Low-temperature Cemex mix intended to reduce cracking risk

The foundations of the new Terminal 2 building at Heathrow are using a new concrete mix that generates considerable lower temperatures than normal.

The mix has been specially developed by Cemex in collaboration with main contractor Laing O’Rourke and incorporates high levels of pulverised fuel ash combined with high range water reducing admixtures to lower the heat generation in the concrete. It is hoped this will minimise the temperature difference between the core of the pour and the surface, reducing the risk of early age thermal cracking.

The new concrete mix incorporates factory blended cement in combination with 40% pulverised fuel ash - a by-product from coal fired power stations - and limestone aggregates to produce a low-heat concrete.

Steve Crompton, technical director of Cemex UK, says: “A traditional concrete mix could easily generate peak concrete temperatures in excess of 700C potentially leading to issues with early age thermal cracking. Given the substantial pours this contract involves, it was a key challenge to minimise the heat generation in the concrete whilst still meeting the strength and durability requirements for the structure”.

The special mix has performed better than expected with the peak temperature recorded on the first pour reaching just 53C despite being placed on one the warmest days of the year so far.

Cemex is supplying over 100,000m3 of concrete for the construction of Terminal 2, with a contract value of £7million.