ETL 1110-2-343
31 May 93
lower strength, and less bond between lifts than
and reduce potential damage caused by water seep-
RCC containing properly processed aggregates.
ing along lift joint surfaces. The permeability of
both conventional and RCC concrete is dependent
d. Fly ash (Pozzolan). Fly ash is used in RCC
on the pore structure and air void system within the
construction to serve three purposes; (1) as a partial
parent concrete and along lift joint surfaces. Con-
replacement for cement to reduce heat generation,
crete with low permeability generally has a low
(2) to reduce cost, and (3) as a mineral addition to
water-cement ratio, is well mixed and consolidated,
the mixture to provide fines to improve workability
is proportioned with adequate paste and mortar to
(Reference 12a). The use of higher fly ash contents
sufficiently fill all voids, and has been properly
may be helpful in obtaining adequate paste content
cured to allow for the continued hydration of
in situations where natural fines are not available,
cement. RCC structures are generally constructed
while at the same time keeping the total Portland
with concrete having much higher water-cement
cement content at relatively low levels. However,
ratios than used for conventional concrete struc-
fly ash slows strength gain, and the use of high fly
tures. The effect that these high water-cement
ash contents in combination with high water-cement
ratios have on pore pressures within the body of
ratios can produce very low strengths at early ages.
hydraulic structures and on long-term durability is
Because of this, RCC strength requirements are
unknown.
generally specified based on compressive strength at
g. Thermal cracking.
1 year rather than at 28 days, as is the practice for
conventional concrete.
(1) The extent of thermal cracking in an RCC
e. Strength and density. Work performed by
structure will be affected by the type and degree of
M. F. Kaplan (Reference 12f) demonstrated that
temperature control used. Temperature is controlled
5-percent air voids due to incomplete compaction
in RCC by selecting cementitious materials that
can result in a 30-percent loss of strength, while
generate the least heat of hydration, by cooling
20-percent air voids can produce a strength loss of
various components of the RCC mixture prior to
80 percent. The more difficult an RCC mixture is
mixing and placing, and by restricting RCC place-
to compact, the more likely it is that incomplete
ments to cool weather seasons.
compaction will occur and that strength will be less
than desired. In some instances, adding water to a
(2) The selection of cement and fly ash propor-
very dry mix may produce a strength gain, because
tions will significantly affect the heat of hydration
the added water increases workability of the mix,
and subsequent thermal cracking. To reduce tem-
thereby reducing air voids. This effect has also
perature rise, low-heat-of-hydration cement and the
been observed with difficult-to-consolidate con-
maximum pozzolan replacement of cement, consis-
ventional concrete mixtures. The chance of obtain-
tent with strength requirements, are generally speci-
ing the desired bond and tensile strength at lift
fied. The potential for thermal cracking in massive
joints is less likely with RCC mixtures that are too
RCC structures should be determined by a thermal
dry to be easily consolidated, or with RCC mixtures
analysis that includes a temperature study to iden-
that are designed with inadequate paste and mortar
tify temperature gradients, and a stress analysis to
volumes. Bonding and permeability at the lift joints
identify the critical tensile stresses that can develop
will be improved with a mortar bedding; however,
within the structure due to temperature gradients.
the permeability of the drier consistency mixtures
Critical temperature differentials are best deter-
may still be unacceptably high. The bond and
mined using computer models that have lift-by-lift
tensile strength at the lift joints for properly pro-
construction sequencing capabilities. Variables
portioned, well-consolidated RCC will approach that
important to the temperature studies include place-
achieved at the lift joints of conventionally placed
ment temperature, placement rate, diffusivity, heat
mass concrete.
of hydration, and external temperature cycles.
Variables important to stress analyses include joint
f. Permeability. Permeability along lift joint
spacing, restraint conditions, modulus of elasticity,
surfaces is generally higher than the permeability of
creep effects, and tensile strength. Many of the
the parent concrete. Joints between layers of RCC
variables described should be verified through labo-
must be well bonded to provide low permeability,
ratory testing and RCC test placements.
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