Why are separation membranes used between concrete pavement slab and sub-base?

The purpose of separation membrane between the concrete pavement slab and the sub-base are as follows:

*The separation membrane reduces the frictional forces between the concrete slabs and the sub-base.The membrane aids the movement of the concrete slab in reference to the sub-base when changes in the level of the moisture and temperature occurs

*It aids in the segregation of sub-base materials from freshly placed concrete.

*The separation membrane also helps in the reduction of cement and water loss in the form of immature concrete.Immature concrete greatly affects the strength of the concrete. It also affects the durability of it.

 A good example of a separation membranes is polythene sheeting which is commonly used.

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I agree with colleague answer / Dinu Mathew Thomas