Civil Engineering Reference
In-Depth Information
Resistivity
Regular Mix
HPC, particularly that formulated with silica fume, has
very high resistivity, up to 20 to 25 times that of normal
concrete. This increases resistance to the flow of electrical
current and reduces corrosion rates. Particularly if dry,
HPC acts as an effective dielectric. Where cracking occurs
in HPC, the corrosion is localized and minor; this is due to
the high resistivity of the concrete which suppresses the
development of a macro corrosion cell.
10%
18%
2%
25%
45%
Cement
Water
Air
Fine Aggregate
Coarse Aggregate
Fines
10%
18%
2%
8%
26%
36%
SCC
Fig. 17-6. Examples of materials used in regular concrete
and self-compacting concrete by absolute volume.
SELF-COMPACTING CONCRETE
Self-compacting concrete (SCC), also referred to as self-
consolidating concrete, is able to flow and consolidate
under its own weight. At the same time it is cohesive
enough to fill spaces of almost any size and shape without
segregation or bleeding. This makes SCC particularly
useful wherever placing is difficult, such as in heavily-
reinforced concrete members or in complicated formwork.
This technology, developed in Japan in the 1980s, is
based on increasing the amount of fine material, for
example fly ash or limestone filler, without changing the
water content compared to common concrete. This
changes the rheological behavior of the concrete. SCC has
to have a low yield value to ensure high flowability; a low
water content ensures high viscosity, so the coarse aggre-
gate can float in the mortar without segregating. To achieve
a balance between deformability and stability, the total
content of particles finer than the 150 µm (No. 100) sieve
has to be high, usually about 520 to 560 kg/m
3
(880 to 950
lb/yd
3
). High-range water reducers based on polycarboxy-
late ethers are typically used to plasticize the mixture. SCC
is very sensitive to fluctuation in water content; therefore,
stabilizers such as polysaccarides are used. Fig. 17-6 shows
an example of mix proportions used in self-compacting
concrete as compared to a regular concrete mix.
In Japan, self-compacting concretes are divided into
three different types according to the composition of the
mortar:
• Powder type
• Viscosity agent (stabilizer) type
• Combination type
For the powder type, a high proportion of fines produces
the necessary mortar volume. In the stabilizer type, the
fines content can be in the range admissible for vibrated
concrete. The viscosity required to inhibit segregation will
then be adjusted by using a stabilizer. The combination
type is created by adding a small amount of stabilizer to
the powder type to balance the moisture fluctuations in
the manufacturing process.
Since SCC is characterized by special fresh concrete
properties, many new tests have been developed to
measure flowability, viscosity, blocking tendency, self-lev-
eling, and stability of the mixture (
Skarendahl and
Peterson 1999
and
Ludwig and others 2001
). A simple test
to measure the stable and unblocked flow is the J-Ring
test, which is a modified slump test. The J-Ring—300 mm
(12 in.) diameter with circular rods—is added to the slump
test (Fig. 17-7). The number of rods has to be adjusted
depending on the maximum size aggregate in the SCC
mix. The SCC has to pass through the obstacles in the
J-Ring without separation of paste and coarse aggregates.
The slump diameter of a well-proportioned SCC is
approximately the same with and without the J-Ring; it is
usually about 750 mm (30 in.). Therefore, the test surface
has to be at least 1000 mm (40 in.) in diameter.
Strength and durability of well-designed SCC are
almost similar to conventional concrete. Without proper
curing, SCC tends to have higher plastic shrinkage
18 mm (0.7 in.)
300 mm (12 in.)
Fig. 17-7. J-ring test. Photo courtesy of VDZ.
