TN 08.23 Screed Drying Times

INTRODUCTION

The estimated amount of time it takes for a screed to dry is dependent upon several factors. These factors need to be considered when creating the time schedule and the relevant amount of drying time required. Provision for adjustment to the schedule should be made to allow for delays in drying due to unfavourable conditions. Foundation slabs and screeds should be allowed to dry out as slowly as possible after initial curing to reduce the risk of curling and cracking. The surface strength of the foundation slab or screed should be enough to restrain the shrinkage stresses during the setting, hardening and drying of the screed.

All drying times are estimated and should be considered very approximate as drying is influenced by ambient conditions, material quality, thickness and surface finish. All screeds should be moisture tested prior to any floor covering being applied.

Ceramic and Porcelain tiles, whilst unaffected by construction water, concrete bases and screeds shrink as they dry producing enough compression force, causing tiles to crack and de-bond. Allowing the base to sufficiently dry prior to installing the tiles will avoid these failures.

FACTORS DELAYING DRYING

• Low temperatures, high humidity.
• The use of curing membranes significantly extends the drying out period.
• Power-float and power-trowelled finishing will extend drying out period.
• Material quality & thickness.

WATER CONTENT

The amount of water used in a screed material is higher than the required amount for hydration of the cement used. This is to create the correct consistency required for installing the material. Therefore, a significant amount of the water used in the mix, needs to be eliminated after curing by the drying out process. Increasing the amount of water in a material increases the drying shrinkage of the material, increasing the risk of cracking. Always keep the water content to the minimum amount enough to ensure full hydration of the material. Water reducing admixtures can be added to reduce the water content whilst maintaining the correct material consistency.

TYPES OF SCREED
• Unbonded A 1:3 to 4.5 cement/sand mortar batched by weight, minimum thickness of 50mm
• Unbonded-modified A 1:3 to 4.5 cement:sand mortar batched by weight, 50-70mm thick, modified with a suitable admixture.
• Bonded A 1:3 to 4.5 cement:sand mortar batched by weight, minimum thickness 25mm; the design thickness should be 40mm.                                                                                                                              • Bonded modified A 1:3 to 4.5 cement:sand mortar batched by weight, 12-60mm thick, modified with a suitable admixture. the design thickness being 40mm.
• Floating A 1:4 to 5 cement, preferably fine aggregate concrete batched by weight, minimum thickness of 75mm, for light loads this may be reduced to 65mm thick. (The use of a steel mesh will greatly reduce the risk of cracking through drying shrinkage). This should be applied over a layer of thermal or acoustic insulation material, e.g. extruded polystyrene foam.
• Floating, modified A 1:4 to 5 cement, preferably fine aggregate concrete batched by weight, minimum thickness of 50mm. (The use of a steel mesh will greatly reduce the risk of cracking through drying shrinkage). Applied over a layer of thermal or acoustic insulation material, e.g. polystyrene foam.
• Semi-dry A cement/sand mortar, 1:3.5 to 4 by volume, minimum thickness 40mm, maximum thickness 70mm, (a greater bed thickness may be achieved with the addition of aggregate 10mm or less, at the rate of 1part cement to 1.5 parts of dry sand, to 3 parts of dry coarse aggregate).
• Underfloor Heating A 1:3 to 4.5 reinforced cement/sand mortar batched by weight, with a minimum thickness of 75mm, laid onto a separating membrane of thermal insulation. See below for minimum screed depth over heating pipes.

MINIMUM APPLICATION THICKNESSES

All screed materials will have a minimum application thickness, refer to the manufacturer for this information. Regardless of what is possible in application, there are minimum thicknesses for screeds depending on their type as seen in the table below.

FORCED VENTILATION OR HEATING

Forced drying of a screed should be avoided. This can result in incomplete hydrations of the cement in the material as well as cracking and curling of the screed. Calcium sulfate-based screeds containing underfloor heating systems may be heated once they are at least 7 days old to accelerate the drying, however the risk of cracking and curling will be increased.

ADMIXTURES

Admixtures should be thoroughly dispersed throughout the screed. There are 6 types of admixture that can be considered for screeds and concrete.
• Water reducing – reduce the water content otherwise required.
• Super plasticizing – increase consistence, material requires little or no compaction.
• Air-entraining – traps small quantity of air to reduce bleeding of free water.
• Accelerating – speeds up setting and hardening in cold weather.
• Waterproofing – use with caution, can impair adhesion to the base. Waterproofing admix is not considered an alternative to damp proof membranes.
• Retarding – delay the setting and hardening.

CURING

Immediately after the surface has been finished by trowelling and when it has hardened sufficiently the screed should be cured continuously for at least 7 days in one of the following ways, followed by continuous air drying prior to fixing tiles.
• Liquid Curing Membrane – these should not be used in screeds to which materials will be directly fixed unless the membrane is to be mechanically removed effectively.
• Waterproof Sheeting – should be kept in close contact with the surface of the screed for the full curing period to provide conditions that minimise evaporation.
Providing conditions that minimise evaporation is key to the success of the screed. The risk of cracking is greatly reduced by ensuring the screed dries out slowly. Buildings should not be heated for 6 to 8 weeks after the screed is laid and then the temperature increased slowly.

DRYING SHRINKAGE

The Concrete Society indicates that as a rule of thumb, a typical slab of span 10m can experience a drying shrinkage of 3.0mm irrespective of design, depth, or amount of reinforcement used. This movement can affect a tiling layer as hair line cracking of the grout joints or cracking and debonding of the tiles.

MOISTURE TESTING

There are two methods used for testing moisture contents in screeds. For an immediate indication of the moisture content, a digital hygrometer should be used. There are various types of digital hygrometer available on the market. It is important to ensure you are using the correct type of hygrometer, as pin pronged hygrometers are calibrated for wood and are not suitable for testing cement-based products. The second method is Carbide Bomb testing where a sample of the full depth of the screed is taken from site and tested in a lab. See table below for relevant moisture contents.

HEATED SCREEDS

Allow cement: sand screeds to dry for a minimum of 3 weeks (at 20°C) prior to turning on the underfloor heating. Heat up the screed slowly at a maximum rate of 5ºC per day to the maximum operating water temperature of 45ºC (i.e. approx. 25 ºC screed temperature) or as recommended by the heating manufacturer and maintained at this temperature for 3 days. The maximum floor temperature should then be used and maintained for a further 4 days. In doing so draughts must be avoided. The floor should then be allowed to cool down to room temperature (approx. 15ºC) before laying floor coverings.

APPROXIMATE DRYING TIMES
All screeds should be moisture tested prior to fixing tiles

BAL FLEXBONE 2EASY
Where tiling is laid on a BAL FLEXBONE 2EASY uncoupling system directly over the structural floor, we can override the drying times to the below. This is due to the perforations within the BAL FLEXBONE 2EASY, that allow residual moisture from the substrate to ventilate, so that the floor assembly can continue to dry whilst in use.

TECHNICAL ADVISORY SERVICE
For free expert guidance on the use of BAL products, or any aspect of ceramic tiling with BAL products, contact the BAL TECHNICAL ADVISORY SERVICE on 01782 591120.

THE BAL 25 YEAR GUARANTEE
BAL products are supplied with a 25 year product guarantee. For further details and/or copies please contact the Company’s marketing department.

NOTE
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