Study on the mechanical properties and freeze-thaw damage evolution of tuff manufactured sand concrete with different admixture incorporation schemes and coarse aggregate gradation adjustments

To investigate the effects of different admixtures and coarse aggregate gradation adjustment on the mechanical properties and freeze-thaw damage of concrete using tuff manufactured sand as the fine aggregate, five concrete groups were designed, including individual and combined incorporation of defoaming agent (DA) and air-entraining agent (AEA), as well as further adjustment of the coarse aggregate gradation based on the combined admixture system. Macroscopic tests and micro- and mesostructural characterization were conducted to analyze the mechanical properties, freeze-thaw damage, and microstructural changes of concrete with different mix proportions, and freeze-thaw damage models were established. The results show that the incorporation of DA alone increased the compressive strength by 14.46% but was accompanied by relatively pronounced freeze-thaw deterioration, whereas the incorporation of AEA alone reduced the compressive strength by 3.24% while mitigating freeze-thaw deterioration. After the combined incorporation of DA and AEA together with adjustment of the coarse aggregate gradation, the compressive and bending strengths of YH5 increased by 11.97% and 11.65%, respectively, and both the mass loss and compressive strength degradation after 200 freeze-thaw cycles were the lowest. The hardened-concrete air-void system results indicate that freeze-thaw resistance is closely related to air-void size and spatial distribution. YH5 exhibited a higher specific surface, a smaller spacing factor, and a higher proportion of small air voids, which was consistent with its relatively favorable freeze-thaw resistance. After 200 freeze-thaw cycles, the porosity of all groups increased, and the relative increase in porosity showed a significant positive correlation with compressive strength damage, indicating a good correspondence between pore structure deterioration and macroscopic mechanical damage. SEM and XRD results further showed that freeze-thaw action mainly caused pore expansion, microcrack development, and deterioration of microstructural integrity, while the main phase types remained essentially unchanged. Among the four damage models, the Gompertz model exhibited the best overall fitting performance, with a mean R 2 of 0.9839, and effectively described the nonlinear accumulation of compressive strength damage with increasing freeze-thaw cycles. These results provide a reference for the freeze-thaw-resistant mix design and damage evaluation of tuff manufactured sand concrete.