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Dams are a vital, but aging, part of our public works
infrastructure. Bringing them up to current standards is a
$40-billion repair job. The challenge is to find cost-effective
repair and replacement methods without sacrificing safety and
reliability.
Officials and engineers are turning to a relatively new material
called roller-compacted concrete, or RCC. Since 1980, RCC-a
specially proportioned, no-slump concrete-has been used successfully
to restore more than 100 dams and to build more than 70 new dams.
RCC has three key properties that make it uniquely suited for
dams: economy, performance, and high-speed construction. It has the
strength and durability of conventional concrete, but at a cost that
rivals earth or rockfill construction.
 RCC can be used to
build new dams or to shore up old ones. It protects dams from
over-topping failure, earthquakes, and erosion.
Roller-compacted concrete has the same ingredients as
conventional concrete: cement, water, and aggregates. But RCC is
much drier.
It can be placed quickly and easily with large-volume
earth-moving equipment.
It's generally transported by dump trucks, spread by bulldozers, and
compacted by vibratory rollers.
 Sections
are built lift-by-lift in successive horizontal layers so the
downstream slope 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. Workers
also place it with motor graders, spreader boxes, and paving
machines.
For existing earth and rockfill dams, RCC acts like an armor
plating to protect them from the erosion of high-velocity water
flows. RCC can also be used to build new or replacement dams. While
it's most economical for large dam projects, RCC is increasingly
used to build small dams for water supply and flood control. Not
only is RCC more durable than earth or rockfill dams, it's
frequently more economical.
RCC has also
proven itself in many other types of applications. Older concrete
and masonry dams can be buttressed with RCC to increase resistance
to earthquake loading and to improve stability to prevent
overturning and sliding.
RCC is used as backfill to support conventional concrete
spillways. Due to its high resistance to abrasion, RCC is also used
to construct stilling basins, build liners for outlet channels, and
form grade-control structures in rivers.
Public Works for Public Safety
As public works programs focus on infrastructure renewal, dam safety
emerges as a prime concern. Today, many of our dams fail to meet modern standards-victims of
neglect, insufficient funding, or age. At risk are the 79,000 dams we depend on for water supply, power,
irrigation, flood control, even recreation.
Statistics tell the story. Many dams, especially earthen
embankment dams constructed before 1960, no longer meet current
hydraulic design criteria. Most dams are designed for a service life of 50 years. Yet today,
30% of these dams
have surpassed the half-century mark.
By 2020, 80% of our nation's dams will be 50 years old, requiring
replacement or major rehabilitation. Failures can be catastrophic, with damage measured in lost lives,
displaced communities, and millions of dollars in destruction.
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