Relevant parameters fitted by Bai model.

The Belt and Road strategy has significantly advanced the scale of infrastructure construction in the Qinghai–Tibet Plateau permafrost area. Consequently, this demands higher requirements on the strength and frost resistance of concrete (FRC) cured under low-temperature and negative-temperature conditions. Accordingly, in this study, tests on the mechanical properties and FRC were conducted under standard curing, 5 °C curing, and −3 °C curing conditions. The pore structure characteristics of concrete subjected to freeze–thaw (F–T) damage (FTD) under different curing methods were analyzed using nuclear magnetic resonance. The study results show that when the air content is constant, the compressive strength of concrete (CSC) tends to decrease with the curing temperature. Moreover, the occurrence of an age lag phenomenon is evident. The compressive strength of concrete cured under standard curing for 28-d was comparable to that achieved by concrete cured at 5 °C curing for 56-d and at −3 °C curing for 84-d. Under the same curing conditions, the CSC decreases with increasing air content. Observations revealed that with the air content in the concrete set at 0.08%, the material’s compressive strength was at its minimum. As the number of F–T cycles increases, the concrete transverse relaxation time (T2) curve shifts to the right, and the proportion of both harmful and multi-harmful pores increases. Based on the same CSC under different curing methods, the FRC under 5 °C curing and −3 °C curing conditions is considerably lower than that under standard curing conditions. Moreover, the FRC exhibits an increasing and then a decreasing trend with increasing air content. Concrete exhibits the best frost resistance when the air content is 3.6%. It was established that an optimal range exists for air content in concrete. If the air content is too low, there is only a slight improvement in the FRC. Conversely, if the air content was excessively high, it leads to a significant decrease in frost resistance. Further, this study establishes an FTD model for concrete under 5 °C curing and −3 °C curing conditions considering the compressive strength factors of concrete under standard curing conditions for 28-d. This study is anticipated to be used as reference for determining the FRC cured under different temperatures.一带一路倡议（Belt and Road strategy）显著扩大了青藏高原多年冻土区的基础设施建设规模。由此对低温、负温养护条件下混凝土的强度及抗冻性（Frost Resistance of Concrete, FRC）提出了更高要求。据此，本研究针对标准养护、5℃养护及-3℃养护三种工况开展了混凝土力学性能与抗冻性试验，并采用核磁共振技术分析了不同养护方式下经历冻融（Freeze–Thaw, F-T）损伤（FTD）的混凝土孔隙结构特征。研究结果表明：当含气量固定时，混凝土抗压强度（Compressive Strength of Concrete, CSC）随养护温度降低呈下降趋势，且龄期滞后现象显著。标准养护28d的混凝土抗压强度，等效于5℃养护56d及-3℃养护84d的混凝土抗压强度。在相同养护条件下，混凝土抗压强度随含气量升高而降低；当混凝土含气量设定为0.08%时，其抗压强度达到最低值。随着冻融循环次数增加，混凝土横向弛豫时间（Transverse Relaxation Time, T2）曲线向右偏移，有害孔与多害孔占比均有所上升。基于不同养护方式下相同抗压强度的前提，5℃养护与-3℃养护工况下的混凝土抗冻性显著低于标准养护工况。且混凝土抗冻性随含气量升高呈先升后降的趋势：当含气量为3.6%时，混凝土抗冻性最优。研究证实混凝土含气量存在最优区间：含气量过低时，抗冻性提升效果有限；反之含气量过高则会导致抗冻性显著下降。此外，本研究基于标准养护28d的混凝土抗压强度影响因子，建立了5℃与-3℃养护工况下的混凝土冻融损伤模型。本研究可为不同温度养护条件下混凝土抗冻性的确定提供参考依据。