Floor screeds and finishes

It is important to distinguish between the curing of a screed and the drying of a screed as these processes are very different.

Curing is the chemical reaction process (hydration) that takes place and is essential for the proper service life of the completed product. The process of hydration uses about one third of the mixing water used in the mix and should take place over a minimum of 4 days but usually 7. Therefore if the water is removed too early (by drying out), full hydration may not take place and the durability of the screed may be compromised. To prevent premature drying it is usual to cover a newly laid screed with polythene for at least the minimum 4 days but preferably 7. (Spray-applied curing membranes may adversely affect adhesion of overlaid flooring and should not be used.)

In general terms, the rate of drying is influenced by the ambient weather or temperature conditions such as relative humidity, exposure to air currents, direct sunlight, etc., the type and thickness of the floor slab itself if there is no separating layer, and the finish applied to the screed.

Failure to control curing properly could, depending on atmospheric conditions, lead to premature drying. However, the surface of the screed is likely to become drier than the main body of the screed with the result that the top surface will shrink more than the lower parts, thus creating curling forces.

Rapid drying also results in cracking. Some cracking is almost inevitable in a large area, but excess cracking, particularly map pattern cracking, indicates poor curing practice. Shrinkage can also become manifest at daywork joints, or, if the screed has been laid in defined bays, at bay joints. (This practice was popular during the 1960s but is generally out of favour today unless underfloor heating systems are used.)

The optimum rate of drying as set out in BS 8203 is 1 day per millimetre of thickness up to about 50mm. Screed thicknesses greater than 50mm take appreciably longer to dry out, more so if they are laid on a relatively new floor slab which in itself has not fully dried. However, optimum conditions may not be achievable on site, with the attendant pressures of programming and the needs of following trades. Applying floor finishes at too early a stage brings the risk of damage and deterioration to the floor finish. Therefore, a number of proprietary quick drying screeds have been developed to enable floor laying within a much reduced time period. However, the advantages of fast drying systems must not be seen as a substitute for proper preparation and laying practice.

If the cracks are wide enough to prevent the aggregate particles bridging and interlocking, there is a risk that the curling forces could break the bond of the screed to the sub-base and permit the lifting of the screed around the perimeter of the bay or crack. Curling is less likely to occur if the screed has been allowed to develop full strength properly and allowed to dry out naturally. The provision of steel mesh within a screed (see the table comparing screed types) will prevent differential curling of screeds at crack positions or at daywork joints, although it will not contribute significantly to preventing the subsequent deflection of screed that has curled, for example at the perimeter of a bay. The mesh will however help to limit cracking. (At one time it was common practice to include chicken wire within the body of a screed. This material has little benefit in controlling cracking or curling and is best avoided in favour of fabric mesh such as D49 or D98.)

Differential curling between adjacent sections of screed will affect rigid or flexible flooring systems according to the severity of the problem. Rigid surfaces such as ceramic tiles are less tolerant of movement, although even flexible sheet materials can be damaged in the longer term by relatively small movements. Importantly, if the screed has curled away from the base sufficiently for it to deflect under load, there is a high likelihood that the screed will crack - again with the risk of damage to the floor finish.

Resistance to applied loadings can be improved by increasing the thickness of the screed, using fine concrete or selecting a proprietary polymer modified screed.

While the section of screed that has curled could in principle be ground down to remove the lipped edge, the fact that it has curled away from the sub-base means that it will no longer be bonded and may sound hollow underfoot. Service life could be compromised. In these circumstances, the provision of a low viscosity epoxy grout may be successful in re-attaching the screed, although this should only be considered where there is reasonable certainty that the screed was of an appropriate quality to begin with, i.e. it was adequately mixed and compacted.

According to CIRIA (Screeds, flooring and finishes: selection, construction and maintenance, Part 5 – Screeds, Report R 184, 1998), the application of an impervious floor covering can sometimes trap moisture within a screed causing it to uncurl, although to specify such treatment would seem to be a fairly high risk policy in case it did not work. The future serviceability of the screed needs to be considered carefully.

Another factor that may influence curling is the lack of proper preparation of the concrete base prior to screeding. Large amounts of dust on the surface, traces of laitance or contaminants or excessive suction may all reduce the bond or prevent it occurring in the first place. Surface preparation is as important as proper curing and must be carried out carefully. Measures include:

• washing the surface with water to control suction;
• grit blasting to remove laitance and loose material;
• mechanical scabbling to provide a key and clean and expose coarse aggregate in the surface;
• provision of a bonding cement/water slurry (although this must not be allowed to dry out before the screed is laid); and
• use of a bonding agent such as epoxy, PVA or SBR (PVA is not recommended in areas that may be subject to wetting).