Effects of Curing Conditions on Freeze-Thaw Resistance of Hydraulic Concrete by Magnetic Resonance Technology

[Objective] This study aims to investigate the effects of different temperature and humidity curing conditions on the freeze-thaw resistance of concrete, and to conduct freeze-thaw cycle tests on cured specimens for quantitative analysis of mechanical properties and internal micropore structure. [Methods] Nuclear magnetic resonance technology (NMR) was used to measure internal pore parameters of concrete under freeze-thaw cycles and to study the evolution of pore distribution with the increasing number of cycles. Based on the NMR test results, the proportions of different pore categories and the changes in porosity during the freeze-thaw cycles were analysed dynamically. [Results] Compared with concrete cured at 20 ℃-95 % RH, whose relative dynamic modulus of elasticity remained generally stable after 300 cycles, concretes cured at 3 ℃-50% RH and 10 ℃-70% RH deteriorated sharply after 200 and 150 cycles, respectively, indicating that insufficient curing temperature and humidity led to complete hydration reactions, thereby severely affecting concrete durability. After different numbers of freeze-thaw cycles, the T2 relaxation time distributions of concrete exhibited good continuity, with the overall pattern consisting of three parts: small pores, medium pores, and large pores (micro-cracks). However, for concrete cured under insufficient conditions, the third peak became markedly pronounced because the reduced temperature and humidity altered the distribution and connectivity of internal pores and micro-cracks, and the freeze-thaw cycles further increased the number of large pores and micro-cracks. According to porosity measurements and pore distribution characteristics, insufficient curing temperature and humidity resulted in inadequate hydration products filling the original pores, resulting in increased porosity. As the freeze-thaw cycles progressed, when subject to osmotic and frost-heave pressures, different pore types suffered damage and interconnection, and new micro-cracks even formed in the interfacial transition zone, causing porosity to increase continuously. Based on the freeze-thaw damage test results, the cumulative damage after different freeze-thaw periods was obtained using the relative dynamic modulus of elasticity as the damage variable. Comparison between test results and the established damage model yielded correlation coefficients all greater than 0.98, and a concrete freeze-thaw damage model was developed using the Boltzmann equation. [Conclusions] Analysis of the relationship between freeze-thaw cycles and cumulative damage shows that as the number of freeze-thaw cycles increases, internal cracks propagate and permeability intensifies, resulting in continuous accumulation of macroscopic damage. Investigating the freeze-thaw damage process of concrete under different curing conditions not only helps improve macroscopic mechanical properties and durability performance by optimising pore structure but also reveals the meso-scale damage evolution mechanism of hydraulic concrete under freeze-thaw action.