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Unique Whitetopping Approach Offers Strength and
Durability in Wyoming
For years, pavement engineers have used ultra-thin whitetopping
(UTW) to rehabilitate old asphalt pavements. In this process, a
thin layer of concrete (three to four inches) is bonded to the existing
asphalt pavement. The new concrete provides an economical, smooth
riding surface that is much stronger and more durable than the old
asphalt pavement.
To be successful, the UTW must be placed over asphalt that does
not show signs of deep pavement distress. If potholes, alligator
cracking or deep fissures exist in the asphalt, the concrete will
not form an adequate bond, resulting in pavement that lacks adequate
support.
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| Existing asphalt pavement on West 8th Streethad
deteriorated too much to accept a traditional ultra-thin whitetopping
surface. |
In early 2003, the City of Sheridan, Wyo., faced just this situation:
a stretch of West 8th Street was in need of restoration, but the existing
asphalt pavement was too deteriorated to provide a good base for a
bonded concrete surface. The city considered excavating the existing
asphalt and base and reconstructing the pavement from the subgrade
up, but deemed the method cost-prohibitive. City engineers met with
cement and concrete industry representatives to evaluate possible
alternatives to full reconstruction and traditional UTW.
Recycled Base
Discussions centered around a procedure called full-depth reclamation
(FDR), a type of pavement recycling that has been used successfully
to rehabilitate old asphalt pavements by pulverizing the existing
surface and base and stabilizing the mixture with portland cement
and water. While FDR is a proven method for base stabilization,
the technique had never been used as the base for a UTW surface.
After considering the alternatives, it was decided to launch a pilot
project to evaluate the benefits of placing concrete over a reclaimed
base.
Pavement design called for 3 1/2 inches of whitetopping with control
joints sawn in a four-foot grid over eight inches of cement-stabilized
reclaimed base, creating a roadway 540 feet long and 34 feet between
curb sections. Typically, road reclaimers/mixers are used to pulverize
and mix in-place the old asphalt surface, base material and cement.
However, for this project, the material was mixed off site.
First, crews used a milling machine to pulverize and recover the
old pavement section to a depth of eight inches, placing the reclaimed
material in a stockpile. The original plan called for blending the
reclaimed material with 5% cement in an off-site central plant pug
mill and trucking it to the project site, but scheduling difficulties
made the equipment unavailable. Instead, the material was blended
with cement in transit mixers and placed on the subgrade. The discharged
material was initially too wet to compact immediately with a roller;
however, after some additional in-place mixing with a grader, the
cement-stabilized material achieved the proper moisture content
for compaction. Crews then placed whitetopping over the base using
a deck-finishing unit.
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| A grader was used to place the new cement-stabilized
base, consisting of the pulverized asphalt and old base material. |
Crews place ultra-thin whitetopping over the
cement-stabilized base. |
The new concrete roadway provides an economical,
smooth, light surface, with excellent strength, durability,
and appearance. |
Pavement Performs
Tests performed after construction showed the cement-stabilized
base had a compressive strength of about 320 psi in 28 days; the
concrete achieved a compressive strength of 5,250 psi in 28 days.
The most important aspect of the pilot project‹the bond between
the whitetopping and the stabilized base‹was evaluated with
core samples taken shortly after construction. The cores showed
excellent bonding between the surface and base layers.
Today, after almost one year of traffic, the section is performing
well, and the City of Sheridan is pleased with the results. City
engineers will continue to monitor the West 8th Street project in
coming years. Those who worked on the pilot project say the process
shows promise for use in other projects where old, deteriorated
asphalt pavement can be reclaimed and surfaced with UTW.
Full-Depth Reclamation with Cement
Deteriorating roads are a constant problem for cities and counties.
That¹s why engineers and public works officials are turning
to a process called full-depth reclamation (FDR) with cement. This
process rebuilds worn-out asphalt pavements by recycling the existing
roadway. The old asphalt and base materials are pulverized, mixed
with cement and water, and compacted to produce a strong, durable
base for the new pavement surface. Full-depth reclamation recycles
the old asphalt and base material for the new road. There¹s
no need to use new aggregate or dispose of old material.
More Information
For more information on ultra-thin whitetopping, visit the Web site
of the American Concrete Pavement
Association.
For local support, tap into the cement industry's network of regional
groups covering the United States. Click
here for contact information.
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