Floor screeds and finishes

In contemporary construction the use of raised access floors in office buildings is almost universal. These floors are often set at 100-150mm above structural slab level, meaning that at core positions the designer is faced either with the option of extending the raised floor into the core or raising the floor up to finished level ready for the application of hard surfaces such as terrazzo (not so popular now) or hard tiled surfaces such as natural stone.

There are various ways in which the core floor level can be raised - the provision of lightweight blocks with a cementitious screed being one method or alternatively a no-fines screed (such as lytag) laid to an appropriate thickness and with the top 13mm finished (while the lytag is still green) with sand and cement. The use of lytag screeds is popular since in many cases the same material is used for the main floor plates in conjunction with permanent steel shuttering.

However, the provision of a 13mm surface layer can cause problems, with delamination occurring between the sand and cement and the base material. Lytag screeds do not have a particularly strong surface and are easily damaged. If the finishing layer exceeds 13mm, shrinkage forces can disrupt the screed base material with the result that not only does the dense material become detached, but a portion of the screed beneath can be destroyed.

Since this problem is most likely to occur in a highly trafficked area, most probably with expensive natural finishes, delamination of these forms of composite screed can be problematic.

Delamination of the screed will result in the loosening of the joints in the floor finish, and edge breakdown of the finished flooring units (stone or tile), as well as the propensity for cracking being reflected through to the finished surface.

As with many screed failures, investigations should concentrate not only on the failed areas, but also adjoining sound parts in order to gauge the propensity of the floor to develop further problems. Remedial work could take one of two forms:

• localised lifting of finishes, removal of defective layers and their replacement; or
• replacement of the entire area to the correct thickness and specification.

If localised repair is considered to be appropriate, it is important to cut out the affected area and to ensure that the remaining screed surface is firm and without friable material. Once all loose material is removed, the surface should be coated with a polymer modified bonding slurry and a suitable polymer mortar - the main concern being to limit the potential of the screed repair material to shrink and permit cracking or risk further delamination.

Repairs in excess of 25-30mm thickness could cause shrinkage problems. In this case, particularly if the damaged area is extensive, it would be better to take up the entire screed to its full thickness (in some cases partial replacement of material to a minimum 50mm over the whole area may be possible) and to replace it with a new no-fines screed, paying particular attention to the need to restrict the surface layer to 13mm.

Replacing the screed with conventional screeds could cause problems in terms of loading and should be avoided unless the additional floor loadings can be justified in structural terms.