The new mix design, developed in close collaboration with Laing O’Rourke as part of its joint venture with Ferrovial Agroman for BAA, incorporates high levels of pulverised fuel ash (pfa) combined with high range water reducing admixtures to lower the heat generation in the concrete to minimise thermal gradients in the structure and reduce the risk of early age thermal cracking.
The innovative concrete mix was developed at CEMEX’s UK National Technical Centre in Southam, Warwickshire over a two month period as part of the £7 million contract secured by CEMEX for supplying over 100,000 cubic metres of readymixed concrete towards the construction of Terminal 2.
The new concrete mix incorporates factory blended cement from CEMEX’s new flagship cement plant in Tilbury, Essex, CEM II/B-V, in combination with pulverised fuel ash which is a by-product from coal fired power stations, and limestone aggregates to produce a low heat concrete.
The relatively high proportion of pfa in the mix has the additional advantage of reducing the overall carbon footprint. The emissions in the CEMII alone are reduced by 25% when compared to CEMI.
CEMEX UK’s Technical Director, Steve Crompton explains the development: “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.
“Following rigorous investigations and testing, we arrived at this new mix design, which incorporates a unique combination of CEM II/B-V with pulverised fuel ash combined with state of the art admixtures manufactured at Cemex’s own admixture factory in Rugby. The result is an easy to place, low heat concrete. The mix performed even better than expected with the peak temperature recorded on the first pour reaching just 53oC despite being placed on one the warmest days of the year so far.”
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