Civil Engineering Reference
In-Depth Information
5
1.0
The effect of mix design, surface treatment, curing, or
other variables on resistance to surface scaling can be eval-
uated by ASTM C 672.
Air content
2%
4%
6%
4
0.8
3
0.6
Sulfate Resistance
2
0.4
Sulfate resistance of concrete is improved by air entrain-
ment, as shown in Figs. 8-9 and 8-10, when advantage is
taken of the reduction in water-cement ratio possible with
air entrainment. Air-entrained concrete made with a low
water-cement ratio, an adequate cement content and a sul-
fate-resistant cement will be resistant to attack from sulfate
soils and waters.
ASTM C 672
deicer scaling test
1
0.2
0
0.2
0.25
0.35
Water to cement ratio
0.3
0.4
0.45
0.5
Fig. 8-8. Measured mass loss of concrete after 40 cycles of
deicer and frost exposure at various water to cement ratios
(
Pinto and Hover 2001
).
Resistance to Alkali-Silica Reactivity
The expansive disruption caused by alkali-silica reactivity
is reduced through the use of air-entrainment (
Kretsinger
1949
). Alkali hydroxides react with the silica of reactive
aggregates to form expansive reaction products, causing
the concrete to expand. Excessive expansion will disrupt
and deteriorate concrete. As shown in Fig. 8-11, the expan-
sion of mortar bars made with reactive materials is re-
duced as the air content is increased.
with poor placing and curing practices can aggravate
scaling. Consult local guidelines on allowable dosages
and practices for using these materials in deicer environ-
ments as they can vary from
ACI 318
requirements.
Air Drying.
The resistance of air-entrained concrete to
freeze-thaw cycles and deicers is greatly increased by an
air drying period after initial moist curing. Air drying
removes excess moisture from the concrete which in turn
reduces the internal stress caused by freeze-thaw condi-
tions and deicers. Water-saturated concrete will deterio-
rate faster than an air-dried concrete when exposed to
moist freeze-thaw cycling and deicers. Concrete placed in
the spring or summer has an adequate drying period.
Concrete placed in the fall season, however, often does not
dry out enough before deicers are used. This is especially
true of fall paving cured by membrane-forming com-
pounds. These membranes remain intact until worn off by
traffic; thus, adequate drying may not occur before the
onset of winter. Curing methods, such as use of plastic
sheets, that allow drying at the completion of the curing
period are preferable for fall paving on all projects where
deicers will be used. Concrete placed in
the fall should be allowed at least 30
days for air drying after the moist-curing
period. The exact length of time for suf-
ficient drying to take place may vary
with climate and weather conditions.
Treatment of Scaled Surfaces.
If sur-
face scaling (an indication of an inade-
quate air-void system or poor finishing
practices) should develop during the
first frost season, or if the concrete is of
poor quality, a breathable surface treat-
ment can be applied to the dry concrete
to help protect it against further damage.
Treatment often consists of a penetrating
sealer made with boiled linseed oil
(
ACPA 1996
), breathable methacrylate,
or other materials. Nonbreathable for-
mulations should be avoided as they can
cause delamination.
Strength
When the air content is maintained constant, strength
varies inversely with the water-cement ratio. Fig. 8-12
shows a typical relationship between 28-day compressive
strength and water-cement ratio for concrete that has the
recommended percentages of entrained air. As air content
is increased, a given strength generally can be maintained
by holding the voids (air + water) to cement ratio con-
stant; this may, however, necessitate some increase in
cement content.
Air-entrained as well as non-air-entrained concrete
can readily be proportioned to provide similar moderate
strengths. Both generally must contain the same amount
Cement
content
With air
Without air
222 kg/m
3
(375 lb/yd
3
)
306 kg/m
3
(515 lb/yd
3
)
392 kg/m
3
(660 lb/yd
3
)
Fig. 8-9. Effect of entrained air and cement content (Type II) on performance of
concrete specimens exposed to a sulfate soil. Without entrained air the
specimens made with lesser amounts of cement deteriorated badly. Specimens
made with the most cement and the lowest water-cement ratio were further
improved by air entrainment. Specimens were 5 years old when photographed
(
Stanton 1948
and
Lerch 1960
).
