Resilient Flooring To Concrete / Sand : Cement Screeds Under 75% RH
Concrete and Sand : Cement screeds are the most common bases over which resilient flooring is laid. Concrete is formed when aggregate is mixed together with dry Portland cement and water, the mixture forms fluid slurry that is easily poured and moulded into shape. The cement reacts chemically with the water and other ingredients to form a hard matrix that binds the materials together into a durable stone-like material that has many uses and must be at least 6 weeks old to enable movement caused by drying shrinkage to have taken place. Traditional Sand : Cement screed basically consists of sand & cement mixed at a ratio of between 3 to 5 parts sand & 1 part cement. In the majority of cases 4 to 1 is quite sufficient. In the past reinforcement was achieved by using Hex wire (chicken wire) or D49 mesh. In the early 90’s Polypropylene Fibres (PPF) started to become very popular, and today PPF is the most common used reinforcement for traditional floor screed. The screed must be at least 3 weeks old to enable movement caused by drying shrinkage to have taken place.
The moisture content of the screed should be ascertained by the use of a hygrometer in accordance with BS 8201:1987 The flooring should not be installed until a reading of not more than 75% is achieved.
The concrete / sand : cement Screed must be clean, dry and free from dust, laitance and any other contaminants which may act as a release agent, preventing the adhesive from bonding to the concrete. If the concrete is dusty, dry brush with a wire brush and remove the loose dust. Brush apply Norcros Prime Bond diluted 1: 4 with water, to reduce the absorbency of the concrete and suppress dust.
Apply a layer of Norcros Pro 10 or Pro 10+ Levelling Compound.
In to a clean pail add (4 to 4.2 litres Pro 10) or (5 litres Pro10+) of water and slowly add the 20kg of Norcros Levelling Compound. Mix with a rotary drill paddle to form a creamy lump free consistency.
The material should be mixed for a minimum of 3 minutes.
N.B. Using excess water may weaken the leveller causing it to separate, crack and de bond.
Pour the mixed material onto the prepared subfloor and allow to flow to give a smooth finish. Minimal work with a smoothing trowel is required. The use of a spiked roller will help eliminate trapped air and smooth out trowel/ flow lines to give a more uniform surface appearance. The mixed material should be applied at a thickness between 0mm to 10mm. For best results, an overall thickness of at least 4mm should be maintained.
Norcros Levelling Compounds are self-smoothing, but should any imperfections remain they can be removed by rubbing with a carborundum stone when dry.
The underlayment will accept light foot traffic typically 4 hours after application.
A 20Kg unit will cover approximately 5 meters square at 3mm of thickness.
Flooring can then be installed as manufacturer’s recommendations.