冻融循环对沥青混合料内水气扩散的影响

水气是沥青混合料水损害的重要来源之一，沥青混合料内部的水气运动不容忽视。大部分沥青路面常年经受冻融循环作用，为探究冻融循环前后沥青混合料内水气运动的变化规律，设计以相对湿度梯度为驱动力的水气扩散试验装置，以密级配沥青混合料AC-20C和开级配沥青混合料OGFC-13为研究对象，测量冻融循环前后沥青混合料水气穿透量随时间的变化情况，并采用菲克第一定律计算水气扩散系数。试验结果表明：5 cm厚OGFC-13试件在常压、20℃、100%~60%湿度差下的水气穿透量与时间呈线性正相关，可获得其扩散系数，而相同条件下的AC-20试件内水气较难穿透，水气穿透量与时间呈不规则变化，无法计算得到扩散系数，OGFC试件水气扩散速率明显大于AC试件，说明空隙结构对混合料水气扩散影响较大，但经历1个冻融循环后2类试件相同时间段内水气穿透量均有显著提高；负压环境下（13.325 kPa）进行试验可以增大AC-20试件的水气扩散速率，使水气穿透量与时间呈线性正相关，从而测量得到冻融循环前后AC试件的扩散系数；经冻融循环作用后，相同时间内2种沥青混合料水气扩散系数均显著增长，OGFC试件连通空隙率明显增大，说明冻融循环作用使沥青混合料内部结构发生变化，水气在沥青混合料内部的运动路径增多，更易进入沥青内部及沥青与集料界面，进一步加速沥青混合料水损害进程。

Effect of Freezing-thawing Cycles on Water Vapor Diffusion in Asphalt Mixtures

Water vapor is one of the important sources of moisture damage in asphalt mixtures, and its movement in asphalt mixtures cannot be ignored. Most asphalt pavements are subject to freezing-thawing cycles throughout the year. To investigate the change of water vapor movement in asphalt mixtures after one freezing-thawing cycle, a test apparatus was developed to measure the penetration of water vapor with time, which was driven by the relative humidity gradients, in open-graded friction course (OGFC)-13 and asphalt concrete (AC)-20 mixtures. Fick's first law was used to estimate the water vapor diffusion coefficient. The results show that the water vapor penetration in 5-cm-thick OGFC-13 mixtures increases linearly with time at atmospheric pressure, 20℃, and 100%-60% relative humidity differential, and the diffusion coefficient of OGFC-13 can be obtained. However, the penetration process in AC-20 mixtures is difficult, and there is no obvious law of penetration and time under the same condition, causing the diffusivity to be unmeasured. This shows that water vapor diffuses faster in OGFC mixtures than in AC mixtures, proving that the void structure of asphalt mixtures has a great influence on water vapor diffusion. Also, the water vapor penetration within the same time period in the two types of specimens increased notably after one freezing-thawing cycle. The rate of water vapor diffusion increases dramatically, and the water vapor penetration in AC-20 mixtures increases linearly with time at a negative pressure of 13.325 kPa, so the diffusion coefficient of AC-20 mixtures can be measured. After one freezing-thawing cycle, the diffusion coefficient of water vapor increased significantly for the two kinds of asphalt mixtures, and the percentage of air-connected voids in the OGFC-13 specimens also increased. This indicates that the freezing-thawing cycle changes the internal voids of asphalt mixtures and increases the water vapor diffusion path, which makes water vapor diffuse into the asphalt and the interface between asphalt and aggregates more easily and further accelerates the process of moisture damage in asphalt mixtures.