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Aggregate ban on the cards

AMENDMENTS to the Highways Agency's specification intended to prevent thaumasite sulphate attack on concrete structures could see the use of limestone aggregates banned in high risk areas.

The new specification has been drawn up following the Government-backed expert group report on TSA, published in January. The amendments are due to be made public in two weeks time.

It is understood Agency concern centres around Table 9.1 of the expert group report, which gives advice on mix designs for TSA resistance in a range of ground conditions. There is no general ban on any particular aggregate or cement type, but some cement types are ruled out as ground sulphate levels increase.

But for structures with a 120 year design life - such as motorway structures - aggregates containing less than 30% calcium carbonate are recommended. For the worst sulphate conditions only aggregates with a calcium carbonate content of less than 12% are backed. But it is known that ground sulphate levels can increase with time due to the oxidation of pyrites in certain clays and this can cause difficulty.

Experts believe one simple way to ensure long-term resistance to TSA that is being considered is to specify only the lowest possible calcium carbonate content - effectively banning limestone.

Aggregate industry sources said that if the Agency went for this option it would increase the cost of motorway structures and cause confusion among concrete specifiers throughout the UK.

Agency senior technical advisor Neil Loudon, its representative on the expert group, refused to give details of amendments to the structural specification at last week's BRE seminar on TSA.

DP

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