Finishing school
November 1st 2005

Installation of Bases and Screeds for Resin Floorings and Coatings - By Cyril Potter, Chief Chemist ARDEX UK Limited

One of the requirements for resin floorings and coatings is the provision of an adequately strong and hard base so that the full potential of the flooring or coating can be achieved.

In most cases a good quality concrete base is the best option, providing it can be finished to the surface regularity required for the resin flooring or coating.

Thin resin floorings and coatings will normally require surface regularity 1 and a surface finish acceptable for the direct application of the resin flooring or coating. In most cases the normally acceptable finish on the concrete base will not be suitable unless the appropriate finishing technique is used so that not only is the required surface regularity achieved but also the surface is 'flat' and smooth enough for the thin resin flooring or coating. The higher standard of surface regularity may not be necessary, depending on the requirements of the resin flooring and end use, providing a smoothly undulating surface is acceptable. Note that the surface preparation technique used will also affect the texture of the surface and the coating that may be used.

Concrete bases

Direct finished concrete surface designed to accept floorings should be installed in accordance with the recommendations of BS 8204-1 and a minimum grade C35 concrete with a minimum cement content of 300 kg/m3 should be used. It is also advisable to use clean aggregates with low content of soluble salts to reduce the risk of osmotic blistering in the resin flooring.

The concrete base installed to receive resin floorings and coatings should have been cured for at least 7 days to prevent rapid surface drying and then any curing membranes should be removed to allow the concrete to dry. Note that some spray-on curing membranes may penetrate into the concrete surface so they should be used with caution in case they inhibit the adhesion of resin floorings and coatings. Other types can prove difficult to remove completely using only shot blasting techniques.

Levelling screeds

Where a levelling screed has to be applied to a concrete base to provide a suitable base for a resin flooring it should be laid normally as a bonded screed.

The concrete base should be sufficiently mature so that most of the drying shrinkage has taken place. The guidance generally given is to cure the levelling screed for 7 days and allow air drying for sufficient time for the screed to reach an acceptable moisture level.

The bonded screed should be a polymer modified cementitious screed and be compatible with the resin flooring or coating and should also provide the required cohesive strength and load bearing capacity. Testing for surface strength is described in clause 7.2 of BS 8204-6, the rebound hammer test requires a reading of not less than 25, whilst the surface tensile strength test (pull off) requires at least 1.5N/mm2.

In some cases there may be limitations, depending on the restraint that has to be provided, on the type and thickness of resin floorings that can be laid on levelling screeds. The manufacturer of the resin flooring will provide advice on such limitations and usually polyurethane resin floorings and the thicker resin floorings require a higher degree of restraint and are normally installed on concrete bases.

BS 8204-3 should be consulted for the application of any polymer modified cementitious levelling screed necessary to prepare the concrete base prior to the application of the resin flooring or coating. Cement sand screeds should be installed in accordance with the recommendations of BS 8204-1 paying particular attention to the use of suitably graded screeding sand and a forced action mixer to produce a workable and compactable mortar within the recommended cement/water ratio.

Screed application

The concrete base should be prepared using mechanised equipment to remove all weak and friable surface residues, surface laitance, etc. The use of scabbling equipment or contained shot blasting equipment with vacuum extraction is recommended. The prepared concrete surface should be vacuum cleaned to remove and dust.

The screed mortar should be mixed in a forced action mixer; free fall (tumble drum) mixers are not suitable. The prepared concrete surface should be treated with a suitable bonding grout/slurry immediately before applying the screed mortar, placing it ‘fresh in fresh’ on the bonding grout. The levelling screed should be finished to the required surface regularity using either a steel trowel or wood float as appropriate for the resin flooring being applied.

If speed of installation is necessary the use of either a rapid setting, hardening and drying levelling screed with a polymer admix, or a pumped cementitious screed with rapid drying and early strength development properties can be advantageous for some resin floorings and coatings. Compatibility of the resin flooring or coating with proprietary systems should be established at the design stage.

Surface Preparation

The surface of a direct finished concrete slab or levelling screed should be suitably prepared to remove all laitance, lime bloom and cement paste, as well as residues of curing agents and other barriers to adhesion. The use of contained shot blasting equipment is particularly useful for this operation since it is quiet and essentially dust-free, selectively removing the cohesively weak residues without unduly roughening the surface. Surface grinding of the concrete is also an option and has some use in removing localised surface irregularities.

For resin floorings thicker than 4mm, where the surface profile of the concrete base is unlikely to show through the resin flooring, a rougher finish to the concrete may be acceptable.

Where a levelling screed has been installed the surface should be clean and dry, any surface laitance being removed by light contained shot blasting.

Priming may be required prior to application of some resin floorings on a concrete base or screed that is sufficiently mature but is too damp for direct application of the required resin flooring.

The above advice should be considered in conjunction with the information from the manufacturer of the resin flooring or coating and the recommendations given in BS 8204-6.

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