EASY TO DIGEST BITE-SIZE INFORMATION ON RAAC
FOLLOWING ON THE PREVIOUS BLOG POST “ CRACK A RAAC “ WHICH SHOWED YOU A SUMMARY OF INFORMATION WIDELY AVAILABLE ONLINE AND PREPARED FOR YOU BY THE DEPARTMENT OF EDUCATION, ISTRUCTE AND CROSS I WOULD LIKE TO DELVE INTO THE MATTERS OF RAAC FROM A MORE ENGINEERING-LIKE PERSPECTIVE BUT STILL WRITTEN IN SIMPLE TERMS FOR EVERYONE TO APPRECIATE.
INTRODUCTION
RAAC stands for Reinforced Autoclaved Aerated Concrete. The description is perhaps wrong and misleading after all AAC (Autoclaved Aerated Concrete) is not concrete in the traditional sense. It is more of an AAG (Autoclaved Aerated Grout) but who would want to buy that right? Concrete sounds much better from the marketing point of view.
The traditional recipe for Concrete consists of Cement + Coarse Aggregate + Fine Aggregate + Water. It is simple but every part of the mixture can be of varying composition itself so perhaps it will be better to rewrite the recipe and name the constituents by the role they play. Cement is the binder that holds the whole rock-like mass together. Aggregate (coarse and fine) are the little stones, the gravel, and the sand that make up most of the mass and most of the strength. Water activates the process. So concrete is Binder + Filler + Activator.
The traditional recipe for grout is Cement + Water = Binder + Activator.
The recipe for the AAC is Binder + Aluminium Powder + Water + Pressure Steam Cooking. The difference between the Concrete and the AAC is that the AAC not only does not have filler but also has voids.
AAC has been widely used since its inception by the Swedish architect Johan Acel Eriksson in the early 1900s. The material has sufficient compressive strength to carry small loads and provides enhanced thermal resistivity compared to traditional materials and is widely used in residential settings. Companies like YTONG or CELCON produce this still today, many inner walls of cavity wall construction are indeed built using such blocks.
The introduction of RAAC in the 1950s has been an attempt to use the material as a load-bearing flexural (beam-like) structural element capable of spanning some distance between the supports, typically 2.5m - 4.5m. Many panels have dilapidated greatly, and many buildings were removed by the 1990s.
WHO LOOKED INTO IT
Building Research Establishment with their information papers IP10_96 and IP7_02 looked into the panels in use of the pre-1990s designation in 1996 and into the design assessment of the panels in 2002.
The International Union of Laboratories and Experts in Construction Materials, Systems and Structures RILEM assessed the material and issued guidance for testing, properties, and design in 1993.
The University of Leeds made tests on the material and their findings show well how the mechanisms of failure and the behaviour of the element made under a controlled environment. It gives a good starting point to understand how the element should behave in situ.
THE PROPERTIES OF THE MATERIALS
Permeable
Low mass, 3-4 times lesser than that of traditional concrete
5-10 times lower compressive strength than that of standard concrete, somewhere at the mortar magnitude 2.5-5N/mm2
Low tensile strength
Low flexural strength
Reduction of strength with moisture
Short elastic range at 30% of strength
Large strain value 0.0035
The low modulus of elasticity is some 15 times smaller than the standard concrete and varies with ambient relative humidity.
No fail arrest mechanism present after microcracking
Possible weak bond between the steel and the material. The flexural and shear resistance is reliant on the mechanical anchorage provided by the welded transverse reinforcement.
The cover does not provide corrosion protection. Bars previously coated with rubber latex and cement mixture then bitumen. Bitumen sometimes delaminates and bonds with the concrete during autoclaving.
With initial moisture content, the material required acclimatisation and reduction of moisture before installation (unlikely occurrence)
Used in sheds, residential buildings, building complexes, schools etc. As is the case now with flooring systems like hollow core slabs or beam and block, RAAC panels were widely available and could have been used by anyone anywhere.
The panels used in walls and floors do not suffer as much as the roof planks. This is often associated with different characteristics of load application in the case of wall panels and composite action with a reinforced screed topping which is often the case in flooring application.
ASSESSMENT
It is vital to understand that the buildings associated with the use of RAAC panels are often old and pre-year 2000 which means asbestos may be ever present. The building owner should identify any such risks in the building before disturbing materials that cover suspected RAAC panels.
The process is staged and should follow a well-laid-down process such as proposed by the Institution of Structural Engineers:
Initial Consultation, Identification and Plan of Survey and Testing.
Record of the dilapidations and risk assessment – for every panel, intrusive investigation may be required.
Specification and Design of remediation measures or/and provision of management strategy
Oversight of the works or periodical reassessment.
ASSURE THE INVOLVEMENT OF A CHARTERED ENGINEER
TYPICAL DEFECTS
PERFORMANCE – high deflection, cracking/spalling, corrosion of reinforcement, deterioration in condition, overloading distress, independent acting panels (no load sharing).
MANUFACTURING – misplaced transverse reinforcement, insufficient anchorage of longitudinal steel, voids around reinforcement, incorrect cover to tension steel
CONSTRUCTION – cutting of panels post-manufacturing, short bearing lengths, missing reinforcement, structural damages due to later works.
REMEDIATION
Monitoring – usually yearly cycles.
Strengthening – emergency propping, enhancement of end bearing, provision of positive or passive support systems.
Replacement – single panels or entire roofs.
REFERENCES:
British Standards Institution (1985) BS8110: Structural Use of Concrete, Part 2: Code of practice for special circumstances. British Standards Institution (1996) Draft prEN 12602: Prefabricated reinforced components of autoclaved aerated concrete. British Standards Institution (1997) BS8110: Structural Use of Concrete, Part 1: Code of practice for design and construction. Building Research Establishment (1996) IP 10/96 Reinforced Autoclaved Aerated Concrete Planks Designed before 1980, CI/SfB (2) H q6. RILEM (1993) Technical Committees 78-MCA and 51-ALC, Autoclaved Aerated Concrete - Properties, Testing and Design. London: E & FN Spon
Campbell P. (2001) Learning from Construction Failures: Applied Forensic Engineering, Caithness: Whittles Publishing.
IStructE (Feb 2022) Reinforced Autoclaved Aerated Concrete (RAAC) Panels Investigation and Assessment. London: IStructE
IStructE (Apr 2022) Reinforced Autoclaved Aerated Concrete (RAAC) Panels Investigation and Assessment – Further Guidance. London: IStructE
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