Civil Engineering Reference
In-Depth Information
0.7
Type II cement, alkali = 1.00%
Class F fly ash
Rhyolitic reactive aggregate
ASTM C 227 mortar bars
0.6
0.5
0.4
0.3
0.2
Failure criterion
0.1
0
0
10
20
30
40
Fly ash dosage, percent
Fig. 1-29. Carbonation destroys concrete's ability to protect
embedded steel from corrosion. All concrete carbonates to
a small depth, but reinforcing steel must have adequate
concrete cover to prevent carbonation from reaching the
steel. This reinforcing bar in a wall had less than 10 mm (0.4
in.) of concrete cover; the ACI building code requires a
minimum of 38 mm (1
1
⁄
2
in.). After years of outdoor exposure,
the concrete carbonated to the depth of the bar, allowing the
steel to rust and spall the concrete surface. (68340)
Fig. 1-28. Certain fly ashes at appropriate dosages can
control alkali-silica reactivity.
See
Farny and Kosmatka (1997)
for more information
on alkali-silica reaction and alkali-carbonate reaction.
Carbonation
ACI 318 building code, provides reinforcing steel cover
requirements for different exposures.
Carbonation of concrete is a process by which carbon diox-
ide in the ambient air penetrates the concrete and reacts
with the hydroxides, such as calcium hydroxide, to form
carbonates (
Verbeck 1958
). In the reaction with calcium
hydroxide, calcium carbonate is formed. Carbonation and
rapid drying of fresh concrete may affect surface durability,
but this is prevented by proper curing. Carbonation of
hardened concrete does not harm the concrete matrix.
However, carbonation significantly lowers the alkalinity
(pH) of the concrete. High alkalinity is needed to protect
embedded steel from corrosion; consequently, concrete
should be resistant to carbonation to help prevent steel
corrosion.
The amount of carbonation is significantly increased in
concrete that has a high water to cement ratio, low cement
content, short curing period, low strength, and highly
permeable (porous) paste. The depth of carbonation in
good-quality, well-cured concrete is generally of little prac-
tical significance as long as embedded steel has adequate
concrete cover (Fig. 1-29). Finished surfaces tend to have
less carbonation than formed surfaces. Carbonation of
finished surfaces is often observed to a depth of 1 to 10 mm
(0.04 to 0.4 in.) and for formed surfaces, between 2 and 20
mm (0.1 and 0.9 in.) after several years of exposure,
depending on the concrete properties, ingredients, age, and
environmental exposure (
Campbell, Sturm, and Kosmatka
1991
). ACI 201.2R,
Guide to Durable Concrete,
has more
information on atmospheric and water carbonation and the
Chloride Resistance and Steel Corrosion
Concrete protects embedded steel from corrosion through its
highly alkaline nature. The high pH environment in concrete
(usually greater than 12.5) causes a passive and noncorrod-
ing protective oxide film to form on steel. However, the pres-
ence of chloride ions from deicers or seawater can destroy or
penetrate the film. Once the chloride corrosion threshold
(about 0.15% water-soluble chloride by mass of cement) is
reached, an electric cell is formed along the steel or between
steel bars and the electrochemical process of corrosion
begins. Some steel areas along the bar become the anode,
discharging current in the electric cell; from there the iron
goes into solution. Steel areas that receive current are the
cathodes where hydroxide ions are formed. The iron and
hydroxide ions form iron hydroxide, FeOH, which further
oxidizes to form rust (iron oxide). Rusting is an expansive
process—rust expands up to four times its original
volume—which induces internal stress and eventual
spalling of the concrete over reinforcing steel. The cross-
sectional area of the steel can also be significantly reduced.
Once it starts, the rate of steel corrosion is influenced by
the concrete's electrical resistivity, moisture content, and the
rate at which oxygen migrates through the concrete to the
steel. Chloride ions alone can also penetrate the passive film
