Theoretical porosity design, mechanical properties, and durability of large-pore sandy recycled concrete

This study used recycled aggregate (RA) and mineral additions to design large-pore sandy recycled concrete (LPSRC) using a design porosity (DP) of 25 %, 20 %, and 15 %. Thus, the related effects of recycled coarse aggregate (RCA), recycled fine aggregate (RFA) and fly ash (FA) on the compressive strength, frost resistance (using slow and fast freezing methods), and sulfate attack properties of LPSRC were systematically studied. Moreover, to reduce the error between the design and theoretical porosity values, the bulk density ratio adjustment method was employed to design the theoretical porosity. The results of binarization analysis demonstrated that the deviation of the actual porosity caused by the sidewall effect was ∼2 %, which should be considered during DP design and performance evaluation. The compressive strength of the LPSRC after 56 curing days increased by 35.9 % on average as the DP was reduced from 25 % to 15 %. In a freeze–thaw cycle test, the slow freezing method was more accurate for evaluating the frost resistance of the LPSRC. Under the same conditions, the mass loss rate of the LPSRC calculated using the slow and fast freezing methods was 3.2 % and 7.62 %, respectively. This distinction is pivotal for establishing reliable frost resistance criteria for LPSRC in cold-weather applications, complementing previous research in the field. In terms of sulfate deterioration resistance, compressive strength corrosion factors decreased with increasing DP. Additionally, damage was mainly concentrated at the contact interface between RCA and the cement matrix, while the aggregate itself showed relatively little damage, suggesting RCA application has a positive effect on improving preparation of the LPSRC. Specifically, as DP increased from 15 % to 25 %, the average reduction in the compressive strength corrosion factors for LPSRC after 30 and 60 times of dry–wet cycle were 9.64 % and 8.37 %, respectively, illustrating a clear correlation between higher DP and reduced sulfate resistance. This study contributed to the LPSRC design and production using RCA, RCF and FA, that positively impacts to the sustainable development of the construction industry and helps to evaluate the effectiveness of RA in macroporous concrete. Moreover, the strategic implementation of design porosity has been shown to optimize the structural integrity and performance of the concrete, achieving a balance between strength and workability, which is crucial for the concrete's durability and its ability to adapt to various environmental conditions.