Optimal Rap Content For Minnesota Gravel Roads

The objective of this project was to provide a better understanding of how various virgin aggregate and recycled asphalt pavement (RAP) mixtures for surface layers affect the performance of gravel or crushed rock roads and, with further analysis, to determine the optimal RAP content for Minnesota gravel roads. This project included a literature review, preliminary laboratory testing, economic and feasibility analysis, and two field studies. Several studies regarding the use of RAP materials for road surfaces were reviewed. Then, laboratory tests on various RAP materials, one virgin aggregate, and mixtures of RAP materials and virgin aggregate were conducted to observe the effect of RAP on the index properties of the materials and the engineering properties of the mixtures. Initially, six test sections were constructed with various surface aggregates in two locations. Virgin RAP-aggregate blends having 15% to 60% RAP contents were placed as surface aggregate. Then, three more test sections were constructed using RAP-aggregate blends having 50%, 70%, and 80% RAP contents. Several field tests, including lightweight deflectometer, dynamic cone penetrometer, scrape, and dustometer tests, were performed to evaluate the test sections. This report provides insights regarding the effect of using RAP material on surface layers to reduce the use of virgin aggregates. It was concluded that the optimal RAP content for unpaved road surfaces changes according to the properties of the materials used, testing methods, and site conditions.