They're called slabs on grade. They're composed of two equally important elements, each with a distinctly different function. But when it comes to floors, the slab part—being what the owner is left to see—invariably draws much more attention than the grade part hidden below. This imbalance of interest has always caused problems for the flooring industry, because:

Rule 10a: The grade is the structure; the slab is just a wear surface.

This reality leads to:

Rule 10b: The grade must be designed, constructed, and maintained so it will support the load indefinitely, whether the slab is there or not.

If Rule 10b can't be followed, then a structural slab—that is, one that must span the grade's deficiencies—must be designed. Slab-on-grade design is fundamentally different from structural slab design, a critical distinction that seems to elude most engineers. Structural slabs, unlike slabs on grade, are expected to crack randomly. Consequently, because they cannot rely upon the grade for support, they must always contain enough continuous reinforcement to carry the shear and tensile loads without any help from the concrete. This column is strictly about slabs on grade and the special perspective required to design and construct them successfully. The design of structural slabs is a separate (and routine) issue covered in any number of textbooks. The surprising, but logical consequence of Rule 10b is:

Rule 10c: In principle, no slab on grade needs to be very thick.

Just how thick depends upon the degree to which real world imperfections are likely to separate the actual construction from the ideal; particularly, how badly Rule 10b is likely to be broken. If the grade approaches perfection, as in the case of some bonded toppings, then a 1½-inch-thick slab will suffice. But when it comes to live load induced cracks, slabs on grade are so forgiving that even in less than ideal conditions, only rarely must they be more than 6 inches thick.

Because Rule 6a (see “More Dog Work” in the March 2010 issue of CC) keeps the character of the grade in jeopardy, because subsequent bad weather can negate the results from standard density tests, and because the loads imposed by the owner will never be nearly as severe:

Rule 10d: A thorough proof rolling in front of the placement is the best method available for assuring the competence of the grade.

Under a loaded 40-ton truck, there should be no visible “pumping,” and all stirring and rutting of the grade should be superficial.

Rule 10e: Means must be kept available during the placement to repair any damage done to the grade.

Finally, as general principles:

Rule 10f: Soil cement makes the best base. The local DOT's crusher run base is the next best alternative. Georgia clay and Florida sand and like materials can be tolerated, but graded stones are unacceptable.

Rule 10g: In rapid drying environments, put the specified plastic sheet under the base. Otherwise, put it under the slab.

Rule 10h: During placement, keep the temperature difference between grade and concrete under 30º F.

Rule 10i: Lose the old 2-inch top sand layer idea, and forget about prewetting the base.