Q.: What would happen if I omitted the crushed aggregate subbase under a warehouse floor? The soil is firm and dry, and in a well-drained location.

A.: Some authorities do not believe that special crushed aggregate subbases are needed under all slabs on grade. In many cases a slab can be cast directly on undisturbed soil or fill that has been properly compacted if there are no water problems.

The concrete mixture is very important, however. Many slabs are constructed in warehouse and industrial buildings with 3000-psi concrete but such concrete doesn't have sufficient strength or wear resistance for such service. Instead, 4000-psi or even 4500-psi concrete should be used.

Let's say it would cost about $2 a cubic yard more for 4000-psi concrete than for 3000-psi concrete. For a 5- or 6-inch slab, that would be a difference of about 4 cents a square foot. It is likely that one could save more than 4 cents a square foot by not using a crushed aggregate subbase.