Screeding systems

The key to faster, tighter building programmes

By George Guesford UK Industrial & Resin Flooring Product Manager Mapei UK LTD.

WITH the ever-increasing emphasis on faster, tighter building programmes, Mapei’s range of fast-drying screed products meets this demand in abundance.

Traditional sand:cement screeds are widely used to create level substrates that are suitable to receive a further decorative topping such as tiles, vinyls, carpets etc yet the time required to allow the screeds to dry is often excessive and doesn’t fit in with current fast-track build programmes.

The standard ‘rule-of-thumb’ measurement used to assess a screeds likely drying time, as outlined in BS 8204-1 2003, 6.11.1, is to allow one day’s drying time for each millimetre of thickness, up to a thickness of 50mm, followed by an increasing time for each millimetre above 50mm, with two days per each mm being the norm.

By this calculation, a standard 50mm thick screed could take 50 days to dry, plus an extra seven days if you include the recommended seven days under polythene for curing purposes. A 60mm thick screed could take over 70 days to completely dry.

The above guide is only a ‘rule-of-thumb’ and can’t be relied on to be accurate; there are many factors outside of the control of the screed installer that will have an influence on the screeds ability to dry. It’s imperitive reliable testing is carried out on the newly installed screed in order to establish an accurate assessment of the actual moisture content before proceeding with the installation of the chosen floorcovering.

There are several different ways that can be used to measure the moisture content of a screed, some more reliable than others.

If we follow the advice given in BS 8203, ‘Code of practice for Installation of Resilient Floorcoverings’, then a non-invasive test such as the one described in that standard (a hygrometer or relative humidity probe), ‘is the only suitable method relevant for use in conjunction with installation of resilient floor coverings’. So, choose your method carefully, lest you get caught out.

If the moisture content remains too high for the chosen floorcovering following the moisture content test, a damp-proof membrane (DPM), a popular way of overcoming the retained moisture issue, can be applied to the surface of the screed.

Surface-applied DPMs work by controlling the release of moisture within the screed into the atmosphere in a controlled manner, releasing it as a vapour rather than in its liquid form. This prevents the escaping moisture from affecting moisture sensitive toppings that may be subsequently applied to the screeds surface.

Whether you’re waiting for the screed to dry to a point where it’s ready to receive the specified topping or applying a DPM, both incur time delays and the application of extra products such as DPMs can add significantly to the overall cost of the install.

A much better option therefore, would be to use a proprietary fast-drying screed, such as a screed containing the Mapei Topcem binder.

Mapei’s Topcem is a hydraulic binder (or cement) that has ‘normal’ setting characteristics, is shrinkage controlled and imparts fast-drying properties to the screed mix.
Topcem binder is defined in BS 8204-1, paragraph 5.1.3 – f, as follows:

5.1.3. – Cement for cementitious levelling screeds
f. – Proprietary cements, designed to provide rapid drying and hardening properties, for which no British Standard exists (reference should be made to the manufacturers for guidance on their use).

Screeds containing the Mapei Topcem binder exhibit many benefits over conventional sand:cement screeds, not just faster-drying properties, as follows:

No curing The screed mix doesn’t require curing under polythene sheeting, a must for conventional screed mixes, where a minimum curing period of seven days is recommended. Protection from direct sunlight and through-draughts for the first 48 hours after installation are all that’s required to allow the proprietary screed mix to set and harden successfully.

Fast-drying Under normal drying conditions, the screed will be dry enough to receive ceramic and porcelain tiles 24 hours after installation, natural stone products after two days and resilient and wood flooring products after four days.

Testing the moisture content of screeds containing the Topcem binder are rarely required, such is the reliability of the screed mix to achieve the published moisture levels in the quoted time periods.

If moisture tests are requested, experience has shown screeds produced from proprietary cements should be tested using the Calcium Carbide Method (CCM), otherwise known as the ’Carbide Bomb’.

Thinner installations Experience has demonstrated that proprietary screed mixes can be laid at thicknesses that are much less that those required for conventional sand:cement screeds. For instance, bonded screeds down to 10mm, unbonded screeds down to 35mm and floating screeds down to 55mm. These reduced thicknesses represent significant savings on material useage, and can also contribute towards other benefits, of design and enviromental impact for instance.

Stronger The design of the binder allows a screed mix to be produced that has a greater compressive and flexural strength than conventional screed mixes, albeit with a reduced binder content than would normally be used in conventional screed mixes.

Early access A rapid strength development, coupled with a stronger mix, allows the floor to be trafficked earlier than conventional screeds, usually after 12 hours in the case of screeds containing the Topcem binder.

Under-layment for resins The greater strength and overall robustness, coupled with the guaranteed low moisture levels of these screeds, make them extremely suitable substrates to receive resin-based flooring products.

Underfloor heating Screeds containing the Mapei Topcem binder are particularly suitable for installation over underfloor heating systems, usually plastic pipes of varying diameter, laid over or incorporated into, insulation boards. Such screeds are easily installed and compacted around and over the pipes without the need for further admixtures, and their subsequent rapid drying allows the heating system to be fully commissioned just four days after completion of the screeds installation. Commissioning should always be carried out in accordance with the heating system manufacturers recommendations.

Of course, all of the above cannot be achieved with a proprietary cement alone, and the overall success of the screed mix to perform as expected is also dependant on the selection of a suitably graded sand, along with adherence to all mixing recommendations, particularly water content. Advice on which types of sand are suitable should be sought from the cement manufacturers.

Another benefit, and one that’s overlooked or, in truth I suspect, rarely considered, is that the rapid strength development and drying properties would allow the proprietary screed to be installed much later in the build programme.

Traditionally the screed tends to get installed over the concrete substrate as soon as is allowable, which tends to be towards the start of the build programme. I suspect this is a ‘traditional’ way of thinking, by installing the screed early, it allows it the maximum amount of time during the build programme for drying to take place.

The reality tends to be that the screed is very rarely protected as it should be and is often allowed to get regular wetting, either from a lack of roof or windows or both, along with further wetting from other following ‘wet’ trades such as plasterers.

This lack of protection also invariably leads to the screed being damaged, often quite badly, as following trades carry out the rest of the construction programme using the newly screeded floor as the base for their various activities, including running heavy, mechanised machinery over the floor.

All too often over the years that I’ve been involved with screeds, I’ve been asked to attend sites to inspect, comment and report on damage inflicted to a screed.

This request is usually made just prior to the installation of the chosen floorcovering, as the floor layers have arrived onsite to install the floorcovering, seen the condition of the screed and refused to carry on until the screed has been repaired.

Invariably the perception is the screed must be faulty so ‘What are you going to do about it?’ Rather than get involved in long conversations about why the screed is now in the condition it is, and although the reasons are usually blatantly obvious though conveniently ignored, why not avoid all of it and install the screed at the end of the programme?

Fast-drying screeds such as those that contain the Mapei Topcem binder can be installed quickly and effectively and be dry enough to receive the most moisture-sensitive flooring materials after just four days.

Would shifting the screed installation to the end of the build programme, when all other works are finished and gone, be so difficult to organise? Food for thought!

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