Reinforced autoclaved aerated concrete (RAAC)

Why was RAAC used?

In the UK, RAAC panels became popular in the 1950s. During the post-war reconstruction period, the huge demand for new homes, factories and civic buildings was outpacing supply.

Traditional building is a hungry consumer of time, money and raw materials. The difficulties of meeting the demand led to a need to find alternative ways to build.

RAAC is insulating, fire resistant − and light weight. This made it easy to use and allowed corresponding savings in the structure that was needed to support it. It could be cut quite easily with power tools, which meant that it was easy to accommodate small changes in design.

Large, ready-made planks or panels could be brought to site, lifted and placed in position quickly and cheaply as builders didn’t have to wait for the concrete to harden.

The advantages of RAAC led to it being used in buildings where the repetitive nature of the design could make the best use of standard-sized parts.

How is RAAC different from normal concrete?

Ordinary concrete is a very heavy and dense material that has been used successfully since Roman times. It is often made by mixing cement, sand and small pieces of stone or crushed rock.

Autoclaved aerated concrete (AAC), however, was first developed in the early 20th century. It is made using smaller ingredients like sand, lime and cement and sometimes pulverised fuel ash – a fine powder that is a byproduct of coal-fired power stations.

By adding a small amount of aluminium powder, a chemical reaction creates thousands of tiny bubbles in the mixture. When this mixture is baked in a steam oven (called an autoclave), it hardens into a lightweight material that can be cut into block shapes for building.

The resulting blocks are light because they are about 70% air. This makes them useful in walls to help reduce heat loss and keep buildings warm.

People sometimes confuse AAC blocks with breeze blocks, but they are not the same.

AAC blocks are recognisable by their light grey or white colour and bubbly texture, rather like an Aero chocolate bar.

They are easy to cut or drill and are widely used in building projects around the world. When used correctly, AAC blocks are safe and pose no risks to health or to the buildings they are part of.

While AAC blocks are primarily used for walls, the same concrete mixture could also be made into larger structural elements, such as floors and roof panels. However, like regular concrete, AAC is not very strong on its own and needs steel rods for reinforcement.

If reinforcement is added we have reinforced autoclaved aerated concrete, or RAAC – the material that is now a source of concern.

How and why does RAAC go wrong?

In 2019 the partial collapse of two school roofs, in Kent and Bolton, changed the industry viewpoint that RAAC would grow old gracefully, and it was realised that RAAC could fail suddenly.

While it's not necessary to understand every detail of how RAAC fails, having a basic grasp of some key engineering terms can help.

Let’s start with two important concepts:

Bending and shear failures are obviously dangerous, but bending tends to happen over a longer period – you may get some warning that things are going wrong.

Shear failure can be sudden – you don’t necessarily get the warning signs first.

Typically, with a flat roof the warning signs might be that large puddles of water are now collecting (termed ‘ponding’). The more water that collects, the heavier it becomes. The roof planks bend a little more and collect more water, which adds more weight – a vicious cycle.

Going back to our example of a plank, the term bearing refers to how much of the plank rests on the chairs.

If the plank rests on the entire seat, then all well and good. But if it’s balanced on the edge of the chair seat there is a possibility that the whole thing will collapse when you stand on it.

The shorter the length of plank sitting on the chair, the greater the pressure the plank exerts. In other words, if you are standing on the plank, your weight spread over a large area is okay, but if it’s spread over a small area, it’s more likely to cause a problem.

RAAC planks are similar – if the bearing width is not sufficient to ensure that the steel-rod reinforcement is correctly placed, shear failure can occur.

When RAAC planks are installed in buildings, they may bend under their own weight and any loads applied to them. Think of a roof weighed down under snow or rainwater, or with lights or ventilation ductwork attached underneath. If the bearing ends of the planks are also insufficient, shear failure can occur.

Were RAAC planks built to be strong enough?

In theory, if you reinforce the lightweight concrete with steel bars, you can control the amount of bending and prevent the concrete failing at the ends of the planks where they are supported by the walls of the building (or by a steel or concrete beam).

However, there are several reasons why RAAC planks have become problematic.

• To prevent shear failure, it was very important to include what is called ‘anchorage’ reinforcement at each end of the planks. Poor quality control in the factory could have resulted in the reinforcement being in the wrong place or omitted.
• RAAC is easy to cut with power tools, allowing builders to cut the planks down if they were too long. However, doing this might mean that vital reinforcement could be cut off altogether.
• If the planks have been allowed to become wet (for example due to a leaky roof), they become weaker and heavier.

Most buildings containing RAAC roof decks will have been re-roofed at least once in their lifetimes. This could have changed the overall weight of the roof or the way the roof reacts to changes in temperature, which could influence long-term performance.

Sadly, due to budgetary constraints, some buildings have suffered poor levels of maintenance over the years, and this can in turn affect the long-term performance of RAAC.

In summary, the main risks for RAAC planks are:

• shear and poor bearing
• bending
• poor maintenance.

Each case must be assessed individually, as not all planks in a roof or floor system are necessarily compromised. In some instances, only one plank may be at risk of failure.

What should I do if RAAC is found?

Finding RAAC in your building isn’t a reason to panic, but it is a sign that action is needed.

A professional assessment is essential to determine the best course of action and ensure the safety and longevity of your property.

With proper management, many RAAC-related risks can be mitigated, allowing you to plan with confidence. Possible solutions include:

• regular monitoring – setting up a schedule of inspections to track any changes in the condition of the RAAC
• reinforcement or repair – strengthening RAAC with additional supports
• replacement – removing and replacing RAAC panels, especially if they are severely deteriorated or located in critical areas.

Can I make a claim on my household insurance policy if RAAC is found?

Most household policies will cover damage arising from an ’insurable event’. Insurable events are things like storm damage, fire or flood – not the existence of a product or material that has been known to fail.

If a RAAC roof is still performing as it was designed to, it has not failed. Therefore, it is unlikely that you would be able to claim on your insurance to replace the roof.

Should a collapse occur, your insurer may cover the accidental damage arising but not necessarily the cost of removing any undamaged or remaining areas.

The situation varies according to the specific wording of your insurance policy, and it’s best to speak with your broker or insurer about it.

What about my mortgage or the prospects for selling?

The presence of RAAC could affect a valuer’s professional opinion of value.

If a valuer has reason to suspect or finds RAAC during their visual (non-intrusive) inspection, they may request an independent specialist report (usually from a chartered building surveyor or structural engineer) before a valuation can be provided. The results of that report could have an impact on their opinion.

A valuer will not make any inspection of the condition (structural or otherwise) of the RAAC elements.

The valuer will also have to conform to the instruction and any policy provided by their mortgage lender clients. Mortgage lending policies do vary throughout the mortgage market and vary from time to time. It is not necessarily the case that a property with RAAC is not suitable for a mortgage.

Make sure you have all relevant information and documentation on your property’s history, including but not limited to details of alterations, repairs and maintenance to areas with RAAC present.

If you are buying or (re)mortgaging a property with RAAC present, ensure this documentation is available, as this will help answer queries from the valuer, lender or legal representatives.

Can I make a legal claim against the builder or architect?

The situation on legal claims is complicated and best discussed with a solicitor.

As most buildings with RAAC were built some time ago, you will probably not be able to claim.

However, the Defective Premises Act may cover you if your building was converted to residential use more recently. The provisions of this Act were extended following the Grenfell Tower tragedy and the cladding crisis that emerged from it.