Residential floors must have adequate strength, surfaces that are hard and free of dusting and they must not crack. Hardness will depend on how densely the surface materials are compacted by finishing and how firmly they are bound by the cement paste. Cracking is mainly a function of how much drying shrinkage takes place and what provisions have been made to accommodate it. Good slabs on ground are made from a mix in which the water-cement ratio is kept low, and they contain as much coarse aggregate as possible at the surface. Compressive strength must be at least 3500 psi. Water reducing admixtures lower the amount of water needed for workability and improve compressive strength at all ages. Retarding admixtures delay set in hot weather to extend available finishing time. Accelerating admixtures increase the rate of strength gain at early ages and in cold weather shorten time that must be spent on the job. Some admixtures combine the water reducing function with that of retarding or accelerating. Jointing or reinforcement with so-called temperature steel are measures taken to control cracking; they may be used singly or in combination. Construction joints made with keys are needed wherever concreting operations must be interrupted during concrete placement. They should not be located within just a few feet of a control joint, however. Preformed keys of steel or fiber-filled asphalt are commercially available. Control joints of about one-fifth the thickness of the concrete should be spaced about 15 to 20 feet apart. Steel or welded wire fabric reinforcement for control of cracking must ultimately be secured midway between the top and bottom of the slab; if it ends up at the bottom it serves no purpose. Satisfactory ways of achieving the proper location are: supporting and tieing up on chairs or small concrete units made for the purpose; placing the lower half of the slab, screeding roughly, laying out the steel and placing the rest of the concrete; and placing the steel on top of the plastic concrete before finishing and then depressing it to the proper level.