Attempting to control where concrete cracks is both a science and an art. Just when you think you know how to control cracking, cracks that defy logic will appear. When you lay out control joints (also called contraction joints) on exterior slabs on grade, plan for drying shrinkage during initial curing, curling due to differential shrinkage, and thermal movement over the life of the pavement.
The American Concrete Pavement Association (ACPA) has established a number of guidelines for planning joint locations.
The three basic ways to construct control joints are placing control joint products into the concrete during placement, using hand groovers to cut control joints into freshly placed concrete, and sawing joints into the concrete after initial set.
Also referred to as expansion joints, isolation joints should be placed wherever pavement abuts other features. Their purpose is to isolate the movement of the pavement. As a general rule, isolation joints should not be placed in the body of the pavement.
Crack widths must not exceed 0.04 inch for good aggregate interlock. Fewer control joints will result in greater crack widths and less load transfer.
The ACPA recommends not using dowels for load transfer in pavements less than 7 inches thick. It also recommends against using reinforcement in slabs on grade in an attempt to eliminate cracking and to control joints. Steel reinforcement can help control differential settlement, and rebar will help minimize crack widths, but slabs with reinforcement will still develop shrinkage cracks.