RAAC (Reinforced Autoclaved Aerated Concrete) has hit the headlines in the UK for all the wrong reasons. Around a hundred schools have been forced to partially or fully close buildings and relocate classes due to a risk of roof collapse.
The whys and wherefores of the government and press response to the crisis will be hotly debated, but the focus will now be on how to make these buildings safe, at least in the short to medium term, to end the distress and disruption to school staff and their pupils.
In spite of the notoriety the material is quickly gaining, RAAC is not dangerous insidious material on a par with asbestos. When first conceived, RAAC represented a real innovation in the construction market and offered some remarkable properties that made it extremely attractive to engineers, specifiers and asset owners. It allowed lightweight and relatively inexpensive roofs with good thermal performance to be constructed quickly and safely.
In addition, RAAC provides an early example of using waste materials to cut the embodied carbon of cementious materials (if you ignore the autoclave part!). RAAC panels are made up of cement, blast furnace slag, pulverised fuel ash or silica flour mixed with water and aluminium powder. This slurry is then cast in a mould, with the steel reinforcement, where the chemical reaction between the aluminium and the other constituents increases the volume of the mixture around fivefold, by the formation of small hydrogen gas bubbles. The hydrogen then evaporates, leaving a cellular concrete mixture before the product is cured in an autoclave for between 8 and 15 hours at high temperature and pressure.
In many ways the story of RAAC is not that dissimilar to that of any other innovative construction material. Take away the sensational headlines around this ‘crumbly concrete’ and what you have is a product that was researched, developed and specified to fill a gap in the market and fix a problem on site. As a materials manufacturer, this remains one of Sika’s primary aims as we develop and test products in our R&D facilities across the globe.
Ignoring the specifics of RAAC, the lessons we can take from this furore from a construction materials point of view can be applied to any material that is used to construct or repair a structure, be it cementitious, ferrous or polymeric:
Specify the correct materials for the application
To specify the correct materials, it is essential that the installation and service conditions and design life are well understood. Installation considerations include access and time constraints as well as interaction with other materials. Service condition considerations include loads placed on the materials, exposure to chemicals, temperature and moisture. For repair projects, it is extremely useful to know the cause of the original damage.
- Curing conditions are critical for cementitious and polymeric products and have a dramatic effect on their cured properties, performance and longevity. Applying high performing materials in winter when the temperature is too low, or surface moisture content too high may lead to material failure, however specialist materials that cure and perform at low temperatures are available.
- In a concrete repair project, the cause of reinforcement corrosion must be understood and the level of risk assessed prior to the selection of a repair system. This can be done by a combination of visual inspection, on site and laboratory testing to determine whether chloride-induced or carbonation-induced corrosion is the culprit. Failure to select the right products is likely to lead to further damage in adjacent concrete, however products like corrosion inhibitors and galvanic sacrificial anodes can prevent further deterioration.
Once the installation and service conditions are fully understood it is essential for engineers and specifiers to engage and work alongside materials manufacturers and construction product suppliers to ensure that the products and systems specified will address the specific requirements of the project, ensuring the design life is met and a sound return on investment.
It is during the specification process that product experts can propose materials that have enhanced or unique properties to improve performance or reduce risk further. They may even be able to make suggestions about construction detailing to improve overall durability and performance.
It appears that at the time of construction, the concept of a RAAC panel was fit for purpose when considering the application and service conditions. What has led to the current situation is poor manufacturing quality, poor installation, lack of inspection and maintenance, wilful damage and poor planning.
Install it properly
You might have the perfect material or system for the job, but incorrect installation or failure to prepare the structure that is receiving the material correctly can risk poor performance or premature failure. In the case of RAAC it was found that in some cases, panels had been cut to size on site, removing much of the transverse reinforcement that they relied on for strength. In other cases, poor placement meant that the panels had insufficient bearing and risked slipping from their supports.
Contractor competence, experience and reputation is essential to ensuring the correct installation of construction products and evidence of this should be sought prior to the work. Materials manufacturers work closely with contractors to ensure they have all of the necessary support and training required to complete the work and often offer approved contractor schemes. In addition, manufacturers almost always offer phone or email technical support and installation guidance in the form of product data sheets, method statements and instructions for use.
Inspect and maintain it
All materials degrade over time. The rate and the mechanism varies, with this being dependent on the material itself and the environment it’s exposed to. There should be no ‘fit and forget’ structural or protective materials on any part of our national infrastructure or public buildings, irrespective of their use.
Regular inspection allows for the early detection of potential issues, monitoring and early intervention where appropriate. This can lead to greater understanding of the structure and allow for timely repair and maintenance interventions, which often incur lower costs in the long run than if problems are left to come to a head. Another benefit of this approach is that is possible to plan and budget for the ‘big ticket’ items in the life of a structure, or even demolition and replacement.
Aside from having no knowledge of the presence of RAAC, reported factors that have contributed to the risk posed have included pooling rainwater leading to increased loads, failed waterproofing leading to accelerated corrosion of the reinforcement and holes cut through panels to accommodate services. All of these issues could have been avoided or addressed by better understanding of the structure and regular inspection.
Ensure material quality
Variable manufacturing quality has been identified as a contributor to the failure of RAAC panels. Poor process control and quality of materials can mean that issues are effectively, and in this case literally, ‘baked in’ to the finished product, resulting in poorer than specified performance from the off, or an increase in the likelihood of deterioration later in life.
Nowadays, quality management standards such as ISO 9001 seek to ensure that the product of a business is of high and consistent quality and will perform as stated. Manufacturers should be able to produce Product Data Sheets and in some cases Declarations of Performance which detail the product’s performance characteristics with relation to national or international standards.
Furthermore, in the wake of the Grenfell Tower disaster, the UK Construction Products Association (CPA) has initiated the Code for Construction Product Information (CCPI), which seeks to promote an urgent and positive culture and behaviour change in the way the construction product manufacturing industry manages and provides information on their products.
Understanding our structures, what they are made from and what they need from us to perform is crucial to their longevity and both financial and environmental sustainability.
At the moment it’s hard to see past the headlines, opinions and disinformation surrounding RAAC, but we have to hope that the resolution of this issue, however it plays out, will lead to a change in the way we approach the life-management of our public buildings.
Sika does not have a solution to the structural remediation of RAAC roof panels. We do, however, offer products for adjacent works such as concrete repair, waterproofing, protective coatings and chemical anchors. If you would like to discuss Sika’s approach to specification and see what we have to offer for building repair and remediation, please email us.