Intended key changes in draft SD1:2005 as compared to SD1:2001
Two key changes have been made to the procedure for ground assessment as compared with the previous edition of SD1:
· The limits of the Design Sulfate Classes based on 2:1 water/soil extract tests on soil have been reduced, making this classification route more conservative. The change stems from findings of several research investigations on ground carried out by BRE and others. Sulfate class limits based on 2:1 water / soil extract tests on soil have been found to be substantially lower than sulfate class based on sulfate in groundwater. The new limits bring sulfate classification based on 2:1 water / soil extract tests into parity with sulfate classification based on groundwater (more details).
· There is no need to take high magnesium levels into account when determining the ACEC class of natural ground in the UK.
The following key changes have been made to the procedure for concrete specification:
· The recommended maximum w/c ratio and minimum cement content have been revised, and a new classification for cements and combinations has been introduced.
· The recommended concrete quality now caters for an ever-present possibility of exposure to an external source of the carbonate required for TSA (principally bicarbonate in groundwater). The concept of Aggregate Carbonate Range is therefore no longer included, since the revised concrete specification simultaneously caters for an internal source from carbonate in aggregates. A further consequence is that starred and double-starred concrete qualities that related to restricted aggregate carbonate content are no longer included.
· The number of APM to be applied at higher sulfate levels has been reduced, in general by two. This follows from a higher level of confidence in the provisions for the concrete.
· The use of the concept ‘Intended Working Life’ replaces that of ‘Structural Performance Level’. This is for harmony with European standards such as BS EN 206-1.