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Bridge blend

I write in response to your article 'Sulphate attack on M5 bridges' (NCE 2 April). I ask, how often has sulphate attack been observed on bridge structures? I then ask, how often has chloride attack on the steel reinforcement been observed in such structures?

If concrete is made using ground granulated blast furnace slag cement then the rate of deterioration due to chloride-induced corrosion of the steel reinforcement is very much reduced. The slag also reduces the risk of sulphate attack to the structure. To use sulphate resisting cement very much increases the permeability of the concrete to chloride and so precipitates chloride attack of the steel reinforcement.

There may just be some merit in adopting the use of an OPC slag cement blend, a part of the requirements of the Department of Transport Standard BD27/86, Materials for the repair of concrete highway structures, for new construction rather than waiting for it to require repairs.

David R White-Cooper, The Hets Laboratory, Horsbury Road, Ossett, Wakefield, WF5 0BZ.

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