Sulfate Contamination
The most widespread reported failures to concrete floor slabs involve the use of partially burnt colliery shale (Red Shale). This was frequently used in construction in the 1950's and 1960's as a fill under floors without a damp proof membrane being laid between the hardcore and concrete. Since 1966, National Building regulations have required that no hardcore containing water soluble sulfates or other deleterious matter be laid under concrete.
Soluble Sulfates leading to Sulfate Attack:
The underside of a ground bearing concrete slab is vulnerable to sulfate attack
where the fill below the slab contains sulfate slats and the slab is not
isolated from the fill by a damp proof membrane. In damp or wet conditions
Sulfates can migrate to the underside of the slab and react with the tricalcium
aluminate found in Portland cement to form bettringite.
Migration takes place when the moisture carries the sulfates through the concrete slab and evaporates at the surface leaving behind the slats. Obviously therefore, any form of barrier to the water evaporation will determine the rate of attack. The reaction produces expansion in that part of the slab in contact with the fill. This reaction is expansive within the concrete the first visible signs are some unevenness in the floor, followed by cracking and possible heave. The upper part of the slab is put in tension leading to a map pattern of cracking. Containment of the slab by walls forces the slab to distort into a domed shape. In the worse cases the walls bounding the slab can be pushed out. This can be visible as walls over-sailing the damp proof course. Where the wall is cavity constructed, the outward movement can push the inner leaf without necessarily moving the latter.
How do I know if my floor is contaminated with sulfate?
The only way to determine the presence of sulfate is to carry out core samples
taken by an independent company acting on our instructions. We would however
point out that a charge is applicable should core sample testing be required.
These samples are then sent away to a laboratory to determine the extent
of the sulfate attack. These finding's are then submitted by us in a report
form detailing the make up of the floor area and recommendations as to
their renewal.
How is this treated?
The existing floor slab including the in fill below the floor slab is removed
complete. Failure to remove the contaminated in fill could result in outward
movement affecting the building structure. The floor slab will then be reinstated
in new inert hardcore, sand blinded with a 1200 gage damp proof membrane,
insulation slabs followed by a concrete floor and a sand/cement screed finish.
How much is it likely to cost?
The cost of replacing sulphate contaminated floor depends on the property
and would be subject to an inspection.
For further details, please refer to British Research Establishment
