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How To Spot Signs of Problematic Reinforced Autoclaved Aerated Concrete (RAAC)

26 September 2023

RAAC, once believed to a viable long term solution to traditional concrete, has recently come under fire as the concrete itself crumbles, and is likely to fail. In some instances, this failure can be sudden and completely catastrophic, so it’s more important than ever to be aware of the signs of crumbling RAAC.

So, if you suspect that a property you own or manage does feature RAAC, then its important to act sooner rather than later.

What signs should you be looking out for when it comes to RAAC?

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What is RAAC?

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First we will go through what RAAC is, and what makes it differ from regular concrete.

RAAC stands for Reinforced Autoclaved Aerated Concrete, and it was primarily used starting in the 1950’s and ending in the 1990’s throughout the UK. You may well have heard about the crisis of RAAC in schools throughout the UK, but RAAC was a commonplace feature in many properties constructed during this time, so the problem won’t be contained just to schools.

RAAC is a much lighter material to use in construction thanks to the way in which its produced. A mix of cement, lime, aluminium flakes, and sand is heated at high pressure, using steam to heat the materials. This process is called autoclaving, and the RAAC produced is much lighter than regular concrete.

To make up for the shortfalls in density and weight compared to regular concrete it was common practise for steel bars to be placed through the RAAC for additional support. These steel bars would not be left exposed, and would instead be encased in a protective shell. This might be made of cement, of latex, or other kind of casing, but the purpose was always the same: to prevent corrosion of these steel bars should water make its way within the RAAC.

RAAC was incredibly common, and could be found on building sites throughout the country thanks to its lower price as well as its relative ease of use.

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What Makes RAAC Problematic?

RAAC has been found to be problematic for a number of reasons, not least of which being that RAAC could potentially fail quite suddenly with catastrophic consequences.

Over time, RAAC can become much more volatile than its more stable alternative found in standard concrete. There are a number of reasons for this, and we can run through them all, but the overarching factor is that RAAC is permeable.

That means that the concrete itself is open to water damage, and this leads to all kinds of problems. First off, the actual permeation of the water can compromise the structural integrity of the RAAC itself, causing the concrete to degrade.

Then there is the danger that the permeating water poses to the supporting steel bars that run through the RAAC itself. If the RAAC is compromised by water, then given enough time that same water can erode the coating on the steel bars, and the steel bars themselves, leaving the brittle and crumbling RAAC without any internal support.

It won’t just erode the steel bars either. The rust causes the steel to expand, which in turn pushes against the RAAC, which can easily crack the surrounding RAAC, and snap the internal steel bars as they push against the concrete – it’s a battle to see which will break first.

Combined, these problems make RAAC into a very unreliable material, especially if it has been installed a number of years ago and the RAAC is reaching the end of its serviceable lifespan. This is why its so important to be aware that RAAC is an unreliable material in a structure, and that you should be aware of any warning signs that you can potentially spot.

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How to Spot Signs of Dangerous RAAC?

RAAC is a material that can in fact display signs of its failure before it becomes totally compromised, and it fails entirely. In fact, thanks to its unique composition and the way in which it does degrade, you may be able to spot some signs.

One way to spot compromised or failing RAAC is if the RAAC itself seems damp. Signs of dampness in RAAC are the very first indication that the concrete might have a problem. That’s because many of the problems inherent in RAAC stem from the presence of moisture, and if you notice that the exterior of any RAAC in a property you own or manage is damp, then it could be a sign that further problems have developed, or are developing within the RAAC.

Another sign that the RAAC might be unstable, or unsuitable for its purpose would be if the concrete begins to crumble. RAAC is often referred to as crumbly concrete, and that’s because it’s made from a fine aggregate rather than a course one. This means that as the lifecycle of the RAAC goes on, it can crumble to the touch – a surefire sign that the RAAC is not suitable.

Then there is if you notice the RAAC sagging. Sagging in RAAC is a sign that the material is not at its full strength, and is now struggling to cope with the weight placed on top of it. The cause of the sagging could be a number of things, it could be that the steel bars within the RAAC have corroded and are no longer capable of bearing the weight being placed upon the RAAC, or it could be that the concrete itself is no longer capable of taking the weight and is struggling to hold together.

Any of these signs are serious, and you should get in touch with a specialist at the first opportunity to avoid disaster if you notice any signs of potential RAAC failure in a property you own.

If you believe that a structure you own or manage may be susceptible to crumbling RAAC, then it’s important to begin the repair and replacement process as soon as possible.

You can get in touch with our team to arrange for a structural repair survey by calling 0800 288 8660, or by requesting a survey below.