Evolution of Mechanical Properties of Phase Change Concrete under High–Low Temperature Cycles and Freeze–Thaw Cycles

In order to solve the problem of deterioration of concrete properties caused by the coupling of high–low temperature cycles and freeze–thaw cycles in northwest China, a new type of high–low temperature phase change concrete (HLPCC) is prepared in this study using high–low temperature composite phase change aggregate (PCA). PCA is prepared through the adsorption of porous materials and the cement surface modification technology and using isovolumetric substitution of sand to incorporate it into the concrete. The thermal properties of the composite phase change aggregate are tested. The physical and mechanical changes of HLPCC are studied with different amounts of PCA under combined temperature and freeze–thaw cycles. The test results show that the phase transition temperatures of high–low temperature PCA are 42.2°C and 5.5°C, respectively, and they are able to undergo a phase transition to absorb and release heat before the ambient temperature reaches 42.2°C and the water freezes. The incorporation of PCA can well enhance the durability of the concrete itself, although it leads to a decrease in the early compressive strength of HLPCC. The prepared HLPCC has excellent resistance to high–low temperature cycles and freeze–thaw cycles after 200 high–low temperature cycles and freeze–thaw cycles, in which the PCA doped with 20% of the HLPCC mass loss rate is only 2.2%, the compressive strength decreases by 7.9%, the relative dynamic elastic modulus remains at 89.1%, and the pore structure degradation is the lightest, the most effective reduction of temperature stresses. Macro- and microanalyses show that PCA inhibits the cracking of concrete due to the internal and external temperature difference through the phase change effect and mitigates the change of concrete volume under the freeze–thaw effect, thus improving the durability of concrete. Research can provide certain theoretical references for improving the durability of concrete in cold regions.