Sulphate attacks to ground floor slabs are often overlooked when undertaking a damp survey, as they can be considered as structural and not a dampness related issue, but without moisture, the reaction cannot take place and can result in serious structural damage to both the floor slab and walls of the building.
A sulphate attack occurs when the infill material beneath the slab contains sulphates and these are taken up into solution by ground moisture, which then migrates into the concrete which forms the floor slab.
The three main infill materials to a solid floor which result in a sulphate attack being Red Ash (Shale), Black Ash, Slag and Grey Fly Ash, although other industrial materials and building rubble etc. could also be used and present a potential problem.
These materials originated from various industrial sources ie. Coal Mines, Steelworks, Foundries, Power Stations etc. all of which were present in the North West of England. As this material was widely available and was virtually free, builders used the material extensively as it's used at that time did not contravene any Building Regulations.
Typically, this issue affects properties constructed in the 1950s and 1960s but can affect earlier structures, was a concrete floor slab has been installed.
Photographs showing the movement to the external elevations, due to the pressure of the expanding floor slab.
This reaction with the concrete causing the slab to expand, and results in lifting, distortion and cracking of the slab referred to as heave. As the slab continues to expand it will apply a pressure to the surrounding walls, which can result in structural movement to the external elevations, normally seen at the damp proof level, as this forms slip plane or weak point in the brickwork, internal walls and stairs constructed off the slab, will be often be affected by movement.
Initially a visual and levelling inspection to the property will be sufficient to recognise there is a sulphate issue , trial holes will need to be excavated to establish the type and depth of infill and if there is water in the subfloor structure, a structural engineer may need to be instructed, subject to the level of damage/ movement to the walls.
There are several options for remedial action for sulphate-attacked concrete floor slabs,
the appropriate course of action depends on the severity of the attack, the perceived risk of future damage and the degree of assurance required for a lender, should the inspection be related to a mortgage.
If damage is sufficient to justify repair, the slab must be broken out and removed, the spoil should not be used as hardcore under the replacement slab. The new floor structure will need to be reinstated to conform to Approved document Part L1b Conservation of fuel and power in existing dwellings and requires the U values of the floors to be improved to an minimum 0.25 W/m2 .