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AARtfully done

Letters

Your article on cement with higher than certified alkalis, leading to alkali aggregate reaction (AAR) risk in the South West, beautifully illustrates how complacency, 30 years after the UK's fi rst AAR at Charles Cross, has led to a recurrence of concerns.

Lafarge, having identified rogue data, seems to be taking a responsible approach. Its problems will be aggravated as cement industry pressure on the Hawkins' Committee reduced the safety margin for variability of cement alkali and content in concrete.

A ominal kg/m 3 alkali', actually gives up to 3.5kg/m 3 average in some concrete pours, with over 4kg/m 3 locally from normal variability and segregation.

Checking the actual risk of damage in structures from this high alkali cement must be based on rigorous IStructE techniques and comparisons with data from the investigation and appraisal of structures.

These evaluate the structural sensitivity, environment, expansive potential and consequences of failure to give an overall measure of the severity of potential consequences of AAR Jonathan GM Wood, Structural Studies & Design, Northbridge House, Chiddingfold, Surrey GU8 4UU.

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