Reinforced autoclaved aerated concrete RAAC

[edit] What is RAAC?

Reinforced autoclaved aerated concrete (RAAC) is essentially what it implies, it is the same autoclaved aerated concrete (AAC) product but containing reinforcement elements which improve tensile strength and allow larger, thinner panels to be made from the same aerated product. Issues have arisen with these products, in particular where RAAC have been installed as structural plank elements.

RAAC playbook, published by the Manufacturing Technology Centre and the High Value Manufacturing Catapult in 2024 defines reinforced aerated autoclaved concrete (RAAC) as: ‘the reinforced version of autoclaved aerated concrete (AAC), a lightweight cellular material typically reinforced by steel bars or mesh to enhance its structural strength. Unlike concrete, AAC does not contain coarse aggregates thus the term ‘concrete’ may lead to confusion. Additionally, the physical and mechanical properties of RAAC greatly differ from those of concrete.

[edit] When was RAAC first used?

RAAC was first developed not long after the commercial development of autoclaved aerated concrete (AAC in the 1930's), the first reinforced AAC products were also developed in Sweden (where AAC was developed) not by the original manufacturer of AAC but by a competitor. The goal of developing reinforced elements was to be able to manufacture an entire building from AAC based systems, including panels and floor elements.

RAAC products were further improved by German firm and began establish themselves in the market by the 1940's. After the war and effectively between 1945 and 1975 mass building programs across Europe searched for modern methods to construct buildings quickly and cost effectively, and as such many innovative new products were developed during this period (known collectively as post-war building materials). AAC and RAAC or lightweight concrete systems became key elements in many of these building programmes, alongside other products developed during the same period such as insulation products, glazing and window frame systems, and a variety of prefabricated and precast systems including lightweight concrete.

[edit] What are the issues with RAAC?

Crumbling concrete describes failure where concrete parts are filling off in chunks and the strength of the overall material is compromised. It can simply be the result of ageing but maybe worsened where the initial pour was not managed correctly, where reinforcement elements are beginning to corrode or where the material is exposed to harsh elements or mechanical damage. This type of failure has started to be associated more closely with RAAC, the reasons vary with specific cases but some RAAC installations were built as early as the 1950's and so maybe seeing issues because the products have surpassed their expected accepted lifespan. Other issues with newer product installations may indeed be as a result of either poor design or fabrication / installation issues often related to the location of the reinforcement and concrete mix.

[edit] What is happening today with RAAC?

In December 2022 the UK Government published 'Reinforced autoclaved aerated concrete (RAAC): estates guidance'. The guidance set out a 5-stage approach to the identification and management of RAAC in educational buildings, where these maybe present in floors, walls and roofs (pitched and flat) of buildings constructed or modified between the 1950s and mid-1990s. This guidance outlined initial steps that should be taken by those responsible for the management of educational buildings, how to procure building professional’s services when specialist advice is needed. It was designed for all parties involved in the identification and management of RAAC, including estates managers and those providing specialist advice, can use this guidance.

In mid 2023 UK ministers Ministers launched a UK government-wide inquiry into the use of crumbling concrete, in particular occurring in reinforced autoclaved aerated concrete (RAAC). Initial indication is that many of the installations at risk are over 30 years old which may be beyond the expected lifespan of the product. Typically these are low-rise flat roofed structures built between mid-1960s and mid-1990s primarily of RAAC blocks.

Towards the end of August 2023, RAAC was increasingly classed as a deleterious material as many school across the UK were unable to open due to safety fears. In the same month the Government published its updated guidance "Reinforced Autoclaved Aerated Concrete (RAAC):Identification guidance"

[edit] Related articles on Designing Buildings

• Aerated concrete.
• Admixtures in concrete.
• Aircrete.
• Aircrete blocks.
• Alkali-activated binder.
• Alkali-aggregate reaction (AAR).
• Alkali-silica reaction (ASR).
• Applications, performance characteristics and environmental benefits of alkali-activated binder concretes.
• Autoclaved aerated concrete.
• Blockwork.
• Cellular concrete.
• Concrete masonry unit CMU.
• Concrete superplasticizer.
• Crumbling concrete.
• Defective Concrete Blocks Grant Scheme.
• Fly ash.
• Formwork.
• Precast concrete.
• Self-compacting concrete.
• Smart concrete.
• Types of concrete.
• Types of concrete specification.

[edit] External links

https://www.aircrete.com/aircrete-news/history-of-autoclaved-aerated-concrete-2/

http://postwarbuildingmaterials.be/material/post-war-building-materials/