What is RAAC?

Reinforced Autoclaved Aerated Concrete (RAAC) is a product combining aircrete with steel reinforcing bars. It is produced in the form of planks which derive their structural strength from the reinforcement bars.

RAAC roof plank failures

Over the last few years SCOSS, the Standing Committee on Structural Safety (part of the Institution of Structural Engineers) have been reporting on the failure of RAAC planks. SCOSS has now been replaced by CROSS UK (Collaborative Reporting for Safer Structures UK). 

The issue would appear to be associated with thin roof slabs in schools, hospitals, residential accommodation and other buildings dating from the 1960-80s with flat roofs consisting of RAAC planks. 

It has been reported that in the 1980s there were many instances of failure of RAAC roof planks installed during the mid-1960s and a large proportion of such installations were subsequently demolished.

Potential causes of RAAC failure

The Construction Leadership Council (CLC) in its Information and FAQs on RAAC, published in November 2023, pointed out that: “When properly designed, manufactured, installed .. and well maintained, RAAC installations are considered as safe as any other form of construction.

“If overloaded or not maintained well, then panels can deflect over time, and this process of deterioration can be exacerbated by water penetration…. Deterioration caused by these factors, particularly when combined with hidden risks from the lack of appropriate bearings, could lead to failure and in the case of poor bearing, sudden failure.”

Several case studies revealed some primary deficiencies e.g. incorrect cover to the tension steel, high span-to depth ratio, insufficient provision of crossbars for providing anchorage for the longitudinal steel, failure in performance of roof water proofing membrane leading to moisture penetration and local corrosion of steel. SCOSS concluded that pre-1980 RAAC planks are now past their expected service life and it is recommended that consideration is given to their replacement. 

To be clear, failures were due to failure in the reinforcing steel aspect of the RAAC and not the aircrete material.

Why aircrete is different

H+H UK wishes to make it clear that any aircrete blocks or storey height wall elements being delivered to the marketplace do not fall into the category of RAAC. All H+H aircrete blocks are manufactured and UKCA marked to BS EN 771-4 and are covered by BBA certification for their intended application, which confirms that “Autoclaved aerated concrete (Aircrete) is a durable material” and “Walls constructed from the products will have durability equivalent to walls of traditional masonry and will fulfil their intended function for the life of the building in which they have been installed”.

Vertical Wall Panels do contain a minimal amount of reinforcement, but this is purely to prevent damage during transportation and installation. The reinforcement does not contribute to the structural performance of the component or building, the elements being designed as though they are large format masonry blocks. All such elements comply with the European Standard EN12602 and have third party certification by official Notified Bodies. In addition, the Vertical Wall Panels and systems being supplied have been assessed and approved both by BOPAS (when used within the i-House system) and  NHBC Accepts.

Customers using the H+H Vertical Wall Panels can be confident that the long-term durability will not be compromised in any way.

H+H blocks strength testing

H+H regularly tests sample products from its manufacturing plants to ensure they meet the compressive strength required for use in building structures.

You can view this process, carried out on newly manufactured blocks, 40-year-old blocks and blocks which have been subjected to abnormally adverse conditions in the video below: