Concrete Cancer & Concrete Spalling Explanation

What is Concrete Cancer?

Concrete Cancer is caused by many factors:

Factors Include: carbonation, moisture, efflorescence and salt.

When concrete cracks, water and water-borne corrosive pollutants penetrate through the cracks causing the steel reinforcement deep inside to rust. Rusting steel then sheds its skin forcing the layers of rust to expand and push away the concrete surrounding it.  In some cases the very presence original untreated steel reinforcement as it expands during the corrosion process is very cause of those cracks that then serve to feed and exacerbate the problem.

This results in large chunks of concrete (constantly causing property and personal injury as in the case of the Franklin D. Roosevelt Freeway on the East River side of NYC and a whole raft of similar decaying infrastructures throughout the world) or smaller pieces (as in the case of domestic/suburban homes, driveways, swim pools, etc. ) cracking and/or falling away [See Photographs] thus allowing steel reinforcement to become even more corroded which leads devastating structural weakness (wear) of both the steel and the concrete.


On the outside of the affected concrete, rust stains are visable, broken concrete or even rusted steel reinforcement protruding through the concrete structure, i.e. roadway, pavement, bridge, pylon, wharf, dam wall, concrete pipe, walkway, eave, awning, facade, garage, flat roofs, balcony edges etc.

However, what is not detectable by the human eye is the spread of the “cancer” throughout the concrete, which will continue unabated if left untreated.

Concrete Cancer and Spalling Repair

Concrete cancer is caused when the steel reinforcing within a concrete slab begins to rust. As steel rusts it can expand up to 7 times its original size causing the surrounding concrete to be displaced and become flakey. As the steel pushes the concrete away, more water and corrosive pollutants come into direct contact with the steel expediting the process. The process is often referred to as ‘concrete spalling’.  The practice of continually washing/hosing, as is being carried out in many domestic situations and natural rain-water / snow melt (containing road salt & petroleum residuals) in civil structures and infrastructures  only serves to exacerbate the problem.

Tell-tale signs of Spalling Concrete

Unless one is a structural engineer, or have seen the signs before, chances are signs of spalling are not recognized and completely disregarded, even when it’s time to organise concrete cancer repair work. Here are a few key signs indicating the need for structural repair.

  • Flaking and cracking concrete (concrete spalling)
  • Chunks of concrete actually separating and completely dislodging
  • Rust stains which seem to leak out from within the concrete
  • Bubbling of concrete render or
  • Leaks which appear in the roof or internal walls

As often as not, observers mistake these signs as general weathering and dilapidation, caused by the elements; but whilst this is true to some extent and may play a part in exacerbating the problem, the real problem lies within the steel reinforcing within concrete itself.

Risks of Spalled Concrete

Whilst all will agree that spalled concrete is an eye-sore, what many people fail to realise is that as time goes on, concrete cancer also represents a potential danger of injury and in some case even death as well as being an OH&S hazard and this can lead to serious and costly litigation/damages issues. Over time, and with increased exposure to the elements, untreated pieces of concrete may fall from the structure. If these fall from heights in high traffic areas, then passers-by will be at risk of injury, and other buildings in proximity, at risk of damage as well.

Spalling and delaminated concrete should be repaired immediately, as deferring the decision to treat it will inevitably lead to increased problems into the future.

How Concrete Cancer Begins

The process can start in many ways, but generally it is due to one of the following:

  • Poorly treated reinforcing steel being used when the concrete is poured
  • The ends of the reinforcing being too close to the surface. As water seeps through the concrete it picks up limestone and other corrosive chemicals.
  • When it comes into contact with the steel it causes oxidation to occur in the form of rust
  • Incompatible metals being used in close proximity to each other, thus causing a reaction which allows water into the slab
  • Stress fractures from bearing weight or general wear and tear allow water to penetrate the concrete and react with the steel
  • Poorly designed/constructed structures trapping/directing or puddling of the elements causing rusting to occur

Furthermore, any attempts to fix the problem for the short-term (such as rendering over the problem) will only exacerbate the issue. Despite, potentially looking better, the rusting process will continue below the surface causing the steel to again displace the concrete and in some cases rust so badly the steel eventually needs replacement. Treatment and repair of the concrete is the best solution.

Concrete Cancer Repairs

The M.A.C. team have a certain expertise in this field, however at the present time we do not undertake the actual remedial processes.   We are a manufacturer and supplier of the remedial product.

Ideally you should request your ‘Repairer’ provide you with a scope of works, a repair methodology and an estimate before they begin the process of repairing your concrete. In a brief summary, the repairer should remove all ‘drummy’ concrete and brick work, grind back the reinforcing steel to remove all rust and treat the steel with anti-corrosive membranes (or replace the steel if required).  Once the steel underneath has been treated they should then reinstate the actual concrete and the visual component (bricks or render) to restore the original facade.  It may also require that the entire building be further protected with a ‘cathodic device’.  The MAC Concrete Cancer Products are one hundred percent compatible with ‘cathodic devices’.

Where the issue has originated in a structure that is exposed to flooding / trafficking of liquids / liquid containment / puddling, it is likely that waterproofing will also need to be undertaken.

Once the rusted reinforcing has been repaired, application of a waterproofing membrane will help prevent the ingress of moisture to the slab and prolong the life of the steel and therefore the structure itself.


“Sadly, as often as not, fixing concrete cancer is just one part of the problem. Depending on the nature of your building, its age or the cause of the original problem, you may need more than just spalling repair. “