Structural Engineer Pocket Book

Building Elements, Materials, Fixings and Fastenings 299
Screeds
Screeds are generally specified by an architect as a finish to structural floors in order to
provide a level surface, to conceal service routes and/or as a preparation for application of
floor finishes. Historically screeds fail due to inadequate soundness, cracking and curling and
therefore, like waterproofing, it is useful for the engineer to have some background knowledge.
Structural toppings generally act as part of a precast structural floor to resist vertical
load or to enhance diaphragm action. The structural issues affecting the choice of screed are:
type of floor construction, deflection, thermal or moisture movements, surface accuracy and
moisture condition.
Deflection
Directly bonded screeds can be successfully applied to solid reinforced concrete slabs as they are
generally sufficiently rigid, while floating screeds are more suitable for flexible floors (such as
precast planks or composite metal decking) to avoid reflective cracking of the screed. Floating
screeds must be thicker than bonded screeds to withstand the applied floor loadings and are
laid on a slip membrane to ensure free movement and avoid reflective cracking.
Thermal/moisture effects
Drying shrinkage and temperature changes will result in movement in the structure, which
could lead to the cracking of an overlying bonded screed. It is general practice to leave concrete
slabs to cure for 6 weeks before laying screed or applying rigid finishes such as tiles, stone or
terrazzo. For other finishes the required floor slab drying times vary. If movement is likely to be
problematic, joints should be made in the screed at predetermined points to allow expansion/
contraction/stress relief.
Sand:cement screeds must be cured by close covering with polythene sheet for 7 days
while foot traffic is prevented and the screed is protected from frost. After this the
remaining free moisture in the screed needs time to escape before application of finishes.
This is especially true if the substructure and finish are both vapour proof as this can result
in moisture being trapped in the screed. Accurate prediction of screed drying times is
difficult, but a rough rule is 4 weeks per 25 mm of screed thickness (to reach about 75%
relative humidity). Accelerated heating to speed the drying process can cause the screed
to crack or curl, but dehumidifiers can be useful.
Surface accuracy
The accuracy of surface level and flatness of a laying surface is related to the type of base,
accuracy of the setting out and the quality of workmanship. These issues should be
considered when selecting the overall thickness of the floor finishes to avoid problems
with the finish and/or costly remedial measures.