(Video) RAAC Expert answers frequently asked questions

During the last few weeks many misunderstandings and questions have been raised to us and in media. Bringing together some of the most common questions we have created a little FAQ video with our RAAC Expert, Christian Stone.

Is reinforced autoclaved aerated concrete inherently dangerous?

No. Aerated concrete is weaker than traditional concrete, around one tenth the compressive strength, so can be broken with much less force. In fact, the material can be pushed into with a screwdriver with just the force of your arm. However, the strength does not just come from the concrete but from the concrete and the steel working together and with RAAC being much lighter than concrete it has less weight to hold up. It was designed as a roof, wall and floor material and there are many cases of RAAC from the 1960’s showing no signs of bending or cracking.

The issues come when the RAAC has one or both of the following problems:

1. The steel cage is not sitting close enough to the end of the panel at the bearing, and/or the wall or beam at the bearing point is too narrow. In such cases there may be inadequate strength at the critical location where the roof panel meets the wall or beam.
2. The panels get wet, especially often. Aerated concrete acts like a sponge and can absorb half its own weight in water making it heavier, increasing the load. The plank also loses 20% of its strength when wet increasing the chance of cracking and bending. Also, when moisture and steel mix, rust often becomes an issue which can lead to cracks in the roof.

Why did they build using this material?

RAAC was a wonder material of the time. RAAC is lightweight, uses fewer building high-carbon materials, is very thermally insulating and could be purchased in many shapes and sizes. This allowed for the cost effective and rapid building of many structures throughout the UK in the post war period. In many ways it is an impressive material, many of the issues come from lower quality control in the 60’s, different building standards, misunderstandings of the material leading to harsh treatment, and maintenance.

Does all RAAC need to be demolished?

Most RAAC will not require demolition. Whilst there are hundreds of thousands, if not millions, of panels in the UK, there have been few reported failures. Some RAAC panels may be beyond help and it may be best to replace the roof entirely, however the majority will either be found to be currently safe or in need of some remediation.

Does it contain asbestos?

RAAC has not been found to contain asbestos, however plaster board, paint and other materials around the planks from the 50s-70s may contain asbestos and are best to leave alone. Many RAAC planks when they fail structurally are unlikely to fall through to the floor. Additional steel straps securing the panel from slipping are found on some RAAC roofs.

Is it just in schools and hospitals?

No. Hundreds of thousands of these planks were produced by two different manufacturers in the UK for decades. It will be found in all kinds of post-war building, from theatres and airports to offices and warehouses.

Will this be easy to fix?

There is more research to be done. Fail-safe work or additional supports are options to reduce the risk of collapse in some cases. A structural engineer should be brought in who can look at each roof in a case-by-case basis. More research is required to develop more effective remediation measures, both in terms of strength, speed of deployment, cost, and environmental impact.

Corrosion has been found in many RAAC roofs and is very likely in planks from the 60’s and 70’s that have experienced water leaks or increased humidity from shower/kettles/etc. CPT has spent years developing corrosion management solutions for RAAC and have implemented the products on thousands of RAAC panels in the UK over the last 18 months.

If you have any questions about corrosion in RAAC and our solutions, please feel free to reach out.