Reinforced Autoclaved Aerated concrete (RAAC) – Basic composition & petrography

From Bradley E. Staniforth, CEng

The recent publishing of the “Reinforced autoclaved aerated concrete: guidance for responsible bodies and education settings with confirmed RAAC” document [1] by the Department for Education is resulting in the potential closure of around 150 schools across England due to potential structural safety concerns. This document in particular has been prompted by heightened concerns following an in situ failure of particular RAAC elements previously thought to be at low risk of deterioration.  Additional research by an NHS commissioned research project through Loughborough university (To which Petrolab contributed some petrographic findings) and risk factor guidance from the IStructE’s RAAC study group [2] have also acted to provide up-to-date guidance and understanding of the associated risks. However failures related to RAAC materials have been identified and investigated as early as the 1990’s in the UK. Production of RAAC materials was halted in the 1990’s following a series of high profile structural failures prompting earlier research and investigation by the BRE [3] and later SCOSS [4].

What is the composition of RAAC ? 

Reinforced autoclaved aerated concrete (RAAC) was a particular form of precast autoclaved aerated concrete (AAC) produced primarily during the 1950’s-1980’s to provide lightweight closed cell components. The material was particularly utilised within the construction of precast planks, flat roofs and walls within public sector buildings. In modern applications precast AAC masonry blocks continue to be popular construction materials in many regions including Europe, the Middle East and across Asia.  RAAC is composed primarily of the AAC plank material requiring embedded longitudinal and transverse steel reinforcement to provide flexual and tensile strength. Compositionally the cementitious component is best described as an aerated mortar material which can comprise blends of cement, lime, silica flour (microsilica), gypsum, blast-furnace slag and/or pulverised fuel ash, a small quantity of finely graded sand and an entraining agent such as aluminium powder. Following mixing the AAC is cured using a high pressure steam curing process known as autoclaving for a normal period of 6-12 hours. During mixing and casting the aluminium powder and lime react with water to form calcium aluminate and liberate hydrogen resulting in the abundant air entrained voids throughout the binder structure. Giving the AAC its characteristic “Aero-bar” texture and very low to low density (~500-1200 kg/m2).

Following mixing the material is autoclaved, this process introduces higher temperature and pressure into the system and the formed cement hydrates undergo a number of further microstructural changes resulting (dominant pozzolan dependant) in a mixed cement paste of poorly crystalline calcium-silicate-hydrate paste (C-S-H) and coarser tobermorite paste [5,6].

What are the main material durability issues?

Due to the high void content of the AAC and high susceptibility to moisture ingress and carbonation, it provided effectively no protection (in the form of a protective alkaline zone) to the embedded reinforcement. This must rely on its own coating system (typically a cementitious grout coating or a latex system however complex composite coating systems have been identified) to inhibit corrosion. Carbonation of the intermixed poorly crystalline C-S-H and tobermorite proceeds in line with typical carbonation reactions resulting in a conversion to calcium carbonate and hydrated silica gel. Carbonation can lead to deterioration such as loss of strength, loss of alkalinity, increase in deflection potential, carbonation shrinkage and increased susceptibility to external processes such as thaumasite & gypsum-form sulphate attack (carbonation providing a Ca-source in partially carbonated samples) and leaching of the cement hydrates. Whilst typically none of these processes on their own lead to failure all may contribute to loss of strength and durability of the RAAC material as a whole. Whilst research on modern AAC materials has progressed significantly in recent years due to the increasing need to better utilise SCM’s, further research is required on historically utilised systems. All of which are now operating far beyond their original 30 year service life expectancy. Whilst much examined plank material appears to be of a relatively sound condition, the material is operating far beyond its intended service life. The presence of factors such as microfracturing and evidence of cyclical moisture ingress into the RAAC suggests that the plank material is now no longer suitable for its purpose. It is suggested that, due to its age and identified deterioration, neither the RAAC plank or crucial reinforcement coating materials could be expected to perform as an effective protective material to the embedded reinforcement which forms the essential strengthening component in this structural system in the longer term. The key deterioration mechanisms leading to strength loss are mostly connected to moisture ingress, carbonation or poor construction detailing/physical alteration of planks and panels.

Further to the petrographically identifiable durability issues detailed above are the physical issues which should be assessed onsite. These include inadequate panel bearings, missing transverse reinforcement, anchorage, cracking, moisture ingress, easily damaged or altered planks (due to the ease of cutting and piercing), excess loading and high deflections. These physical attributes form the highest risk factors currently associated with structural collapse and failure, but can be exacerbated by the petrographically observable durability issues. These major issues are currently best summarised within the IStructE guidance documents [4].

Petrographic analysis

With regards to examination of RAAC materials, analysis, identification and management are best undertaken by suitably experienced and qualified onsite structural engineer-lead investigation. However analogue to investigations in calcium-aluminate cement concrete (more commonly HAC concrete) petrography can serve a number of applications, mainly definitive materials identification (identification of the steam cured poorly crystalline C-S-H to tobermorite identifiable through optical petrography, SEM/EDX analysis and XRD analysis) where plank compositions cannot be confirmed on site and comparative examinations to identify evidence of moisture ingress and active deterioration (particularly where the engineer suspected the degree of moisture ingress may have further altered and weakened the cement paste in key load-bearing planks).

As always if in doubt over a materials in situ performance expert advice from suitably qualified professionals should always be sought.

[1] DEPARTMENT FOR EDUCATION, Guidance, Reinforced autoclaved aerated concrete: guidance for responsible bodies and education settings with confirmed RAAC, August 2023, UNITED KINGDOM

[2] INSTITUTION OF STRUCTURAL ENGINEERS, Reinforced Autoclaved Aerated Concrete (RAAC) panels: Investigation and assessment – Further Guidance, March 2023, UNITED KINGDOM

[3] BUILDING RESEARCH INSTITUTE, BRE IP 10/96 Reinforced autoclaved aerated concrete planks designed before 1980, 1996, BRACKNELL, UNITED KINGDOM

[4] STANDING COMMITTEE ON STRUCTURAL SAFETY, SCOSS Alert May 2019 – Failure of reinforced autoclaved aerated concrete (RAAC) planks, 2019, UNITED KINGDOM

[5] TAYLOR, H. F. Cement chemistry (Vol. 2, p. 459). 1997. London: Thomas Telford

[6] POOLE, A. B., & SIMS, I. Concrete petrography: a handbook of investigative techniques. Second edition. 2016. Crc Press.

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