AGS – Association of Geotechnical and Geoenvironmental Specialists

The Building Research Establishment (BRE) has recently revised Special Digest 1 “Concrete in aggressive ground”. This new edition (SD1: 2005), funded by The Concrete Centre, was published as a single volume in June, following the completion of a four year research programme on combating the thaumasite form of sulphate attack (TSA) . There are two key changes to the procedure for assessing the ground;

1. The limits of the design sulphate classes based upon 2:1 water/soil extract tests on soil have been reduced to bring sulphate classifications based on 2:1 water/soil extract tests and on groundwater into parity. This will result in some sites being rated as more aggressive to concrete than hitherto.

2. High magnesium levels are no longer taken into account for natural ground.

There are five key changes to the procedure for the specification of concrete;

1. The recommended maximum water/cement ratios and the minimum cement contents have been revised.

2. A new classification for cements and blended cements has been introduced to harmonise with European standards.

3. The recommended concrete quality now caters for the inherent possibility of exposure to an external source of the carbonate required for TSA.

4. The number of additional protective measures to be applied at higher sulphate levels has been reduced, in general by two.

5. The use of the concept ‘intended working life’ replaces that of ‘structural performance level’ to harmonise with European standards.

BRE has told FPS that the take up of its revised SD1 would likely be slow as it has received no funding to mount a promotional campaign. It is relevant to note that currently some of the on-line information services are still offering just the SD1:2003 version, some 6 months after publication of the latest edition. Also, even if professionals do know of the existence of SD1:2005, they may not feel obliged to use it at the moment as the current edition of BS 8500 Concrete refers to SD1:2003. BS 8500 will unlikely to be updated before the latter part of 2006.

It is often not clear which version of SD1 has been used to classify the ground and the concrete.

The members of the Federation of Piling Specialists (FPS) are supplied with numerous site investigation reports and Engineer’s Particular Specifications every working day. Currently it is often not clear in these which version of SD1 has been used to classify the ground and the concrete. It is the exception when clear reference is made to say SD1: 2005.

FPS requests that in future all site investigation reports and Particular Specifications make clear reference to SD1: 2005. Our Clients can then be confident that piling concrete is provided in accordance with the latest requirements by eliminating the potential for misunderstanding to enter into the specification process.

Grout is a different material to concrete and SD1:2005 is only applicable for concrete.

In addition, FPS also requests that where the foundation solution may comprise minipiles, ground anchors, soil nails, grouting, base or shaft grouted piles, permanent sleeves to piles, or cross-hole sonic logging of piles, i.e. any case where grout is likely to be used either in lieu of, or in addition to, concrete, that reference is not made to SD1: 2005 for these options. This is due to the fact that grout is a different material to concrete and SD1: 2005 is only applicable for concrete.

Tony Suckling
Technical Development Manager Stent Foundations Ltd

Chair Technical Committee
Federation of Piling Specialists

All too often Site Investigation work is seen solely to provide soil strength parameters to enable economical foundation design. There is a need for the site investigation industry to make clients more aware that the aggressive nature of the ground should also be accurately determined if adequate precautions are to be taken in the design of a durable concrete for use in the foundations.

The problems associated with the thaumasite form of sulfate attack (TSA) have been well documented and in August 2001 BRE Special Digest 1 was published. Part 1 of the digest is particularly relevant to the site investigation industry. Without the necessary soil and ground water testing to determine the extent of those aggressive chemicals present at a particular site the concrete cannot be designed in accordance with best practice. It is where possible beneficial to have results from both groundwater and soil samples.

Many Site Investigation reports are issued without fundamental site-related parameters to enable the adequate design of the concrete.

The site assessment procedures should vary depending on whether the site can be defined as natural ground, brownfield containing industrial wastes or pyritic ground, reference to BRE Special Digest 1 should be made for full details.

In general it will be necessary to determine the water soluble sulfate in 2:1 water/soil extracts and the pH in 2.5:1 water/soil extracts. Many Site Investigations where they report any chemical testing only show an occasional soluble sulfate result which is often inadequate to determine the Design Classification for the concrete mix. Where the sulfate in the soil extract exceeds 3.7 g/l SO4 or in the groundwater sample exceeds 3.0 g/l SO4 it is necessary to also determine the Magnesium content. The mobility or otherwise of the groundwater on site also has an affect and should be established.

Where a site is brownfield it will generally be necessary to obtain the Chloride and Nitrate content in both the soil and groundwater samples if the aggressive chemical environment for the concrete is to be accurately determined. Where Pyritic ground conditions are anticipated more substantive testing is required to enable the total potential Sulfate and hence the concrete design requirements to be determined, for full details reference should be made to BRE Special Digest 1.

It should be apparent from the above that greater consideration needs to be given to determining the aggressive chemical environment at the site investigation stage than is currently the case, to determine site-related parameters for strength in one site investigation and then undertake further work at a later date to enable the Aggressive Chemical Environment for Concrete to be determined is no way for the industry to improve its standards or its advice to clients.

It should also be noted that BRE Special Digest 1 has superseded BRE 363.

D.Brightman Technical Manager, Rock & Alluvium

Association of Geotechnical & Geoenvironmental Specialists

BRE SD1:2005 – Implication for SI & Specifications

SITE INVESTIGATION SHOULD BE FOR CONCRETE DURABILITY IN ADDITION TO SOIL STRENGTH PARAMETERS