Friction between concrete and sub-base
Friction between the underside of a concrete slab and the sub-base will reduce the slabs capacity to move. This may result in tensile stresses that cause cracking of the slab.
The expression used to calculate the force required to overcome friction is;
Ff = wu, where Ff is the frictional force, w is the weight of the slab and u is the coefficient of friction.
Typically the value of u between concrete and sub-base is 0.7. This can be improved to 0.2 by having a plastic sheet between the concrete and sub-base.
Ref: Knapton J. Ground bearing concrete slabs, Thomas Telford, 2003