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A layer of waterproof (or separation) membrane is normally placed between sub-base and concrete slab for the following reasons:
(i) It prevents the loss of water from cement paste which affects the strength of concrete slab.
(ii) It enhances the movement of concrete slab relative to sub-base layer and reduces the frictional forces developed at their interface.
(iii) It avoids the possibility of active aggressive agents from soil water being attached to the concrete slab.
This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.