Civil Engineering Reference
In-Depth Information
Seawater Exposures
Concrete has been used in seawater exposures for decades
with excellent performance. However, special care in mix
design and material selection is necessary for these severe
environments. A structure exposed to seawater or seawater
spray is most vulnerable in the tidal or splash zone where
there are repeated cycles of wetting and drying and/or
freezing and thawing. Sulfates and chlorides in seawater
require the use of low permeability concrete to minimize
steel corrosion and sulfate attack (Fig. 1-34).
A cement resistant to moderate sulfate exposure is
helpful. Portland cements with tricalcium aluminate (C
3
A)
contents that range from 4% to 10% have been found to
provide satisfactory protection against seawater sulfate
attack, as well as protection against corrosion of reinforce-
ment by chlorides. Proper concrete cover over reinforcing
steel must be provided (see ACI 318). Water-cementitious
material ratios should not exceed 0.40. In northern
climates, the concrete must be properly air entrained with
at least 6% air. High-strength concrete should be consid-
ered where large ice formations abrade the structure. See
Stark (1995 and 2001)
,
Farny (1996)
, and
Kerkhoff (2001)
.
Fig. 1-35. White secondary ettringite deposits in void. Field
width 64
µ
m. (69547)
fate within 24 hours (
Klemm and Miller 1997
). At this stage
ettringite is uniformly and discretely dispersed throughout
the cement paste at a submicroscopic level (less than a
micrometer in cross-section). This ettringite is often called
primary ettringite.
If concrete is exposed to moisture for long periods of
time (many years), the ettringite can slowly dissolve and
reform in less confined locations. Upon microscopic exam-
ination, harmless white needle-like crystals of ettringite can
be observed lining air voids. This reformed ettringite is
usually called secondary ettringite (Fig. 1-35).
Concrete deterioration accelerates the rate at which
ettringite leaves its original location in the paste to go into
solution and recrystallize in larger spaces such as air voids
or cracks. Both water and sufficient space must be present
for the crystals to form. Cracks can form due to damage
caused by frost action, alkali-aggregate reactivity, drying
shrinkage, thermal effects, strain due to excessive stress, or
other mechanisms.
Ettringite crystals in air voids and cracks are typically
two to four micrometers in cross section and 20 to 30
micrometers long. Under conditions of extreme deteriora-
tion or decades in a moist environment, the white ettringite
crystals can appear to completely fill voids or cracks.
However, secondary ettringite, as large needle-like crystals,
should not be interpreted as being harmful to the concrete
(
Detwiler and Powers-Couche 1997
).
Heat Induced Delayed Expansion.
Heat induced delayed
expansion (HIDE)—also called delayed ettringite forma-
tion (DEF)—refers to a rare condition of internal sulfate
attack* in which mature concretes undergo expansion and
Fig. 1-34. Concrete bridges in seawater exposure must be
specially designed for durability. (68667)
Ettringite and Heat Induced
Delayed Expansion
Ettringite, one form of calcium sulfoaluminate, is found in
all portland cement paste. Calcium sulfate sources, such as
gypsum, are added to portland cement during final grind-
ing at the cement mill to prevent rapid setting and improve
strength development. Sulfate is also present in supple-
mentary cementitious materials and admixtures. Gypsum
and other sulfate compounds react with calcium aluminate
in cement to form ettringite within the first few hours after
mixing with water. Most of the sulfate in cement is
normally consumed to form ettringite or calcium monosul-
* Internal sulfate attack refers to deterioration mechanisms occurring in
connection with sulfate that is in the concrete at the time of placement.
