· by Michael Blair, Nashville's Concrete
Every concrete slab shrinks as it cures and moves with temperature afterward. It cannot do either without cracking, so the job of a control joint is to create a weak line where that crack will form, straight and out of sight at the bottom of the groove. A slab that has cracked randomly is almost always one where the joints were too far apart, too shallow, or cut too late.
What a control joint is
A control joint is a groove that reduces the thickness of the slab along a line, either tooled into the wet surface or sawn after the concrete has set. Because the slab is thinner there, the shrinkage crack forms at the bottom of the groove rather than somewhere else. From the surface it looks like a clean line. The crack is underneath.
It is different from an expansion or isolation joint, which is a full-depth separation with a compressible filler, used where a slab meets a foundation, a wall, a post or another slab that moves independently. A driveway needs isolation at the garage floor and the street apron, and control joints across its length.
Spacing and depth
The usual guidance is that joints should be spaced at no more than two to three times the slab thickness in feet: a four-inch slab wants joints roughly every eight to twelve feet, and closer is safer. The panels between joints should be close to square. A long narrow panel, more than about one and a half times as long as it is wide, cracks across the middle regardless of the joints at its ends.
Depth is the detail most often skimped. A joint needs to be about a quarter of the slab thickness to work, an inch on a four-inch slab. A shallow decorative groove a quarter inch deep does nothing, and the crack forms wherever it likes. Sawn joints that were cut too shallow to save blade wear are a common reason a new driveway cracks beside its own joints.
- Spacing: no more than two to three times slab thickness, in feet
- Panels near square; length no more than one and a half times width
- Depth: at least a quarter of the slab thickness
- Joints through every inside corner and re-entrant angle
Timing, and Middle Tennessee weather
Tooled joints go in while the concrete is still plastic. Sawn joints have a window: the concrete has to be hard enough not to ravel at the cut but the cut has to happen before the shrinkage cracks start, which on a hot windy day here can be within hours of the pour. A crew that comes back the next morning to saw has often missed it.
Our summer conditions, hot with a drying wind and sometimes a sudden drop in temperature with an afternoon storm, accelerate shrinkage. Early-entry saws that can cut within an hour or two of finishing exist for exactly this reason. Ask when the joints will be cut, and expect the answer to be the same day.
Corners, and why cracks start there
Concrete cracks from stress concentrations, and an inside corner is the worst one. Where a walk meets a driveway at a right angle, where a slab wraps a column or a step, where a drain box interrupts a patio, the crack runs from the corner diagonally across the slab. A joint should run from every inside corner so the crack has somewhere to go.
Once a slab has cracked randomly there is no putting a joint in after the fact. The crack can be routed and sealed to keep water and freeze-thaw out, which is worthwhile, but it will always be visible. Getting the joint layout on paper before the pour is the whole job.