TradesHow this works
Pick what the slab is for and the tool sets a thickness and reinforcement to match. From the thickness it lays out control joints at the right spacing and depth, counts the panels for your slab, and warns if any panel comes out too long and skinny. Concrete cracks. Control joints decide where.
These are field rules of thumb for flatwork. A structural or load-bearing slab should be sized by an engineer. Cut or tool the joints early, before shrinkage starts pulling the slab apart.
Common questions
How far apart do control joints go?
About 2 to 3 times the slab thickness in inches, read as feet. A 4 inch slab gets joints roughly every 8 to 12 feet, and it is smart to stay on the closer end for driveways, patios, and walkways. Joints cut a quarter of the way through the slab.
Why does panel shape matter?
Long skinny panels crack across the middle no matter how you cut the perimeter. Keep any panel under a 1.5 to 1 length-to-width ratio. If your spacing leaves a stretched panel, add an intermediate joint to square it up.
Rebar or fiber?
They do different jobs. Fiber and welded wire control shrinkage cracking and are fine for patios and walkways. A rebar grid adds load capacity, which is what a driveway or garage that carries vehicles needs.
Sources: ACI 302 / 330-style rules of thumb: joint spacing 2-3x thickness (in) in feet, depth 1/4 thickness, panel ratio <= 1.5. Engine version 1.0.0. Risk tier 1.