Floor screeds and finishes

Osmosis is the flow of water from a more diluted solution to a more concentrated one when separated by a semi-permeable membrane. A semi-permeable membrane is one which allows water molecules to pass through, but which prevents larger molecules of contaminants to pass. In flooring, the surface zone of concrete immediately below the floor finish acts as the semi-permeable membrane.

For osmosis to occur, 3 conditions are required:

• a concentration of water soluble material - this could come from resins used in the flooring, contamination or from the concrete base;
• a semi-permeable membrane; and
• a source of water - such as from the concrete base (even apparently dry concrete will have 3-5% free water by weight; sufficient for osmosis to initiate).

The formation of blisters ranging from a few millimetres to 50mm or so in resin flooring is usually a symptom of osmosis. The blisters can be up to 12mm high and may contain aqueous fluid under pressure, although older cases may be dry.

Osmosis can occur in floors coated with thin (up to 6mm) resin finishes, perhaps 3 months to 2 years after laying. Osmosis in bonded rubber or PVC floors is not unknown, but rarer.

Osmosis in resin floor finishes

Remedial work involves the cutting out of the affected area and its replacement. The surface will need to be properly prepared prior to treatment - often using 'blast track' equipment - a form of grit blasting with an enclosed machine. Double priming the surface or using trowel applied, thicker coatings are alternative methods of treatment.