Concrete cancer – what is it and what does it mean?

February 2, 2015

Concrete cancer is a term you’ll often hear in the building industry.  And, just as cancer is something you don’t want to discover in yourself, neither do you want to find it in your concrete.  So what is it, and what does it mean?

Just like cancer in humans, cancer in concrete starts out as an invisible disease.  By the time it starts to display symptoms and you actually find out about it, it’s been present and growing for a long time.  However, human cancer and concrete cancer share another common aspect:  If you find it and treat it early, you’ll have a much better chance of success.

Ironically, despite the name, concrete cancer isn’t actually a problem with the concrete.  Rather, it’s a problem with the steel reinforcement that is encased within the concrete.  Concrete slabs, beams, and columns all work and support load by combining concrete and steel in composite action.   Steel reinforcement is to concrete what a skeleton is to a human body – it’s the strong element that helps support loads and gives strength.

Unfortunately, steel rusts.  It is prone to corrosion.  When pouring a concrete member (such as a slab), it’s critical that the steel reinforcing bars are encased in the concrete with sufficient cover around them to protect the steel from the elements.  Concrete is actually permeable, and so moisture and airborne salts (particularly near the ocean) can penetrate the concrete surface and permeate down to the encased steel.  Once the moisture reaches the encased steel (particularly if the moisture contains salts), you have a recipe for corrosion.

Once the steel starts to rust, it expands and deforms.  The expanding steel can often break or blow out the concrete, causing pieces of the concrete slab to fall away.  This is referred to as “spalling”.  Naturally, this then increases the steel’s exposure, and the problem is exacerbated.  Whilst the problem may initially appear to be an aesthetic or cosmetic concern, the reality is more sinister.  The steel bars resist the tension forces in the concrete, and the cross-sectional area of each reinforcing bar works hard to resist the load it has to carry.  As the steel rusts away, the effective cross-sectional area of the bar reduces, until – eventually – there is not enough steel remaining intact to resist the forces.  At that point, the slab (or beam or column or stair, etc) is now greatly weakened and at risk of sudden failure or collapse.

This exterior staircase in the Sydney suburb of Paddington was poured with insufficient cover to the bottom reinforcement. The rusting bars have expanded, breaking off the entire soffit of the stair flight.[/caption]

Engineers work to prevent concrete cancer initiating by designing the concrete appropriately for the surrounding environment and its respective exposure condition.   For example, a concrete slab that is wholly enclosed in a building out in Sydney’s west (say, Penrith) will be subject to a very different environmental condition than a slab that is external, exposed to the elements, and within a few hundred metres of breaking surf in Sydney’s east.  So it’s horses for courses.  As such, the Australian design code for concrete, AS3600, gives strict requirements as to the concrete strength that needs to be specified, and the amount of cover that has to be provided to the steel reinforcement – all depending on the environment.  Areas in aggressive environments (such as suburbs by the sea) require a stronger grade of concrete, and require that the steel reinforcement is placed further within the slab, so as to increase the protective cover of concrete around the bars.  Conversely, concrete in benign areas can afford to be of a weaker grade, and the bars don’t need as much protective cover of concrete to surround them.

This concrete downturn beam to a balcony in Drummoyne featured reinforcing bars placed too close to the surface, thus triggering corrosion that broke off the concrete.

Unfortunately, this awareness of the durability of concrete and the risk of corrosion is something that was less well understood by the construction industry in decades gone by.   Builders were notorious for placing the bars with inadequate cover, and engineers probably weren’t particularly vigilant at picking this up at the time of the pre-pour inspection.  As such, apartment buildings and other concrete structures constructed in the 1950’s and 1960’s (even into the 70’s and 80’s) were typically built with very little cover to the reinforcement, and thus the steel started to corrode over time.  Another catalyst was the use of magnesite toppings on the floor slabs, which – over time – leached chloride ions into the concrete, thus triggering significant corrosion of the reinforcement, as evidenced in the photograph below.   There are literally hundreds, if not thousands, of apartment buildings all over Sydney with concrete cancer in varying degrees of progress.  It’s an issue that keeps the Remedial division of Partridge incredibly busy.

This extreme concrete cancer was actually initiated by the magnesite floor topping. Rain water came in through the balcony door, wetting the magnesite that then leached aggressive chloride ions into the concrete, leading to major corrosion issues.

So how can you detect if your building has concrete cancer?  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 (referred to as spalling concrete).

Remember, concrete cancer is really just rusting of steel reinforcement.  So, once it’s detected, all you need to do is treat and repair the steel.  Whilst that sounds easy enough, the reality is that it is a very slow, tedious, time-consuming, and expensive process.  It requires that the concrete be broken back and removed around the reinforcement, so as to fully expose each affected bar.   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.  The next step is to thoroughly coat the steelwork with a protective epoxy coating.  Clearly, we can’t push the bar “further back” to improve its coverage, so we instead rely on protective coatings to act as a barrier.  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, you can have confidence that the concrete will then continue to perform satisfactorily and do its job in the years to come.

These days, builders or building specifiers (such as engineers or architects) occasionally take additional steps to further reduce the risk of concrete cancer occurring further down the track.  This is good practice in areas that are particularly high-risk (such as beach-side suburbs) or with buildings that feature exposed concrete facades.  One of the most commonly adopted practices today is to add a waterproofing admixture to the concrete at the time of batching and pouring.  There are two main products on the market – Caltite & Xypex.  These two products work in different ways and utilise different technology, but both serve to ensure that moisture and salts don’t penetrate through the concrete matrix and reach the steel.   Another – albeit more expensive solution – is to galvanise all of the reinforcement.  Galvanising effectively adds a protective coating to the steel, so that it is resistant to moisture and chemical attack.

Just like cancer in humans, when it comes to concrete cancer, prevention is the best cure.  But, should you own a building that is already exhibiting signs, the best course of action is to act quickly and treat the problem.  At present, Partridge looks after about 50 to 75 buildings every year that are showing signs of concrete cancer.  Our Remedial division carries out inspections and prepares reports on concrete cancer in buildings, and even sets out the repair methodologies and oversees and project manages the repair process.   If you’ve got suspicions about your building, or you’d like further information on this, please don’t hesitate to contact our Remedial division.

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