Research on Concrete Strength Growth and Micromechanism under Negative Temperature Curing Based on Equal Strength Theory

In order to study the influence of different casting temperatures and curing conditions on the hydration reaction, strength, and micropore structure of concrete cured in a low negative temperature environment, a hydration heat test of high-speed railway durability concrete in a permafrost area was carried out. The external temperature was $-5°\mathrm{C}$, while the casting temperature was either 5°C, 10°C, 15°C, or 20°C. A concrete compressive strength test was carried out, in which the dosage of air-entraining agent used was 0.00%, 0.05%, 0.10%, 0.15%, or 0.20%. Based on a mercury injection test, the mechanisms of concrete pore structure formation under the variation of casting temperature and air-entraining agent content were analyzed. The age of equal strength under different casting temperatures and different dosages of air-entraining agent was obtained, based on the theory of equal strength. The results of our research show that compared with standard curing, the hydration reaction of cement was significantly inhibited and the hydration rate and degree were decreased under the condition of constant $-5°\mathrm{C}$ curing. Under negative temperature curing, the micropore structure of concrete underwent obvious deterioration. The air introduced by the air-entraining agent formed dispersed and nearly spherical bubbles in the concrete, changing the bubble spacing and improving the antifreeze performance of the concrete at low temperatures. The concrete showed an obvious age lag phenomenon under negative temperature curing. Low negative temperature cured samples with 20°C, 15°C, 10°C, and 5°C casting temperatures met the equal strength requirements in 42, 49, 56, and 63 days, respectively. These research results provide guidance for future concrete design efforts and high-speed engineering construction activities in frozen soil areas.