This report presents the findings from laboratory and field studies on the effectiveness of interlayer systems to mitigate reflective cracking in asphalt overlays over cracked asphalt pavements. In order to obtain the necessary background information for this project, an extensive literature review was first performed. This literature review provided insights into the availability, design, selection, construction, and performance of interlayer systems. In general, the literature review found that interlayer systems do have the ability to delay reflective cracking from three to five years and can help water-proof the pavement, potentially leading to a reduction in subgrade moisture and improved performance over the pavement life. The downside of interlayer treatments, however, is that they tend to increase project costs slightly, may introduce new sources of variability into the paving project during construction, and can lead to increased or new distresses in overlays in certain situations.
During the course of this research, a survey was performed that asked North Carolina Department of Transportation (NCDOT) Division personnel to quantify their experiences with reflective cracking mitigation interlayers. Taking into consideration this acquired survey information, several interlayer systems were selected to be used in a field trial project in North Carolina to evaluate the various systems' construction and performance. A flexible pavement section of US 1 in Moore County was selected for these trials, and five interlayer research segments were placed that incorporated three geosynthetics, a chip seal, and a control (tack coat only) segment. During the course of the field construction, several issues were encountered, which imply that forming conclusions about the performance of these materials over time may be problematic. Based on information obtained from both the literature review and the field construction, construction guidelines were developed in order to aid construction personnel and engineers in understanding the factors and challenges that are present when utilizing interlayer systems in a construction project.
Additionally, these materials also were subjected to laboratory testing in order to quantify critical factors that affect the materials' performance and ultimately to develop a method that can rank their suitability as reflective crack-mitigating interlayers. Three test methods were used for this analysis: a scaled-down wheel load test known as the reflective cracking test (RCT), a notched beam fatigue (NBF) test on layered asphalt samples, and a direct shear test.
The RCTs demonstrated the importance of having high interfacial bond strength between the interlayer system and the other pavement layers, and that pavements with high relative displacements across existing cracks or joints represent an extreme condition that should be avoided in the field. The NBF tests provided additional information about the mechanisms of reflective cracking, how vertical crack growth is affected by the presence of an interlayer, and further demonstrated the importance of interfacial bond strength and the potential for horizontal cracking. Ultimately, these two tests provided useful information about the mechanisms present in these samples and provided some indication of material suitability; however, development of objective criteria that could be used to rank samples or predict pavement life in the field require further testing and analysis. The direct shear tests helped to confirm that interlayer systems do reduce the shear strength between pavement layers and demonstrated that field variability during construction can affect the bond quality of pavement layers. The direct shear test showed the potential to be used as a screening test to ensure that interlayer systems meet minimum shear strength requirements for a given application.
Due to the limited ability of the laboratory tests to quantify pavement service life, no extensive life cycle cost analysis could be performed. However, a simplified framework is presented so that engineers can obtain a ballpark estimate as to the cost-effectiveness of interlayer systems in the field. Finally, the project selection guideline was developed based on the information obtained from the literature.
