THAUMASITE IN CONCRETE STRUCTURES: SOME UK CASE STUDIES

Thaumasite is a calcium sulphate carbonate silicate hydrate formed from the reaction of sulfates with silicate and carbonate in concrete exposed to cold wet conditions. The reaction can lead to softening, expansion and cracking of concrete. Unlike conventional sulfate attack, in which the calcium hydroxide and calcium aluminate hydrates react with sulfates to form gypsum and ettringite, in the case of thaumasite formation the calcium silicate hydrates in the cement paste can also be attacked. As a consequence, even concrete containing sulphate-resisting Portland cement and designed in accordance with the recommendations of BRE Digest 363 may be affected.

Following the identification of the thaumasite sulfate attack (TSA) to a bridge foundation in Gloucestershire in February 1998, over forty other sites have been investigated by Halcrow to confirm the extent of this form of sulfate attack. These investigations have found thaumasite formation in a variety of member types, concrete grades and exposure conditions. In some cases the sulfate reaction products were found to be almost pure thaumasite, whilst in other cases it was mixed with gypsum and ettringite. The pattern of deterioration has also varied widely from thaumasite formation without disruption to severe TSA related deterioration requiring repair and replacement of members.

This paper discusses four case studies in England and Wales, encompassing buried and nonburied members, mixes with Portland cement and sulfate-resisting Portland cement and disturbed and undisturbed ground. The paper discusses some key factors and features associated with the deterioration observed in each case in order to provide practical guidance to engineers and infrastructure owners and managers.

• INTRODUCTION

• CASE STUDIES

• CONCLUSIONS

• REFERENCES