Research on frost resistance and life prediction of fly ash manufactured sand concrete under negative temperature curing

According to the principles of green and environmental protection development, manufactured sand fine aggregate and industrial solid waste fly ash (FA) have been increasingly applied in hydraulic concrete engineering. However, there is still insufficient understanding of the frost resistance of manufactured sand concrete containing FA in high-altitude cold regions. This study designed manufactured sand concrete with different FA content (0%, 10%, 15%, 20%) and conducted freeze-thaw cycle tests on concrete cured under -5°C and 20°C to investigate the impact of FA dosage on the frost resistance and microstructure of manufactured sand concrete in high-altitude cold regions. Compressive strength tests have been conducted at the curing age. The frost resistance was evaluated at the macroscopic level using surface morphology changes, mass loss ratio, and relative dynamic elastic modulus (RDEM); at the microscopic level, nuclear magnetic resonance (NMR) was employed to analyze pore distribution, and scanning electron microscopy (SEM) was used to observe the interfacial structure. The results indicate that negative temperature curing (NC, -5°C) concrete compressive strength has an "age lag" phenomenon, and its strength at 49 days is close to that of standard curing (SC, 20°C) concrete at 28 days under the same FA content. Throughout freeze-thaw cycles, the frost durability of NC concrete is weaker than SC concrete, and the mass loss ratio of concrete decreases and then increases, while the RDEM exhibits a declining trend; NMR and SEM analyses indicated that an appropriate quantity of FA could optimize the pore structure of the concrete and reduce the proportion of large pores and cracks, porosity decreased and then increased with rising FA dosage, under two curing temperatures, the highest frost resistance of concrete is achieved when the FA content is 15%; the specimens T-5-FA15 and T20-FA15 had the lowest porosities of 2.23% and 1.97%. Finally, utilizing the Weibull distribution function, a durability deterioration damage model for manufactured sand concrete was established. It was determined that the specimens T-5-FA15 and T20-FA15 suffered the least damage at two curing temperatures, with a frost durability life of 79 and 107 years.