Civil Engineering Reference
In-Depth Information
concrete from freezing by this method are foolhardy. In-
stead, proven reliable precautions should be taken during
cold weather (see
Chapter 14
, Cold-Weather Concreting).
When used, calcium chloride should be added to the
concrete mixture in solution form as part of the mixing
water. If added to the concrete in dry flake form, all of the
dry particles may not be completely dissolved during
mixing. Undissolved lumps in the mix can cause popouts
or dark spots in hardened concrete.
The amount of calcium chloride added to concrete
should be no more than is necessary to produce the de-
sired results and in no case exceed 2% by mass of cement-
ing material. When calculating the chloride content of
commercially available calcium chloride, it can be
assumed that:
1. Regular flake contains a minimum of 77% CaCl
2
2. Concentrated flake, pellet, or granular forms contain
a minimum of 94% CaCl
2
An overdose can result in placement problems and
can be detrimental to concrete. It may cause: rapid stiffen-
ing, a large increase in drying shrinkage, corrosion of rein-
forcement, and loss of strength at later ages (
Abrams 1924
and
Lackey 1992
).
Applications where calcium chloride should be used
with caution:
1. Concrete subjected to steam curing
2. Concrete containing embedded dissimilar metals,
especially if electrically connected to steel reinforce-
ment
3. Concrete slabs supported on permanent galvanized-
steel forms
4. Colored concrete
Calcium chloride or admixtures containing soluble
chlorides
should not be used
in the following:
1. Construction of parking garages
2. Prestressed concrete because of possible steel corro-
sion hazards
3. Concrete containing embedded aluminum (for exam-
ple, conduit) since serious corrosion of the aluminum
can result, especially if the aluminum is in contact
with embedded steel and the concrete is in a humid
environment
4. Concrete containing aggregates that, under standard
test conditions, have been shown to be potentially
deleteriously reactive
5. Concrete exposed to soil or water containing sulfates
6. Floor slabs intended to receive dry-shake metallic
finishes
7. Hot weather generally
8. Massive concrete placements
The maximum chloride-ion content for corrosion protec-
tion of prestressed and reinforced concrete as recommended
by the
ACI 318
building code is presented in Table 6-3. Re-
sistance to the corrosion of embedded steel is further
improved with an increase in the depth of concrete cover
Table 6-3. Maximum Chloride-Ion Content for
Corrosion Protection of Reinforcement*
Maximum water soluble
chloride-ion (CI
-
) in
concrete, percent by
Type of member
mass of cement
Prestressed concrete
0.06
Reinforced concrete exposed to
chloride in service
0.15
Reinforced concrete that will be
dry or protected from moisture
1.00
in service
Other reinforced concrete
construction
0.30
* Requirements from
ACI 318
tested per ASTM C 1218.
over reinforcing steel, and a lower water-cement ratio. Stark
(1989) demonstrated that concretes made with 1%
CaCl
2
•
2H
2
O by mass of cement developed active steel corro-
sion when stored continuously in fog. When 2% CaCl
2
•
2H
2
O
was used, active corrosion was detected in concrete stored in
a fog room at 100% relative humidity. Risk of corrosion was
greatly reduced at lower relative humidities (50%).
Gaynor
(1998)
demonstrates how to calculate the chloride content of
fresh concrete and compare it with recommended limits.
Several nonchloride, noncorrosive accelerators are
available for use in concrete where chlorides are not
recommended (Table 6-1). However, some nonchloride
accelerators are not as effective as calcium chloride.
Certain nonchloride accelerators are specially formulated
for use in cold weather applications with ambient temper-
atures down to -7°C (20°F).
CORROSION INHIBITORS
Corrosion inhibitors are used in concrete for parking
structures, marine structures, and bridges where chloride
salts are present. The chlorides can cause corrosion of steel
reinforcement in concrete (Fig. 6-16). Ferrous oxide and
ferric oxide form on the surface of reinforcing steel in
concrete. Ferrous oxide, though stable in concrete's alka-
line environment, reacts with chlorides to form complexes
that move away from the steel to form rust. The chloride
ions continue to attack the steel until the passivating oxide
layer is destroyed. Corrosion-inhibiting admixtures chem-
ically arrest the corrosion reaction.
Commercially available corrosion inhibitors include:
calcium nitrite, sodium nitrite, dimethyl ethanolamine,
amines, phosphates, and ester amines. Anodic inhibitors,
such as nitrites, block the corrosion reaction of the chlo-
ride-ions by chemically reinforcing and stabilizing the
passive protective film on the steel; this ferric oxide film is
created by the high pH environment in concrete. The
