Screeds

THE NEED TO DRY SCREEDS AND TOPPINGS BEFORE COVERING

By Robin Harold-Barry, BSc(CEng), MICE, MASCE, MICT of Concrete Consultancy Ltd

This article may be freely used but acknowledgment is requested.

Cement based screeds and toppings require water to give them the right consistency for laying and for the chemical process of hardening. The water should be retained for at least three days at normal temperatures to allow a reasonable proportion of the potential strength to be gained before drying commences. Thorough drying is then necessary if an impermeable covering is to be stuck to the floor with a WATER SUSCEPTIBLE ADHESIVE. This is particularly so if the room is not warmer than the floor slab or if any moisture could be trapped. Water vapor will migrate through even the highest quality concrete from the hotter side and condense on the cooler side where it will soften a water susceptible adhesive.

Drying is a slow process. The accepted rule of thumb rate is 1 mm per day for the first 50 mm of thickness from the drying surface, and slower thereafter.

EPOXY ADHESIVES are available which will harden in the presence of water and are not water susceptible. These, though rather more expensive, can be used as soon as the screed or topping is strong enough to resist the stress induced by any thermal movement in the covering, normally after about three days.

It is sometimes said that water vapour pressure causes blisters in thermoplastic coverings. This is rarely the case, unless solar radiation produces very high temperatures in a saturated slab. The vapour pressure at 60 °C is only 20% of that at 100 °C which is one atmosphere. The more likely cause is the relatively high coeficient of expansion of the covering leading to buckling over points of low adhesion. Water is often drawn into the resulting voids.

CEMENTATIOUS ADHESIVES used for hard tiles such as marble, ceramic, terazzo or mosaic also harden in the presence of water, indeed they require water to harden. The sooner they are laid the better it is for curing.

A screed must always receive a covering because it will not stand up to normal wear and tear on its own. A screed is a semi dry mix of cement and 5 mm concreting sand in the proportions of between 1 to 3 and 1 to 4.5. This gives a very permeable material in which there is not sufficient cement paste to fill the voids between the sand grains even if they are well compacted together. The advantage is minimal drying shrinkage because of the touching sand grains. If more water is added to make a cement paste which would fill or over fill the voids then the mix would be too weak and would have excessive shrinkage.

A topping does not necessarily have to receive a covering because it is strong enough to resist wear on its own. A grano topping, consisting of 1 part of cement to 1 part of concreting sand to 2 parts of 10 mm aggregate by weight, gives high wear resistance. The water content should be just sufficient to give a workability which allows full compaction. This results in a water / cement ratio of about 0.42 and a potential 28 day strength of over 60 N/mm². The thickness of this topping should not exceed 40 mm to avoid debonding from the base concrete. Good preparation of the base is also essential which should include vigorously scrubbing into it a thin layer of cement grout just before laying the topping.

A weaker concrete can be given high wear resistance by sprinkling a 1 to 1 mix of dry sand and cement onto the surface while it is workable. The concrete is first floated to give a flat surface and any free water is removed or allowed to evaporate. Half of the 'dry shake' is sprinkled on and troweled. 2.5 kg/m² will give a 1 mm thickness. The second half is then sprinkled on and troweled several times at intervals to give the desired finish. Total thicknesses of 2 mm to 5 mm are normal. Extra wear and skid resistance can be obtained by using a harder mineral such as carborundum instead of sand.