Civil Engineering Reference
In-Depth Information
soluble chloride ion level at which steel reinforcement
corrosion begins in concrete is about 0.2% to 0.4% by mass
of cement (0.15% to 0.3% water soluble). Of the total chlo-
ride-ion content in concrete, only about 50% to 85% is
water soluble; the rest becomes chemically combined in
cement reactions (
Whiting 1997
,
Whiting, Taylor, and Nagi
2002
, and
Taylor, Whiting, and Nagi 2000
).
Chlorides can be introduced into concrete with the
separate mixture ingredients—admixtures, aggregates,
cementitious materials, and mixing water—or through
exposure to deicing salts, seawater, or salt-laden air in
coastal environments. Placing an acceptable limit on chlo-
ride content for any one ingredient, such as mixing water,
is difficult considering the several possible sources of chlo-
ride ions in concrete. An acceptable limit in the concrete
depends primarily upon the type of structure and the
environment to which it is exposed during its service life.
A high dissolved solids content of a natural water is
sometimes due to a high content of sodium chloride or
sodium sulfate. Both can be tolerated in rather large quan-
tities. Concentrations of 20,000 ppm of sodium chloride
are generally tolerable in concrete that will be dry in serv-
ice and has low potential for corrosive reactions. Water
used in prestressed concrete or in concrete that is to have
aluminum embedments should not contain deleterious
amounts of chloride ion. The contribution of chlorides
from ingredients other than water should also be consid-
ered. Calcium chloride admixtures should be avoided in
steel reinforced concrete.
The
ACI 318
building code limits water soluble chlo-
ride ion content in reinforced concrete to the following
percentages by mass of cement:
Prestressed concrete
OTHER COMMON SALTS
Carbonates of calcium and magnesium are not very solu-
ble in water and are seldom found in sufficient concentra-
tion to affect the strength of concrete. Bicarbonates of
calcium and magnesium are present in some municipal
waters. Concentrations up to 400 ppm of bicarbonate in
these forms are not considered harmful.
Magnesium sulfate and magnesium chloride can be
present in high concentrations without harmful effects on
strength. Good strengths have been obtained using water
with concentrations up to 40,000 ppm of magnesium chlo-
ride. Concentrations of magnesium sulfate should be less
than 25,000 ppm.
IRON SALTS
Natural ground waters seldom contain more than 20 to 30
ppm of iron; however, acid mine waters may carry rather
large quantities. Iron salts in concentrations up to 40,000
ppm do not usually affect concrete strengths adversely.
MISCELLANEOUS INORGANIC SALTS
Salts of manganese, tin, zinc, copper, and lead in mixing
water can cause a significant reduction in strength and
large variations in setting time. Of these, salts of zinc,
copper, and lead are the most active. Salts that are espe-
cially active as retarders include sodium iodate, sodium
phosphate, sodium arsenate, and sodium borate. All can
greatly retard both set and strength development when
present in concentrations of a few tenths percent by mass
of the cement. Generally, concentrations of these salts up
to 500 ppm can be tolerated in mixing water.
Another salt that may be detrimental to concrete is
sodium sulfide; even the presence of 100 ppm warrants
testing. Additional information on the effects of other salts
can be found in the references.
0.06%
Reinforced concrete exposed to
chloride in service
0.15%
Reinforced concrete that will be dry or
protected from moisture in service 1.00%
Other reinforced concrete construction 0.30%
ACI 318
does not limit the amount of chlorides in
plain concrete, that is concrete not containing steel.
Additional information on limits and tests can be found in
ACI 222
,
Corrosion of Metals in Concrete.
The acid-soluble
and water-soluble chloride content of concrete can be
determined by using ASTM C 1152 and C 1218.
SEAWATER
Seawater containing up to 35,000 ppm of dissolved salts is
generally suitable as mixing water for concrete not
containing steel. About 78% of the salt is sodium chloride,
and 15% is chloride and sulfate of magnesium. Although
concrete made with seawater may have higher early
strength than normal concrete, strengths at later ages
(after 28 days) may be lower. This strength reduction can
be compensated for by reducing the water-cement ratio.
Seawater is not suitable for use in making steel rein-
forced concrete and it should not be used in prestressed
concrete due to the risk of corrosion of the reinforcement,
particularly in warm and humid environments. If seawa-
ter is used in plain concrete (no steel) in marine applica-
SULFATE
Concern over a high sulfate content in mix water is due to
possible expansive reactions and deterioration by sulfate
attack, especially in areas where the concrete will be
exposed to high sulfate soils or water. Although mixing
waters containing 10,000 ppm of sodium sulfate have
been used satisfactorily, the limit in Table 4-3 should be
considered unless special precautions are taken.
