Case Studies: Full-Depth Report

Roads & Bridges / March 26, 2007

Full-depth reclamation (FDR) of failed asphalt pavements using cement provides many advantages over other methods of road rehabilitation, and Spokane County in northeastern Washington state has been using the advantages to full benefit. The county first used FDR with cement in the late 1990s and is currently maintaining a steady program of five to six miles of FDR construction per year. According to County Engineer Ross Kelley, their reclamation program allows them to gradually improve the quality of their road network at a very reasonable cost.

Being located in the foothills of the Rocky Mountains and only 100 miles from the Canadian border, Spokane County experiences the type of weather conditions that can cause serious pavement damage. In the spring, the warmer weather brings melting conditions that can cause serious road deterioration. This brings the need for truck restrictions on rural roads that are not built to handle heavy loads during the spring conditions. These road restrictions cause interruptions for commerce and industry that can affect the local economy.

The FDR process results in a reconstructed cement-treated base, which greatly increases the strength of the pavement structure and does not weaken during seasonal changes. In areas with frost conditions, the cement-treated base will not experience moisture movement, so frost is not a problem in the base. For frost in the subgrade, the stabilized base tends to raise and lower as a platform, reducing the effects of frost heaves. Spokane County has not noticed any frost problems with roads that have been reconstructed with the FDR process. In fact, they are programming the FDR work in order to build routes of cement-stabilized all-weather roads that will not have to undergo the spring load restrictions.

The county developed a very effective pavement design that incorporates 8 to 10 in. of cement-treated base. The typical design for the base is 400 psi compressive strength in seven days, usually requiring about 4% cement.

To complete the pavement structure, a 1- to 3-in. layer of crushed stone is used on top of the base for leveling and as a mat for the surface, which consists of a “triple-shot” bituminous surface treatment. This surface treatment is constructed by placing three light bituminous surface treatments.

On all of their FDR projects, Spokane County has been involved as a partner in the construction process, thereby reducing costs and having more control over the final product. The county will typically contract out the pulverization of the existing pavement, placement of cement and mixing of the base materials and will complete the grading, compaction and surfacing with county forces.

The all-inclusive cost (including contingencies and engineering) of a typical FDR project in Spokane County is around $140,000 per mile, including the surfacing. The portion of the project spent on just the FDR process (pulverization and cement stabilization to build the cement-treated base) is approximately one-fourth to one-third of the total, typically in the range of $3 to $4 per sq yd.

“We’re real happy with the results of the process,” said Howard Hamby, pavement manager for Spokane County’s Public Works Department. The county goes through a thorough evaluation of the costs of pavement design alternatives and feels that the substantial increase in pavement strength from FDR with cement will lead to much better long-term pavement performance.

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