Sulphate attack on ground floor slabs is a very serious problem which can cause structural damage to the main walls of a building. The problem occurs when the fill material (hardcore) beneath the slab contains sulphates and these migrate into the concrete. The sulphates react with the concrete causing it to expand. This results in heave of the slab and structural damage to the external walls as the slab pushes them out. Eventually the concrete may disintegrate. When the slab heaves, any internal walls built off the slab will be lifted and may cause damage to the structure above.
For the sulphates to migrate into the slab, there must be moisture present. Migration is encouraged if conditions allow the moisture to be drawn through the slab by evaporation from its surface. Migration is prohibited if there is an adequate damp-proof membrane between the fill and the slab. Although a high level of sulphate may be present, there may be no sulphate attack if the fill is dry and/or there is an effective damp-proof membrane. However, moisture contents can vary and increase to very high levels if an undetected water leak occurs. Also damp-proof membranes can deteriorate with time. Whenever sulphates are present there is a potential problem for the future.
Hardcore containing sulphates is now banned from use below floor slabs, and in any case there is now a requirement for a damp proof membrane which would prevent sulphate attack.
Typically the problem houses are those built by local authorities in the 1950's and 1960's in mining areas. This is because colliery shale, which often contains sulphates, was used for the hardcore. Sometimes the colliery shale would come from tips which had caught fire and this is where the names 'red ash' or 'burnt shale' come from.
It should be noted that there are other sources of sulphates in hardcore, and there are other causes of heave to ground floor slabs, all of which must be understood and considered as possibilities when investigating such problems.
A sulphate report entails breaking one or more holes through the ground floor slab and digging down to determine the hardcore thickness. A sample may then be taken for laboratory analysis. If a damp proof membrane is present it cannot be fully reinstated, and any surface finishes may be affected. It is therefore necessary for the owners to agree, in writing, to the investigations. The laboratory analysis will take about 2 weeks, and a further week is required to complete the report.
A sulphate report also includes observations on any movement, cracking or distortion to the external walls around damp proof course level, the internal ground floor walls (which may be built off the slab) and the slab itself. The condition of the concrete is examined, the presence or otherwise of any damp proof membrane is recorded and the dampness of the hardcore is noted. These are all factors which either indicate a sulphate problem or which could affect the risk of sulphate attack.