Crumbling Classrooms: The Science behind Weakening School Foundations

by Harvey Xu

A recent rise in school closures across the country due to raising concerns about the safety of infrastructure has caused widespread anger and frustration in regards to children’s quality of education. As of now, 100 buildings have closed, forcing a transition to remote learning in many schools, but not for the usual reasons. Instead, the widespread use of Reinforced Autoclaved Aerated Concrete (RAAC) in school construction has pushed the government to shut all building infrastructure using this material. With an estimated 30 year life span, the National Audit Office reported this year that over 600 buildings in schools may have to close to maintain the safety of staff and students. 

What is RAAC?  RAAC is a type of building material that gained popularity due to its low cost, light weight, and purportedly good insulating properties. It has been extensively used in the construction of schools and other buildings, particularly between the 1960s and 1980s. RAAC is produced by curing a mixture of sand, lime, and cement in an autoclave, which subjects the material to high-pressure steam. This process generates a porous concrete that is lighter and less dense compared to traditional concrete. Reinforcement is then added to increase its tensile strength.

On the surface, RAAC might seem like a constructional marvel. It's cheap, easy to work with, and offers advantages like good thermal insulation. However, what makes RAAC a favourable choice for budget projects is also what makes it a ticking time bomb in terms of structural integrity. 

The porosity of RAAC is its primary drawback. Unlike traditional concrete, which is denser and more robust, the porous nature of RAAC makes it susceptible to moisture infiltration. Moisture weakens the structural integrity of the concrete over time causing it to lose its load-bearing capabilities. Traditional concrete is primarily composed of calcium silicate hydrates which is a remarkably stable compound formed from the hydration of Portland cement. RAAC, on the other hand, contains a more significant proportion of weaker compounds like calcium hydroxide. These weaker bonds are less durable and more prone to degradation over time.  Reinforcement Corrosion Reinforcing material, typically steel bars, are embedded into the aerated concrete to improve its tensile strength. However, RAAC's porous nature makes it easier for water to reach the steel causing it to rust and corrode. Corroded steel loses its tensile strength which makes the reinforced concrete weak and susceptible to structural failure. In regions where weather is extreme or highly variable, the impact on RAAC could be even more drastic which leads to quicker wear and tear. The school closures are an immediate response to the looming threat that RAAC poses, but they raise several long-term questions:  

How can the existing schools be reinforced to prevent further closures? 

What alternative materials should new schools use to ensure long-lasting structural integrity? 

How can school administrators and policymakers execute this crucial transition without compromising the quality of education?