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Spokane County Builds All-Weather Roads Using FDR
with Cement
 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
1990’s, and is currently maintaining a steady program of 5-6
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.
Building All-Weather Roads with Cement
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.
Effective Pavement Design
The county developed a very effective pavement design that incorporates
8” – 10” of cement-treated base. The typical design
for the base is 400 psi compressive strength in 7 days, usually
requiring about 4 percent cement. In situations where the soil conditions
are poor, the county prefers to keep the strength of the base unchanged,
and increase the thickness from 8” to 10”.
To complete the pavement structure, a 1” – 3”
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 3 light bituminous surface treatments. The leveling course
also acts as an effective “stress relief” layer, to
prevent reflective cracks in the pavement surface. The county is
currently evaluating the thickness of the leveling course, and recently
built some sections with 1”, 2”, and 3” thicknesses
and will monitor their performance. They are also considering the
use of a paved asphalt concrete surface (instead of the triple surface
treatment) for roads that carry high levels of traffic.
Preserving the Road Template
Increasing the strength of the pavement structure
using conventional methods (such as using an additional 8”
– 10” of stone base) requires changing the road cross-section,
because the additional road elevation must be accompanied by building
up the shoulder slope. This can be expensive because of the additional
material and construction required to change the cross-section.
Using FDR provides the capability of “building the pavement
down”; strengthening the existing pavement using the materials
already there, and eliminating the need for changing the road elevation.
Partnering the Construction Process
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 - $4 per square yard. “We’re real
happy with the results of the process” says 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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