Sulfate Contamination
I) Conventional or Ettringite Form of Sulphate Attack
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 without the use of a separating damp proof membrane. The legacy has been continuing occurrence of damage to floor slabs and abutting walls.
Soluble Sulphates leading to Sulphate Attack:
The underside of a ground bearing concrete slab is vulnerable to sulphate attack where the fill below the slab contains sulphate salts and the slab is not isolated from the fill by a damp proof membrane. In damp or wet conditions Sulphates can migrate to the underside of the slab and react with the tricalcium aluminate found in Portland cement to form ettringite. The formation of ettringite can be destructively expansive since it has a solid volume greater than the original constituents and typically grows as myriad acicular (needle shaped) crystals that can generate high internal stresses in concrete.
Migration takes place when the moisture carries the sulphates through the concrete slab and evaporates at the surface leaving behind the salts. 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.
II) The thaumastite Form of Sulphate Attack (TSA)
This form of sulphate attack was first recognised in the UK in the 1990’s and has since been found in several floor slabs and in oversite concrete. The reaction product is the mineral thaumastite which is a calcium silicate carbonate sulphate hydrate.
Deleterious levels of TSA require the following:
- a significant concentration of water-soluble sulphates
- concrete which contains calcium silicate hydrates, as mostly derived from cementitous calcium solicate phases in Portland cements
- a source of carbonate, generally from a limestone aggregate
- a ph of 10 or higher, such as found in non-carbonated concrete
- persistent wetness
- low temperature (generally below 15oC)
Since the calcium silicate hydrates provide the main binding agent in Portland cement, this
form of attack weakens the concrete as well as causing some expansion and, in advanced
cases, the cement paste matrix is eventually reduced to a mushy, incohesive mass.
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.
Marston Timber Preservation Ltd can arrange for a test to be carried out and following a site visit are able to carry out all remedial treatments including arranging for the Local Authority to inspect our reinstatement of the floor slab so that Building Regulations are being adhered to.
For further details, please refer to British Research Establishment
