Quick Search

Reinforced autoclaved aerated concrete

Report ID: 5100

Report Overview

Extract from 12th SCOSS report 1999 Reinforced autoclaved aerated concrete (RAAC) has been used for at least 25 years in the UK in the form of planks for roofing, and also panels for walls, floors and internal partitions. These units were manufactured and widely used in the UK until 1982, when production was discontinued for commercial reasons. A limited amount (around 1 million cubic metres) is imported from other countries in Europe and used in a small number of new developments. This compares with continuing widespread use in many other parts of the world. For example, approximately 2000 million cubic metres per year are manufactured in Germany. The material is extensively used in Japan as walling units in steel framed structures because of its good behaviour in seismic conditions.

Report Content

Extract from 12th SCOSS report 1999
Reinforced autoclaved aerated concrete (RAAC) has been used for at least 25 years in the UK in the form of planks for roofing, and also panels for walls, floors and internal partitions. These units were manufactured and widely used in the UK until 1982, when production was discontinued for commercial reasons. A limited amount (around 1 million cubic metres) is imported from other countries in Europe and used in a small number of new developments. This compares with continuing widespread use in many other parts of the world. For example, approximately 2000 million cubic metres per year are manufactured in Germany. The material is extensively used in Japan as walling units in steel framed structures because of its good behaviour in seismic conditions.

Currently produced roofing planks are designed with a lower span/depth ratio than that used in the 1970s. There is not thought to be any likelihood of UK manufacture starting again because of the high costs of setting up a suitable manufacturing plant.

In 1995, concern was expressed in the Verulam column in The Structural Engineer about the structural performance of this form of construction following inspections of cracked units in school roofs (1).Although BS8110: Part 2: Section 6(2) has rules for designing RAAC, investigations reported by the BRE (3) had concluded that RAAC planks could not be expected to have a useful life of much more than 30 years. Subsequently a proposal was made for the removal of the reference to RAAC in BS8110 on the basis that its inclusion gives this form of construction ‘an unjustified respectability’, ‘the impression that it can be used for permanent structures’ and that safety is in question. This concern was brought to the attention of SCOSS.

Autoclaved aerated concrete is a relatively weak material compared to normal dense concrete and, in particular, has a low capacity for developing bond with embedded reinforcement. In addition, the material provides little protection to reinforcement against corrosion. The reinforcement is therefore given a protective coating (a form of latex was used up to about 1980; more recently a bitumen coating has been used) during manufacturing to enhance durability. Such coatings, however, tend to reduce bond. To assist in safeguarding against bond failure, reinforcement cages are welded and incorporate transverse bars. Overall the result is that RAAC planks tend to creep – deflecting over time and cracks may occur on soffits, and the reinforcement may corrode.

In 1994, the then Department of Education asked BRE to inspect a number of school roofs in Essex. The results were reported in BRE Information Paper IP10/96.(3) This report, which was limited toRAAC planks designed before 1980, concluded that ‘there is no evidence so far to suggest that RAAC planks pose a safety hazard to building users’. The Department of Education sent a warning to all schools referencing the Information Paper, and advising inspection and assessment of roofs incorporating RAAC planks. As a result Essex County Council is known to be inspecting some 60 school buildings. The BRE investigation did not therefore suggest a need for SCOSS to examine the topic at that time.

BRE has subsequently conducted loading tests on RAAC planks of recent manufacture using bitumen-coated reinforcement in top and bottom mats that are linked together. In some tests failure was sudden at very low deflection. These tests on imported planks raised questions on the draft European Standard prEN 12602 (4) which is at an advanced stage of preparation. The DETR has therefore commissioned design studies to examine the issues and to provide a basis for comments on the prEN. DETR has also had discussions with the Belgian manufacturers. BSI Committee B/525/2, which is responsible for BS8110, was contacted by SCOSS to ask if consideration is being given to amending or withdrawing BS8110: Part 2: Section 6.

The main issues relate to:

  • whether the assumptions made in design concerning bond, modulus of elasticity and resistance to corrosion are sufficiently conservative in view of observed deflections, cracking and bond slip.
  • the mechanism of failure over time, eg. whether moisture movement may adversely affect structural performance.
  • whether excessive deflections and/or corrosion may lead to an unsafe condition and failures without warning, particularly in modern imported planks.
  • the suitability of RAAC planks for use in ‘permanent’ construction.

BRE has concluded that pre-1980 RAAC planks in roofs do not appear generally to present a safety hazard as they gradually deteriorate over time. SCOSS concurs with this conclusion. Inspections of existing roofs of this type have been recommended. The recommendation has been drawn to the attention of schools.

The adequacy of the current structural design of RAAC planks has recently been examined by DETR and subsequently amendments to BS8110 and prEN 12602 have been agreed that remove references to RAAC in these standards.

Recommendation:
Owners of both school and non-school buildings that have pre - 1980 RAAC plank roofs should arrange for these roofs to be inspected if this has not been done since 1994, although generally the deterioration of RAAC planks does not jeopardise structural safety.

References
1. Verulam. The Structural Engineer, 21 February 1995.

2. BS 8110. Structural use of concrete. Part 2: 1985. Code of practice for special circumstances. London, BSI, 1985.

3. Reinforced autoclaved aerated concrete planks designed before 1980. BRE Information Paper IP 10/96.

4. prEN 12602, Prefabricated reinforced components of autoclaved aerated concrete, London, BSI.


Email Updates

How to Report

Online submission:
Submit by post: