Civil Engineering Reference
In-Depth Information
to 85 MPa (10,000 psi to 12,000 psi) to specify a maximum
delivery temperature of 18°C (64°F) (
Ryell and Bickley
1987
). In summertime it is possible that this limit could only
be met by using liquid nitrogen to cool the concrete.
Experience with very-high-strength concrete suggests that
a delivery temperature of no more than 25°C (77°F), prefer-
ably 20°C (68°F), should be allowed. In addition to liquid
nitrogen, measures to cool HPC in the summer may involve
using ice or chilled water as part of the mix water. The spec-
ifier should state the required delivery temperature.
In HPC applications such as high-rise buildings,
column sizes are large enough to be classed as mass con-
crete. Normally, excessive heat generation in mass con-
crete is controlled by using a low cement content. When
high-cement-content HPC mixes are used under these
conditions, other methods of controlling maximum con-
crete temperature must be employed.
Burg and Ost (1994)
recorded temperature rise for 1220-mm (4-ft) concrete
cubes using the mixtures in Table 17-5. A maximum
temperature rise of 9.4°C to 11.7°C for every 100 kg of
cement per cubic meter of concrete (1°F to 12.5°F for every
100 lb of cement per cubic yard of concrete) was meas-
ured.
Burg and Fiorato (1999)
monitored temperature rise
in high-strength concrete caissons; they determined that
in-place strength was not affected by temperature rise due
to heat of hydration.
crete (Mix 4) with a water to cementing materials ratio of
0.22 was frost resistant.
Sidewalks constructed in Chicago in the 1920s used
25-mm (1-in.) thick toppings made of no-slump dry-pack
mortar that had to be rammed into place. The concrete
contained no air entrainment. Many of these sidewalks are
still in use today; they are in good condition (minus some
surface paste exposing fine aggregate) after 60 years of
exposure to frost and deicers. No documentation exists on
the water to cement ratio; however, it can be assumed that
the water to cement ratio was comparable to that of
modern HPCs.
While the above experiences prove the excellent dura-
bility of certain high-performance concretes to freeze-
thaw damage and salt scaling, it is considered prudent to
use air-entrainment. No well-documented field experi-
ments have been made to prove that air-entrainment is not
needed. Until such data are available, current practice for
air-entrainment should be followed. It has been shown
that the prime requirement of an air-void system for HPC
is a preponderance of air bubbles of 200 µm size and
smaller. If the correct air bubble size and spacing can be
assured, then a moderate air content will ensure durability
and minimize strength loss. The best measure of air-
entrainment is the spacing factor.
Chemical Attack
Freeze-Thaw Resistance
For resistance to chemical attack on most structures, HPC
offers a much improved performance. Resistance to
various sulfates is achieved primarily by the use of a
dense, strong concrete of very low permeability and low
water-to-cementing materials ratio; these are all charac-
teristics of HPC. Similarly, as discussed by
Gagne and
others (1994)
, resistance to acid from wastes is also
much improved.
Because of its very low water-cementing materials ratio
(less than 0.25), it is widely believed that HPC should be
highly resistant to both scaling and physical breakup due
to freezing and thawing. There is ample evidence that
properly air-entrained high performance concretes are
highly resistant to freezing and thawing and to scaling.
Gagne, Pigeon, and Aítcin (1990)
tested 27 mixes using
cement and silica fume with water-cementing materials
ratios of 0.30, 0.26, and 0.23 and a wide range of quality in
air-voids systems. All specimens performed exceptionally
well in salt-scaling tests, confirming the durability of high-
performance concrete, and suggesting that air-entrainment
is not needed.
Tachitana and others (1990)
conducted
ASTM C 666 (Procedure A) tests on non-air-entrained high
performance concretes with water-cementing materials
ratios between 0.22 and 0.31. All were found to be
extremely resistant to freeze-thaw damage and again it
was suggested that air-entrainment is not needed.
Pinto and Hover (2001)
found that non-air-entrained
concrete with a water to portland cement ratio of 0.25 was
deicer-scaling resistant with no supplementary cementing
materials present. They found that higher strength port-
land cement concretes needed less air than normal
concrete to be frost and scale resistant.
Burg and Ost (1994)
found that of the six mixes tested
in Table 17-5 using ASTM C 666, only the silica fume con-
Alkali-Silica Reactivity
Reactivity between certain siliceous aggregates and alkali
hydroxides can affect the long-term performance of
concrete. Two characteristics of HPC that help combat
alkali-silica reactivity are:
(1) HPC concretes at very low water to cement ratios
can self desiccate (dry out) to a level that does not allow
ASR to occur (relative humidity less than 80%).
Burg and
Ost (1994)
observed relative humidity values ranging
from 62% to 72% for their six mixes in Table 17-5. The low
permeability of HPC also minimizes external moisture
from entering the concrete.
(2) HPC concretes can use significant amounts of
supplementary cementing materials that may have the
ability to control alkali-silica reactivity. However, this
must be demonstrated by test. HPC concretes can also use
ASR inhibiting admixtures to control ASR.
HPC concretes are not immune to alkali-silica reac-
tivity and appropriate precautions must be taken.
