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Report backs quality concrete against TSA

Continued from page 3.

GOOD QUALITY, well-compacted low water/cement ratio concrete is the first step towards fighting Thaumasite Sulphate Attack, the expert group concludes.

Water-reducing admixtures can help achieve this objective, but beyond this, the key factors have been identified as correct aggregate and cement selection, plus careful consideration of the material to be used as backfill.

The expert group's report* concluded that precautions were only appropriate where soil and groundwater sulphate conditions were equivalent to Classes 3 to 5 of BS 5326: Part 1. As sulphate concentrations increase, the percentage of CaCO3 in the aggregates becomes more critical.

The report categorises aggregates into three ranges, with Range A materials having CaCO3 contents of between 2% and 12% and Range C covering aggregates containing between 30% and 100%.

Limestone filler cements, which may contain up to 20% ground limestone, are restricted to Class 1 sulphate conditions by the expert committee recommendations. Where limestone additions are below 5% there is no evidence of additional risk of TSA, the report concludes.

But evidence was found that significant additions of blastfurnace slag or pulverised fuel ash could significantly increase resistance to TSA, as has been found to occur in the more common ettringite form of sulphate attack.

All the committee's recommendations are summarised in Table 9.1 of its report, which gives minimum cement contents and maximum water/cement ratios for every likely combination of ground conditions and aggregate and cement types.

But the expert group concludes that 'there is little information available on the protection afforded by commercially available coatings and tankings'. And it warns against the use of backfill materials with high sulphate or sulphide contents.

*The thaumasite form of sulphate attack: Risks, diagnosis, remedial works and guidance on new construction. Copies are available from the Department of the Environment, Transport and the Regions. Tel (01709) 891318.

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