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FAQs > Compaction and density of RCC
Q: Why is compaction of RCC so important?
 Similar
to conventional concrete, RCC must be consolidated or in the case
of RCC, compacted in order to achieve the desired performance characteristics.
The degree of compaction of RCC has a direct role on its ultimate
strength and durability. Because of RCC’s very dry consistency
and reduced workability, adequate compaction of RCC can be more
difficult to achieve than with conventional concrete. Compaction
of RCC depends upon many variables including materials used, mixture
proportions, mixing, transporting and placement methods used, compaction
equipment, lift thickness and time of compaction.
The best performance characteristics are obtained when the RCC
is reasonably free of segregation and compacted throughout the entire
lift at, or close to, maximum density. Research studies including
the one below have shown that the strength of RCC drops appreciably
as the density drops. (Figure 1).
Figure 1. Strength vs. density for various RCC mixtures
(Schrader)
Data from the figure indicates that for compacted densities between
97 to 100% of theoretical air-free density (TAFD), the compressive
strength of RCC showed little variation. However, as the density
drops below 96%, significant strength loss occurs. This is why most
specifications require a minimum density of at least 98% of maximum
density, based on a modified Proctor density (MPD). (Note: 98% of
MPD is not equivalent to 98% of TAFD because there are some voids
– typically 0.5 to 2% - in the MPD).
 In
obtaining the specified density, it’s important to recognize
that delays in compaction, segregation of material, inadequate compaction
equipment, too thick of lifts, and insufficient water in the mixture
are some of the issues that may lead to reduced density and subsequent
strength loss. On the other hand, over-compaction can lead to a
weaken RCC surface by loosening of the material directly under the
roller.
The recommended approach is to determine an optimum rolling pattern
that will result in the specified minimum density being met in the
least amount of time and passes. This can be determined either during
a trial placement or early in the construction process. Additional
rolling that will not result in an increased density should be avoided.
Reference:
Schrader, E.K., “Roller-Compacted Concrete for Dams –
State of the Art”, International Conference on Advances in
Concrete Technology, Athens, Greece, May 1992.
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