A control joint, also called a contraction joint, is a groove tooled or sawn into a slab to make a weakened line, so the crack that shrinkage is going to cause happens in a straight line where you put it.
Concrete shrinks as it dries, and a slab that's held by the ground and its edges will crack somewhere. You can't stop the crack. You can decide where it goes.
Depth. ACI 302.1R's guidance is at least one-fourth of the slab thickness, so a 4-inch slab gets a joint at least an inch deep. Too shallow and the crack wanders past it.
Spacing. ACI 302.1R gives 24 to 36 times the slab thickness. For a 4-inch slab that's 96 to 144 inches, or 8 to 12 feet. The shortcut is 2 to 3 times the thickness in inches, read as feet. Keep panels close to square. Long, skinny panels crack across the middle.
Placement. Put joints at inside corners, where a slab narrows, and lined up with joints in adjoining slabs or curb. Cracks start at re-entrant corners, so a joint there saves you a diagonal crack.
Timing. Tool the joints while the concrete is plastic, or saw them once the slab is hard enough not to ravel but before it cracks on its own. That's often within hours on a hot day. Early-entry saws can cut sooner than conventional saws. Saw-cutting is a separate trip, so price it.
Don't mix it up with an isolation joint. That's a full-depth break, usually with filler board, that separates a slab from a building, a column or an existing slab so they can move independently. Put the joint spacing on the proposal. The concrete flatwork guide covers pricing it.