Evaluation of Aggregate Dispersion Uniformity of Reclaimed Asphalt Mixtures Using DIP Technique

In view of environmental concerns and rising price in pavement materials, the use of reclaimed asphalt pavement (RAP) in asphalt mixtures has gained in popularity recently. However, the aggregate dispersion uniformity of reclaimed asphalt mixtures is difficult to be evaluated due to its complex material composition. In this study, a new method of square region division was adopted based on the digital image processing (DIP) technique for evaluating the aggregate dispersion uniformity. The shear gyratory compactor (SGC) specimens of reclaimed asphalt mixtures were prepared and then cut into six horizontal cross sections. A digital camera was used to photograph each cross section. After obtaining the cross-sectional images, the microstructure of the asphalt mixtures was extracted by DIP technology, including background trimming, image enhancement, threshold segmentation, and image morphology. The inscribed square within the image was selected and equally divided into sixteen unit grids. Based on the normal distribution function of area ratio of aggregates to each unit grid (Pi value), the probability of Pi value happening in specific range was proposed as a dispersion uniformity index (DUI). The effects of RAP content, RAP agglomeration, and asphalt type on the aggregate dispersion uniformity were taken into account in the laboratory experiment. The results show that the square region division method is validated to be effective and the proposed index can well reflect the variation of aggregate dispersion uniformity under different test conditions. The RAP agglomeration shows an adverse effect on the aggregate dispersion uniformity. This effect becomes greater as the increase of RAP content. Nevertheless, a significant improvement in the dispersion uniformity of aggregate can be acquired with the replacement of virgin asphalt by foamed asphalt.