Characterization of Air Voids Distribution in the Open-Graded Asphalt Mixture Based on 2D Image Analysis

The air voids distribution has a significant effect on the pavement performance related distresses such as rutting, cracking, moisture damage and permeability. However, most studies only quantified the air voids by average content, ignoring the heterogeneity inside the materials. This study focuses on the heterogeneity of air voids distribution inside the open-grade asphalt mixture based on 2D image analysis. Equivalent aperture is proposed to measure the area of each air void. Results showed that along both vertical and horizontal sections inside the open-grade asphalt mixture, the number of voids with 0~2 mm equivalent aperture would have a great impact on the total number of voids, while large amounts of small voids would not significantly affect the total voids number. Additionally, voids with 0~4 mm equivalent aperture account for the largest proportion, and when the equivalent aperture is beyond 4 mm, the number of voids would decrease as the equivalent aperture increases, regardless of the void areas. Furthermore, in both vertical and horizontal sections, as the equivalent aperture increases, the speed to accumulate voids area would firstly increase and then decrease. As the equivalent aperture increases, the contribution to the total voids area would increase accordingly, and it would approach the peak when the equivalent aperture reaches about 8 mm.