Civil Engineering Reference
In-Depth Information
on the reinforcement; they combine with iron ions to form a
soluble iron chloride complex that carries the iron into the
concrete for later oxidation (rust) (
Whiting 1997
,
Taylor,
Whiting, and Nagi 2000
and
Whiting, Taylor and Nagi 2002
).
The resistance of concrete to chloride is good; however,
it can be increased by using a low water-cement ratio (0.40),
at least seven days of moist curing, and supplementary
cementitious materials, such as silica fume, to reduce
permeability. Increasing the concrete cover over the steel
also helps slow down the migration of chlorides.
Other methods of reducing steel corrosion include the
use of corrosion inhibiting admixtures, epoxy-coated rein-
forcing steel (ASTM D 3963 or AASHTO M 284), surface
treatments, concrete overlays, and cathodic protection.
Epoxy-coated reinforcing steel works by preventing
chloride ions from reaching the steel (Fig. 1-30). Surface
treatments and concrete overlays attempt to stop or reduce
chloride ion penetration at the concrete surface. Silanes,
siloxanes, methacrylates, epoxies, and other materials are
used as surface treatments.
Impermeable materials, such as most epoxies, should
not be used on slabs on ground or other concrete where
moisture can freeze under the coating. The freezing water
can cause surface delamination under the impermeable
coating. Latex-modified portland cement concrete, low-
slump concrete, and concrete with silica fume are used in
overlays to reduce chloride-ion ingress.
Cathodic protection methods reverse the corrosion
current flow through the concrete and reinforcing steel.
This is done by inserting a nonstructural anode in the
concrete and forcing the steel to be the cathode by electri-
cally charging the system. The anode is connected to the
positive pole of a rectifier. Since corrosion occurs where the
current leaves the steel, the steel cannot corrode if it is
receiving the induced current.
Chloride present in plain concrete (not containing
steel) is generally not a durability concern.
Corrosion of nonferrous metals in concrete is dis-
cussed by
Kerkhoff (2001)
.
Chemical Resistance
Portland cement concrete is resistant to most natural envi-
ronments; however, concrete is sometimes exposed to
substances that can attack and cause deterioration. Concrete
in chemical manufacturing and storage facilities is especially
prone to chemical attack. The effect of sulfates and chlorides
is discussed in this chapter. Acids attack concrete by dissolv-
ing cement paste and calcareous aggregates. In addition to
using concrete with a low permeability, surface treatments can
be used to keep aggressive substances from coming in contact
with concrete.
Effects of Substances on Concrete and Guide to
Protective Treatments
(
Kerkhoff 2001
) discusses the effects of
hundreds of chemicals on concrete and provides a list of
treatments to help control chemical attack.
Sulfate Attack
Excessive amounts of sulfates in soil or water can attack
and destroy a concrete that is not properly designed.
Sulfates (for example calcium sulfate, sodium sulfate, and
magnesium sulfate) can attack concrete by reacting with
hydrated compounds in the hardened cement paste. These
reactions can induce sufficient pressure to disrupt the
cement paste, resulting in disintegration of the concrete
(loss of paste cohesion and strength). Calcium sulfate
attacks calcium aluminate hydrate and forms ettringite.
Sodium sulfate reacts with calcium hydroxide and calcium
aluminate hydrate forming ettringite and gypsum.
Magnesium sulfate attacks in a manner similar to sodium
sulfate and forms ettringite, gypsum, and also brucite
(magnesium hydroxide). Brucite forms primarily on the
concrete surface; it consumes calcium hydroxide, lowers
the pH of the pore solution, and then decomposes the
calcium silicate hydrates (
Santhanam and others 2001
).
Thaumasite may form during sulfate attack in moist
conditions at temperatures usually between 0°C and 10°C
(32°F to 50°F) and it occurs as a result of a reaction between
calcium silicate hydrate, sulfate, calcium carbonate, and
water (Report of the Thaumasite Expert Group 1999). In
concretes where deterioration is associated with excess
thaumasite formation, cracks can be filled with thaumasite
and haloes of white thaumasite are present around aggre-
gate particles. At the concrete/soil interface the surface
concrete layer can be “mushy” with complete replacement
of the cement paste by thaumasite (
Hobbs 2001
).
Like natural rock formations such as limestone,
porous concrete is susceptible to weathering caused by salt
crystallization. These salts may or may not contain sulfates
and they may or may not react with the hydrated
compounds in concrete. Examples of salts known to cause
weathering of field concrete include sodium carbonate and
sodium sulfate (laboratory studies have also related satu-
rated solutions of calcium chloride and other salts to
concrete deterioration). The greatest damage occurs with
drying of saturated solutions of these salts, often in an envi-
ronment with specific cyclic changes in relative humidity
and temperature that alter mineralogical phases. In perme-
able concrete exposed to drying conditions, salt solutions
Fig. 1-30. Epoxy-coated reinforcing steel used in a bridge
deck. (69915)
