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Frequently Asked Questions
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FAQs > Edge
Compaction of RCC
Q: What methods are available for edge compaction
of RCC?
A: Proper compaction must be achieved
to insure good performance of RCC. This is especially important
at RCC edges where the material is generally exposed to the elements
and to the erosive forces from water flows and sediment loads. There
are two methods of design/construction of RCC lift edges; formed
and unformed.
Formed Edges
Proper edge compaction and adequate RCC density can be achieved
when properly proportioned mixes are placed against forms without
excessive segregation. This requires more “workable”
RCC mixes than for an unformed surface. Mixes containing higher
cementitious contents and having lower Vebe times tend to be more
workable as compared to dryer mixes with low cementitious contents.
Enhanced workability is required for consolidation of RCC against
the formed surface to produce a smooth finished surface and to minimize
rock pockets.
 Typically,
large vibratory compactors are used to compact the RCC to within
a foot or so from the edge form. Small vibratory compactors may
be used to compact the RCC near the forms if the forms are built
to resist the compaction forces without excessive deformations.
On most projects, hand operated compactors are used to compact RCC
in areas that are not accessible to vibratory rollers.
Unformed Edges
 Unformed
RCC lift edges are sometimes used for emergency spillway chutes
and for downstream faces of gravity dams. They are usually less
expensive and take less time to construct. Compaction is performed
by heavy vibratory rollers. However, the unrestrained face would
remain not fully compacted near the outside edge. This zone of lower
density should be (1) considered as “sacrificial concrete”
by the designer or (2) compacted by hand-operated equipment or modified
machinery capable of compacting the sloped edge and adjacent RCC.
The edge of the sacrificial concrete typically has the appearance
of rough, irregular shape concrete. If a smoother finish is desired,
the sacrificial concrete should be compacted or trimmed to give
a more uniform surface. Compaction of the exposed RCC face will
increase the RCC density and would reduce raveling. However, scattered
rock pockets would still be encountered.
Techniques for Edge Compaction
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| Jumping jack used to compact edge. |
Vibratory plate compactor for edge compaction. |
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| Vibratory plate compactor attached
to small track hoe. |
Additional detailed information concerning RCC compaction for formed
and unformed edges can be found in PCA’s publications EB218,
Design Manual
for RCC Spillways and Overtopping Protection and EB400,
Facing Systems
for Roller-Compacted Concrete Dams and Spillways.
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