Effect of Freeze-Thaw Cycles on Carbonation Behavior of Three Generations of Repeatedly Recycled Aggregate Concrete

Multiple recycling of waste concrete has attracted widespread attention. This study presented the carbonation behavior of repeatedly recycled aggregate concrete (RRAC) used in a micro-frozen region. The effects of freeze-thaw cycles on the carbonation depth of three generations of RRAC with 25%, 75%, and 100% of replacement rate were evaluated. All RRAC specimens after different numbers of freeze-thaw cycles were rapidly carbonated for 28 d indoors to test the carbonation resistance of concrete. The results suggested that the carbonation depth of RRAC subjected to freeze-thaw cycles is higher than that in the non-freeze-thaw condition. This is because the freeze-thaw damages cause the internal structure of RRAC to become porous and and prone to cracking, thus providing convenient channels for CO2 to react with the alkali in the cementitious materials. With the growth of replacement rate or recycling number, RRAC reveals serious freeze-thaw damage and inferior carbonation resistance, which is due to the continuous deterioration repeatedly recycled concrete aggregate (RRCA) quality. However, when the replacement rate was 25%, the carbonation depth for the third generation of RAC was comparable to the second generation of RAC at a 75% replacement rate, and even the first generation of 100% RAC. To ensure better carbonation resistance durability of multiple recycling RAC, the low replacement rate of RRCA should be considered. For the third generation of RAC with the 100% replacement rate, its highest carbonation depth after freeze-thaw cycles was 9.16 mm, which still met the design requirements for structural use in a micro-frozen region. This indicates that it is feasible for three generations of RRAC to be used in the micro-frozen environment and that RRAC has great engineering application potential and promotional value.