Performance Based Evaluation of Cost Effective Aggregate Options for Granular Roadways

Project status

In progress

Start date: 04/15/16
End date: 10/31/18


Principal investigator:

Co-principal investigators:

About the research

Approximately 68,400 miles of granular roadways exist in the 114,000 mile road network in the State of Iowa. Operation and maintenance of these granular roadways costs roughly $270 million annually. The sustainability of granular roadways is very important to the rural economy, since these roads provide access to rural land and enable the transportation of agricultural products. The timing of the transportation of these products is often tightly constrained by economic and weather considerations. Any interruption in access via these granular roadways can thus have a significant impact on agricultural productivity and the economy of Iowa. Heavy traffic loads and freeze/thaw cycles during the winter and spring seasons can cause extensive damage to unpaved roads. Such damage leads to many problems such as material loss, gradation change, loss of crown, surface erosion, rutting, and potholes. The rate of deterioration (or damage) is directly correlated to the quality of the granular aggregate materials used in the design of unpaved roads.                                                                                                     

Performance and long-term sustainability of granular roadways are dependent to a considerable degree on the quality of the aggregate materials used, which varies considerably from one source to another across Iowa. There are a wide range of granular material sources in Iowa, each producing different qualities, supply amounts and prices. County engineers and Iowa DOT personnel have observed considerable differences between regions of the state with respect to the level of performance that granular materials are able to provide. They have reported that quarries located in certain counties have higher quality granular materials offered at prices similar to others of lesser quality. For instance, materials in northeast Iowa (e.g., Ordovician and Silurian bedrock) perform better and degrade more slowly than those in the southern regions of the state (e.g., Pennsylvanian bedrock). Thus roads in some counties require more maintenance efforts and therefore higher costs in comparison to other counties. A similar contrast occurs for the cost of new construction of granular roads.

County engineers and their employees invest considerable effort in managing and maintaining granular roads. However, to date there are no readily available tools to evaluate the costs versus field performance of granular road materials.  When maintenance and construction of granular roadways is costly due to use of low quality materials, it is necessary for counties to spend a considerable portion of their budget (sometimes up to 28% of the total county budget) just to purchase granular materials (excluding placement and maintenance) to replace those lost during the service life of a granular road. The problems commonly encountered with unpaved roads are, (1) unsuitable material usage, (2) inadequate material distribution, (3) surface deterioration through aggregate loss, (4) surface abrasion, (5) ineffective drainage, (6) insufficient road maintenance. The proposed study aims to test the problems associated with reasons 1 and 4.  

In this project, the research team proposes to conduct laboratory and field tests to examine the link between quality and performance of granular aggregate materials used in unpaved road designs, using materials collected from various quarries in Iowa. Based on the laboratory test results, field test sections will be constructed using materials with different quality and aggregate sources. The field performance (abrasion resistance, freeze/thaw resistance, density, material loss, modulus, gradation change) of sections built with different quality materials will be compared. Then, a comprehensive cost-performance analysis will be conducted to evaluate the cost effectiveness of the different materials to determine whether it is economically advantageous to transport good quality aggregate materials from quarries located in different counties.

The results of this study are expected to improve the performance, economics, and service lifespans of unpaved roads in Iowa. This study will develop design charts that may help county engineers and planners predict the performance of unpaved roads based on results of a few common laboratory tests such as abrasion and sieve analysis tests. In addition, this research will investigate whether transportation of high quality aggregates across county lines can provide an economical solution for Iowa counties, especially those in the western part of the southern two or three tiers. The results and recommendations of this project will provide a foundation for further development of decision-making tools to evaluate the relative cost-effectiveness of granular materials available in Iowa based on their quality and performance.



  • Iowa Department of Transportation
  • Iowa Highway Research Board