Why are control joints used in slabs on grade but not on structural floors?
Slabs on grade require control joints to permit horizontal movement (shrinkage) without random cracking. Control joints are permissible because of the subgrade support. On structural slabs it is necessary for the concrete to be continuous between supports, thus making it impossible to use control joints. Here cracking is distributed uniformly and unobjectionably by the distribution of the reinforcing steel.