Investigating Concrete Performance: Compressive Strength and Pore-Structure Evolution in Simulated Permafrost Conditions of the Qinghai–Tibet Plateau Zone

As construction continues, a large number of concrete engineering facilities will be put into use in permafrost areas, which poses a challenge to the long-term service of concrete materials. To investigate the evolution of strength and pore structure of concrete under the cold permafrost environment, concrete compressive strength tests and mercury intrusion tests were performed at four molding temperatures (5°C, 10°C, 15°C, and 20°C) under continuous $-5°\mathrm{C}$ curing and standard curing conditions. Under negative-temperature curing, the strength of concrete at an early age reached about 80% that of standard curing; however, its strength at a later age was comparable to that at 28 days under standard curing. Under negative-temperature curing, the most probable pore size and porosity at 28 days of age were larger than those under standard curing. Increasing the molding temperature was beneficial for the promotion of the internal hydration reaction of concrete, refining its microscopic pore structure, and promoting its strength development. The strength of concrete with different porosities and average pore diameters could be predicted better using the Atzeni model or the bivariate Hasselmann model. Based on maturity theory, an equivalent age model of concrete under a permafrost temperature environment was established. The prediction deviation of the model was controlled within $\pm 5\%$, so the model can be used to predict the time-dependent strength of concrete in permafrost zones.