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Key points in assessing TSA

Further to your article 'Responding to thaumasite form of sulfate attack' (GE April 1999), we would like to raise some issues which we have identified when assessing potential sulphate using the DETR Expert Group report on thaumasite sulfate attack (TSA).

We appreciate the author's difficulty in deciding what to include and what to leave out of his article. Nevertheless, when considering the potential for TSA there are a few key points, largely from the DETR report, which perhaps have not been given a sufficiently high profile in the various articles and discussions in circulation.

The problem of oxidation of sulfides into sulfates, thus raising the severity of the exposure conditions, is not specific to TSA.

The same conditions in concrete containing non-carbonate aggregates are likely to result in conventional (ettringite) sulfate attack if the sulfate class is not raised to take due account.

Oxidation of sulfides into sulfates need only be considered where the ground is sufficiently disturbed to allow rapid oxidation and will be subsequently in contact with the concrete (or close enough that groundwater from it could move to the concrete). Typically this would be cutting and filling to terrace a site or excavation and backfilling, such as frequently happens during a site reclamation. Under natural conditions this process is extremely slow. Simply exposing a new face, such as in a trench or a bored pile in previously undisturbed ground, would not provide the necessary conditions for rapid oxidation. If the construction method is known it may not be necessary to assess the potential sulfate class. On the other hand, where the construction method is not known, it may be necessary to reclassify the site anyway.

Oxidation of sulfides into sulfates is not a necessary condition of TSA. Concrete in class 3 or class 4 sulfate conditions and made with sulfate- resisting Portland cement and limestone aggregates, for example, could suffer TSA in cold, wet conditions, even if designed to the current BS5328 or BRE Digest 363 recommendations. Concrete designed to BS5328/BRE Digest 363 may need to be supplemented by additional measures such as drainage and/or surface protection even when the sulfate class has been correctly identified.

Our own recent investigation has identified several standard methods used by chemical laboratories for determining total sulfur. However, it appears that not all the methods indicate maximum total sulfur present. When specifying total sulfur tests for calculation of potential sulfate content it is thus important to specify a suitable test method when scheduling the test.

The article also raises the question of whether measures such as ground drainage or a protective coating can be used as an alternative to specifying concrete which will withstand high sulfate levels as stated. Our reading of the DETR report suggests that these measures are additional to the specification of sulfate resistant concrete. We hope this information is of use to your readers.

Melanie Woodburn, Bryan Marsh and Jim Johnson

Ove Arup & Partners, London

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