Concrete Cancer

I conducted an inspection on 2nd October and found a prime example of
concrete cancer to an entrance verandah slab- see photo.
Concrete cancer is usually caused by placing the steel reinforcement too close
to the external slab edges- usually less that 50mm. Water penetration causes
the concrete reinforcement to rust and expand which in turn creates stresses
on the surrounding concrete which can then spall (break away). The initial
cause of concrete cancer is usually water penetration. When calcium oxide
reacts with water that penetrates the concrete it forms a solution of calcium


Concrete cancer can be treated in some structures. In order to repair the
damaged areas, the spalled concrete must be removed and any exposed steel
must either be replaced or cleaned and treated with rust inhibitor. The area is
then repaired to the original concrete profile by coating the exposed area with
Bondcrete and using cement mortar, epoxy mortar or concrete, depending on
the size of the damage and the structural requirements.
Cracks are repaired using suitable epoxy resins, special mortars and injection
techniques. This process is referred to as ‘crack injection’ and may constitute a
negative membrane. Negative membranes will not prevent water from
entering the concrete, merely shift the water’s direction through the slab.
Treatment of concrete cancer must incorporate proper waterproofing or risk
being a temporary solution.
Concrete cancer is increasingly common in structures which have not been
sufficiently waterproofed. As builders cut costs on waterproofing membranes,
the problems are increasing. The incidence of concrete cancer is particularly
high in countries such as Australia where liquid applied membranes are still
commonly used. Liquid membranes are often used inappropriately, and lead to
water penetrating into the concrete. If early symptoms including the presence
of calcium stalactites beneath the slab and visible rust from the slab are
apparent, a stop leak specialist should be called immediately to assess the
potential for damage.

If you have any questions or concerns about this or any other building matter, you can call Matt King at BPI Building and Pest Inspections on 07 4151 1613 visit or email us on

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