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FDR with Cement Tames Urban Jungle Streets
“…we were able to reduce the asphalt section
by 1 to 3 inches and saved another million dollars over and above
the savings already realized by FDR.”
Project designer Greg Lyman, Farr West Engineering,
Reno, Nev.
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| Sierra Street after reconstruction using FDR
with cement. |
Recently, the Regional Transportation Commission (RTC) in Reno, Nevada,
which plans and implements the ground transportation program in Washoe
County, completed the rehabilitation of three downtown streets using
full-depth reclamation (FDR) with portland cement. Through the use
of a Pavement Condition Index system it was discovered that portions
of three streets—Mira Loma Drive, Sierra Street, and Hunter
Lake Drive—were experiencing base failures and in need of immediate
repair.
Issues considered while determining which rehabilitation strategy
would work best included project-specific limitations such as the
maintaining of curb lip elevations, the presence of cobbles in the
subgrade, insufficient or contaminated base, and shallow utilities.
The urban setting of these streets and their associated high traffic
volumes only added to the challenge. In addition, a limited budget
required that the solution be one that minimized construction costs
while meeting pavement structural requirements.
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| Sierra Street prior to reconstruction. |
Lumos and Associates, a multi-disciplinary consulting firm located
in Reno, served as both the engineer and construction manager on
these projects. After it was determined which structural section
would be required to support projected traffic loading, rehabilitation
options were evaluated, including full removal and replacement,
partial removal and replacement incorporating geotextiles, thick
hot-mix asphalt (HMA) layers, and FDR using cement. FDR proved to
be the winner, meeting structural criteria for the roadways while
also saving time, money, and reducing construction traffic.
Project designer Greg Lyman, of Farr West Engineering in Reno,
says, “Had we gone with a full remove and replace option,
we would have spent $4.5 million (15% more) than the $3.9 million
cost, even when utilizing synthetic reinforcement.” After
applying some value engineering concepts such as future maintenance
costs and life span for the roadways, Lyman adds, “When you
look at the costs per square foot for just the roadway structural
sections of the projects, the savings are even more dramatic with
FDR saving more than 50 percent over the full removal and replacement
alternative."
A simple design process was followed that involved digging test
pits to obtain representative samples of surface, base, and subgrade
materials from the roadways; pulverizing the HMA in the lab; blending
the HMA, base, and limited native materials to a representative
gradation; adding cement; preparing specimens at optimum moisture
and maximum dry density using the modified Proctor test; and testing
them for unconfined compressive strength. Four specimens each were
prepared at 2%, 4%, and 6% cement and were tested for strength after
2, 7, and 28 days. The emphasis on early strength of these specimens
was due to the urban nature of the project and the fact that the
roadways had to be paved and open to traffic as soon as possible.
A cement content of 3.5% by dry weight of the pulverized material
was found to meet the strength and durability requirements established
by the RTC.
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| Pulverizing failed flexible pavement in downtown
Reno, Nev. |
The FDR work was performed by Sierra Nevada Construction, Reno,
Nevada, and started with the pulverization of the existing roadway
to a depth at or slightly below the final bottom of the treatment
elevation. Then 5 to 6 inches of pulverized material was removed
to ensure that, once the compacted reclaimed/treated base section
was completed, the final surfacing course would match existing curb
and gutter elevations. Dry cement was spread directly onto the pulverized
materials with a cement spreading machine that produced little to
no dust. The pulverized, reclaimed material was blended with the
cement and, with the addition of water, was brought to optimum moisture.
This material was then graded to the appropriate plan lines, grades,
and cross-sections, and compacted to a minimum of 95% of the established
modified Proctor density.
The RTC mandated that all the FDR roadways had to be repaved within
7 days due to events planned in the downtown core and to minimize
disruption to the traveling public. Because the cement in these
roadways was designed to achieve the highest balance between strength
and durability, there were some concerns that shrinkage cracking
might occur that could reflect back up through the newly placed
HMA surfacing. In order to minimize the potential for any reflective
cracking of this nature, the roadways were “microcracked.”
Microcracking is the application of several vibratory roller passes
to a cement-stabilized base after a short curing stage to create
a fine network of cracks. This fine cracking is intended to prevent
wider, more severe cracks from forming. Each section of roadway
was pulverized, blended with cement, shaped, and compacted in one
day, and then moisture cured for 48 hours. The microcracking process
was performed at the 2-day mark on each roadway, and all sections
were paved within 12 hours of microcracking. This process allowed
the roadways to be back under traffic in half the time required
by the RTC—an added bonus in an urban area.
The stiff, durable cement-treated FDR base layer provided such
a stable deck that it allowed the expensive HMA thicknesses to be
reduced by 1 to 3 inches, decreasing asphalt quantities by as much
as 30%. Lyman says the projects went smoothly, and FDR cut construction
time from eight weeks to six weeks. Reduced construction time allowed
for quicker back-to-normal traffic loads and less traffic congestion
due to construction barriers. Environmentally, FDR with cement also
cut down on the amount of virgin natural resources needed and the
amount of hauling and fuel used on the projects.
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