The most widely reported failures of concrete floor slabs occur when Red Shale has been used in construction. This was frequently used in the 1950s and 1960s as an under floor fill between the hardcore and concrete, in floors without a damp proof membrane.
In damp or wet conditions, the underside of the concrete floor is vulnerable to sulphate attack, with needle-shaped ettringite crystals growing and causing high internal stresses. Moisture can carry the sulphates through the concrete slab, causing them to evaporate at the surface, leaving behind salts.
If the floor contains a significant concentration of water-soluble sulphates and some kind of barrier, such as non-carbonated concrete or concrete containing calcium silicate hydrates, evaporation cannot take place and damage will inevitably occur. This may be an unevenness in the floor, followed by cracking, or distortion into a domed shape if the floor slab is contained by walls. In the worst cases, the walls may be pushed out.
Since 1966, National Building Regulations have stated that no hardcore containing water soluble sulphates should be laid under concrete without the use of a separating damp proof membrane.
We can arrange a site visit and test the floor to ascertain the extent of the sulphate attack and the level of remedial treatment that is required. This may involve removing the existing floor slab and infill below, and replacing it with a new inert hardcore, sand blinded with a 1200 gauge damp proof membrane, and insulation slabs, followed by a concrete floor and a sand/cement screed finish.