Civil Engineering Reference
In-Depth Information
can rise to the surface by capillary action and subse-
quently—as a result of surface evaporation—the solution
phase becomes supersaturated and salt crystallization
occurs, sometimes generating pressures large enough to
cause cracking. If the rate of migration of the salt solution
through the pores is lower than the rate of evaporation, a
drying zone forms beneath the surface and salt crystalliza-
tion occurs in the pores causing expansion and scaling
(
Mehta 2000
). Both aggregate particles and cement paste
can be attacked by salts.
For the best defense against external sulfate attack:
(1) design concrete with a low water to cementitious mate-
rials ratio (around 0.4), and (2) use cements specially
formulated for sulfate environments, such as ASTM C 150
(AASHTO M 85) Type II or V cements, C 595 (AASHTO M
240) moderate sulfate resistant cements, or C 1157 Types
MS or HS. The superior sulfate resistance of ASTM C 150
Type II and V cements is shown in Fig. 1-33.
1.0
Sulfate attack and
salt crystallization are
more severe at locations
where the concrete is
exposed to wetting and
drying cycles, than con-
tinuously wet exposures.
This is often seen in
concrete posts where the
concrete has deteriorated
only a few centimeters
above and below the soil
line. The portion of con-
crete deep in the soil
(where it is continuously
wet) is in good condi-
tion (Fig. 1-31 and 1-32).
However, if the sul-
fate exposure is severe
enough, with time even
the continuously moist sections can be attacked by sulfates
if the concrete is not properly designed.
Rating:
1.0 = no deterioration
5.0 = severe deterioration
2.0
ASTM Type I
ASTM Type II
ASTM Type V
3.0
Fig. 1-31. Sulfate attack is often
the most severe at the location
of the most wetting and drying,
which is usually near the soil
line. Here concrete posts have
been attacked by sulfates near
the soil line. Also see the inset
in Fig. 1-32. The concrete is in
better condition deep within the
soil where it is moist. (43093)
4.0
5.0
0.3
0.4
0.5
0.6
0.7
0.8
Water-cement ratio by mass
Fig. 1-33. Average 16-yr ratings of concrete beams in sulfate
soils for three portland cements at various water-cement
ratios (
Stark 2002
).
Fig. 1-32. Concrete beams after seven years of exposure to sulfate-rich wet soil in a Sacramento, California, test plot. The beams
in better condition have low water-cementitious materials ratios, and most have sulfate resistant cement. The inset shows two
of the beams tipped on their side to expose decreasing levels of deterioration with depth and moisture level. (66900, 58499)
