Design sulfate class and why buried concrete depends on it

A design sulfate class, written DS-1 through to DS-5, is the figure that tells your engineer how aggressive the ground chemistry at your site is towards buried concrete. It comes from soil and groundwater sampling rather than a desk study, and it feeds the aggressive chemical environment class, which in turn sets the concrete specification for foundations, ground slabs and buried drainage. On a cleared plot where demolition rubble has been pushed into the made ground, that single classification often decides whether ordinary foundation concrete will do.

Planning officers rarely ask about it by name. Building control will.

What sulfate attack does to buried concrete

Sulfates dissolved in soil water react with the hydrated aluminate phases in Portland cement to form ettringite. The reaction expands. Buried concrete cracks, softens at its face and loses cover over the reinforcement, and none of it is visible from above ground until something settles.

There is a second and nastier mechanism. The thaumasite form of sulfate attack needs sulfate, a source of carbonate, groundwater that moves and a cool ground temperature, and it goes after the calcium silicate hydrate that gives concrete its strength rather than only the aluminates. Affected concrete turns to a pale mush with no structural value. Findings of that kind on motorway bridge foundations in Gloucestershire in the late 1990s led to the Thaumasite Expert Group report and, eventually, to the rewriting of UK guidance into BRE Special Digest 1.

So the classification is not an academic exercise. It is the reason two identical houses, 400 metres apart, can need different concrete.

How the design sulfate class is derived

The class reads water-soluble sulfate concentration and places it in one of five bands. For soil samples the measurement is taken on a 2:1 water to soil extract; for groundwater it is measured directly. DS-1 sits at the benign end and DS-5 at the hostile end. Magnesium becomes relevant at the top of the range, which is why the highest classes carry an "m" qualifier where magnesium concentration is high enough to change the chemistry of the attack.

Pyritic ground is handled differently, and this catches people out. Where sulfides are present and could oxidise once the ground is opened up and aerated, the assessment works from total potential sulfate rather than what happens to be soluble on the day of sampling. Excavation creates the problem. Pyritic clays, colliery spoil and some Lias deposits all behave this way.

Nor is the design value simply the worst result from one trial pit. BRE Special Digest 1 sets out how to treat the spread of results across a site, and the current edition should be read rather than a half-remembered rule of thumb.

From design sulfate class to a concrete specification

The DS class on its own specifies nothing. Combine it with the pH of the soil or groundwater and with the character of the site, and you arrive at the aggressive chemical environment for concrete class, AC-1 to AC-5. Two factors drive the jump between bands. Whether groundwater is static or mobile, and whether the ground is natural or brownfield. Brownfield ground is assumed to behave worse at the same measured sulfate level, because disturbed fill is less predictable than an undisturbed horizon.

From there, BS 8500-1 converts the AC class into a design chemical class, DC-1 to DC-4, with any additional protective measures. Intended working life forms part of that step, so a 100 year design life pushes the specification harder than a 50 year life in the same ground.

Sampling and testing that will stand up

The analytical suite is short and inexpensive next to the rest of a site investigation. Water-soluble sulfate, pH, magnesium where sulfate is raised, and total potential sulfate where sulfides are suspected. The cost sits in getting the samples, not in the laboratory schedule.

Sample positions matter more than sample numbers. You want material from the depth at which concrete will actually sit, which means the formation level of the foundations and the underside of the slab, not a topsoil scrape. Where groundwater is shallow enough to bathe the foundations, a water sample is worth more than three soil samples. A single borehole on a plot that has been filled unevenly will under-report, and the fix is more trial pits rather than more analysis per pit.

This work usually sits inside a wider ground investigation. If the site has an industrial history the same fieldwork should serve the chemical risk assessment for human health, which is where a land contamination assessment comes in, and the geotechnical side feeds a geotechnical assessment covering bearing capacity, settlement and slope stability. Commissioning one set of trial pits for all three is obvious and still gets missed.

Land contamination work under BS 10175 and a ground gas or sulfate assessment both need a suitably qualified and experienced author. That is not a formality. The local planning authority will expect the report to be signed by someone competent, and PlanScope's draft is a starting point for that person rather than a replacement.

Made ground and demolition fill as the usual source

Gypsum is calcium sulfate. Plasterboard, plaster render and old floor screeds are full of it, and when a building comes down and the rubble is spread across the plot as a working platform, the sulfate goes into the ground with it. A site that would have classified as DS-1 in its natural state can read several bands higher once a century of demolition arisings has been mixed in.

That is a sequencing problem as much as a technical one. Sampling before demolition tells you about the old ground. Sampling after an uncontrolled site clearance tells you about whatever was left behind. Where demolition is part of the scheme, the prudent order is to record the ground beforehand, control what happens to the arisings, and then sample again at formation level. Demolition itself may need its own consent route, and prior notification of demolition explains when that bites.

Imported fill deserves the same scrutiny. A clean validation certificate for a 6N or 6F capping layer does not say anything about sulfate unless sulfate was tested, and it frequently was not.

Carrying the classification through to building control

Record the classification where someone will find it. The ground investigation report should state the DS class, the ACEC class, the assumed working life and the resulting DC class with any protective measures, each with the sample results behind it. Foundation drawings and the concrete specification should then quote the DC class rather than a bare strength class, because a C30/37 mix means nothing about chemical resistance on its own.

Approved Document C of the Building Regulations covers site preparation and resistance to contaminants, and the inspector will want to see that the ground chemistry was assessed rather than assumed. On a plot with a demolition history, expect the question. Having the answer in writing, with sample locations on a plan, usually closes it in one exchange.

Two practical habits help. Quote the edition of Special Digest 1 and of BS 8500-1 you worked to, because specification tables change between editions. And keep the laboratory certificates with the report rather than in a separate folder, so a reviewer can trace the class back to the raw numbers without asking.

Enter your postcode on PlanScope to see which of the 66 reports a site with made ground is likely to need, then send the technical drafts for professional review.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.