Roller-compacted concrete ( RCC) or rolled concrete

Roller-compacted concrete ( RCC) or rolled
concrete is a special blend of concrete that has
essentially the same ingredients as conventional
concrete but in different ratios, and increasingly
with partial substitution of fly ash for Portland
cement . [2] RCC is a mix of cement/fly ash,
water, sand, aggregate and common additives,
but contains much less water. The produced mix
is drier and essentially has no slump . RCC is
placed in a manner similar to paving ; the
material is delivered by dump trucks or
conveyors , spread by small bulldozers or
specially modified asphalt pavers , and then
compacted by vibratory rollers .
In dam construction, roller-compacted concrete
began its initial development with the
construction of the Alpa Gera Dam near Sondrio
in North Italy between 1961 and 1964.
Concrete was laid in a similar form and method
but not rolled. [3][4][5] RCC had been touted in
engineering journals during the 1970s as a
revolutionary material suitable for, among other
things, dam construction. [6] Initially and
generally, RCC was used for backfill, sub-base
and concrete pavement construction, but
increasingly it has been used to build concrete
gravity dams because the low cement content
and use of fly ash cause less heat to be
generated while curing than do conventional
mass concrete placements. Roller-compacted
concrete has many time and cost benefits over
conventional mass concrete dams; these include
higher rates of concrete placement, lower
material costs and lower costs associated with
post-cooling and formwork .
Dam applications
For dam applications, RCC sections are built lift-
by-lift in successive horizontal layers resulting in
a downstream slope that resembles a concrete
staircase. Once a layer is placed, it can
immediately support the earth-moving
equipment to place the next layer. After RCC is
deposited on the lift surface, small dozers
typically spread it in one-foot-thick layers
(300mm). [7]
The first RCC dam built in the USA was the
Willow Creek Dam on Willow Creek, a tributary
in Oregon of the Columbia River . It was
constructed by the Army Corps of Engineers
between November 1981 [8] and February
1983. [6] Construction proceeded well, within a
fast schedule and under budget (estimated $50
million, actual $35 million). On initial filling
though, it was found that the leakage between
the compacted layers within the dam body was
unusually high. This condition was treated by
traditional remedial grouting at a further cost of
$2 million, which initially reduced the leakage
by nearly 75%; over the years, seepage has
since decreased to less than 10% of its initial
flow. Concern over the dam's long-term safety
has continued however, although only indirectly
related to its RCC construction. Within a few
years of construction, problems were noted
with stratification of the reservoir water, caused
by upstream pollution and anoxic
decomposition, which produced hydrogen
sulfide gas. Concerns were expressed that this
could in turn give rise to sulfuric acid, and thus
accelerate damage to the concrete. The
controversy itself, as well as its handling
continued for some years. In 2004 an aeration
plant was installed to address the root cause in
the reservoir, as had been suggested 18 years
earlier. [6]
In the quarter century since Willow Creek,
considerable research and experimentation
have yielded innumerable improvements in
concrete mix designs, dam designs and
construction methods for roller-compacted
concrete dams; by 2008, about 350 RCC dams
existed world-wide. [9] Currently the highest
dam of this type is Longtan Dam, at 216 m,
with Diamer-Bhasha Dam planned at 272 m.