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Cement-Treated Base Case Study
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Florida Pavements Say “No” to Cracks
By: Greg Halsted
 Over
the years, Tampa Bay, Florida area residents have had to contend
with high temperatures, rain, and hurricanes in the summer but they
have not had to worry about the durability of some of their roadways.
Many subdivision streets, parking lots, and some County roads in
Hillsborough, Pasco, Pinellas, and Polk Counties have been constructed
using a soil-cement mixture, or cement-treated base (CTB), that
show absolutely no signs of failure after more than eight years
of traffic and Mother Nature.
These counties have roadways that were constructed as far back
as November 1996 that today show virtually no rutting or cracking
in the thin asphalt surfacing. “The reason for the longevity
of these roadways without any indications of distress in the paving
is attributed to the Croc Rock that lies underneath them,”
says Alan Payne, president of Florida Soil Cement, LLC based in
Riverview, whose company supplies the CTB manufactured under the
name Croc Rock to the Tampa Bay market.
One of the bad raps that cement-stabilized roadway bases get is
the frequent appearance of small cracks in the asphalt surfacing.
These types of bases, including CTB, can be the source of shrinkage
cracks that can occur in the stabilized base layer during curing,
which can reflect up through the asphalt surface. These cracks in
the CTB layer can result in stress concentrations and cracking in
the asphalt surface layer. The surface cracks tend to follow the
same pattern as the cracks in the base, and are commonly referred
to as “reflection” cracks. These reflective cracks are
typically only cosmetic in nature and can be present for many years
without the need for significant and costly maintenance. However,
Payne is supplying CTB for roadways that exhibit no indications
of reflective cracking at all.
 Roadways
in these four Tampa Bay area counties are typically constructed
from either Florida Department of Transportation (FDOT) approved
unconsolidated limerock, crushed concrete, or soil-cement (CTB).
In fact, Payne’s Croc Rock is actually FDOT limerock
blended with portland cement at a central plant, or pugmill, and
delivered to the roadway where it is then spread on the prepared
subgrade. “The big advantage to using Croc Rock is
that there is an extremely high water table around these parts and
CTB utilizing 100% FDOT limerock and cement gives us pavements that
don’t rut, pothole, or crack,” says Payne. “By
mixing in the cement, we take an approved roadway construction material
and make it better. With the extremely limited supply of crushed
concrete; Croc Rock becomes the natural choice for a durable
and economic roadway building material.”
Jeff Joaquin, Vice President of Kearney Construction Company, Incorporated
located in Tampa agrees. “There is not another base material
which compares with Croc Rock based on its durability,
consistency, availability, and ease of installation. Kearney utilizes
Croc Rock on every project where CTB is specified and we
have also had projects where we have convinced the owner and the
geotechnical firm to switch from the original designed base to this
product. From a standpoint of productivity, Croc Rock allows
Kearney the ability to finish the product the same day it is installed
which greatly reduces the time and energy spent on re-grading and
finishing when compared to other base products.”
Due to the high water table and the lack of “hard”
rock that does not absorb moisture in these situations, engineers
are typically specifying roadway bases of either crushed concrete
or soil-cement, manufactured using material that has a minimum Limerock
Bearing Ratio (LBR) of 100. The soil-cement is extremely cost competitive
with crushed concrete and typically is a better choice when construction
and long term maintenance costs are taken into consideration. As
municipalities and developers become educated about the benefits
of cement-treated limerock, and visually observe the lack of reflective
cracking in old CTB roadways, the demand for cement-treated limerock
is increasing.
 The
success of Payne’s CTB roadways is the direct result of the
quality control he puts into the testing, production, and construction
on each project. Limerock samples are obtained from the mine and
blended in the lab with Type I portland cement to prepare specimens
for unconfined compressive strength testing after 7 days of curing.
A minimum strength of 300 psi is desired and is usually obtained
with the addition of only 2 to 2.5 percent cement by dry weight
of the limerock. This combination of dense graded limerock and extremely
small amounts of cement typically results in CTB that has densities
between 110 and 115 pcf with optimum moisture contents between 12
and 15 percent. When homogenously blended in Payne’s ARAN
pugmills, approximately 600 tons per hour of this non-cracking base
material is produced.
During construction, the CTB is delivered from the mixing plant
to the roadway in rear-dump trucks and spread on the subgrade by
either a bulldozer or front end loader. It is then shaped to grade
and the desired thickness (typically 6 inches compacted) by a motor
grader. Initial compaction is achieved through multiple passes of
a smooth drum roller in the static mode with finish rolling performed
by a pneumatic-tire roller to seal the surface. A minimum field
density of 98 percent of the established laboratory modified Proctor
test is required for acceptance. After density is obtained, the
base is then primed to aid in curing for between 7 and 14 days,
depending on the County, before the final application of a thin
1.75-inch hot-mix-asphalt surfacing. It is roadways constructed
in precisely this manner that have resulted in crack-free pavements
for the better part of a decade!
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| CTB is delivered from the mixing plant to
the roadway in rear-dump trucks. |
CTB is shaped to grade and the desired thickness
by a motor grader. |
Final application of a thin 1.75-inch hot-mix-asphalt
surfacing. |
More Information
For local support, tap into the cement industry's network of regional
groups covering the United States. Contact information is available
at www.cement.org/local.
For an additional CTB case study on Ocracoke Island, click
here.
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More About CTB:
CTB Case Studies
CTB Research In Progress
Other SC Sites:
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