Background to the revision
One of the key drivers for revision of BRE Digests dealing with concrete in aggressive ground since the 1990s has been a growing recognition of the occurrence the thaumasite form of sulfate attack (TSA) in UK buildings and structures.
It has long been known in the UK that concretes made with Portland cements are vulnerable to attack by sulfates in the ground. For many years it was considered that the affected components of the concrete matrix were the calcium aluminate phases and calcium hydroxide, and that the minerals formed by this ‘conventional’ form of sulfate attack were ettringite and gypsum. Guidance on designing concretes to resist conventional sulfate attack was developed in a series of BRE Digests, the most recent of which was Digest 363 Sulfate and acid resistance of concrete in the ground, the first edition of which was published in 1991.
In the early 1990s, the thaumasite form of sulfate attack (TSA) became recognised as a separate mechanism affecting concrete in the UK. The distinguishing features of this are that it
· occurs preferentially at low temperatures (below 15°C, such as is typically found in the ground);
· requires availability of carbonate ions in addition to sulfate ions, with sources including limestone aggregate and bicarbonate in groundwater;
· targets the calcium silicate phases within hardened cement paste, potentially reducing concrete to a mush.
As part of its studies, BRE investigated three cases of TSA in the concrete foundations to domestic properties on Lower Lias Clay in the in the Cotswold area of England. In all three cases the concrete contained carbonate-bearing (limestone) aggregates. It was concluded that the concrete had suffered attack despite it satisfying the recommendations of the then-current version of Digest 363 in respect of Sulfates Class 3 ground conditions. It was apparent that the Digest needed to be revised to counter the risk of TSA occurrence and, in particular, to take into account the contribution made by carbonates. Accordingly, in 1996a new version of Digest 363 was issued which drew attention to the risk of TSA in concretes containing internal calcium carbonate and promised further guidance based on on-going research.
Subsequently, in 1998, several cases of TSA were identified in the foundations to motorway bridges in Gloucestershire. As in the previous cases, the concrete contained carbonate-bearing aggregates. The most severe occurrence resulted in severe concrete deterioration to a depth of up to 50 mm, exposing steel reinforcement to corrosion. The high profile of these cases ensured a co-ordinated national review, culminating in 1999 with a report from a Thaumasite Expert Group (TEG) set up by Government. The TEG Report gave interim guidance on specifications to minimise the risk of TSA in new construction and on the management of existing structures affected by TSA. It also gave recommendations for further research on occurrence of TSA and mitigating measures. Following publication of the TEG Report, BRE guidance was revised to incorporate the interim recommendations. This was published in 2001 as Special Digest 1: Concrete in aggressive ground (SD1:2001).
In the four years since 1999, much of the research recommended by the TEG Report has been completed. Key outcomes in respect of the mechanism of TSA and concrete specification have been:
(i) A confirmation that the carbonate required for TSA may come from a source other than aggregates in the concrete. In particular, it can come from bicarbonate dissolved in groundwater.
(ii) A clarification of the performance of various compositions for concrete recommended in SD1:2001 for aggressive sulfate conditions.
Together with other findings, such as deficiencies in guidance for ground assessment, the new knowledge has prompted the current major revision of SD1.