It has been dubbed a “concrete crisis.”
More than a hundred schools have been ordered to close across the United Kingdom due to concerns about the risk of collapse in a particular type of building material, called reinforced autoclaved aerated concrete.
The product is widely used in Australia too for residential, commercial and infrastructure applications. Should we be concerned?
Cosmos asks infrastructure engineer and concrete innovation enthusiast Dr Xuemei Liu from the University in Melbourne.
What is autoclaved aerated concrete?
Conventional concrete comprises water, aggregates (like sand, gravel and crushed rock) and a binding agent called Portland cement.
Liu says the main difference between conventional concrete and autoclaved aerated concrete (AAC) is the lack of course aggregate, and the addition of a foaming agent that reacts to create small bubbles throughout the material.
The result is a much lighter construction material, around a fifth of the density of normal concrete. AAC is often produced in pre-cast panels, and commonly used for walls, for cladding and sometimes roof structures.
Like regular concrete, when AAC is used in a load-bearing capacity it is usually reinforced with steel.
Why is it used?
Liu says AAC has a lot of benefits: it’s relatively strong, a great material for insulation and sound proofing; and has high fire resistance.
And because the material is less dense, it has lower greenhouse gas emissions by volume than regular concrete.
What are the risks?
If properly designed, Liu says, conventional concrete can last anywhere from 50 to more than 100 years. Whereas the service life of properly maintained AAC is more like 30 years.
Conventional concrete has better durability in part because the denser structure prevents water from penetrating.
Liu says AAC is more porous and brittle, which means moisture can more easily get inside and over a longer timeframe, potentially corrode the steel reinforcements.
An additional factor in the UK is the age of public school buildings, some dating back to the 1950s. This means the reinforced AAC is reaching the end of its service life, Liu says.
“It doesn’t mean the structure will collapse if properly maintained,” she says, but authorities do need to monitor for changes, make sure there’s no water ingress and that no new loads have been added to the structure.
According to Liu, the risks also depend on what the AAC is being used for. In walls or cladding the material can be more easily replaced at the end of its service life.
Whereas in a load-bearing capacity, like a roof or floor, there’s concern if the material’s performance has weakened or deteriorated due to environmental conditions like water, or building refurbishments and large loads causing the material to deform.
What is the situation with AAC in Australia?
In Europe, early use of AAC dates back almost a century. It has been widely used in construction since the 1950s. AAC manufacturing quality and structural design has improved over time.
Meanwhile In Australia, AAC has only been used for about 30 years, and mainly in walls or cladding.
AAC is “still a good material to use”, Liu says, provided the material has properly manufactured, designed, used and maintained according to the relevant standards.