Control joints should be cut about one fourth the thickness of the slab during the finish-troweling process. Concrete shrinks slightly as it cures and will crack at regular intervals. You want the cracks to occur at the control joints. Space the joints about every 4 ft. on walks and at least every 10 ft. on larger slabs. If the concrete hardens before you can cut the grooves, cut them the next day with a circular saw and diamond blade.
Concrete will cure harder and stronger if it has a chance to dry slowly. In cool and moderate conditions, cover it with plastic. Under hot conditions, above 80 degrees F, frequently water it down with a garden hose to keep it moist. If kept wet, concrete will reach about 90 percent of its maximum strength after about a week.