I conducted an inspection on 2nd October and found a prime example of concrete cancer to an entrance verandah slab – see photo.
What Causes Concrete Cancer?
Concrete cancer on the Gold Coast 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 hydroxide.
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.