Full-Depth Reclamation with Cement
roads are a constant problem for cities and counties. That’s
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 either an asphalt or concrete surface.
Full-depth reclamation uses the old asphalt and base material for
the new road. There’s no need to haul in aggregate or haul
out old material for disposal. Truck traffic is reduced, and there
is little or no waste.
Recycling saves money
and natural resources
Full-depth reclamation uses the materials from the deteriorated
asphalt pavement, and, with the addition of cement, creates a new
A surface consisting of a thin bituminous chip seal, hot-mix asphalt,
or concrete completes the road. The recycled base will be stronger,
more uniform, and more moisture resistant than the original base,
resulting in a long, low-maintenance life. And most important, recycling
costs are normally at least 25% to 50% less than the removal and
replacement of the old pavement.
A wise choice
Conserving virgin construction materials through recycling with
cement makes smart economic and strategic sense. A century of modern
growth and urbanization in America has depleted once plentiful aggregate
supplies. Frequently, aggregates either come from distant quarries
at great expense or from local sources offering only marginal quality.
Continuing to exhaust these valuable resources to rebuild existing
roads only propagates and accelerates the problem.
Additionally, if old asphalt and base materials are not recycled,
they must be disposed of or stockpiled, increasing transportation
costs and utilizing valuable landfill space. In some locales, old
asphalt can no longer be landfilled. Environmental laws are becoming
stricter, thus adding to the expense of mining new materials and
Recycling with cement makes the reconstruction of old roads a largely
self-sustaining process. The original “investment” in
virgin road materials becomes a one-time cost, which is renewed
periodically, through cement stabilization and addition of a new,
thin surface course.
Old asphalt, new foundation
Stabilizing the old asphalt surface, granular base, and underlying
subgrade soil with cement creates a strong foundation for the pavement.
Usually, there is little need for material to be removed or added.
The old, brittle asphalt, when pulverized, becomes a “black
gravel” that will bond to hydrated cement readily. The removed
material may be suitable for recycling into a new asphalt surface.
In case the existing asphalt pavement does not meet the aggregate
requirements for a good stabilized base, additional aggregates can
be readily incorporated into the recycled aggregate during construction.
and construction: Simple and fast
The basic procedure is simple. The complete recycling process can
be finished in one day, and local traffic can return almost immediately.
The procedure includes the following steps:
Thickness Design. Pavement thickness can be determined
by using PCA’s Thickness
Design for Soil-Cement Pavements (EB068). Other methods,
such as the American Association of State Highway and Transportation
Officials (AASHTO) Guide for Design of Pavement Structures can also
Site Investigation. The site should be investigated
to determine the cause of failure. Cores or test holes should be
used to determine layer thicknesses and to obtain samples of thematerial
to be recycled (which can include asphalt surface, base course aggregate,
Lab Evaluation. Material samples from the site
should be pulverized in the lab to create an aggregate-soil mix
that will be similar to that expected from the recycling process.
The mix design procedure is the same as that performed for soil-cement.
Refer to PCA publication EB052 Soil-Cement
Laboratory Handbook. This includes the determination of
maximum dry density and optimum moisture content. If unconfinedcompressive
strength is used to determine cement content, a 7-day strength of
300 to 400 psi (2.1 to 2.8 MPa) is recommended.
and Pulverization. Depending on the construction equipment
available, and the thickness of the existing pavement, the roadway
may need to be scarified (ripped) before it can be pulverized. Some
equipment, however, is capable of pulverization without scarification
first. Quality full-depth recycling is usually performed using equipment
especially designed for this purpose. The depth of pulverization
is usually 6 to 10 in. (150 to 300 mm), which on secondary roads
will typically include all of the surface and base, plus some part
of the subgrade. To achieve the proper gradation after pulverization,
more than one pass of the equipment may be necessary. The particle
distribution should have 100% smaller than 2 in. (50 mm) and 55%
passing a No. 4 (6-mm) sieve.
Shaping and Grading. The pulverized material is shaped
to the desired cross-section and grade. This could involve additional
earthwork in order to widen the roadway. Final base elevation requirements
may necessitate a small amount of material removal or addition.
Spreading Cement. Cement is spread in a measured amount on the surface
of the shaped roadway, in either dry or slurry form.
Application. Water is added to bring the aggregate-soil
mixture to optimum moisture content (water content at maximum dry
density as determined by ASTM D558), either in front of the pulverizer/reclaimer
or in the mixing chamber.
Mixing. The aggregate-soil-cement-water mixture
is combined and blended with the pulverizing/mixing machinery. Multiple
passes of the mixer may be required to achieve a uniform blend of
The mixture is compacted to the required density of at least 96%
of standard Proctor density (ASTM D558). The compaction is usually
performed with smooth-wheeled vibratory rollers. A pneumatic-tired
roller may follow to finish the surface. Final compaction should
take place no more than 3 hours past initial mixing of the cement.
The field density and moisture are monitored for quality control
The surface is kept moist by periodically applying water to the
surface, to make sure it does not become dry. This is done continuously
through the curing period until the base can support traffic without
deforming. The application of the prime coat should occur as soon
as possible to ensure that moisture is sealed inside the base.
Pavement Surface. The new pavement surface consisting
of a chip seal, hot-mix asphalt, or concrete is constructed to complete
the recycling process.
tack coat prior to paving
Quality Control. Recycling with cement follows
the same basic procedures used for normal soil-cement operations.
The success of a recycling project depends upon the careful attention
to the following control factors:
- Adequate pulverization
- Proper cement content
- Proper moisture content
- Adequate density
- Adequate curing
More About FDR:
How FDR Works
Start with a Good Foundation
FDR Research In Progress
FDR Recent Projects
Other SC Sites:
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