New BRE Special Digest 1: consultation

The third edition of BRE Special Digest 1 (SD1:2005), was published 17 June 2005.
inside of office

Changes to sulfate classification

The current and intended new limits for sulfate classes based on 2:1 water/soil extraction test are as follows:

Sulfate class

Current limits

g/l SO4

New limits

mg/l SO4





1.2 – 2.3

500 - 1500


2.4 –3.7

1600 - 3000


3.8 - 6.7

3100 - 6000




The consequence of this adjustment will be to make the ground classification based on soil tests more conservative, eg some soils that were previously classified as DS-2 would now be considered as being DS-3.

The change stems from findings of numerous research ground investigations carried out by BRE and others on BRE concrete trial sites and locations where TSA has occurred. In the majority of cases, the sulfate class limits based on soil extract tests were both lower than sulfate class based on sulfate in groundwater and were also low when compared to the actual occurrence of TSA. The new limits bring sulfate classification based on 2:1 water / soil extract tests into parity with the groundwater based tests.

A review of the historical background to sulfate assessment has thrown light on how the current discrepancy came about. [1]  Guidance for concrete in the ground in terms of sulfate classes based on sulfate in water/soil extracts was introduced by BRS in 1968 in Digest 90. Initially 1:1 water/ soil extract was used and the limits were arbitrarily set at 1.2, 2.5, 5.0 and 10.0 g/l SO3, ie approximately doubling at each step. No field data would appear to have been available for correlation with sulfate classes based on sulfate levels in groundwater. These ball-park limits were converted in 1975 in BRE Digest 174 to 2:1 water / soil extract limits to facilitate easier filtration of the sulfate-containing solution from clays. They were further changed in BRE Digest 363:1991 from SO3 to SO4 units. It is unfortunate that, through these changes, they gained an apparent precision that has imparted greater confidence in the derived classification than is warranted.

 [1] Longworth T I. Development of guidance on classification of sulfate-bearing ground for concrete. Concrete. Vol 38, No 2, February 2004, pp 25 –26.