ETL 1110-2-343
31 May 93
high-velocity flows, it may be economically prudent
to accept some cavitation damage in lieu of strict
surface tolerance requirements.
(3) Abrasion erosion. Spillway aprons, stilling
basins, and many other hydraulic structures may
suffer surface erosion due to abrasion. Concrete,
whether RCC or conventionally placed, cannot
withstand continued abrasive action from silt, sand,
gravel, rocks, construction debris, or other water-
borne debris without experiencing severe erosion
problems. RCC mixtures with a low water-cement
ratio and large-size aggregates are expected to pro-
vide erosion resistance equal to a conventional
concrete with similar ingredients. In circumstances
where abrasion erosion or cavitation erosion is
severe, a steel lining may be chosen to minimize
maintenance and repair work. The embedments or
anchorages required with steel linings do not lend
themselves to RCC construction. Therefore, when
steel linings are used, conventional concrete, placed
to a depth sufficient to encapsulate the liner anchor
system, is used over the RCC.
(4) Surface treatment for high-velocity flow
conditions. RCC can be used for paving open
Figure 1-14. Precast concrete panels
channel inverts, for bank stabilization and erosion
protection, and for other flow channelization pro-
jects, provided flow velocities are less than 25 fps.
The surface tolerance control obtained with RCC
construction is not suitable when flow velocities
exceed 25 fps. RCC construction may be consid-
ered for spillways, stilling basins, and other flow
channelization projects, where velocities exceed
25 fps; however, a conventionally placed surface
concrete screeded and floated to meet specified
tolerance requirements must be used. Conventional
concrete overlay details are shown in Figure 1-16.
Typical conventional concrete applications in RCC
dams include spillways, spillway caps, spillway
bucket and stilling basins.
Figure 1-15. Unformed (D/S) surface finish
j. Galleries, joints, waterstops, and other spe-
cial features. Galleries, joints, waterstops, and
damage at flow velocities as low as 40 fps (Refer-
other special features that are easily incorporated
ence 12c). RCC surfaces cannot be held to the
into conventional concrete placements are much
same close tolerances as conventionally placed
more difficult when RCC construction methods are
concrete with formed, slipformed, or screeded sur-
used. Designers must carefully consider the
faces. Therefore, a conventional concrete topping
construction details for such features when RCC is
or facing may be required over RCC placements
specified, to minimize the impact of these features
where the surface will be exposed to flowing water.
on the RCC placement process and to assure that
Duration of flow, however, is also a factor. For
these features will perform as intended. Galleries in
structures with infrequent, short-duration,
dams must be located to provide adequate space for
1-12
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