During the design stage of most buildings, little if any attention is given to floors on grade. Owners and their architects and engineers are concerned primarily with space utilization, functioning utilities, energy conservation, adequate clearances and aesthetics. The quality of concrete floors on grade is usually taken for granted. As a result the owner often ends up with a floor that is inferior to the one he expected. However, if the owner knows what he needs in a floor on grade, he can plan and specify the requirements realistically so the floor performs as intended.
What makes concrete floors curl, crack, settle unevenly, dust, erode or scale? A basic understanding of concrete as a material can help avoid the causes of undesirable floor performance. One characteristic of fresh concrete that can affect the performance of floors is bleeding. If the bleed water leaves the concrete by evaporation or by some physical or mechanical means the total amount of water in the concrete is reduced, resulting in a lower water-cement ratio and thereby improving the quality of the concrete. However, if the bleed water does not leave the surface, an increase in the water-cement ratio will result near the concrete surface. Thus the weakest, most permeable, and least wear-resistant concrete will be at the top surface where the best concrete is needed.
The functions of a floor on grade are to provide a smooth, easily cleaned and maintained wearing surface and to transmit the loads to the soil beneath the floor. The following elements discussed in this article determine proper floor performance: subbase, isolation joints, control joints, slab thickness, surface finish, surface tolerances, durability, and dusting.