Homogeneity evaluation of hot in-place recycling asphalt mixture using digital image processing technique

Abstract

Hot in-place recycling (HIR) allows 100% reuse of reclaimed asphalt pavement (RAP) directly on a construction site. However, the homogeneity of the HIR is difficult to be evaluated due to the complexity of the material composition and construction process. In this study, in view of the homogeneity of the HIR can be reflected by the mixing quality of virgin and RAP aggregates, the homogeneity of virgin and RAP aggregates was investigated. Firstly, the Marshall test specimens of an HIR asphalt mixture were prepared and saw-cut, and the microstructure of each cross section was extracted using digital image processing (DIP) technique, including image enhancement, iterative threshold segmentation and maximum between-class variance (OTSU threshold), retention of the green channel, and image morphology. Then, each cross section of the specimen was divided into 36 regions of equal area by an annular segmentation with sector segmentation. Based on the coefficient of variation in the area ratio of the virgin aggregate to RAP aggregate, a homogeneity coefficient of HIR asphalt mixture was proposed. Another nine groups of Marshall specimens were prepared and cut, and the homogeneity coefficient of each group was calculated. The results showed that the virgin and RAP aggregates in the cross section could be effectively identified due to different gray values by retaining the green channel. The characteristics of the virgin and RAP aggregates could be comprehensively described using annular segmentation combined with sector segmentation. The related macro qualitative test showed that the aggregate in the asphalt mixture could be accurately extracted by the threshold segmentation method based on annular segmentation combined with the OTSU method. The homogeneity could be accurately and quantitatively evaluated by a homogeneity coefficient based on the area ratio of the virgin and RAP aggregates in each cross section. An improvement in the homogeneity of the virgin and RAP aggregates was achieved with an increase in the heating temperature of RAP and mixing time. However, no further improvement was noticed as the heating temperature and mixing time increased. In short, the quantitative homogeneity evaluation index of virgin and RAP aggregates presented in this study, reflecting the homogeneity of HIR asphalt mixture, is helpful for controlling the quality of the HIR asphalt pavement.