Cracking Behavior And Fracture Mechanism Of Cold Recycled Mixes With Emulsion

Cold in-place recycling (CIR) is a technique for recycling reclaimed asphalt pavement (RAP) by using emulsified bituminous binder or other binding agents and adding appropriate amount of additives without the application of heat. The CIR process is done with a number of pieces of equipment including tanker trucks, milling machines, crushing and screening units, mixers, pavers, and rollers. The CIR technique has been gradually accepted and applied in multiple areas of China. It is usually used as the lower layer of highway pavement in China. The performance of CIR mixtures with the typical pavement structure incorporating cement-stabilised macadam base shows a satisfactory resistance to high-temperature deformations and reflective cracks. The success of CIR performance in the field typically lies in the resistance to reflective cracking from underlying concrete pavement. Therefore, the cracking behavior of CIR mixtures should be well understood in order to capture the mechanisms of cracking.The compaction of cold recycled mixtures in the field varies significantly in different working conditions. It is therefore necessary to study the compaction characteristics of cold recycled mixtures. In this study, the primary compactibility energy index (PCEI) and secondary compatibility energy index (SCEI) of CIR were defined from the gyration compaction curve to evaluate the effect of compaction temperature, asphalt emulsion content, moisture content, aggregate gradation, and initial cure time. The compaction parameters of CIR were obtained and analyzed statistically. The results from this study indicate that the regression of Ndes is linear relative to the compaction temperature and asphalt emulsion content. The effect of compaction temperature is much more significant than the other factors. Based on the effect of moisture content on PCEI and SCEI, a new method to determine the optimum moisture content of CIR is provided.The micro structure of CIR mixtures with emulsion was investigated by processing a series of sectional images from an X-ray CT scanner. The self-adaptive algorithm developed by Otsu was used as the segmentation method to confirm the optimum thresholds. Then, the air voids were extracted from asphalt mastic and coarse aggregates after the image segmentation. The uniformity index (UI) was derived to describe the distribution of coarse aggregates. The aggregate orientation was obtained by calculating the average angle of inclination and the coherence of orientation. The morphological indices, such as the flat and elongated ratio (FER), angularity index (AI), and surface texture index (STI) were used to address the shape properties of CIR mixtures. The volume, size, and number of air voids in CIR specimens were statistically analyzed considering different aggregate gradations and compaction methods. The thickness and distribution of asphalt mastic was used to characterize the asphalt mastic.A newly developed Arcan configuration was utilized to simulate five levels of Mixed-Mode cracking. The peak load, crack angle, and three types of fracture energies were obtained to compare the Mixed-Mode cracking characteristics of CIR mixture. Utilizing Digital Image Correlation (DIC) allowed for full-field displacement/strain fields to be captured, along with the more traditional readings from attached gauges. The results indicate that in the process of crack initiation, the distribution of vertical deformation is relatively balanced across the whole specimen face. However, as the crack propagates after the peak load, the vertical deformation mainly distributes around the cracking. A common feature of five strain maps for Mixed-Mode cracking is that the strains near crack tip are higher than other strains, which results in the crack initiation. However, the crack path formed along the higher strains changes significantly with different levels of Mixed-Mode cracking. The selection of crack path makes the crack propagating in a twisting and complicated way.With the micro structure of CIR mixtures, the particle flow codes in three-dimensions (PFC3D) were used to establish the discrete element model (DEM) of Arcan configuration. The displacement-softening model which provides bonded behavior of asphalt mixture was improved. In the model, the strength is reduced as a function of applied displacement. The properties of displacement-softening model were confirmed. The cracking mechanism of CIR mixtures was fully analyzed considering the effects of micro structure and mechanical properties in the DEM method. Compared with hot mix asphalt (HMA) mixture, the CIR mixture has some advantages in the aspects of failure strain and stress in fracture. It shows a good resistance to reflective cracking.The fatigue behavior of CIR mixtures was evaluated in the semicircular bending (SCB) and indirect tensile (IDT) fatigue tests with the application of digital image correlation (DIC) technique. The results indicate that the CR mixture has longer fatigue life and larger tensile strain at the failure point than the HMA mixture at lower stress levels. In addition, the contour plots of the horizontal tensile strain of CR specimen demonstrate the cracking path and the three stages of fatigue process.Overall, this research provides a solid foundation for future research in exploring reflective cracking characteristics of asphalt mixtures. It can be used to fully understand the mechanism of CIR mixtures and improve the resistance to reflective cracks.