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TALKING POINT

Revisions to a BRE document still leave geotechnical engineers with issues to resolve says Tony Suckling.

The BRE recently revised Special Digest 1: Concrete in aggressive ground (GE August 05).

This new edition (SD1: 2005) was published as a single volume in May, following completion of a four-year research programme.

Key changes to the procedures for assessing the ground and for the specification of concrete are described at www. projects. bre. co. uk/sd1/changes.html. Though both comprehensive and flexible, the previous editions of SD1 had become long and complex and one objective of the latest edition is to simplify the guidance for practitioners.

While the changes are commendable, the piling industry still has issues with certain aspects of the document. In particular, limits for sulfate classes based upon 2:1 water/soil extract tests on soil have been dramatically lowered to bring them into parity with sulfate classifications based on groundwater.

The initial reaction to this lowering was dismay, with piling contractors expecting ground classifications to increase by at least one class.

However, Federation of Piling Specialist (FPS) members have been monitoring the consequences of this adjustment since May and, at the moment, it appears this may not happen - provided the SD1:2005 procedures for assessing the ground are applied pragmatically.

It is important to note there is no evidence of any widespread problems with pile concrete durability in the UK.

The reuse of piles is also becoming increasingly common. Therefore, piling contractors would resist major changes to their proven pile concrete mixes, which are designed for their workability, as well as for strength and durability.

Use of very high cement replacement pile concrete mixes (eg greater than 81% ground granulated blastfurnace slag (GGBS) replacement), as used for the infill piles in secant pile walls, are not permitted in ground where the sulfate class is greater than DS-1.

According to SD1:2005, secant pile walls using these mixes will not be allowed for permanent works, which has an obvious impact on their economic viability. Civil and Marine (a supplier of GGBS) recently employed BRE to investigate this issue in more detail. Work is still ongoing but it appears these mixes will prove to be durable in ground where the sulfate class is DS-3, and possibly DS-4.

Grout is not concrete! Only the ground assessment part of SD1:2005 is valid when grouts are being used. Grout is a fundamentally different material to concrete. For mini piles it is common to use a 1:1 sand: OPC mix with a cement content in excess of 850kg/m 3 and a water/cement ratio of 0.45. For ground anchors and soil nails it is common to use a neat OPC mix with a cement content in excess of 1300kg/m 3 and a water/cement ratio of only 0.4.

Importantly, these grouts are commonly mixed using a high shear colloidal mixer, which wets about 75% of the cement particles, compared to about only 25% achieved by the paddle mixers used by the ready mix concrete companies.

This superior mixing action produces grouts of high density and low permeability, thus improving durability.

Ground engineering contractors have recently experienced difficulties with mini piles and ground anchors in ground classified as being DS-3. The concrete specification part of SD1:2005 has been incorrectly enforced here and these grouts have been rejected for not incorporating either sulfate resisting cement or a blended cement.

Similar to concrete piles, there is no significant evidence of any widespread problems with grout durability in the UK. Are similar grout specification difficulties experienced by tunnelling and mine infilling contractors?

The principal liaison between BRE and industry to date has been with the concrete community, not the geotechnical community. A meeting was recently held between FPS and BRE to discuss these matters and Hilary Skinner of BRE welcomes comments from any other ground engineering specialists regarding SD1:2005 (by email to skinnerh@bre. co. uk).

Tony Suckling is technical development manager of Stent Foundations and chair of the Federation of Piling Specialists' Technical Committee.

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