Before moisture sensitive floorings and floor coverings are laid it is essential to ensure both that the floor is constructed to prevent moisture reaching them from the ground and that sufficient of the water used in the construction is eliminated.

Usually the flooring or floor covering is fixed directly to the concrete base slab or onto a levelling screed laid above it. In either case the amount of water used in the screed material or concrete is more than that required for hydration of the cement used because extra water is normally required to give an adequate level of consistence to the screed material or concrete. After the curing period, it is essential that the excess water be allowed to evaporate. The time taken for this to happen should be taken into account at the planning stage. Estimated drying times are necessarily only very approximate as drying is influenced by ambient conditions, concrete quality, thickness and surface finish. Of these factors, thickness and finish are the most important.

With cement sand levelling screeds, one day should be allowed for each millimetre of thickness for the first 50 mm, followed by an increasing time for each millimetre above this thickness. It is therefore reasonable to expect a levelling screed 50 mm thick, drying under good conditions, to be sufficiently dry in about 2 months.
Concrete, being less permeable, takes longer to dry. In practice it has been found that even under good drying conditions concrete bases 150 mm thick often take more than a year to dry from one face only. Moderate and heavy use of power-float and power-trowel finishing methods further delay drying. Suspended concrete slabs cast on permanent metal decking or other impermeable materials have similar drying times to those laid over damp-proof membranes. The use of curing membranes significantly extends the drying out period. For slabs that can dry from both sides, about half the thickness can be considered to dry downwards.
Where levelling screeds are laid directly onto concrete bases without a damp-proof membrane between them, account should be taken of the time needed to dry the total thickness of base and screed. Where moisture-sensitive floorings or floor coverings are to be laid, schedules should be arranged to permit extended drying times for concrete bases. If this cannot be done, then it is crucial at the design stage to specify the use of a sandwich damp-proof membrane between the base concrete and the levelling screed.
Accelerated drying of concrete bases and levelling screeds by forced ventilation or heating should not be used. This can result in incomplete hydration of the cement and cracking and curling of the screed.
Proprietary systems based either on admixtures for normal concrete bases and screeds or special cements are available to produce early drying concrete and levelling screeds.

Interior Screed Ltd are the UK’s leading floor screeding contractors specialising in all types of floor screeding systems. Established in 2010 and working with many of the biggest and most prestigious names in the residential and commercial building sector’s we install screeds into building developments of all sizes. From complex large-scale commercial projects to residential and mixed-use contracts, we carry a comprehensive range of floor screeding products and services designed to offer our customers a one-stop floor screeding facility.

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