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The Right Choice for Safe Dams
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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