Concrete cancer – what you need to know

Concrete cancer – what you need to know

The tragic collapse of the Morandi Bridge in Genoa, Italy, in 2018 brought the term “concrete cancer” into the public’s focus once more. Cancer is a scary term and a deadly disease, so is concrete cancer just a click-baiting buzz word or a genuine concern?  Do you need to know how to fix concrete cancer?  We’ll look at that below as well…

Concrete cancer shares two traits with human cancer:

  1. Prevention is better than cure.
  2. If detected and treated early, the odds of a better outcome improve tremendously.

But there’s one key difference between concrete cancer and human cancer, and that’s the simple fact that concrete cancer is entirely 100% preventable. Sadly, as humans, we can lead a healthy lifestyle and do all the right things, yet still develop one of the many forms of cancer.  Building with concrete, on the other hand, is a far more predictable caper, and we can self-determine and safeguard against it.

First, let’s clear up a few myths and explain the terminology…..

Concrete cancer is possibly a misnomer, as the source and catalyst of the problem is not actually the concrete, but the embedded reinforcing steel that is cast inside the concrete.  Concrete structures are reinforced with steel: Reinforcement bars, stirrups, cages, strands and cables. Steel, as we all know, is prone to corrosion and rusting if not treated or prepared properly.  Concrete cancer is the mechanism whereby the embedded steel in the concrete starts to rust. Rusting causes the steel to expand and swell which, in turn, starts to crack and break off the surrounding concrete at the surface. (“Spalling” is the term used to describe the concrete breaking and falling away from the structure).

Without wanting to get too technical, concrete structures support their loads by working simultaneously in tension and compression. The reinforcement handles the tension, whilst the concrete resists the compression. Concrete cancer is a double hit: As the steel rusts, the amount of “good steel” capable of handling the tension reduces, and so the structure gets weaker. Similarly, as the concrete spalls and breaks away, there is less material available to handle the compression, which also weakens the structure. If the amount of intact steel and concrete reduces to the point where there’s no longer enough sound material to resist the applied loads, then the structure will fail – i.e. it will collapse. And, as we’ve seen in many instances, the collapse is sudden and catastrophic. For the long-term health of any concrete structure, the key is therefore to prevent the steel from corroding in the first place. Prevent the corrosion and you prevent the concrete cancer!

 

Concrete cancer occurring to a suspended balcony
Rusting reinforcement at the edge of this balcony slab has caused the concrete to break off. (Photo courtesy Structural Reporting)

 

In order for steel to corrode, two things must be present: Water and oxygen. As such, the fundamental objective in the building game is to prevent water and oxygen permeating through the concrete matrix and reaching the embedded steel. Concrete is a permeable material, and so there are three primary tools or tricks that engineers rely upon to keep the water and oxygen away from the steel reinforcement:

  1. We specify and provide a strong, durable, and dense concrete.
  2. We specify sufficient cover to the reinforcement so that it is far enough below the surface where water and oxygen shouldn’t penetrate.
  3. We detail and specify the concrete, its reinforcement, and the placement and curing methods used on site to prevent shrinkage and possible cracking of the concrete. (Obviously, if the concrete cracks, water and oxygen can more easily penetrate down through the crack and reach the reinforcement).

The first two items are easy in theory, because they’re simply part of the structural engineer’s specification. We might specify 40MPa concrete instead of 25MPa concrete, and we might specify 45mm cover to the reinforcement instead of 20mm. And the engineer should then carry out the appropriate calculations and provide details and drawings to illustrate how and where the reinforcement should be placed to resist the shrinkage strains and prevent the risk of cracking as the concrete dries.

However, the rubber doesn’t hit the road until we’re on site: It falls entirely to the builder to ensure the above specifications are achieved, and that sound and proven placement and curing methodologies are employed on the day of the pour, and in the days following whilst the concrete cures. Any shortcut, oversight, or departure from the specification has the potential to compromise everyone’s good intentions and lead to excessive shrinkage and cracking of the concrete.

Another problem on site is keeping the steel adequately secured in place during the pouring process. The steel fixer might set up all the reinforcement and initially provide the minimum cover specified, but it’s not uncommon for the steel to be knocked or displaced on the day of the pour as the concreters walk across the bars and heavy line pumps get dragged across the deck. If no one is on the ball to rectify this as it happens, the concrete can harden with the reinforcement left too close to the surface – and the time bomb starts ticking.

Unfortunately, this awareness of the durability of concrete and the risk of corrosion was 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 insufficient 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 of apartment buildings, which – over time – leached chloride ions into the concrete, thus triggering significant corrosion of the reinforcement.

Concrete cancer to a balcony
Concrete cancer has caused extreme damage not just to the underside of the balcony slab, but also to the supporting column! (Photo courtesy AWS Services)

 

It follows also that some structures and environments are more prone to concrete cancer than others. Steel doesn’t like chlorine, and corrosion is tremendously accelerated when there are chloride ions – that is, salts – in the picture. As such, structures near the coast or large bodies of salt water are more prone to concrete cancer, as the airborne salts can also make their way through the concrete and reach the reinforcement. This is why apartment buildings in Sydney’s eastern suburbs typically battle concrete cancer, whilst it’s less of an issue in Sydney’s west.

The Australian building codes seek to address and prevent concrete cancer by stipulating the minimum concrete strengths and covers required for the building – all governed by the site’s location; its proximity to salt water; and whether the structural member (e.g. a slab or column) is exposed to the elements, or whether it’s entirely an interior item and thus better protected. Again, however, the theory and specifications are all for nought if the covers aren’t achieved on site, or if the concrete cracks due to poor detailing by the engineer or poor curing techniques by the builder.

A fourth tool to combat potential concrete cancer is becoming more commonly used, and this is the use of waterproofing admixtures being added to the concrete as it is batched at the plant. These products reduce the permeability of the concrete and aim to stop water travelling through the concrete matrix and reaching the steel. Such technologies were rarely used (or even available) for structures built in the 1960’s through to the 1980’s, but they have certainly gained popularity in Australia in more recent years. However, whilst this is great in theory, their effectiveness is compromised if the concrete cracks and water/salts can simply travel through the fissures. It again puts the onus on the structural engineer to design and detail the reinforcement accordingly.  Another advancement in more recent times is the development of Glass Polymer Fibre reinforcement, where the reinforcement is a form of glass and thus cannot rust.

A big issue for many people, particularly strata committees and Owners’ Corporations is how to fix concrete cancer?  Once concrete cancer is detected, there are accepted and proven remediation methodologies to arrest and repair the situation. Such methodologies are tedious and typically expensive to carry out, as they involve breaking back the concrete to expose the steel; cleaning or replacing the corroded steel; and then reinstating the concrete with a cementitious repair mortar. Obviously, the less steel needing to be treated, the less invasive and complicated the repair becomes – hence the earlier comment that there is tremendous incentive to identify and treat the cancer early. Or, to use a well-known phrase: A stitch in time saves nine!   There are specialist contractors and consultants who undertake concrete cancer remediation works; a structural engineer who works in the remedial engineering space should be able to refer you to an appropriate operator.

Cheers,
AD

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