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CRACK A RAAC - CROSS!

SUMMARY OF THE CROSS AVAILABLE INFORMATION IN ONE PLACE


IT IS WEDNESDAY THE 20TH OF SEPTEMBER 2023 AND THE BRITISH EDUCATIONAL SECTOR CONTINUES TO BE SHAKEN BY THE IMPACT OF THE UNCANNY PRESENCE OF REINFORCED AUTOCLAVED AERATED CONCRETE PANELS IN THEIR SCHOOLS.

IT APPEARS THE REMAINING PART OF THE BUILDING OWNERS WITH BUILDING STOCK FROM THE 1950S TO 2000S BEGIN TO REALISE THEY MAY ALSO BE IMPACTED.



INTRODUCTION


RAAC TRANSLATES TO REINFORCED AUTOCLAVED AERATED CONCRETE THAT SHOOK THE UK EDUCATIONAL SECTOR WITH THE LAST-MINUTE DIRECTIVE TO CLOSE ROOMS AND SCHOOLS WHICH SUFFER ITS PRESENCE, A FEW DAYS BEFORE THE KIDS RETURN TO SCHOOL AFTER THE SUMMER BREAK. THIS DIRECTION CAME AS A CULMINATION OF PROLONGED NEGLECT BY ALL PARTIES INVOLVED.


SINCE THE GOVERNMENT'S DECLARATION OF 31 AUGUST 2023 THERE HAS BEEN A LOT OF BUZZ AROUND THE TOPIC WITH MANY MISCONCEPTIONS. THE BELOW SUMMARY OF THE MATERIAL AVAILABLE IN CROSS WEBSITES HAS BEEN COLLATED HERE WITH MY COMMENTARY FOR THE EASE OF REFERENCE.


THE MATERIAL IS LISTED CHRONOLOGICALLY WITH THE OLDEST PUBLICATION FIRST AND THERE IS ALSO A COMPREHENSIVE LIST OF LINKS THAT YOU CAN FOLLOW TO FIND OUT MORE ABOUT THE TOPIC.


THIS BLOG IS THE FIRST ONE OF TWO THAT WILL BRING YOU CLOSER TO THE SUBJECT OF RAAC. HERE I START WITH THE MATERIAL AVAILABLE ON THE CROSS PLATFORM. CROSS STANDS FOR COLLABORATIVE REPORTING FOR SAFER STRUCTURE. I WILL END WITH THE BLOG ON THE MATERIAL PROPERTIES, USE AND ASSESSMENT.




CROSS REPORTS


FAILURE OF PRECAST RAAC SLABS DURING CONSTRUCTION:

1 October 2007 - https://www.cross-safety.org/uk/safety-information/cross-safety-report/aerated-concrete-slabs-55


What: RAAC panels, 4.5m span, 175mm thick with 3-4mm reinforcement,

Building type: Steel Portal Frame Shed

When: 1970s

How: Failed under people walking on it seriously injuring persons involved.

It has been reported more slabs failed under test conditions on the ground.

Why: Poor workmanship and lack of quality assurance in manufacturing. Highly variable execution of panels and reinforcement placement within the same batch order. Wires stopped short of the bearing. Increased coalescence at the lines of reinforcement.

Conclusion: RAAC is a highly variable material with a strength more likely proven by empirical testing than can be relied upon by assumptions and scientific methods.



RAAC – CONCERNS OF THE TIME

February 1999 - Structural Safety 1997–99: Review and recommendations

https://www.cross-safety.org/sites/default/files/1999-01/scoss-review-1997-99.pdf


What: Any type of RAAC construction – design codified in BS 8110 however lifespan given in past BRE reports as 30 years therefore concluded not suitable for permanent structures. Widely manufactured and used in Germany and Japan. Building type - schools and other buildings.

When: 1997-1999 RAAC designed pre-1980s (latex-cement coated reinforcement) inspected by BRE in ESSEX following an advisory inspection order issued to local councils, no critical red flags made.

How: General mechanisms of failure highlighted, poor bond and weakness of the material.

Why: The Standard for RAAC was withdrawn from BS8110 after further testing for European Standard prEN 12602 which showed some brittle failures at low deflections (bitumen-coated reinforcement)

Conclusion: The behaviour of the proposed precast solution has not been known mid to late 20th century although a scientific method has been applied successfully to its initial design. The testing regime of a novated product is a must to establish the durability of the product. An initial strength assessment is not enough to ensure the longevity of the proposal.


FAILURE OF REINFORCED AUTOCLAVED AERATED CONCRETE (RAAC) PLANKS

1 May 2019 - https://www.cross-safety.org/uk/safety-information/cross-safety-alert/failure-reinforced-autoclaved-aerated-concrete-raac-planks


What: RAAC PLANKS, span - unknown, details - unknown,

Building type: School

When: 2018 but reference was made to many failures in 1980s of 1960s constructions.

How: Shear cracking adjacent to the support and possible reinforcement short of the support.

Why: Likely, wires stopped short of the bearing.

Conclusion: Order for a widespread inspection of the affected buildings.


REINFORCED AUTOCLAVED AERATED CONCRETE ROOF PLANKS - SHARING EXPERIENCES


What: Roof Planks in local authority buildings – surveyors experience, span - unknown, details – unknown.

When: Following the 1 May 2019 report.

How: The majority of dilapidations experience concern excessive deflections approaching that of span/100 and generally concentrate at the mid-span

Why: The strain of the material is the greatest where the concrete begins to spall. It has been observed that most of the high-deflection planks had full transverse anchorage at the support with only occasional situations of full longitudinal bar only. None of such planks were short of support. The high variability of the quality of the planks was also noticed.

Conclusion: The assessment needs to be carried out on a case-by-case and plank-by-plank basis due to the poor-quality assurance of the product and no possible correlation of results between the planks of even the same production batch.




FAILURE OF RAAC PLANKS IN SCHOOLS


What: Roof Planks in School, span - unknown, details - unknown

When: 2017

How: Overloading and bad structural alteration – bearing failure. Also noted that the panels appear to be deteriorating with time.

Why: Inadequate maintenance of ancillary elements and badly executed intervention into the structural fabric of the school undermining bearing length.

Conclusion: Bearing failure generally leads to a sudden collapse of the panel. There is little load margin of safety provided for the panels.



REINFORCED AUTOCLAVED AERATED CONCRETE IN ROOFING IN SCHOOLS


What: Identification Guidance issued by the Department for Education for education estates built or modified between the 1950s and 1990s.

When: Now

How: Preliminary Assessment by the building owner leading to the assessment by a suitably qualified structural engineer.

Why: Multiple brittle failures of the RAAC planks were recorded in the educational sector.

Conclusion: If you operate a building from the 1950s to the 1990s check what it is made of. Mind asbestos before you do and take action to protect your occupants.




ISTRUCTE RAAC STUDY GROUP & MEMBERS WITH RAAC EXPERIENCE


What: Formation of The ISTRUCTE RAAC Study Group

When: 2021

How: Publication of findings and information on best practice.

Why: To inform the profession and wider public on the issue, assessment, and remediation.

Conclusion: See the website for more information.





REINFORCED AERATED AUTOCLAVED CONCRETE PLANKS FOUND ON PITCHED ROOF OF 1990S HOSPITAL BUILDING

26 August 2022 - https://www.cross-safety.org/uk/safety-information/cross-safety-report/reinforced-aerated-autoclaved-concrete-planks-found-1125


What: Hospital building, span - unknown, details - Pitched Roof

When: Building Constructed Early 1990s

How: Confirmation of construction type

Why: To validate the materials used.

Conclusion: The use of RAAC extended past the boundary of the 1950s-1990s and in fact may still be used. The most concern that existed is connected with the planks designed pre-1980s. It is believed that the planks made to better British and European Standards and perhaps even CE marked should be of better quality, but all are suspect of similar defects unless proven otherwise.



CONSTRUCTION PERIOD OF RAAC PLANKS

20 February 2023 - https://www.cross-safety.org/uk/safety-information/cross-safety-report/construction-period-raac-planks-954


What: Building, span - unknown, details – structural issues with panels

When: 1998

How: Don’t know

Why: To validate the use post 1990.

Conclusion: The use of RAAC extended past the boundary of the 1950s-1990s and in fact may still be used. The most concern that existed is connected with the planks designed pre-1980s. It is believed that the planks made to better British and European Standards and perhaps even CE marked should be of better quality, but all are suspect of similar defects unless proven otherwise.


RAAC PLANKS IN A 1970S SHOPPING CENTRE


What: Large Shopping Centre – External Storerooms and Loading Bays, span - unknown, details – roof planks, deflected planks requiring replacement.

When: 1975

How: Deflection

Why: Planks were widely commercially available and may be present in many more building types.

Conclusion: The RAAC planks were widely available as precast proprietary product and may be present in many building types other than these under the Government’s remit.



RAAC ASSESSMENT UNDERTAKEN BY PERSONS NOT SUITABLY EXPERIENCED


What: Wrong people at the wrong place.

When: Current

How: The Desk study revealed previous attempts to justify RAAC with EC-based concrete calculations showing a lack of appreciation of the difference in the material properties and likely behaviour. Furthermore, no consideration was put to all other aspects of RAAC failures concluding the panels fit for purpose.

Why: To advise the public that it is important to engage with Chartered engineers to produce risk assessments whenever the RAAC panels are identified on site.

Conclusion: The person responsible for the running of a building should ensure that the person undertaking the inspection of their structure is qualified to do so and produces an appropriate Risk assessment as directed by the IStructE guidance.




REINFORCED AUTOCLAVED AERATED CONCRETE: GUIDANCE FOR RESPONSIBLE BODIES AND EDUCATION SETTINGS WITH CONFIRMED RAAC


What: Identification Guidance issued by the Department for Education for education estates built or modified between the 1950s and 1990s.

When: Now

How: Preliminary Assessment by the building owner leading to the assessment by a suitably qualified structural engineer.

Why: Multiple brittle failures of the RAAC planks were recorded in the educational sector.

Conclusion: Restrict access to the RAAC present areas until mitigations are in place.



 

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