Iowa Department of Transportation
Iowa Highway Research Board
Jeramy Ashlockjashlock@iastate.edu email >
Faculty Affiliate, InTrans
About the research
The goal of this project was to investigate the cost-benefits of using high-quality clean (large size) aggregates for construction and maintenance of granular roadways located in areas with low-quality aggregates. Granular road surface aggregates were collected from four different quarries to build seven test sections. Three of these sections were built with typical granular roadway aggregates (Class A) and four sections were built by mixing locally available Class A material with high-quality clean (large size) aggregates.
Test sections were constructed in Decatur County on a relatively stiff subgrade. This site was also selected since its locally available material deteriorates significantly when subjected to freezing and thawing during the winter and spring seasons. Construction and maintenance procedures are detailed, and the costs of aggregate, hauling, and equipment for construction and maintenance are documented in this report.
Extensive laboratory and field tests were performed after construction, as well as before and after three seasonal freeze/thaw periods from 2016 to 2019, to monitor the performance of the demonstration sections. Temperature sensors were embedded at 1 ft increments in the subgrade to monitor ground temperatures up to a depth of 7 ft. These sensors were used to determine the duration and depths of ground freezing and thawing in addition to the number of freeze/thaw cycles at the test site.
A benefit-cost analysis was performed using the documented construction and maintenance costs for service life scenarios of 20, 30, 40, and 50 years. The benefit-cost ratio was calculated for each test section for different scenarios based on the performance measures including surface material and thickness loss, gravel-size material loss, average fines content, total breakage, gravel-to-sand ratio, stiffness, shear strength, surface roughness, and dust production. Performance measures were categorized into three overall mechanistic performance-based groups and their benefit-cost ratios were compared.
Overall, results of this study showed that the test sections constructed with a mixture of local Class A and Lime Creek Formation (LCF) and of local Class A and Crushed River Gravel (CRG) Clean aggregate were the most cost-effective ones. Moreover, sections consisting of aggregate mixtures had better performance under freeze/thaw cycles than that of the locally available Class A material and the mixtures of locally available Class A and clean aggregates. Depending on the distance to higher quality clean aggregate sources, mixing locally available Class A materials with lower quality aggregates can improve the performance of local materials and result in less frequent maintenance.