Mix-Proportion Design Methods and Sustainable Use Evaluation of Recycled Aggregate Concrete Used in Freeze-Thaw Environment

Recycled aggregate concrete (RAC) has been gradually used in practical engineering owing to the maturity of waste-concrete recycling technology. However, the disposal of waste RAC in different environments remains challenging. Second-generation recycled concrete aggregates (second RCAs) prepared by crushing waste RAC can further promote the sustainable development of the construction industry, in which the mix-proportion design method of RAC used in different environments is key in determining the feasibility of recycling second RCA. In this study, to investigate the usability of second RCA, five typical concrete mix-design methods were selected to prepare RAC with a service life of 50 years for freeze-thaw (F-T) environments in cold regions: the ordinary concrete design method, high-performance method, saturated surface-dry aggregate design method, equal mortar volume method (RAC-E), and overall calculation method (RAC-A). The frost resistance of RACs and the physical properties of second RCAs were investigated, and the economy of recycling waste RAC with different mix-proportion design methods was evaluated via value engineering analysis. The results showed that after 350 F-T cycles, the mass loss and relative dynamic modulus of elasticity of RAC-E and RAC-A were better than those of specimens prepared via other methods. Compared with the original RCA, the second RCAs obtained using waste RAC in the F-T environment performed better, and their water absorption was reduced by 23%. A value analysis of the RAC-waste RAC-second RCA circulation system showed that RAC-A exhibited the highest value coefficient. Both the equal mortar volume and overall calculation methods can be used for the mix design of RAC in the F-T environment; however, the latter is a better method owing to its higher value coefficient and lower calculation error.