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Sulphate (red ash) attack?

Sulphate (red ash) attack.

Sulphate attack is the deterioration of concrete over a period of time, effecting it’s strength and appearance. This can be caused either by a fault in the concrete when it was first poured and set (i.e. the presence of chemical compounds now known to react and brake up the structure of the concrete) or through water ingress into the concrete, causing a chemical reaction that breaks up and weakens the concrete.

Sulphate Attack is a chemical attack on the cement paste that binds the concrete together. Sulphate salts are carried in the form of aqueous solution into the concrete when the salts react with part of the cement creating larger crystals than existed originally. This increase in crystal size causes the concrete to expand pushing the external walls outwards and causing the concrete to buckle and crack.

What causes sulphate attack and who is effected?

Post war, building materials were in short supply and material such as burnt colliery shale, red ash and blast furnace slag and demolition brick hardcorewere used as fill material beneath concrete slab floors. Little was known about their composition, with many of the materials used containing sulphate. It was also not common until the 1960’s to use a DPM, such as a polythene sheet, beneath a concrete floor slab which leaves properties built between 1940 and 1970 particularly potentially at risk from sulphate attack.

Another factor that can cause the breakdown of concrete and increase the chance of sulphate attack is the post-curing of the concrete itself. When the concrete is poured, the cooling should be controlled to prevent premature dry-out. This controlled cooling reduces the risk of crack formation, leading to improved durability by decreasing the permeability of the concrete slab.

Geographically, areas located in and around old coal fields and related industrial centres (for example the Midlands, North East of England and the Valley Of Scotland) are most at risk, however cases of sulphate attack have been reported to the British Research Establishment across the UK.

These days, the chance of sulphate attack can be minimised dramatically by the use of special sulphate resisting cements within concrete and the use of Damp Proof Membranes (DPM), a practice that is now commonly used in construction of concrete floor slabs, for example.

Recognising and diagnosing sulphate attack

Concrete floor slabs that expand horizontally in a dome shape (the edges of the slab bear the weight of retaining walls and are generally restrained from expansion), crumble, crack or show stress fractures are tell tale signs of potential sulphate attack. Movement of walls (pushed out by the expanding floor slab), the deterioration of the mortar between the brick courses (often to powder) and visual changes to the surface of concrete or cement surfaces (for example cracks, holes or the severe breakdown) are also good indications of potential sulphate attack.

Expert advice should be sought if you suspect sulphate attack. A qualified civil engineer can diagnose and offer remedial advice to help ensure damaged concrete and effected structures are repaired. Alan Wood & Partners have a dedicated team of experienced civil engineers who’s specialist expertise covers sulphate attack alongside other civil engineering disciplines. Alan Woods’ sulphate attack specialsts operating throughout the UK from offices in Hull, Lincoln, Scarborough, Sheffield and York.

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15 August 2020
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