A quality combined discipline Geo-Environmental Site Investigation should determine the contamination status of the site, but also, accurately quantify the geotechnical parameters for future foundation, road and pavement design. Whilst most geotechnical parameters are solely numerical indicators of physical soil behaviour, an area which often goes misinterpreted either due to technical misunderstandings, or cost critical investigation scopes, is the geo-chemical properties of soils and rock which is paramount to ensuring the longevity of built environment structures.

One consideration which is crucial to future development costs is accurately determining the Design Sulphate (DS) and Aggressive Chemical Environment for Concrete (ACEC) classification.

The difference in price per unit of concrete between a Design Sulphate Class 2 and DS-3 grade can be significant, sometimes enough to make or break a project, so it’s important to ensure that this construction phase risk item is appropriately assessed.

Conversely, getting the classification wrong can result in the incorrect specification of mix resulting in the degradation of concrete over the medium to long term, causing structural issues requiring costly remediation.  

In this blog we’ll discuss how to accurately determine the DS and ACEC classification to ensure risks and liabilities for developers are minimised.

‍How To Accurately Determine the DS and ACEC Concrete Classification‍

Stage 1: Desk Study

The degradation of concrete associated with sulphates in the ground relates to the sulphate oxidation, not as an isolated element but often as a compound, forming the mineralogy of the particular soils and rocks.

 The most common example would be the presence of pyrite (Fe­S₂). There are other compounds which may need consideration, which are summarised in the table below:

The oxidation of these compounds forms an acidic product which in turn can then attack concrete, which is of an alkaline composition.

As an example, the oxidation of pyrite results in Sulfuric Acid. This arises during the excavation of foundations, exposing geology to the open air. Similarly, this can occur during Earthworks cut and fill and requires consideration. The risk of sulphates being present in the ground during an earthworks setting may also influence any modification or stabilisation techniques, such as the use of lime. The reaction between calcium-based products and sulphate can cause damaging soil expansion as a result of chemical reaction.

Sulphurous geochemistry is present in the majority of UK soils, both of a natural and of a man-made nature.

Guidance set out in BRE SD1 Concrete in Aggressive Ground (2005) maps the natural strata across the country that may be susceptible.

However, there are exceptions. Even if soil deposits are not typically associated with sulphurous geochemistry, where they are formed through glacial and riverine processes, there could be sulphurous bedrock mineralogy which has been eroded and deposited by the above processes which  needs to be considered. In addition, where contamination is present within soils, this can also result in elevated Sulphate and other contaminants such as Chloride, Magnesium, Nitrate and Ammonium which all have the potential to react and form an acidic by-product.

For this reason, we recommend that the risk posed by sulphates in the ground needs to first be assessed during a detailed desk study. This would include determining the likelihood of contamination and the anticipated geology to determine the risk from Sulphurous geochemistry.

 Typical geologies to look out for which may need further testing and assessment are:

• Contaminated Made Ground (resulting in chlorine, ammonium, magnesium and nitrate deposition – oxidation to become an acid);
• London Clay;
• Kimmeridge Clay / Oxford Clay;
• Mercia Mudstone Group (particularly the upper weather horizons where this is recovered as clay);
• Weald Clay Formation;
• Devensian Till (particular where Till overlies coal measures bedrock);
• Localised Alluvium (particularly where rivers have cut into coal measures bedrock);
• Coal field geology i.e. Pennine coal measures (all iterations);
• Coal seams; and
• Any marine mudstone or siltstone geology.

Stage 2: Intrusive Site Investigation

Once a ground model has been developed during a Phase 1 Desk Study, this can be used to determine the appropriate scope of testing on the soils during intrusive Site Investigation to determine the DS and ACEC classification.

As a reminder, soil analysis determines the Design Sulphate (DS) classification, whilst groundwater testing determines the ACEC classification. In addition, Sulphates are only a risk if exposed to oxygen. If oxidation is not viable, e.g. no exposure of site soils to open air, then there may be minimal to negligible risk.

Where there is a risk of oxidation in sulphurous soils (i.e., presence of pyrite), the available fraction of sulphate in soils that has the potential to oxidise needs to be assessed, rather than just water soluble sulphate. These terms refer to Total Potential Sulphate (TPS) and Oxidisable Sulphate (OS).

On pyritic sites, you may have low water-soluble sulphates indicating DS-1, but if you have a high Total Potential Sulphate, this may bump up the classification to a higher grade. Without the right testing suite, this may not be identified.

To provide a general overview, there are three main scenarios which may be identified during any desk study, where the following testing would be required:

The Brownfield Land Scenario is also applicable where pyrite is considered to be absent.

If the correct suite of testing / required assessment is not clear on your site, please get in touch and we can discuss how best to determine the most appropriate testing as sometimes a particular scenario may not fit the generalised overview presented above.

Conclusions

Accurately determining the Design Sulphate (DS) Aggressive Chemical Environment for Concrete Class (ACEC) classification can have significant cost implications on a development project due to the price differences between concrete mixes. In addition, an incorrect classification can result in degradation of concrete, posing medium to long term risks to a developer and, in a worst-case scenario, result in structural risks to the built environment.

At Roberts Environmental Ltd we have considerable experience in carrying out accurate assessments to determine suitable concrete classifications that are safe to use within future developments.