The paper will give part of the results and conclusions of a research carried out by the Belgian Building Research Institute over the last years. This part of the research had as goal to determine the influence of the following parameters on the size of differential shrinkage of concrete slabs on grade : • the friction between the sub-base and the concrete slab • the presence of a vapour retardant on one side of the slab • the presence of reinforcement in the top or the bottom part of the concrete slab One of the conclusions drawn is that shrinkage at early age is more impeded by the presence of the vapour retardant then by the presence of armature. At later age (when the slab is older than 91 days until at least 184 days) the reverse has been noticed. An other conclusion was that, although the friction coefficient observed on slabs cast on dry sand was smaller, the best possible sub-base to reduce the amplitude of the shrinkage gradient will be sand with a PEsheet. This because sand on building sites is seldom completely dry and casting concrete on dry sand has negative influences on the mechanical properties of the bottom side of the concrete slab.