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CROSS Safety Report

Aerated concrete slabs

Report ID: 55 Published: 1 October 2007 Region: CROSS-UK, CROSS-AUS, CROSS-US

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A reporter discusses the failure of precast reinforced autoclaved aerated concrete (RAAC) slabs during construction.

Key Learning Outcomes

For building managers/owners:

  • If a building has a flat roof, be aware of what it is constructed of

  • If you are unsure of the form of construction, it is advised to carry out an inspection. If RAAC is suspected, a structural assessment should be made.

Full Report

Find out more about the Full Report

The Full Report below has been submitted to CROSS and describes the reporter’s experience. The text has been edited for clarity and to ensure anonymity and confidentiality by removing any identifiable details. If you would like to know more about our secure reporting process or submit a report yourself, please visit the reporting to CROSS-UK page.


In about 1970, a reporter was responsible for a fairly large steel framed portal shed type building with 175mm thick proprietary precast autoclaved aerated concrete slabs spanning 4.5m between rafters. The joints between units had a small keyway filled with fine concrete and a short continuity bar (about 8mm diameter) over supports. During the erection process, some of the slabs failed whilst being walked on and two men fell and were severely injured.

Surprising features of slabs

The shattered slabs showed some surprising features. Reinforcement was two wires, about 3-4mm diameter, latex coated with cross wires welded at the ends only. Some of the wires stopped short of the bearings by 200mm. The slabs had been cast on edge and showed a continuous line of bubbles along the reinforcement, so there was not much bond.

Slabs which did not have reinforcement visible at the ends were replaced, possibly around 30%. There were also simple load tests on some slabs at ground level and some of these failed. It was clear to the reporter at the time that that the span was too great for safety.

Slabs which did not have reinforcement visible at the ends were replaced, possibly around 30%

The reporter provided a copy of correspondence in Verulam (Journal of the Institution of Structural Engineers), dated February 1995, about reinforced aerated autoclaved concrete units which had cracked in a school roof. It was stated by a correspondent that because of the lack of protection afforded to reinforcement by the concrete, the wires were coated with latex and cross bars welded to the main bars provided a mechanical anchorage

Unsuitable concrete

The author of the letter said that a well known advisory agency had investigated such slabs and concluded that they would not be expected to have a useful life of much more than 30 years. This led him to believe that this type of concrete is not suitable for being reinforced in order to employ it in structural units. BS 8110 Part 3 had rules for designing reinforced autoclaved aerated concrete and the author felt this was wrong in that it gave the impression that the material could be used for permanent structures.

Attached to the report to CROSS was a letter dated 1994 from the same agency saying that materials such as those described, when reaching an age of some 25-30 years would exhibit deflections of the order of 50-60mm (although no mention was made of span). Inspection of the longitudinal rebar in the units is expected to reveal medium to heavy rusting with loss of bond. The bond of the longitudinal bars ultimately relies on the anchor bars welded across the ends of the longitudinal bars.

Replacement of slabs

The slabs can be expected to continue to creep-deflect under load, but sudden collapse is not expected to occur. Water ponding in depressions on a roof will of course add to the creep-deflection problems. It was recommended that slabs should be considered for replacement when they have reached the stage described above as they are outside the serviceability states of deflection and durability.

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