Q. I’m preparing to pour a 2700 square-foot circular driveway in south Florida. I plan to use fiber concrete, without rebar or mesh, and to pour the driveway in slabs. Each slab will be about 300 square feet (15x18 feet) and 4 inches thick, except at the street where the city requires it to be 6 inches thick. Do I need to have control joints in the slabs and, if so, how far apart in a 15-foot-wide driveway? I really don’t want any but don’t want cracking, either.

A. The general rule is to multiply the thickness of the slab by 24 to 36 to get joint spacing. In other words, thickness in inches times 2 to 3 equals spacing in feet. Therefore, for a 4-inch slab, spacing should be from 8 to 12 feet. And panels should be as nearly square as possible, although an aspect ratio under 1.25 is fine (18/15=1.2). Fibers will not have any effect on the spacing of control joints but will help to reduce plastic shrinkage cracking. Based on this, we would recommend that you put a joint dividing each poured section in two in each direction, creating 9x7.5-foot panels. For more info on this, check ACI 302, ACI 360, or Designing Floor Slabs on Grade by Boyd Ringo and Bob Anderson. Joint spacing can be increased a bit with larger coarse aggregate, thicker slabs, and lower-slump concrete.