6 May 2003 – The Structural Engineer| 15 technical update: thaumasite F ollowing the identification of the thaumasite form of sulfate attack (TSA) on the concrete foundations to a number of bridges on the M5 motorway in 1998, the then Minister for C onstruction, Nick Raynsford, established the Thaumasite Expert Group (TEG) under the chairmanship of Les Clark, Professor of Struct ural Engine ering , Univer sity of Birmingham. It was asked to report on the nature and threat of this phenome- non and to provide interim guidance on its avoidance. The Report of the TEG 1 was published by the Department of Environment, Transport and the Regions (DETR) in January 19 99, and a summary of the structural implica- tions of TSA was published in The Structural Engineer the following month 2 . At the Mi nister ’s requ est, the Report was followed up in Spring 2000 by a One -Y ear Review 3 of the Report’s factual statements and guidance in the light of new developments on TSA. A second review 4 of the Thaumasite Expert Group Report has now been undertaken following 3 years experi- ence up to March 2002. Following the demise of DETR, this revi ew was accepted by the Office of the Deputy Prime Minister (ODPM) in January 2003. The principal sections of the Review are summarised in a paper in the journal Concrete prepared by the Group 5 . This article g ives a brief summary of the main conclusions of the Review and highlights a structural issue. Current impacts and awareness The TEG Report continues to be well received by the construction industry and is broadly viewed as well balanced, safe and robust. Its publication ha s not had a significant adverse impact on the various stakeholder sectors. In respect of existing buried concrete construction, the greatest impact of TSA to date has been on the Highways Agency (HA). The HA is undertaking a strategic assessment of structures in its care to check for TSA. Its experience has shown that where the risk factors identified in the TEG Report are all present, the prevalence of TSA is high, although the severity varies. Where necessary , remedial measures involv- ing concrete repair and protection have been undert aken, providing assurance of serviceability and safety . The TEG is concerned that no other owners of existing structures with buried concrete elements have under- taken a similar assessment. The expe- rience of the HA indicates that there could be a large number of structures with some degree of attack to buried concrete. It is therefore important that owners of key structures do assess the risks and take necessary measures. New guidance A substantial body of new guidance has been published in 2000 – 2002. This has evolved from the TEG Report but has taken into account industry consultations and new research find- ings. Publications include a new BRE Special Digest 1: Concrete in aggressive ground 6 , Amendments to BS 5328: Concrete 7 , Amendments to BS 882: Specification for aggregates from natural sources for concrete 8 , new BS 8500: Concrete – complementary British Standard to BS EN 206-1 9 , an d a new BCA Guide: Concrete resistant to chemical attack 10 . There have been a few problems in interpreting and/or implementing the intentions of the guidance given in the TEG Report in some situations. These problems have been addressed through publication of the new guidance listed above and through discussions with the relevant industry bodies. In partic- ular , problems were encountered b y the manufacturers of precast concrete construction products, including those for precast concrete pipes and tunnel linings, in complying with the TEG Recommendations. These ha ve been resolved by the inclusion in the new BRE SD16 of specific design guidance for these products. Guidance in preparation Key items of guidance which have been prepared for publication in 2003 are: • A second e dition o f BRE SD1 t o align included cements or combination groups with the new BS EN 197- 111 11 classification. • A revisi on of the NHBC Stan dards , Chapter 2.1, Concrete and its rein- forcement 12 . New field cases of TSA About 50 new cases of TSA have been identified since the publication of the TEG Report and 30 since the publica- tion of the first review in 2000. Most of these were in the foundations of bridges founded on Lower Lias Clay (Charmouth Mudstone Formation), found as a result of HA investigations in Gloucestersh ire, Somerset and Wiltshire. The remaining ca ses have occurred in a wide range of structures, buildings and environmental condi- tions. Most of these new cases have taken place in conditions that were anticipated in the TEG Report. Significant features which have emerged from inspection of these new cases of TSA are: • Three fu rther so urces of su lfate in the ground have been identified leading to TSA: Kimmeridge Clay , Rheatic Mudstone and railway ash. • Two new fie ld cases o f TSA i n buried concrete containing siliceous aggre- gates, reinfor cing a concern expressed in the TEG Report that concretes containing little or no carbonate in aggregates can be affected by TSA if an external source of carbonate ions is available, for example from groundwater . This is of significance since currently the guid- ance for specification of concrete presumes that restriction of the carbonate content of aggregate is an effective measure for combating TSA. Present understanding of the problem is insufficient to underpin amendment of the guidance , but further research is underway . • Investigations have conc luded that bituminous waterproofing applied to some of the buried concrete exposed to sulfates may have prevented the onset of TSA. Research on occurrence and mitigation of TSA Recommendations for future research given in the TEG Report are being largely met by projects initiated by the BRE, the universities and industry. Initial findings of particular signifi- cance are: • The mix design s recommended i n the TEG Report are robust. • Signif icant deterioration in some concretes not meeting the recom- mendations in the TEG Report have been observed in both laboratory and field trials after as little as 3 years exposure to sulfate Class AC-3 conditions. • Mixes cont aining g round gra nulate d blastfurnace slag (ggbs), in sufficient proportions, continue t o demonstrate excellent resistance to TSA. New guidance and progress on thaumasite control A summary the findings o f the Thaumasite Expert Group Report: ‘Review of structural aspects after 3 years experience’