The separation membrane between concrete pavement slab and sub-base has the following functions:

(i) It aids in reducing the frictional forces between concrete slab and sub-base and helps to movement of concrete slab with respect to sub-base owing to changes in temperature and moisture.

(ii) It prevents the loss of cement and water in immature concrete which significantly

affects the strength and durability of hardened concrete.

(iii)It avoids the mixing up of sub-base materials and freshly placed concrete.

Polythene sheeting, which is a waterproof material, is commonly used as separation

membrane.

1.It avoids the possibility of active aggressive agents from soil water being attached to the concrete slab.

2.It prevents the loss of water from cement paste which affects the strength of concrete slab.

3. It enhance the movement of concrete slab relative to sub-base layer and reduces the frictional forces developed at their interface

Hi Amjad,

Concrete pavements can be designed for different bonding conditions i.e completely bonded, partially bonded, and Unbonded concrete pavements. In the case of unbounded rigid pavements, as separating layer polyethylene sheet is used between pavement quality concrete (PQC) layer and subbase layer to form unbonded condition. Technically, the primary function of this polyethylene layer is just to maintain unbonded condition between the layers so that the frictional stresses developing due to the contraction and expansion of concrete slabs are reduced significantly.

Also, as Mr. Abbas and Kundan suggested, it will reduce the percolation of water from PQC to the underlaying layer which may help to maintain the water-cement ratio (W/C ratio) and strength of concrete.