The quality and serviceability of a concrete floor or slab depend both on the materials in the concrete and on the quality of the work done in placing, finishing and curing. The concrete mix should be so designed that the materials given proper workmanship will make a highly serviceable end product. The size, gradation and quality of both the fine and coarse aggregate, the amount and type of cement, the quantity of mixing water, and (if required) the amount and type of admixture, are all very important. Although instruction on materials and mix proportion is beyond the scope of this special issue, any craftsman or tradesman should come to understand the nature and properties of the material with which he works.
There is one basic procedural rule for concrete finishing that underlies all the others that are to follow: Do not perform any finishing operation while there is bleed water or excess moisture on the surface. This rule is based on the absolute need to avoid separating the sand from the cement in the surface layer.
The corollary of the basic rule stated above is: Any finishing operation performed while there is bleed water or excess moisture on the surface will ruin the surface. It will cause either scaling or dusting. Darbying or bull-floating the surface while there is bleed water on it, for example, remixes the water into the cement fines causing them to segregate in a horizontal plane with cement on the top and sand underneath. If these sand and cement fines are not thoroughly remixed during the subsequent floating operation the surface will definitely scale. Floating while there is excess moisture on the surface will weaken the strength of the cement paste and cause a dusting surface.