SCREED WITH SPEED Mandy Searle, Head of Technical Services at Norcros Adhesives,
looks at some solutions when it comes to tiling on to anhydrite screeds in time-sensitive installations.
Anhydrite screeds, also referred to as calcium sulphate or gypsum-based screeds, are now increasingly popular for larger flooring installations. Their use has grown dramatically in the building industry over the last few years because they offer a number of benefits over sand:cement screeds. The principal advantage is that they are pumpable and can therefore be laid more quickly and easily than traditional sand: cement screeds, making them more cost-effective, especially for larger areas. They will self-level and offer minimal shrinkage, making them especially suitable for use with underfloor heating systems, where they will fully encapsulate all pipework. Up to 2,000m2
can be laid in a single day – compared to 100-150m2
of anhydrite screed which
is the maximum possible with conventional sand:cement screeds, making them particularly suitable for use on large commercial contracts. Additionally anhydrite screeds have a low manufacturing carbon footprint and can offer a saving in excess of 95% of the carbon emissions associated with the manufacture of traditional sand: cement screeds.
There are, however, a number of disadvantages associated with this type of screed, the most significant of which is the extended drying time. Typically sand:cement screeds should be allowed to dry for a minimum of 21 days before they are able to receive a tiled finish as per the recommendations contained within BS5385-3:2003. To achieve the required maximum relative humidity of 75%, the minimum drying times quoted for anhydrite screeds before tiling can commence are one day per mm up to 40mm thickness, with an additional two days per mm thickness above 40mm according to BS8204-7:2003.
In ideal drying conditions therefore a 40mm thick screed should be allowed to dry for a minimum of 40 days, at 50mm thickness the minimum drying time would be 60 days and at 60mm thickness a minimum of 80 days drying time would be required before the screed was sufficiently dry to receive a tiled finish.
24 | ADHESIVES & SEALANTS
In an attempt to reduce the number of failures experienced in the industry, Norcros Adhesives worked together with both a major national house builder and screed manufacturer Gypsol to prepare a generic specification for the industry. This provided a safe method of tiling to anhydrite screeds using cement-based adhesives, but did not improve the drying times required before tiling could commence.
Using a gypsum-based adhesive means that tiling can be carried out when the screed reaches 85% relative humidity, rather than the 75% RH, which is required when fixing tiles in a cement-based adhesive. A number of tile adhesive manufacturers are now offering a tile to gypsum product.
However the latest innovation is a fast-track system which is a complete game changer in terms of the drying times required. These new preparation systems coming on to the market make it possible to fix safely tiles using a cement- based adhesive on to anhydrite screeds which have a relative humidity as high as 95%, which can normally be achieved in just seven days.
This reduction in drying time can reduce the time required for the overall build-up of the floor surface by at least ten weeks, which clearly has a very significant impact on timescales and costs on-site.
These latest generation systems work by taking a system approach, comprising all the required elements – a primer, a moisture suppressant and a gritted primer to provide a ‘key’ on to which the ceramic tile adhesive can form a strong bond.
The adoption of one of these new systems will allow contractors to derive all the benefits of rapid and easy pouring of an anhydrite screed, without involving delays on the job caused by having to wait for an extended drying period.
www.norcros-adhesives.com www.tomorrowscontractfloors.com
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