Civil Engineering Reference
In-Depth Information
ROLLER-COMPACTED CONCRETE
Other water control RCC applications include use as
an emergency spillway or overtopping protection for
embankment dams, low permeable liner for settling ponds,
bank protection, and grade control structure for channels
and riverbeds.
Roller-compacted concrete (RCC) is a lean, no-slump,
almost dry concrete that is compacted in place by vibra-
tory roller or plate compaction equipment (Fig. 18-11).
RCC is a mixture of aggregate, cement, and water; sup-
plementary cementing materials such as fly ash also have
been used. Cement contents range from 60 to 360 kg per
cubic meter (100 to 600 lb per cubic yard). Mixing is done
with conventional batch mixers, continuous mixers, or in
some instances tilting-drum truck mixers.
Applications for RCC fall into two distinct categories—
water control structures (dams) and pavements. While the
same term is used to describe both types of concrete use, the
design and construction processes are different.
Pavements
The uses for RCC paving range from pavements as thick as
one meter (one yard) for the mining industry to city streets,
paved surfaces for composting operations, logging, truck
stagging areas, and warehouse floors.The procedures for
construction of an RCC pavement require tighter control
than for dam construction (
Arnold and Zamensky 2000
).
Cement content is in the same range as conventional con-
crete, 300 to 360 kg/m
3
(500 to 600 lb/yd
3
), and compres-
sive strength is of the same order, 30 to 40 MPa (4000 to
6000 psi). The nominal maximum aggregate size is limited
to 19 mm (
3
⁄
4
in.) to provide a smooth, dense surface. For
even better surface textures, a 16 mm (
5
⁄
8
in.) maximum size
aggregate is recommended.
The zero slump mix is usually produced in a contin-
uous flow pugmill mixer at production rates as high as 400
tons per hour. It is possible to mix RCC in a central batch
plant, but the plant must be dedicated to RCC production
exclusively, because the material tends to stick to the inside
drums. Specifications usually require that the mix be
transported, placed, and compacted within 60 minutes of
the start of mixing; although ambient weather conditions
may increase or decrease that time window.
RCC is typically placed in layers 125 to 250 mm (5 to
10 in.) in thickness using an asphalt-type paving machine.
High-density paving equipment is preferred for layers
thicker than 150 mm (6 in.) since the need for subsequent
compaction by rollers is reduced. Where a design calls for
pavement thickness greater than 250 mm (10 in.), the RCC
should be placed in multiple layers. In this type of con-
struction, it is important that there be a minimum time
delay in placing subsequent layers so that good bond is
assured. Following placement by a paver, RCC can be
compacted with a combination of vibratory steel-wheeled
rollers and rubber-tired equipment.
Curing is vitally important in RCC pavement construc-
tion. The very low water content at the initial mixing stage
means that an RCC mix will dry out very quickly once it is
in place. Continuous water curing is the recommended
method, although sprayed on asphaltic emulsion, plastic
sheeting, and concrete curing compounds have been used
in some cases. Pavement projects have had design com-
pressive strengths of about 35 MPa (5000 psi) with field
strengths in the range of 35 to 70 MPa (5000 to 10,000 psi)
(
Hansen 1987
)
High-performance roller compacted concrete for
areas subjected to high impact and abrasive loading were
developed in the mid-1990's. These mixes are based on ob-
taining the optimum packing of the various sizes of aggre-
gate particles, and the addition of silica fume to the mix
(
Marchand and others 1997
and
Reid and Marchand 1998
).
Water Control Structures
RCC can be used for the entire dam structure, or as an
overtopping protection on the upper section and on the
downstream face. The nominal maximum aggregate size
can range up to 150 mm (6 in.). The zero slump mix is
produced in a high-capacity central-mixing plant near the
site and delivered by truck or by conveyor belt. Cement
content is usually lower than that used in a conventional
concrete mix, but similar to that of mass concrete. Com-
pressive strengths ranging from 7 to 30 MPa (1000 to 4500
psi) have been obtained from roller-compacted concrete in
dam projects. The RCC mix is transported by trucks and
conveyor belts and spread by grader or bulldozer,
followed by rolling with vibratory compactors. No forms
are used. On some projects the upstream face is surfaced
with higher strength conventional air-entrained concrete
or precast panels for improved durability.
RCC dams have the advantage of allowing much
steeper slopes on both faces than an earth fill dam. In addi-
tion to the advantage of using less material, the dam is com-
pleted and placed in service earlier, usually at a significant
savings in overall cost compared to an earth fill structure.
Fig. 18-11. Vibratory rollers are used to compact roller-
compacted concrete. (64401)
