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We encountered these blown sills on this apartment block. These are becoming quite common, make your property look rough and can really be repaired with minimal mess and non-intrusively. So why did these sills blow in this way and what does blow mean? Well blow simply means the sill has split apart along its length due to an internal swelling. With many older buildings, materials break down over the years and this apartment block is no exception. Inside these sills there is an amount of steel reinforcing – steel is frequently used in concrete products to reinforce or give strength. Many houses use concrete lintels too (Thats the strong piece above the window that stops the brickwork above the window or door from falling down), and these can suffer a similar fate – blowing.

The material that is primarily breaking down in this case is not the concrete, rather the steel reinforcing within it. The concrete breaking down in this blown sill is a byproduct of a more fundamental issue and a curse we often come up against – Rust. Rust is a type of corrosion. It happens to iron and its alloys when it is exposed to air or water for a long time. Rust slowly decomposes iron into other chemicals, because of a reaction with the oxygen molecules. Both air and water are needed for rust and it swells. In this case, it swells to split the sill wide open and if left unremedied can be extremely dangerous. Large sections of concrete could fall onto pedestrians below as both the upper and lower layer of the sill breakdown at the same time.

A sill is made where concrete is poured around a steel reinforcing structure – in this case the concrete has gained porosity over the years giving rust the two things it needs to decompose – air and water.

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