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CROSS Safety Report

Reinforced autoclaved aerated concrete roof planks - sharing experiences

Report ID: 874 Published: 1 April 2020 Region: CROSS-UK, CROSS-AUS, CROSS-US

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Overview

A reporter has surveyed a number of buildings with reinforced autoclaved aerated concrete (RAAC) roof planks and wants to share their experiences to help others.

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 reinforced autoclaved aerated concrete (RAAC) roof planks is suspected, a structural assessment should be made.

  • If RAAC planks are present, Section 6 of the Alert on Failure of RAAC Planks provides advice on managing planks

For all those with experience of RAAC planks:

  • Consider confidentiality sharing your experience with CROSS similar to this reporter to allow others to learn from

  • Contact technical@istructe.org if you are interested in joining the Institution of Structural Engineers’ RAAC Study Group

Full Report

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Our secure and confidential safety reporting system gives professionals the opportunity to share their experiences to help others. If you would like to know more, please visit the reporting to CROSS-UK page.

Following the publication of the Standing Committee on Structural Safety (SCOSS) alert on the Failure of Reinforced Autoclaved Aerated Concrete (RAAC) Planks, a reporter has been tasked to undertake a multi-site review of reinforced autoclaved aerated concrete (RAAC) roof planks for a local authority. They have surveyed a number building with RAAC roof planks and want to share their experiences and conclusions in the hope that it can help others working on similar projects.

Condition of RAAC planks

The condition of the planks varies, generally in proportion to the deflections as set out in the Building Research Establishment (BRE) guidance. They have found planks approaching the 1/100 deflection level and these have generally suffered noticeably from water ingress, cracking and bottom face spalling to the rebar level, as well as reinforcement surface-corrosion.

Cracking has generally been worst at mid-span and often seems to follow the line of the reinforcement, both longitudinal and transverse. The reporter considers these condition issues to require rectification but do not consider that they lead to an enhanced risk of sudden shear failure.

One possible remediation solution

They are considering a timber joist and plywood deck wedged below the RAAC planks into the webs of the existing steel beams, effectively making the RAAC planks structurally redundant, and have successfully trialled this method in a small area.

They are considering a timber joist and plywood deck wedged below the RAAC planks into the webs of the existing steel beams, effectively making the RAAC planks structurally redundant, and have successfully trialled this method in a small area

They say that the advantage of this approach is that it prevents, or at least catches, any further spalling of the concrete face of the RAAC planks and does not require wholesale removal of the planks. It can also, to an extent at least, be fitted around existing services. This method was used along with the recommendation to repair the RAAC planks to prevent future water ingress.

Potential for sudden shear-failure

Typical end bearing for the planks is 40-50mm on either compound lattice beams, steel I beams or load-bearing masonry walls. They have broken out small areas of sample planks to identify reinforcement above the supports (which cannot be determined by covermeter methods due to the presence of the steel beams). From these investigations, they have found the following cases:

     a) Welded transverse bar beyond the face of the supports. They consider these panels to be safe from sudden shear failure.

     b) Straight bars projecting beyond the face of the support but insufficient to be considered effectively anchored. They consider these to be at an elevated risk of sudden failure, particularly where the condition issues show cracking parallel to the supports. As a minimum, they are recommending the following measures until strengthening or replacement of the planks is undertaken:

          i) no access to the roof without crawling boards

          ii) no access to the roof during operational hours

          iii) room closure during snow events

          iv) regular visual rechecks following access or snow events

     c) Bars terminating before the face of the supports. They consider these panels to be in the highest risk category and recommend immediate action i.e. closure and propping followed by strengthening or replacement of the planks.

The reporter found that the condition issues described above do not correspond to the shear reinforcement anchorage but are more likely related to creep, to age and possibly to historic water ingress. For example, on one site, where planks were found to have high deflections along with historic ponding and leakage, only a few planks were found to be in case b) above with none in case c). For another site where the planks were found be in better condition, around a third of the planks were found to be in case c).

Varying quality control of reinforcement in planks

Different batches of planks were found to have varying quality control over the placement of the reinforcement and that often the quality control has been poor relative to the short bearing lengths typically used. They have also noted different ages of panels within the same building with different reinforcement dispositions (the replacement panels were due to a fire in the early life of the building).

Panels have also been cut, resulting in the transverse bars being cut off. However, there were also bespoke shorter units with the transverse bars in position.

Expert Panel Comments

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This report is one of several that was received following the publication of the Standing Committee on Structural Safety (SCOSS) alert on Failure of RAAC Planks in May 2019. These confirm that there are considerable areas of roofing consisting of RAAC planks in use in public buildings in the UK.

It appears that not all of these have been identified, so structural engineers and building professionals need to be aware of the situation and, when possible, check for RAAC on large flat roofs built around the 1960-80s. The Local Government Association, the Department of Health and Social Care, and the Department for Education have advised owners to check their premises and make inspections to ensure that they know what they own, and if RAAC is suspected, to have structural assessments made.

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