【正文】 he matrix of the concrete or (b) _TF‘ for thaumasite formation where thaumasite is present in preexisting voids in the concrete, but there is no deterioration evident (a latent stage in development of TSA). The Report also contained practical information on the requirements and methods for identification and testing of concrete and soils in existing structures, to determine if thaumasite is present. Particularly important was the strong message to clients and designers of new structures to undertake a thorough soil survey as part of the design process. This has always been undertaken on road building projects and major civil engineering contracts, but less so on smaller works, and in the domestic property market. Chapter 9 of the Report provided detailed specification requirements for new construction to minimise the risk of thaumasite sulfate attack, based on the approach already adopted in BRE Digest 363 [1], namely by classifying the sulfate conditions in the soil adjacent to the designed concrete, and by setting a series of restrictions and options for the concrete mixes for each classification. However this approach was extended to adopt a ―package‖ of measures to deal with all ground classifications. It sought to leave as many options open to designers, and particularly to the contracting and concrete supply industries, and all such options were deemed to be equally effective. There was also a desire to keep all guidance and remendations as simple as possible, and user friendly. The Report remendations consisted of detailed materials requirements for concrete mixes and this was allied to a risk based strategy based on different structural performance levels, depending on the required service life and usage of the structure. There was an overarching assumption that the soil/ground water classification had been correctly assessed, and the remendations in chapter 6 of the Report had been followed, and so consequently the concrete requirements were appropriate to the existing and anticipated future ground conditions. The remended concrete options followed on from this correct sulfate classification of the soils, with due allowance for any sulfides present. Other related guidance involved consideration of the construction operations and the need, where appropriate, to provide additional drainage around structures, to avoid creating sumps, and where possible not to use reworked sulfate/sulfide bearing backfills. The concrete materials requirements embodied various controls on cement content, and free water/cement ratios, but adopted a new classification of aggregates by defining different carbonate ranges depending on the amount of carbonate present in the fine and coarse aggregate fractions. In terms of the structural performance level the philosophy adopted was one based on banding of structures into high, normal and low performance, representing a range from long service life structures to low performance for short service life structures, and structures with massive buried concrete foundations or those with slender or critical buried elements. This recognized that thaumasite attack in reality was assumed to be a relatively slow process, and may be insignificant for structures with only a short design life. More restrictive measures would be required when dealing with longer service lives for structures and critical/sensitive buried elements, and conversely less severe requirements imposed where structures had short anticipated service lives. Since there were a number of areas where the dearth of research data prevented authoritative remendations for materials, it was agreed that it would be prudent to adopt a _multilayered‘ protection approach, whereby a number of additional protective measures would be instigated, in addition to the material requirements, in appropriate situations. They would represent an important first line of defence and consisted of the adoption of surface protection and drainage requirements, as well as the choice of lower carbonate range aggregates in certain circumstances. Overall the Expert Group Report provided a number of important messages. The number of structures potentially at risk was not thought to be large, and the structural consequences not generally serious. The deterioration would generally provide early warning signs above ground where significant thaumasite sulfate attack was occurring below ground. There was also not thought to be any significant problem for domestic properties. The Report, if followed would minimise the risk of thaumasite sulfate attack, but also pointed out that not all the answers were known and that further research was needed. It also acknowledged that there was a need to update various documents and in particular BS 5328 [3] and BRE Digest 363 [1]. 4. Highways Agency guidance for new construction In parallel with the work of the Thaumasite Expert Group the Highways Agency set down its own advice for new construction, based on the Expert Group requirements. Interim Advice Note 25 [4] was issued, which laid down requirements to be adopted for highway structures, to minimise the risk of thaumasite sulfate attack in new construction. In particular it assumed that in most cases highway structures would be deemed to be high performance level, as befitting the required 120 year service life. The Advice Note also provided some more specific guidance on drainage around structural foundations and the type of protective coatings appropriate for application to buried concrete in highway structures. 5. Highways Agency strategy for existing structures However as well as dealing with new construction, the Highways Agency also needed to address the issues of the occurrence of thaumasite in existing structures and particularly whether it was