A mix for a floor slab must be designed to produce workable and readily finished concrete having the following qualities: low permeability, good wear, abrasion resistance, and sufficient durability to withstand ground exposure conditions. The concrete must have a minimum strength of at least 3,000 psi at 28 days, regardless for aggregate quality or air content. The maximum size of coarse aggregate for non-reinforced floors should not exceed one-third of the slab thickness. For reinforced structural floors the maximum size should not exceed three-quarters of the minimum clear spacing of the reinforcing bars. As a general rule natural aggregates should not be larger than one and one-half inches; lightweight aggregates, not larger than 1 inch. The proportion of fine aggregate present in a floor mix should be the minimum allowable for workability. With floor concrete, placability and finish ability should have priority over more easily measured properties such as slump and compressive strength. Good mix design for floor concrete is governed by two main factors: the water content and the ratio of fine to coarse aggregate. For any given set of materials and placing conditions, the performance of a finished floor is inversely related to the water/cement ratio of the mix. Using the minimum quantity of water to produce a desired slump is therefore highly important. This quality will be influenced by a number of factors, mainly the maximum size of the coarse aggregate, the aggregate grading and their particle shape and texture, and the air content. The use of large, maximum-size aggregate will reduce the mixing water needed. Use of an air-entraining admixture is advantageous for all floor concrete. These admixtures reduce the mixing water requirement and bleeding, increase plasticity, and generally facilitate proper finishing. An air content of about 2 percent, by volume, is recommended for normal aggregate concrete. If lightweight aggregates are used, the air content should be between 4 to 7 percent for 3,000 to 4,500 psi concrete, and 3 to 6 percent for higher strength concrete.