· by Michael Blair, Nashville's Concrete
Concrete shrinks as it cures and keeps moving with heat and moisture for as long as it is down. The Federal Highway Administration’s TechBrief on early-entry sawing says joints are there to prevent the random cracking those forces cause, by sawing a weakened plane so the crack forms along it instead. When a slab cracks at random anyway, the troubleshooting tables in the National Concrete Pavement Technology Center’s practice manual list joints spaced too far apart, cut too shallow or sawn too late among the causes, alongside poor curing and the slab’s friction against its base. Both were written for highway pavement rather than driveways, so we treat their numbers as limits, not targets.
What does a control joint do?
A control joint is a groove that thins the slab along a straight line. The pavement center’s manual describes joints started in plastic or green concrete and shaped afterward, and its advice is to tool the joint or use an early-entry saw as early as possible. Because the slab is thinner at the groove, the shrinkage crack forms at the bottom of it rather than somewhere else. From the surface you see a clean line. The crack is underneath.
An isolation joint does a different job. The same manual’s glossary defines it as a joint that lets the concrete on either side move independently in three directions, and points to its older name, the expansion joint. On a driveway we pour, control joints run across its length, and the top end, where it meets the garage floor, is handled the way our post on garage slab problems and what the code asks for lays out.
How far apart and how deep should control joints be?
For spacing, the pavement center’s manual passes on the American Concrete Pavement Association’s recommended maximum of 24 times the slab depth for pavement on a granular base. That is two feet per inch of thickness, or eight feet between joints on a four-inch slab. Shape counts too. For slabs that crack, the manual’s advice is to add intermediate joints where a slab is too wide for its thickness and length and to keep a reasonable length-to-width ratio, so we lay panels out as close to square as the shape of the pour allows.
Depth is the part nobody can check once the job is finished. The FHWA TechBrief gives a typical saw-cut depth of one-quarter to one-third of the slab thickness, which on a four-inch slab is an inch to an inch and a third, and notes that some guidance accepts a shallower cut of at least an inch only when the sawing is done early. A cut that is too shallow may not relieve the stress, the pavement center’s manual warns, and the random crack forms anyway. Its checklist for shallow joints is mundane: the saw’s depth setting, a worn blade, and an operator pushing the saw fast enough that it rides up out of the cut.
- Spacing: no more than 24 times the thickness, about eight feet on a four-inch slab
- Panels laid out as close to square as the pour allows
- Depth: one-quarter to one-third of the slab thickness
- On our layouts, a joint out of every inside corner
Timing, and Middle Tennessee weather
Tooled joints go in while the concrete is still plastic. Sawn joints have a window, and the FHWA TechBrief describes both edges of it: saw too soon and the concrete ravels at the cut, saw too late and residual stress builds until the slab cracks at random. It puts that window at roughly 4 to 12 hours after paving, varying a good deal with conditions, while the pavement center’s manual says it can be as short as two to four hours. A saw that shows up the next morning is outside both ranges.
We plan our summer pours around the manual’s table of what shortens the window: hot sunny weather, high wind with low humidity, a sudden rise in temperature, and a sudden drop or a rain shower. Lighter early-entry saws exist for exactly this, and the FHWA TechBrief says they can go on the concrete at a much earlier age, often as soon as workers can walk on it; in a Texas study it cites, the one-inch early-entry cuts were made about 2 to 3 hours after placement. The rest of the forecast is in our post on the weather that ruins a concrete pour. Ask any contractor when the joints will be cut, and expect the answer to be the same day.
Corners, and why cracks start there
Neither pavement reference deals with the inside corners a house lot is full of, so this part is our own practice rather than a cited rule. Where a walk meets the driveway at a right angle, where a slab wraps a step or a column, where a drain box sits in a patio, the inside corner is the first place we look on a slab that has cracked, and every layout we draw runs a joint out of each one so a crack starting there has a line to follow.
A slab that has already cracked at random is a different job, and we do not cut new joints into it to fix the crack. We work out whether the crack is still moving, which our post on which concrete cracks matter walks through, and the repairs are on our concrete repair page. That is why the joint layout goes on paper before the forms go up. If you have a pour coming, ask us for a free estimate and the joint plan comes with it.
Sources
- Federal Highway Administration, TechBrief: Early-Entry Sawing of Portland Cement Concrete Pavements (HIF-07-031, 2007) — Joints prevent random cracking from temperature, moisture and shrinkage by a weakened plane; saw depth typically one-quarter to one-third of thickness, at least 1 in. only if sawn early; window roughly 4 to 12 hours, too soon ravels, too late cracks; early-entry saws as soon as workers can walk on it, Texas cuts at about 2 to 3 hours
- National Concrete Pavement Technology Center, Iowa State University, Integrated Materials and Construction Practices for Concrete Pavement, 2nd ed. (2019) — Tooled or early-entry sawn joints; isolation joint definition; ACPA maximum spacing of 24 times depth; reasonable length-width ratio; shallow cuts allow random cracks, depth setting, blade wear, saws riding up; sawing window two to four hours; weather that shortens it; causes of early-age cracking including curing and base friction
FAQ
Where Control Joints Go and Why — questions
Why did my new driveway crack next to the control joints?
Because the joints did not control the crack. The likely reasons are a cut that was too shallow, under about a quarter to a third of the slab depth, a cut made after stress in the young concrete had already built up, or panels too long or too wide between joints. Federal pavement guidance lists all three among the causes of random cracking, along with poor curing. The slab cracked where it wanted to instead.
How far apart should control joints be on a patio?
For a four-inch slab, about eight feet at most, which is the 24-times-thickness maximum the American Concrete Pavement Association recommends for pavement. Keep the panels as close to square as the layout allows, and on our patios a joint also runs out of every inside corner. Spacing wider than that maximum is one of the causes of random cracking named in the guidance.
Can control joints be added to an existing slab?
The pavement guidance we work from treats sawing as part of the pour, done within hours, before stress builds in the young concrete, and it does not cover cutting joints into an older slab. If a slab poured in the last day or two was never jointed, call the contractor straight away, because late sawing is what that guidance ties to random cracking. On a slab that has already cracked, we do not cut new joints to fix it; we check whether the crack is still moving and repair it on that basis.
When should concrete control joints be cut?
The same day, within hours of the pour. Federal pavement guidance puts the sawing window at roughly 4 to 12 hours after placement, and as short as two to four hours in hot, sunny or windy weather. Early-entry saws can start once workers can walk on the slab, while a saw that arrives too late leaves random cracks.