Mapei and Floor Screeds

Topcem Pronto is factory produced pre-blended screeding material that allows rapid drying and fast installation of subsequent floor coverings.

Floor screeds serve the purpose of providing a sound level base onto which a floor finish, such as carpet, timber, vinyl or ceramic tiles can be laid.

The screed material itself generally consists of a filler (usually sand) and a binder, which can be cement, that is mixed and laid in accordance with the British Standard BS:8204.

The standard itself is very prescriptive, and the production standards for the constituent materials also ensure a high degree of quality and consistency.

However there are still a surprising number of problems and common questions encountered within the flooring industry concerning screed and related flooring issues.

The most frequently asked question relates to the time it takes for a screed to dry sufficiently to become dimensionally stable and dry enough to receive a floor covering, without residual moisture causing subsequent damage.

In order to answer this type of question we need to take into consideration the water content of the screed and the factors that govern this as well as the rate of hydration and the drying characteristics of the screed within the actual site conditions that it is placed.

Obviously it would be beneficial to start with the lowest possible water content consistent with achieving correct placement and compaction.

The two factors that control this are:.

1.

The size and shape of the sand particle used.

2.

The type and grade of cement.

Most specifications call for a suitable screeding aggregate (sand) and the most widely available material under EN 13139 Standard 'Aggregates for Mortar', is a 0/4 Medium aggregate.

There is, however, a better choice with a 0/8 Coarse aggregate for screeds, although this is only produced in limited quantities and will generally cost more.

The benefit of using a coarse aggregate comes from its lower surface area which requires less cement and water to coat it.

Also, the more open texture of the screed allows better rates of drying due to evaporation.

The second point relates to the cement or binder used.

The cement standard BS EN 197 now allows a range of different strength grade cements to be produced, some containing fillers, such as slag or fly ash.

Each type of cement will have its own water demand and rate of hydration.

In general terms the higher grade cements >32.5, have the higher water demands and faster rates of hydration, but this is not always the case.

The best advice is to find a reputable source of Portland Cement complying with the CEM 1 standard that gives a consistently good performance.

The next point to consider, following the selection of aggregate and cement, is to maximise the performance of the cement within the screed by using a proprietary admixture to promote good dispersion and fast hydration.

The rate of hydration for most cement types takes around 28 days to achieve approximately 90-95% completion.

After this period the screed is less likely to shrink.

However, evaporation is still likely to have an effect, and it is here that the actual site conditions have an influence.

Ideally sites should be dry and well ventilated, but very often it is not the case and screeds can take longer to dry.

In order to address the above issues of material selection, hydration rates, the effect of site conditions and mixing methods Mapei have developed a range of screeding products, including Topcem Pronto.

Topcem Pronto is factory produced pre-blended screeding material that allows rapid drying and fast installation of subsequent floor coverings.

For example, ceramic tiles may be laid after only 24 hours regardless of how thick the screed is laid.

Other products in the range include Topcem Binder, SBR Additives and fast drying screed admixtures.

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