A flat concrete substrate is one of the most important pre-requisites for the application of unbonded screeds or floating screeds. The purpose of keeping a screed unbounded is to allow the screed to slip on a polythene membrane, providing a suitable screed construction for depths of over 50mm. Taking into consideration the fact that the screed will naturally slip on the substrate, it is generally recommended that a 10mm polyethylene strip is laid to cover all perimeters and penetrations for absorbing the movement from the moving screed.
It is also important to make sure that the concrete substrate is free of stepped or uneven areas ; as such surfaces can lead to restricted movement and causes weak points and areas of greater stress in the floor system. This is often seen as the major cause for random, uncontrolled cracking of screeds.
CSC recommends that a pre-levelling system is used in order to achieve the best results, under such circumstances. A self-smoothing compound is generally prescribed as the ideal solution for reducing the surface imperfections.
However, this can result in considerable variations in the initial budget. But, it is important to consider that this cost variation is generally much lesser than the cost incurred for repairs at to the final floors finis, at a later stage of construction.