Control joints in fiber concrete?

Concrete Construction, Dec, 2002

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.