Civil Engineering Reference
In-Depth Information
5
Magnesium chloride deicers have come under recent criti-
cism for aggravating scaling. One study found that mag-
nesium chloride, magnesium acetate, magnesium nitrate,
and calcium chloride are more damaging to concrete than
sodium chloride (
Cody, Cody, Spry, and Gan 1996
).
The extent of scaling depends upon the amount of
deicer used and the frequency of application. Relatively
low concentrations of deicer (on the order of 2% to 4% by
mass) produce more surface scaling than higher concentra-
tions or the absence of deicer (
Verbeck and Klieger 1956
).
Deicers can reach concrete surfaces in ways other
than direct application, such as splashing by vehicles and
dripping from the undersides of vehicles. Scaling is more
severe in poorly drained areas because more of the deicer
solution remains on the concrete surface during freezing
and thawing. Air entrainment is effective in preventing
surface scaling and is recommended for all concretes that
may come in contact with deicing chemicals (Fig. 8-6).
A good air-void system with a low spacing factor
(maximum of 200 micrometers) is perhaps more important
to deicer environments than saturated frost environments
without deicers. The relationship between spacing factor
and deicer scaling is illustrated in Fig. 8-7. A low water to
portland cement ratio helps minimize scaling, but is not
sufficient to control scaling at normal water-cement ratios.
Rating:
0 = no scaling
5 = severe scaling
4
3
2
ASTM C 672
deicer scaling test
1
0
0
200
400
600
800
µ
Spacing factor,
m
Fig. 8-7. Visual rating as a function of spacing factor, for a
concrete mixture with a water to cement ratio of 0.45 (
Pinto
and Hover 2001
).
Fig. 8-8 illustrates the overriding impact of air content
over water-cement ratio in controlling scaling.
To provide adequate durability and scale resistance in
severe exposures with deicers present, air-entrained con-
crete should be composed of durable materials and have
the following: (1) a low water to cementitious materials
ratio (maximum 0.45), (2) a slump of 100 mm (4 in.) or less
unless a plasticizer is used, (3) a cementitious materials
content of 335 kg/m
3
(564 lb/yd
3
), (4) proper finishing
after bleed water has evaporated from the surface,
(5) adequate drainage, (6) a minimum of 7 days moist
curing at or above 10°C (50°F), (7) a compressive strength
of 28 MPa (4000 psi) when exposed to repeated freeze-
thaw cycling, and (8) a minimum 30-day drying period
after moist curing if concrete will be exposed to freeze-
thaw cycles and deicers when saturated. Target air con-
tents are discussed in “Recommended Air Contents” at
the end of this chapter.
Normal dosages of supplementary cementitious
materials should not effect scaling resistance of properly
designed, placed, and cured concrete (Table 8-2). The
ACI
318
building code allows up to 10% silica fume, 25% fly
ash, and 50% slag as part of the cementitious materials for
deicer exposures. However, abuse of these materials along
2.0
0.4
Air content
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
0
2%
4%
6%
0.3
ASTM C 672
deicer scaling test
0.2
0.1
10
20
30
40
50
Number of cycles
Fig. 8-6. Cumulative mass loss for mixtures with a water
to cement ratio of 0.45 and on-time finishing (
Pinto and
Hover 2001
).
Table 8-2. Deicer Scaling Resistance (Visual Ratings) of Concrete with Selected Supplementary Cementing Materials
Mixture
Control
Fly ash (Class F)
Slag
Calcined shale
Calcined shale
Mass replacement
of cement, %
0
15
40
15
25
Scale rating at
25 cycles
1
1
1
1
1
Scale rating at
50 cycles
2
2
1
2
1
Concrete had 335 kg of cementing materials per cubic meter (565 lb/yd
3
), a Type I portland cement, a water to cementing materials ratio of 0.50,
a nominal slump of 75 mm (3 in.), and a nominal air content of 6%. Test method: ASTM C 672. Results are for specific materials tested in 2000
and may not be representative of other materials. Scale rating: 1 = very slight scaling (3 mm depth maximum) with no coarse aggregate visible,
2 = slight to moderate scaling.
