Corrosion and Concrete
C
oncrete is compatible with a
great number of foreign materials. As proof of this, take a look at
the floors of almost any manufacturing plant to see how concrete
has resisted the effects of many materials placed in contact with it or
spilled on it. As is the case with all
other construction materials, howe ve r, there are some materials
which will corrode if they are
brought into contact with concrete.
Let’s examine a few of the materials that will corrode on contact with
concrete. A great deal of the corrosion will stem from alkaline solutions in the concrete. These alkaline
solutions are inescapable because
of the chemical composition of
portland cement. It is this chemical
composition—and sometimes the
chemical makeup of the aggregate—that forces a reaction between concrete and certain metals
or glass.
Aluminum
Aluminum is one of the most
chemically active metals. Many aluminum alloys are even more susceptible to corrosion in concrete
than is pure aluminum. In concrete
there are two factors which can result in corrosion severe enough to
Aluminum is compatible with concrete
if it is well protected.
cause extensive deterioration of the
metal. One is the alkalies which actively attack aluminum. If concrete
which is to come into contact with
aluminum is allowed to dry out before the aluminum contact is
made—and then is kept dry after
contact has been made—little or no
corrosion will take place. However,
if aluminum is to be buried in concrete, for example as a conduit in a
slab-on-grade floor, these precautions will be impossible and trouble
is like to occur. Aluminum is also reactive with the halogen salts, one of
which is calcium chloride. Steps
should be taken, therefore, to make
sure that calcium chloride—or an
admixture containing it—is not
used in these cases. Steps should also be taken to prevent the development of electrolytic action because
this would speed up corrosion. It is
usually a good idea to provide a protective coating around any aluminum objects that will come in
contact with concrete. High-alumina cement concrete does not attack
aluminum because a film of hydrated aluminum oxide is formed and
this film appears to be stable in this
environment.
Lead
Although lead is commonly used
as a protective covering for less stable metals, it sometimes corrodes
appreciably in concrete. Characteristically, when lead is exposed to air
it rapidly develops a protective film
of lead carbonate. Once this film has
been developed, it is highly resistant
to corrosion. Unfortunately lead
embedded in concrete cannot develop this protective film and an unprotected lead pipe will often deteriorate to the point where leaks
develop. Usually bituminous felt or
bituminous coatings protect lead
from corrosion. Like aluminum,
lead does not corrode in high-alumina cement concrete.
Zinc
Zinc, like lead, develops a protective covering when exposed to the
atmosphere. This coating makes
zinc highly resistant to corrosion.
However, when embedded in concrete, this film cannot develop. In
addition, zinc is susceptible to the
alkaline solutions present in concrete. Zinc galvanized objects may
be embedded in concrete since the
layer of zinc is quite thin and any resultant corrosion will not be troublesome. Solid zinc objects, however, should not be buried in concrete
without a protective covering because the concrete will probably
spall and crack, especially under
damp conditions.
Copper
Copper often is embedded in
concrete and ordinarily will not give
any trouble. However, if chlorides—
such as calcium chloride—are present in the concrete, corrosion can
result. Although this corrosion
would probably not be extensive it
might cause breaks in small copper
wires.
Cadmium
Cadmium-plated parts will resist
corrosion admirably under normal
conditions, but, when buried in
concrete, severe corrosion often occurs. Once again it is the alkaline solutions that are the culprit. To protect cadmium against corrosion, the
metal should be given the same protective coating as aluminum.
Glass
Although glass is highly resistant
to most acids, some glass is attacked
readily by alkalies. There have been
cases where the glass in windows
glazed directly into concrete wall
units has deteriorated at the contact
points. The severity of this damage
depends upon the alkalies set free in
the concrete and the amount of
moisture present. The amount of
free alkalies in concrete in turn depends primarily upon the alkali
content of the cement used in a mix,
although both the aggregate and the
water could contain substantial
amounts of alkalies. Coatings are
available to protect glass in contact
with concrete. Glass aggregates
have been developed for exposed
aggregate work that are said to be
guaranteed to be non-reactive with
any portland cement.
General considerations
The amount of corrosion caused
by embedding certain metals in
concrete will depend upon how well
specific construction and design
practices are followed. Some materials will corrode so severely that attempts to prevent this corrosion in
the field will be of little use. But corrosion in some materials can be
avoided or retarded by following a
few rules.
Steel, for example, does not ordinarily react with concrete, but some
engineers have experienced severe
corrosion. This usually occurs be-
cause strong galvanic corrosion
cells are created at the pipe surface
when a steel pipe comes into contact with an electrically conducting
electrolyte that has variations in either composition or concentration.
It is not a good idea to lay pipe directly on a sand, earth or vermiculite base layer and then cast the
concrete over it. The pipe in this situation makes contact with the base
material and this contact can set up
a flow of corrosion-causing current.
Corrosion results from the difference in voltage between the concrete and the metal. If you have ever
experienced any problems of this
type you probably noticed that the
leaks occurred at the bottom of the
pipes.
This contact between the pipes
and the base material can be avoided in several ways:
The pipes can be supported on
small concrete blocks or chips.
The pipes can be raised on steel
reinforcing bar supports.
A layer of concrete can be cast
and allowed to harden or partially
harden before positioning the pipes
on the slab. The remaining concrete
is then cast. This technique probably would be restricted to fairly thick
slabs with low-slump concrete.
Wood blocks should not be used
because they can cause corrosion.
Aggregate size is also a consideration. If the clearance below
pipework will be, for example, 1 1/2
inches, the maximum aggregate size
should be considerably less than 1
1/2 inches to insure that honeycombing will not occur under the
pipes.
As can be deduced from this short
study of corrosion and concrete,
certain materials should not be
placed in or adjacent to concrete
unless the possibility of corrosion
has been thoroughly investigated.
PUBLICATION #C640035
Copyright © 1964, The Aberdeen Group
All rights reserved