Improving Unbound Aggregate Roads through a Comparison of the Performance of Materials

Open Access
Lombardi, Danielle Maria
Graduate Program:
Civil Engineering
Master of Science
Document Type:
Master Thesis
Date of Defense:
May 29, 2012
Committee Members:
  • Barry Earl Scheetz, Thesis Advisor
  • Unbound Aggregate Roads
  • Full Depth Reclamation
  • Driving Surface Aggregate
Unbound aggregate roads, or unpaved roads, generally do not hold up to the expectations of the general public. This forces agencies to often pave roads unnecessarily. Wearing course materials used for unpaved roads and the subgrade they are placed on need to be improved in order to see an improvement in these roads. Research was conducted to prove if there is a benefit to using Driving Surface Aggregate (DSA), a material developed specifically for use as a wearing course, over another commonly used aggregate gradation, PennDOT 2A, designed for use as a base course. The performance of DSA and PennDOT 2A under scaled loading with the one-third Model Mobile Load Simulator (MMLS3) was compared in terms of rutting. The materials were evaluated in the laboratory at a lightweight traffic setting (simulating passenger vehicles, pick-up trucks and SUVS) and at a heavy traffic setting (simulating tri-axial and semi-trailers). The MMLS3 was also used in the field on the lightweight traffic setting to evaluate the materials in an as-placed setting. DSA consistently performed better than PennDOT 2A with lower average rut depths, although the predictability of the rut depth formation should be studied in more detail. In considering subgrade weakness, a rehabilitation practice called full-depth reclamation (FDR) has been used to increase roadbed stiffness in paved roads. The impact of using this method for unpaved roads has been evaluated. In situ stiffness testing using the lightweight deflectometer (LWD) and condition assessments of four roads in Erie County, Pennsylvania were compared to evaluate the potential of FDR as a practice to improve the subgrade for unpaved roads. A road consisting of FDR with a tar and chip surface treatment had the highest overall stiffness when compared to a FDR road without a surface treatment, an aggregate-surfaced road and an unsurfaced road. Both FDR roads showed higher stiffness over the spring-thaw period proving potential benefit to adopting this method as an unpaved road practice; however, future research is needed to evaluate the performance of a FDR road with an aggregate surface. All four roads showed decreasing condition ratings over the spring-thaw period, but the FDR road with a tar and chip surface treatment required maintenance over the summer months, highlighting one of the major financial downfalls of paving a road.