-RAC. The water absorption and apparent density of the 2nd-RCAs were better than those of RCA. The quality of the 3rd-RCAs has reached class III standards. The emergy sustainable index of the recycling systems under different RAC cycle replacement modes was greater than 2. The N/F ratio of RAC-100 is 0.33/0.50, RAC-100-100 was only 0.22/0.69. Increasing the RCA replacement rate and the number of recycles can increase the utilization rate of renewable resources and effectively alleviate the environmental pressure generated by the concrete industry. In areas where buildings are often damaged by freeze-thaw cycles, it is recyclable to use RCA instead of natural coarse aggregate to prepare concrete, and it is beneficial to improve the frost resistance of concrete. In practical engineering, the suitable RCA replacement rate and cycle mode can be selected by considering the performance, economic and environment.