Roller-Compacted Concrete Carries a Heavy Load

Concrete Roads Article December 28, 2000
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Roller-compacted concrete (RCC) pavements were
first used in Canada during the mid-1970s when dry land log
sorting became mandatory for the forest industries of British
Columbia. RCC is a durable paving material that carries heavy
loads, developed as a fast, economical construction method for
pavement projects such as heavy-duty parking and storage areas.
More recently, it's been used in conventional intersection,
street and highway paving.

What is RCC?

RCC is a stiff,
zero-slump concrete mixture with the consistency of damp gravel
comprised of local aggregates, portland cement and water. The
mixture is placed and roller compacted with the same commonly
available equipment used for asphalt pavement construction. The
process requires no forms, finishing, surface texturing or joint
sawing and sealing.

RCC has low water content, requiring it
to be mixed in a continuous flow system, usually a pugmill,
instead of a ready-mixed truck. A dump truck transports freshly
mixed RCC to the construction site where workers place the
mixture in layers, called lifts, using a conventional asphalt
spreader. Lifts, which range from 8 to 12 in. in thickness, are
then compacted using vibratory steel-wheel and pneumatic-tire
rollers. Immediately after workers complete compaction, water is
applied as a fine mist to cure the concrete.

Because of its
low water-cement ratio, RCC typically has high strengths similar
to, or even greater than, conventional concrete. RCC's
high-strength properties combined with ease of construction and
high rate of production make RCC an alternative to flexible
pavement. Additionally, more than 20 years of exposure as
logging roads in cold climates have demonstrated that RCC has
adequate resistance to freezing and thawing.

Intersection
repair

The first RCC/asphalt inlay intersection rehab
project was performed in 1993 in Edmonton, Canada. Standard
General of Birmingham, Ala., an RCC contractor, performed the
840-sq-yd project.

This year, Edmonton, will pave about
31,900 sq yd using about 20,000 cu yd of RCC. More than eight
Canadian cities now use RCC and asphalt inlays for
intersections.

Typically, intersections are paved using 8
in. of RCC and 2 in. of asphalt. The RCC material boasts a
strong 4,000 psi.

It is useful on intersections that have
heavy traffic loads or those susceptible to rutting. Crews mill
out about 10 in. of existing asphalt traffic lanes to begin the
process. With this rehab technique, it is necessary only to
replace the traffic lanes, not the curb and gutter. Also the
length of each lane replaced varies, depending on the ruts in
the roadway. The longer they are, the longer the RCC pavement.

Crews mill about 8 in. of asphalt out during the week using
ramps, barricades and flaggers to keep traffic flowing through
the intersection, according to Tim McLaughlin of Standard
General. On Friday night, the city closes half the intersection.
Crews remove the remaining 2 in. of asphalt overnight.

Each
side of an intersection is usually about 3,200 sq ft. "We come
in Saturday morning at 7 a.m. and put the RCC down," says
McLaughlin. "That takes until 1 p.m."

Once the RCC is
placed, crews spray on a tack coat of asphalt emulsion. "Then we
change the trucks from the RCC plant to the asphalt plant," says
McLaughlin. "We have an asphalt paver on site, so the same crew
places both products."

By 4 p.m., the 2-in. asphalt layer is
down. It cools until about 7 p.m. when the road reopens to
traffic on the rehabilitated side. "Then they switch the
barricades and let traffic on the road," says McLaughlin.
"Starting Saturday night, we repeat the same process on the
other side."

By about 7 or 8 p.m. Sunday, traffic is flowing
normally. A 6,400-sq-ft intersection rehab is completed in one
weekend.

"This is useful especially in industrial areas
where you have a lot of people coming on weekday mornings," says
McLaughlin. "A lot of municipalities look at it because they can
have a 4,000-psi product in and traffic flowing again in 24
hours."

McLaughlin says another advantage to RCC inlays is
that cities don't have to rehab the entire intersection; they
can stop at the driving lanes. Also, once complete, the pavement
is maintained the same way as regular asphalt pavement.

This
process has caught on in Canada. The cities of Fort McMurray,
Fort St. John, Edmonton, Calgary, and Regina have used RCC
pavement to rehab intersections.

RCC also is emerging as a
cost-effective, high-performance base for conventional highway
and street pavements. A thin layer of asphalt normally covers
the surface to ensure a smooth ride at street and highway
speeds. In Edmonton, Canada, the city paved a two-lane, 1,800-ft
stretch of a street with 8 in. of RCC over a 6-in.
cement-stabilized subgrade to carry increased loads from heavy
trucks accessing a new waste processing facility.

Other
Advantages

RCC pavements have other advantages that helped
make them an attractive option to city officials and planners.
RCC will not rut from high axle loads or shove or tear from
turning or braking.

It will not soften from heat generated
by composting or hot summer sun. RCC resists degradation from
materials such as diesel fuel. RCC pavements also help yield the
highest quality compost because they can withstand high
temperatures generated by natural composting while minimizing
contamination sources.

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