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Concrete cancer is a term you’ll often hear in the building industry. So, what is it, and what does it mean?
By the time you become aware that concrete cancer exists, it’s been present and growing for a long time. However, if found early and treated there is a much better chance of success.
Concrete slabs, beams, and columns are designed using a combining concrete and steel reinforcement to support the loads being imposed on them. Unfortunately, steel rusts and is prone to corrosion so it’s critical that the steel reinforcing are encased in the concrete with sufficient cover around them to protect the steel from the elements.
Concrete is actually permeable, so moisture and airborne salts can penetrate the concrete surface and permeate down to the encased steel. If the moisture reaches the encased steel it can start to corrode.
Once the steel starts to rust, it expands and deforms causing the concrete to break or blow out and fall away. This is known as “spalling”.
The size and cross-sectional area of the steel reinforcement bars are designed to resist the forces they must carry. As the steel rusts away, the effective cross-sectional area of the bar reduces, until there is not enough steel remaining intact to resist the forces.
The Australian Standard for concrete, AS 3600-Concrete Structures, gives strict requirements as to the concrete strength that needs to be specified, and the amount of cover that must be provided to the steel reinforcement depending on the environment the concrete member is exposed to.
One of the first tell-tale signs is the presence of ferrous stains on the surface of the concrete. If the steel is rusting within the concrete, rusty water can bleed off the steel towards the surface, leaving brown-tinged stains on the surface. If the corrosion has advanced to the point where the steel is expanding, it will then typically start to cause the concrete to burst or break off.
As concrete cancer is just rusting of steel reinforcement, all you need to do is treat and repair the steel. It requires the concrete to be chipped back and removed from around the reinforcement, to fully expose the affected bars. A typical repair procedure is to wire-brush the bars to remove all traces of rust and to take them back to bare, shiny metal. This is a very slow, tedious, time-consuming, and expensive process.
Then the steel reinforcement needs to be thoroughly coated with a protective epoxy coating. Once all the affected steel bars have been adequately coated, the concrete can then be repaired with a suitable repair mortar product. Providing the repair process is carried out thoroughly and appropriately, the concrete should continue to perform satisfactorily.
One of the most common practices used today to reduce the risk of concrete cancer occurring further down the track is to add a waterproofing admixture to the concrete at the time of batching and pouring.
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