National Concrete Pavement Technology Center

Local agencies can spend anywhere from $500,000 to $1.5 million per year just putting down fresh aggregate on gravel roads in Iowa, where about 60% of the road network is gravel or otherwise unpaved.

An Institute for Transportation (InTrans) research project, now in its second phase, aims to help local agencies implement improved design methods and recycle existing degraded surface materials to reduce those costs.

The Gradation Optimization spreadsheet tool allows agencies to reach a target gradation and plasticity to maximize performance and durability. The Microsoft Excel-based program lets agencies optimize the proportions of existing materials and two to three available quarry materials.

“The set of recommended testing, design, and construction procedures developed in the Phase I study can provide secondary roads departments with more cost-effective solutions for building or reconstructing granular road systems with improved performance and durability,” said Jeramy Ashlock, who is the principal investigator on both phases of the project.

The initial study included a series of laboratory tests to quantify the influence of gradation and index properties on the mechanical performance of commonly used materials for granular-surfaced roads and shoulders. Once the optimum gradation and plasticity were determined, test sections were constructed and tested over a seasonal freeze-thaw period to assess performance.

Based on the laboratory and field evaluations, a performance-based free design method for determining the gradation and plasticity of granular surface materials was developed, which led to the Gradation Optimization tool.

The spreadsheet tool developed as part of the Phase I field performance study has been distributed to Iowa county engineers through the Iowa County Engineers Service Bureau website, and several counties have begun using the tool and providing feedback. The tool is also available on the Phase I project page.

The goal of the second phase of the project, which is expected to be completed next year, is to expand the Gradation Optimization tool to include a wider range of material blends and material types, including crushed river gravel, higher strength limestone, and others used in different regions of Iowa.

More information about the Phase I research and the spreadsheet tool are available on the project page, which also links to the Phase II project page.

Additionally, Ashlock recently demonstrated the spreadsheet tool and provided an overview of the research as part of an Iowa Local Technical Assistance Program (LTAP) webinar, which is available at this link.