What is a sulphate attack on a concrete floor?

Sulphate attack on a ground floor concrete slab is a very serious problem.

One of the fundamental causes is leakage of sulphates into the concrete floor slab from the material below. Once the sulphates and concrete are in contact, a chemical reaction takes place. This reaction produces crystals, which force the concrete slab to expand. The reason sulphate attack can be so serious is that the expansion of the concrete puts pressure on the external walls, pushing them outwards and destabilising them. The concrete slab also starts to disintegrate, weakening it and reducing the ability to support the building structure which is sitting onto of the weakened concrete slab

 The process of sulphate attack can take several years for the damage to be visible. This is because at first, any extra volume produced by the sulphate attack is absorbed by the concrete reaction that initially fills up tiny pores within the concrete slab itself. It is only once these tiny pores are full that the concrete shows visible signs of becoming lumpy and expansion becomes more obvious.

Which properties do building surveyors consider to be high risk?

Sulphate attack to flooring is a more likely to be problem in post-war properties (i.e. those built between the 1940s and early 1970s) than in houses from other periods.

A timber shortage after the Second World War meant that builders tended to use concrete ground floor slabs instead of the more traditional suspended timber floors. Hardcore beneath the concrete slabs was made up of clinker (broken brick) from demolished houses and furnace ash from industrial processes. Such hardcore materials were cheap and plentiful, but high particularly in sulphates.

Concrete floors constructed in this era also had particularly low cement contents because concrete was mixed on site, with excessive amounts of water. This is an issue with adds the likeliness of sulphate attack. High cement content concrete has a greater resistance to the sulphate attack than the low cement concrete used in this period.

Sulphate attack cannot happen without water present to transport sulphates from the hardcore into the concrete slab above. Moisture can come from ground water or nearby drains or service pipes. This can be especially prevalent where the drainage is old and prone to damage over the years. A damp-proof membrane between the hardcore and concrete can prevent this (but it should be noted that these can deteriorate and leak, so there is always some risk if high-sulphate hardcore is present). Although supposedly standard practice, damp-proof membranes were rarely used in the post-war period. There was an extraordinary about of building work in in Birmingham and Coventry following the war, and a scarcity of building materials at that time in these regions. Surveyors are therefore particularly keen to check for sulphate attack risk and damage when carrying out surveys in these areas.

What do surveyors look for during a building survey?

If sulphate attack is suspected, or the house is thought to be at high risk, a surveyor may recommend an invasive sulphate test. This will involve excavating a trial hole to assess the condition of the concrete, determine what type of material is below the concrete and find out if a damp-proof membrane is present. Concrete and hardcore samples will be tested on-site for moisture levels and in a lab for sulphate content and cement levels.

Even if sulphate attack has not occurred, if the concrete tests show that it has a cement level lower than that needed to resist attack, and the material below has a high sulphate content, replacement of the floor slabs and potentially the hardcore below may be recommended to prevent structural damage in the future. This is particularly likely if there is no damp-proof membrane. For example, if a water source becomes present in the future e.g. leeching of water from old clay drains, then the property could be at risk.