RAAC: How to identify it and what to do 

RAAC: How to identify it and what to do 

In this article we look at what is Reinforced Autoclaved Aerated Concrete, or RAAC, how to identify it, and what you can do if your building has been constructed with it. 

What is RAAC? 

RAAC is a form of concrete developed in Sweden in the 1920’s and introduced in the UK in the 1950’s. It was commonly used in construction of public sector buildings until the mid-1990’s in our country. 

RAAC is a lightweight ‘bubbly’ material commonly found in roofs, floors, and walls. It is made with cement, blast furnace slag, and pulverised fuel ash or silica flour. These are mixed with water and aluminium powder. The mixture is moulded and formed into planks under high pressure and heat. 

What is the lifespan of RAAC? 

RAAC has an estimated lifespan of 30 years and is now liable to collapse, which has significant safety consequences. 

Why does RAAC concrete fail? 

The structure of a RAAC-built building is susceptible to failure if exposed to moisture, as the steel reinforcement within it expands and rusts it weakens the concrete over time. 

Why is RAAC a problem? 

In recent months, there has been a rise in cases of RAAC being the leading cause of structural defects, particularly in public sector buildings. 

This has already happened in two schools, where RAAC panels in the roof suddenly collapsed. Both buildings were operational at the time and their roofs appeared to be in good condition. 

As collapse can happen with little to no notice, The Department for Education, The Standing Committee on Structural Safety (SCOSS), and the Institution of Structural Engineers have published documents which highlight the importance of identifying whether buildings have been built with RAAC - like this one. 

It is crucial that those responsible for the management and maintenance of buildings with suspected RAAC act appropriately to ensure the building is safe for public use. As structural engineers operating across Kent, Sussex, Essex, and London, at BSF, we are qualified to identify RAAC and provide a structural assessment to inform necessary action. 

How do you identify aerated concrete? 

RAC panels are light-grey or white with smooth undersides - only their inside looks ‘bubbly’. For this reason, RAAC planks may look the same as precast concrete. Moreover, in many buildings, they are hidden above false ceilings, which is why it is important to seek the help of a qualified and experienced structural engineer to identify them. 

In some cases, the presence of RAAC may be evident due to levels of flanking or bowing of individual RAAC planks. In other cases, site sampling is needed. This can be done via non-intrusive site sampling (with minimal damage) or destructive site sampling, where small pieces of the panel are removed for thorough investigation. 

It is also important for a qualified structural engineer to assess the building as a whole, as many buildings containing RAAC may also contain asbestos. This has crucial implications on the safe identification and removal of RAAC. 

The four steps to RAAC identification 

At BSF Consulting Engineers, we have worked on a number of schools and hospitals across Kent, Sussex, Essex, and London. We are well-placed to help any building owner with this process. 

When completing a RAAC survey, we follow a four-step process. 

Step 1 – Site Visit & Identification 

Once you get in touch about a RAAC concern, we arrange a site visit, undertake investigative work and provide expert advice. Depending on the type of building, this can take one to two days. 

Step 2 – Structural Report 

Next, we provide a structural report on our findings and suggest recommendations for the next steps. Every building provides its own challenges, and our engineers use their expertise to propose tailored solutions. 

Step 3 – Remedial Works 

Then our team of engineers propose and design any remedial works needed. These initially consist of temporary works to leave the building in a safe state, prior to any permanent design works. 

Step 4 – Monitoring 

If our findings indicate the presence of RAAC, which shows no sign of deflection, cracking or fatigue, BSF can offer a monitoring service to keep tabs on the structural integrity of your building. 

As you can see, identifying RAAC is not a straightforward process. Given the high safety risk involved, it is crucial you appoint qualified structural engineers to carry out a full survey. 

Are you worried your building was built with RAAC? 

If you own a building with suspected RAAC, get in touch with us today. We can help you assess the safety of your building and schedule any remedial works needed. 

We are based in Canterbury and offer structural engineering services in Kent, Sussex, Essex, and London. 

Image Marco Bernardini