Abstract
This paper aims to characterize the three-dimensional (3D) internal structure evolution of asphalt mixtures under freeze–thaw cycles. Asphalt mixtures with three levels of design void content (3%, 5%, and 7%) were prepared in the laboratory. Subsequently, X-ray computed tomography (CT) tests were conducted to capture two-dimensional (2D) images of the internal structure of samples before and after freeze–thaw testing. A set of image processing techniques for reconstructing 3D images of the internal structure were utilized to extract the internal structure properties, which were then used to analyze the changes in the air void distributions and to evaluate the internal structure evolution under freeze–thaw cycles. 3D images reconstructed from X-ray CT images illustrated a dramatic degradation in the internal structure after cyclic freeze–thaw exposure. The change in internal structure occurs mainly in three ways: (1) expansion of existing individual voids, (2) combination of two separated air voids, and (3) generation of new voids. In addition, the parametric analysis of the three-dimensional reconstructed voids revealed that the asphalt mixture void ratio increased with the number of freeze–thaw cycles, while the larger the initial void content, the more pronounced the increase in the specimens. Therefore, asphalt mixture freeze–thaw resistance should be optimized in relation to the design void content.
Highlights
Waterproofing layers are essential to prevent surface water from infiltrating into the high-speed railway subgrade to ensure its stability and bearing capacity, especially in the seasonally frozen regions to prevent subgrade frost [1,2,3,4]
After 5, 15, 25, and 35 freeze–thaw cycles, damaged asphalt mixture specimens were collected for X-ray computed tomography (CT) testing to identify changes in the internal structure
The solid ellipse indicates air void volume increase during freeze–thaw cycles, and the dotted ellipse relates to the connection between existing air voids
Summary
Waterproofing layers are essential to prevent surface water from infiltrating into the high-speed railway subgrade to ensure its stability and bearing capacity, especially in the seasonally frozen regions to prevent subgrade frost [1,2,3,4]. Practicing engineers have suggested that the air void should not exceed 5% based on road construction experience in highway engineering. This is not a scientific conclusion, since there are significant differences between waterproofing layers and road pavements. The internal structure of asphalt mixtures is sensitive to freeze–thaw cycles, resulting in an increased permeability and subsequent damage to the subgrade, especially in cold regions [9]. In order to guide asphalt mixture waterproofing layer design in cold regions, the evolution of internal structure inside asphalt mixture exposed to freeze–thaw cycles should be explored first
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