Resilient Flooring To Heated 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.
Minimum drying times for new concrete/ sand: cement screeds before installation of heating/ tiles are:
Concrete = minimum 6 weeks
Sand: cement screed = minimum 3 weeks
Once dried the heating should be switched on and the temperature raised at a rate of 5oC per day to operating temperature. The operating temperature should then be maintained for 2-3 days before cooling down to room temperature, i.e. above 15oC and below 20oC. This is to ensure any movement in the screed has taken place prior to the levelling compound being applied.
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 7 5% RH 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 levelling compound from bonding to the screed. If the screed is dusty, dry brush with a wire brush and remove all the loose dust. Brush apply Norcros Prime Bond diluted 1:4 with water and allow to dry, to reduce the absorbency of the screed and suppress dust.
Apply a layer of Norcros Pro10 or Pro10+ Levelling Compound.
Into a clean p ail add (4 to 4.2 litres Pro 10) or (5 litres Pro 10+) of clean fresh water and slowly add the 20kgs of Norcros Levelling Compound. Mix using a rotary paddle drill to form a creamy lump free consistency.
The material should be mixed for a minimum of 3 minutes. NB: Using e xcessive water may weaken the leveler causing it to separate, crack and debond
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 the trowel / flow lines to give a more uniform surface appearance. The mixed material should be applied at a thickness between 0mm to 10mm. the 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 foot light 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 manufacturers recommendations. The underfloor heating can be switched on a minimum of 14 after completion at its lowest operating temperature, raising by 5 degrees per day until operating temperature is achieved.