Civil Engineering Reference
In-Depth Information
1100
resistant with a low water-cement ratio. Fig. 8-5 illustrates
the effect of water-cement ratio on the durability of non-
air-entrained concrete.
Concrete elements should be properly drained and
kept as dry as possible as greater degrees of saturation
increase the likelihood of distress due to freeze-thaw
cycles. Concrete that is dry or contains only a small amount
of moisture in service is essentially not affected by even a
large number of cycles of freezing and thawing. Refer to
the sections on “Deicer-Scaling Resistance” and “Recom-
mended Air Contents” in this chapter and to Chapter 9 for
mixture design considerations.
1000
Non-air-entrained mixtures
Air-entrained mixtures
900
800
700
600
500
400
300
200
Deicer-Scaling Resistance
100
1
2
3
4
5
6
7
Air content in concrete, %
Deicing chemicals used for snow and ice removal can
cause and aggravate surface scaling. The damage is pri-
marily a physical action. Deicer scaling of inadequately
air-entrained or non-air-entrained concrete during
freezing is believed to be caused by a buildup of osmotic
and hydraulic pressures in excess of the normal hydraulic
pressures produced when water in concrete freezes. These
pressures become critical and result in scaling unless en-
trained air voids are present at the surface and throughout
the sample to relieve the pressure. The hygroscopic (mois-
ture absorbing) properties of deicing salts also attract
water and keep the concrete more saturated, increasing
the potential for freeze-thaw deterioration. However,
properly designed and placed air-entrained concrete will
withstand deicers for many years.
Studies have also shown that, in absence of freezing,
the formation of salt crystals in concrete (from external
sources of chloride, sulfate, and other salts) may con-
tribute to concrete scaling and deterioration similar to the
crumbling of rocks by salt weathering. The entrained air
voids in concrete allow space for salt crystals to grow; this
relieves internal stress similar to the way the voids relieve
stress from freezing water in concrete
(
ASCE 1982
a
nd
Sayward 1984
).
Deicers can have many effects on concrete and the
immediate environment. All deicers can aggravate scaling
of concrete that is not properly air entrained. Sodium chlo-
ride (rock salt) (ASTM D 632 or AASHTO M 143), calcium
chloride (ASTM D 98 or AASHTO M 144), and urea are the
most frequently used deicers. In the absence of freezing,
sodium chloride has little to no chemical effect on concrete
but can damage plants and corrode metal. Calcium chlo-
ride in weak solutions generally has little chemical effect
on concrete and vegetation but does corrode metal.
Studies have shown that concentrated calcium chloride
solutions can chemically attack concrete (
Brown and Cady
1975
). Urea does not chemically damage concrete, vegeta-
tion, or metal. Nonchloride deicers are used to minimize
corrosion of reinforcing steel and minimize groundwater
chloride contamination. The use of deicers containing am-
monium nitrate and ammonium sulfate should be strictly
prohibited as they rapidly attack and disintegrate concrete.
Fig. 8-4. Spacing factor as a function of total air content in
concrete (
Pinto and Hover 2001
).
The air content of concrete with 19-mm (
3
⁄
4
-in.) maximum-
size aggregate would be about 6% for effective freeze-
thaw resistance.
The relationship between air content of standard
mortar and concrete is illustrated by
Taylor (1948)
.
Pinto
and Hover (2001)
address paste air content versus frost
resistance. The total required concrete air content for dura-
bility increases as the maximum-size aggregate is reduced
(due to greater paste volume) and the exposure conditions
become more severe (see “Recommended Air Contents”
later in this chapter).
Freeze-thaw resistance is also significantly increased
with the use of the following: (1) a good quality aggregate,
(2) a low water to cementing materials ratio (maximum
0.45), (3) a minimum cementitious materials content of
335 kg/m
3
(564 lb/yd
3
), (4) proper finishing and curing
techniques, and (5) a compressive strength of 28 MPa
(4,000 psi) when exposed to repeated freeze-thaw cycles.
Even non-air-entrained concretes will be more freeze-thaw
100
80
60
Water to
cement ratio
0.30
0.35
0.40
0.45
0.50
40
20
ASTM C 666
0
0
50
100
150
200
250
300
350
Number of freeze-thaw cycles
Fig. 8-5. Durability factors vs. number of freeze-thaw cycles for
selected non-air-entrained concretes (
Pinto and Hover 2001
).
