Cement-Treated Base Case
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Cement-Treated Base Reaches New Heights on South
Carolina Road
The latest equipment, materials, and methods for building
cement-treated base (CTB) will save 20% in costs and shave five
months off construction time for the expansion of South Carolina
Route 5 (SC-5).
 Funded
by a county transportation tax program known as “Pennies for
Progress” as well as state transportation funds, SC-5 in York
County needed upgrading to a four-lane road from the current two-lane
route. The original pavement design consisted of 8 inches (200 mm)
of graded aggregate base (GAB) course and 3.5 inches (90 mm) of
hot laid asphalt aggregate base course (asphalt base) overlain by
intermediate and surface asphalt mixes. Site-Prep approached Eagle
Construction of Newberry, SC, to value-engineer cement-stabilized
aggregate base course (CSABC) as a replacement for the GAB and asphalt
base.
Departments of transportation have long recognized CSABC as an
economical and durable alternative to traditional base materials.
CSABC can be produced by the road-mix method or plant-mix method
with both producing an equally robust base.
Site-Prep proposed an 8-inch (200 mm) CSABC as equal structural
replacement. When the CSABC base design was compared by cost to
the “bid design” a savings of approximately $900,000
would be realized for the 238,000 yd2 (199,000 m2)
placement, not including cost savings attributed to the time saved
in a one–lift base construction. Also, Site-Prep recommended
using a high-density paver to place the CSABC. This method had been
successfully used on a previous South Carolina Department of Transportation
(SCDOT) project and was highly praised for its speed, versatility,
grade control features and the compactive effort provided. CSABC
construction began in late summer of 2007.
 Cement
stabilization can be successfully applied to aggregates, soils and
recycled materials to contribute to the structure of a pavement
system. CSABC has two general types: plant-mixed or road-mixed (in-place).
Plant-mixed CSABC utilizes a centralized pugmill to produce a homogeneous
mix of water, aggregate and portland cement that is subsequently
hauled and placed along a project. Pugmills vary in size and most
used for CSABC production are portable in nature.
Road-mixed CSABC differs only in the production of the mixture.
The aggregate used in the production of the CSABC is placed and
compacted on the roadway width and length at a predetermined depth.
Portland cement is then spread by mechanical variable rate spreaders
directly on the surface of the aggregate and then incorporated into
the aggregate layer by typical stabilization pulverization equipment.
Portland cement is incorporated usually one inch less than the placed
thickness to prevent possible subgrade contamination into the CSABC
layer during mixing operations. The pulverization equipment and/or
a water truck adds water for hydration of the cement and compaction.
Compaction and finishing operations are usually the same for plant-mixed
and road-mixed CSABC. Portland cement contents of CSABC generally
range from 1.5 to 4.0% by dry weight of the roadway materials with
finished thicknesses of 6 to 12 inches (150 to 300 mm) used. Unconfined
compressive strengths of CSABC are typically in the 300 to 800 psi
(2.1 to 5.5 MPa) range for a 7-day test with modified Proctor densities
specified from 95 to 100%. CSABC needs curing to some extent and
is usually accomplished with a “wet” cure or sealed
with a bituminous coat or curing compound.
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