Frequently Asked Questions
Q: In the design stage for a new high rise condominium
project the architect has suggested the use of aluminum railings
for the balconies. I had heard that this material may have some
risks for this type of application. Can you provide some guidance
on this issue?
A: Aluminum is an amphoteric material,
meaning it will react chemically with either an acid or an alkali.
When portland cement is hydrating it releases free calcium hydroxide
which is a very strong alkali. The pH of a fresh concrete mix can
be more than 13. Consequently any material that reacts chemically
with an alkali should be carefully considered if it is to be embedded
in or placed adjacent to fresh concrete.
Aluminum posts and flashings are commonly used adjacent to or embedded
in concrete that will be dry in service. While there is a reaction
between the aluminum and the fresh concrete, the reaction slows
or stops as the concrete hardens and dries. In addition it is common
for the aluminum to be coated with a protective barrier to prevent
the alkalinity of the concrete from reaching the aluminum surface.
this material is to be used in an exterior application such as a
balcony the risks of problems increases considerably for two reasons.
First the concrete is exposed to multiple wet cycles that will transport
alkalinity in the concrete to the surface of the aluminum which
continues to corrode the aluminum. Secondly, steel reinforced concrete
used as the structural component of the balcony would produce a
galvanic cell (a galvanic cell is created when dissimilar metals,
each having a different thermodynamic instability, are placed in
contact with each other) leading to an accelerated rate of corrosion.
In general placing aluminum in or next to concrete for exterior
applications should be avoided due to the increased risk of corrosion
and the accompanying concrete cracking issues caused by the increased
volume of the metal as it corrodes.
Note that chloride based accelerators should never be used in concrete
that contains any steel reinforcement or metal embeds.
Corrosion of Embedded
Material Other Than Reinforcing Steel (RX198)
Linberg, R.I., “Aluminum
in concrete,” Concrete Construction
aluminum and concrete controversy,” Concrete Construction
More information on corrosion.