The cracking of concrete base slabs and levelling screeds is mainly caused by restraint to early thermal contraction and drying shrinkage. Restraint can be reduced by including a slip membrane under the slab. Curling due to differential drying through the thickness can lead to cracking under load, especially in the case of levelling screeds. Cracking and curling are further influenced by the following factors.

a) Water content. Increasing the water content of a screed material increases the drying shrinkage of concrete slabs and levelling screeds, increasing the risk of cracking. The quantity of water used should be kept to a minimum but sufficient to ensure thorough compaction. Water reducing and superplasticizing admixtures enable the water content to be reduced while maintaining the required level of consistence.

  • b) Aggregates. Some aggregates have higher than average drying shrinkage properties and should be avoided owing to a greater risk of cracking.
  • c) Reinforcement. The provision of reinforcement does not prevent, but can control, shrinkage cracking. It is seldom used in bonded screeds, but if used in unbonded or floating screeds it limits the width of cracks and so provides some advantage. Reinforcement does not prevent curling but where it passes through joints it will minimize steps. Where reinforcement is used, it should be placed in about the middle third of the levelling screed, and if required may extend across bay joints. To ensure adequate compaction and bond around the steel fabric, the use of a slightly wetter screed material may be considered.
  • d) Curing. Concrete slabs and levelling screeds should be kept moist continuously for at least 7 days, starting as soon as possible after laying, to enable them to develop sufficient tensile strength to resist the stresses caused by restraint to shrinkage that occurs as concrete dries. Shrinkage can continue for several months.
  • e) Drying. Slabs and levelling screeds should be allowed to dry out as slowly as practicable after curing to reduce the risk of curling. Accelerated drying by forced ventilation or heating should not be used as this can result in incomplete hydration of the cement and cracking and curling of the screed.

The possibility of damage to the floor finish over joints or cracks, such as rippling of thin sheet and tile flooring, can be caused by a reversal of curling due to the redistribution of moisture within the thickness of the screed once it is covered. This risk can be reduced by covering the dry screed with impermeable sheeting for about 7 days before laying the flooring.

  • f) Hot weather. Where floors are placed in very hot weather or without cover from the sun the subsequent contraction on cooling increases the risk of cracking. The risk can be reduced by constructing these elements under an overhead covering, although care should be taken to avoid any wind funnelling effect, which could induce rapid drying.

Levelling screeds
If the levelling screed is not adequately bonded to the base slab, curling can occur, which can subsequently lead to the screed cracking under load. Joints in the base that could open should be carried through the levelling screed, otherwise reflected cracking can form at these positions.
Damp-proofing ground-supported floors
Concrete bases and levelling screeds in contact with the ground without a damp-proof membrane should not be expected to keep ground moisture back either in liquid or vapour form. Ground moisture can pass through until it either disperses by free evaporation from the surface of the flooring or reaches a less permeable material under which it tends to accumulate.

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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