Floor screeds and finishes

From the discussion on screed curling it can be seen that proper curing, drying and preparation are important factors in the successful laying of a screed. However, appropriate mix selection and water cement ratio are also vital, as is the method of mixing the materials. Some proprietary polymer-modified pre-mixed screed are available for uses where curing and drying time must be constrained (for example, where fitting out operations do not permit the normal curing and drying time).

The water to cement ratio is important.

• Sufficient water is needed to ensure hydration.
• Too little water will affect strength.
• Too much water will make the screed difficult to place and to level, and will increase the risk of drying shrinkage and therefore cracking and curling.

Usually, the aggregates should be batched by weight rather than volume and when mixed be capable of being moulded in the hand but not so as to release water when squeezed.

Mixing method

The method of mixing the screed can play a large part in its future serviceability. If the mortar is too dry (aside from hydration problems), it is possible that only the surface layer will become properly compacted, such that the top 5-10mm appears to be properly formed, but beneath this compaction and density is poor. Depending on the future use of the building, the poorly compacted material may eventually fail under compression giving rise to undulations in the floor or shallow depressions - elephant's footprints (Building elements: floors and flooring – performance, diagnosis, maintenance, repair and the avoidance of defects (2nd edition), BRE Report 460, 2003). Such problems will often occur in areas of point loading, such as beneath chair legs or equipment stands. Once collapse occurs, further traffic causes the perimeter of the collapsed area to break down and extend the area of damage.

Given the variability of site mixed screeds (both in terms of mix and compaction) collapse of a screed can be very localised. While early breakdown can occur, it is equally possible that a screed that has seemingly performed well for many years may suddenly deteriorate given perhaps a change in layout of the floor, different degrees of usage or change in use.

The problem is more likely to become manifest in floors that are covered with flexible sheet materials. Hard surfaces such as tiles are not immune (if the deficiencies are over a large enough area) but generally they help to spread point loads somewhat and may permit a screed to perform slightly better in terms of loading than would otherwise be the case.

A second feature of poor mixing is the existence of balls of cement powder - particles of cement that have clumped together in the mixer. These result from there being either insufficient water to disperse them properly throughout the mix, or simply the mixer being incapable of permitting even distribution. A conventional mixer mixes wet materials satisfactorily, but the configuration of the blades in the mixing drum is such that they will not break up the clods of material that form with granular materials in damp conditions. For this reason, special forced action trough and paddle or pan mixers are needed.