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Thaumasite sulphate attack spreads

THAUMASITE SULPHATE attack has been identified on a highway structure outside the high risk CheltenhamGloucester area where the problem was first uncovered in early 1998.

Delegates attending a meeting of the Concrete Society in Reading, Berkshire, tonight will hear of the discovery of the rare form of concrete deterioration on a bridge foundation backfilled with burnt colliery shale.

Previous cases were all associated with iron pyrites-rich Lower Lias clay backfill, mainly in the foundations of highway structures on the M5.

British Cement Association associate director of concrete performance Don Hobbs will be the main speaker at the Concrete Society event. He told NCE this week that 25 overbridges on the M5 and three sets of house foundations had been found to be suffering from TSA.

'But all these were on Lower Lias clay. So far, there have only been two cases outside the Lower Lias area - the bridge with the shale backfill and a concrete slab on sulphate-rich fill, ' Hobbs added.

None of these latest examples of TSA attack is believed to be as serious as that found on the infamous Tredington-Aschurch overbridge on the M5 (NCE 2 April 1998).

But TSA is particularly worrying to infrastructure owners, as it occurs where initial site investigations found sulphate levels in the ground were low.

Limestone aggregates are particularly vulnerable if the pyrites decompose and sulphate levels soar.

For more details contact the British Concrete Association on (01344) 762676

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