Controlling shrinkage cracking and curling of screeds

January 2011

Among the most common issues that screeders are faced with during the drying of screeds are- shrinkage, curling and cracking. It is the inherent property of all concrete slabs and screeds made with Portland cement and aggregates to shrink during the process of drying. But, since the drying process takes place from the top surface down for standards screeds, the upper surfaces usually shrink first, resulting in a tendency for the screeds to curl upwards at the edges.

The extent of shrinkage cracking and curling of screeds are generally influenced by:

  • Mix Proportions
  • Bond Strength
  • Thickness of screed
  • Curing
  • Drying times

Mix Proportions: Excessive amount of cement or water in a screed mix can be as harmful to the screed as insufficient cement and water. When screed mixes which contain less than the specified amount of cement or water a screed may not develop the required​ strength, screed mixes with excessive cement or water can be more prone to curling and cracking. It is therefore advisable to conduct a BRE drop hammer test in case of any doubts regarding the mix proportions, mixing or compaction.
Bond Strength: The tendency of screeds to curl upwards at the edges during drying is generally resisted by the bonding to the subsurface and the thickness of the screed. Bonded screeds are therefore found to have the least risk of curling and cracking compared to unbonded screeds, and this is one of the major reasons why fully bonded screeds are preferred where heavy duty usage or rigid flooring is required.
Unbonded screeds and floating on the other hand are found to have a higher risk of curling and cracking at joints and perimeters. This can however be minimized by reinforcing the screed joint with a steel mesh or ties, at mid-depth of the screed.  Polypropylene fibres are generally used for diffusing stress created during curing and reducing the impact of crack formation.
Thickness of screed: The thickness of the screed also plays a major role in controlling curling and shrinkage cracking. The tendency to curl is generally found to be lower in thick unbonded screeds than thinner ones as thicker screeds have more strength to resist the curling forces.
Curing: Curing is required on standard screeds to prevent excess evaporation of moisture from the surface. As these screeds generally dry from the top down, a polythene layer is laid on top to reduce the moisture evaporation during this period and allow the screed to gain strength. This helps to reduce curling by preventing the top layer from drying too quickly before the bottom layers.
However, the practice of curing isn’t widely adopted nowadays owing to environmental and health and safety issues posed by the use of polythene sheets.
Drying times: Nearly one third of the mixing water used in a screed is used for hydrating the cement. Most of the remaining water must be allowed to escape before the installation of moisture sensitive floor finishes. Though this could take quite long (110 days at 75mm thick, based on a controlled environment of 20C and 55% RH) for standard screeds, it is important not to resort to forced drying as it is another major cause that leads to excessive drying shrinkage, causing cracking and curling.
The best alternative under such circumstances, for time sensitive projects would be to go for a modified screed like FlexiDry. FlexiDry fast drying floor screed yield very quick drying times through a controlled drying process along the full depth of the screed, preventing curling or cracking of the screed. These screeds require a low water / cement ratio to achieve plastic consistency, allowing quick drying and also present the advantage of doing away with the process of polythene curing completely.