Sulphate affected floor issues are caused by the use of industrial waste materials used as hardcore infill under concrete floors in domestic properties built between 1940s and 1970s.

This material originated from various industrial sources such as  coal mines, steelworks, foundries, and power stations. As this material was widely available and cheap, builders used the material extensively in the construction of foundations and at that time there was no legislation against its use, it was widely accepted throughout the construction industry

Many years later it became apparent that this infill material could cause damage to the concrete floors due to the sulphate present in these materials. The sulphate in the hardcore infill when combined with moisture can cause the sulphate to migrate into the concrete floor slab causing cracking and possible upward movement of the floor slab and, in extreme cases, cracking and lateral movement of the brickwork below dpc. The technical explanation of the process is that the sulphates react with the Tricalcium Aluminate in ordinary Portland cement (OPC) to form Ettringite, a crystal that expands as it grows, in the process often causing  substantial damage to the concrete floor slab. Moreover there can be consequential structural damage as heave within the floor can displace internal walls built off the floor or indeed displace brickwork in the outer perimeter walls.

In 1967 the Building Regulations were updated to restrict the use of industrial materials as hardcore infill. At the same time a 500g polythene membrane was introduced to provide a barrier between the concrete floor slab and the hardcore infill. The minimum requirement to comply with current Building Regulations for new build or replacement floor construction is a now 300 microns (1200g) polythene.

Although the use of industrial waste materials which had high sulphate levels was outlawed in the 1967 Building Regulations, it’s use can still be found in properties built up to circa 1970. Surveyors may therefore request tests for properties built up to circa 1972.The Surveyor may not see any evidence of possible damage caused by sulphate action but will still ask for a test to be carried out due to the age of the property.

The way that the process works to determine the level of sulphate in the hardcore infill is by taking a sample of the hardcore infill from under a floor in the property. This will determine whether the hardcore is dry or damp and whether there is a polythene membrane present between the concrete floor slab and the hardcore infill. The hardcore sample is then sent to an independent approved laboratory for analysis using a 2:1 water/soil extract test and the result of that, together with the information taken from site, is incorporated into the report.

It is best to sample both the concrete and the hardcore because the hardcore is often the prime culprit and the source of sulphates. Obviously, if there was a DPM installed then this barrier between the hardcore and the concrete slab significantly reduces the risk of sulphates coming into contact with the concrete. However, the concrete itself could contain aggregate with a high sulphate content, hence why both the concrete and hardcore are sampled and analysed for sulphates.