Relationship between thickness distribution characteristics of asphalt mortar and self-healing behavior of asphalt mixture

Abstract

The microstructure of the asphalt mortar considerably affects the macroscopic selfhealing behavior of the asphalt mixture;however,there are insufficient indexes that explain this relationship.Therefore,an image analysis method was proposed in this study to characterize the microstructure of the asphalt mortar,and the effect of the complex mortar thickness distribution on the self-healing properties of the asphalt mixture was investigated.First,the calculation method for the mortar thickness was optimized,and new morphological indexes were proposed to quantify the distribution of asphalt mortar,namely,average mortar thickness（Tm）and standard deviation of the mortar thickness distribution（SDt）.Then,two-dimensional images of six selected asphalt mixtures with different gradations and nominal maximum aggregate sizes（NMAS）were obtained and analyzed to verify the effectiveness of the proposed image analysis method.Finally,the self-healing index（H）of the asphalt mixture was determined by using semi-circular bending（SCB）fatigue tests with and without rest time.The relationship of H with the proposed microscopic indexes was investigated.Both the gradation type and NMAS were found to affect the mortar thickness distribution.The poor correlation between the macroscopic volumetric indexes:voids in the mineral aggregate（VMA）and voids filled with asphalt（VFA）,and the microscopic indexes of asphalt mortar verifies the limitation of the volumetric properties for characterizing the microstructure of asphalt mortar in mixtures.Both the average thickness and distribution uniformity of the asphalt mortar affected the healing index.The comprehensive index,Tm/SDt,was strongly correlated with the healing index,indicating that it can be used as an effective index to evaluate and predict the self-healing efficiency of asphalt mixtures. 沥青混合料自愈合行为与沥青砂浆的细观分布特征关系密切，但目前相关判据不足。为此，采用数字图像处理技术定量表征沥青砂浆的细观结构，并分析其与混合料自愈合行为的关系。首先，优化沥青混合料砂浆厚度计算的方法，提出基于图像分析的砂浆平均厚度Ｔｍ 和砂浆厚度分布标准偏差ＳＤｔ 指标；其次，对６种不同 沥青混合料砂浆的二维图像进行统计分析，验证细观砂浆厚度参数的有效性；最后，通过有无间歇时间的半圆弯曲疲劳试验获取沥青混合料的自愈合指数Ｈ，并进行宏细观指标的相关性分析。结果表明：级配类型和最大公称粒径均对沥青砂浆的空间分布有重要影响；矿料间隙率（ＶＭＡ）和沥青饱和度（ＶＦＡ）与砂浆细观参数之间不存在明显相关关系，证实了宏观体积参数在表征沥青砂浆细观分布特征方面的不足 ；沥青混合料的自愈合指数同时受砂浆平均厚度和空间分布均匀性的影响；综合指标Ｔｍ／ＳＤｔ 与宏观愈合指数Ｈ 之间有很好的相关性，以有效表征沥青混合料的自愈合效率。