Economical design of floors is only possible to the extent that the designer understands such aspects as floor thickness, concrete strength, surface hardness, and accommodation of shrinkage. He must be able to satisfy the performance requirements of the floor from a realistic appraisal of these factors and any special other requirements. The thickness requirements of floors on grade are established on the basis of what is needed to resist the bending loads to which they are subjected with a practical minimum of cracking. Subgrade support plays an important role in the reaction to loading of slabs on grade. For many ordinary floors on grade thicknesses can be arbitrarily taken from recommendations based upon experience. Structural floors are designed, according to many procedures, on the basis of flexural strength. The compressive stresses in floors are small compared to tensile stresses and in many design procedures the tensile strength is used. The concrete mixes used for floors, however, are specified partly in terms of compressive strength. The reason for this is that compressive is widely used as a measure of concrete quality and can be used as a guide to the rate at which the body of concrete is increasing in hardness during curing and later. For any given combination of materials, compressive strength is directly related to flexural tensile strength and to wear resistance. Under normal conditions high compressive strength usually means long wear. The surface hardness in any floor depends on the kind of traffic to which it will be subjected. The hardness of the surface will depend of the strength of the concrete mix, the timing of the finishing operations, the number of steel trowelings, the adequacy of the curing and how soon the curing is begun.