Mesoscale Evaluation Index for the Homogeneity of Hot In-place Recycling Asphalt Mixture

The pavement can be rehabilitated directly on a construction site using hot in-place recycling (HIR). However, the homogeneity of HIR asphalt mixture is difficult to control owing to the complex material and complicated construction process. In this study, the evaluation index for homogeneity of HIR asphalt mixture was investigated by analyzing the aggregates of the mixture. First, a digital camera was used to capture images of the cross-sections of the HIR asphalt mixture specimen, and the microstructure of virgin and RAP aggregates in the cross-section was extracted using digital image processing technique. Then, each cross-section of the specimen was divided into 36 regions of equal area using an annular segmentation with sector segmentation. Based on the area ratio of the aggregate and deviation degree of the virgin aggregate, the homogeneity coefficient of aggregates, D, and homogeneity coefficient of virgin aggregates, H, were proposed, respectively. Finally, an orthogonal experiment with four factors was carried out to analyze the reliability and trend of variation of these two homogeneity coefficients. These factors included the heating temperature and mixing time of RAP, heating temperature of virgin asphalt mixture, and mixing time of virgin and RAP mixture. The results showed that the characteristics of aggregate in the cross-section of asphalt mixture could be comprehensively described using annular segmentation combined with the OTSU threshold segmentation method. The virgin and RAP aggregates in the cross-section could be effectively identified owing to different gray values by retaining the green channel. The related macro qualitative test of the homogeneity of the cross-section of the specimen initially showed that these two homogeneity coefficients were reliable. The orthogonal experiment results show that the heating temperature of RAP and the heating temperature of virgin asphalt mixture had a significant effect on D, whereas the heating temperature of RAP and the mixing time of virgin and RAP mixture had a significant effect on H. Moreover, the homogeneity of HIR asphalt mixture was improved by increasing the heating temperature of RAP, heating temperature of virgin asphalt mixture, and mixing time of virgin and RAP mixture, which is consistent with previous research. The reliability of these two homogeneity coefficients was verified again. © 2020, Editorial Department of China Journal of Highway and Transport. All right reserved.