Those straight lines cut across new concrete are there on purpose. Control joints do not stop cracking, they decide where it happens.
A control joint, also called a contraction joint, is a planned weak line in a concrete slab. It is cut with a saw, tooled into the surface with a groover or formed with an insert, and it reduces the thickness of the slab at that spot. When the concrete shrinks as it hardens and dries, stress builds, and the slab cracks along the weakest line. A control joint makes that weakest line the one you chose.
The goal is a neat, straight crack hidden at the bottom of a joint, rather than a jagged crack running across the middle of a panel. A well-jointed slab can still crack, but the cracking is usually tidy and hard to see.
Concrete shrinks. Mixing water in excess of what the cement needs leaves the slab as it hardens and dries, and the slab gets slightly smaller. Because the slab is held by the ground beneath it and by anything it is attached to, shrinking creates tension, and concrete is weak in tension.
Reinforcement such as wire mesh or rebar does not prevent shrinkage cracking. Its job is to hold the two sides of a crack tightly together once it forms. Joints and reinforcement work as a team: joints choose the location, and reinforcement keeps the crack from widening.
For a deeper look at the other causes of cracking, including settlement and curing problems, read why concrete cracks.
Two rules of thumb are used across the industry. First, a control joint is typically cut to about one quarter of the slab thickness. A 4-inch slab would have joints about 1 inch deep. Second, spacing in feet is often limited to about two to three times the slab thickness in inches. For that same 4-inch slab, joints would be roughly 8 to 12 feet apart.
Layout matters as much as spacing. Panels should be close to square, since long narrow rectangles tend to crack across the middle. Joints should run into corners and edges, such as a column, a drain or a re-entrant corner, because cracks naturally start at corners that stick into the slab.
| Slab thickness | Joint depth (about 1/4) | Spacing (about 2 to 3 times thickness in feet) |
|---|---|---|
| 4 inches | About 1 inch | About 8 to 12 feet |
| 5 inches | About 1.25 inches | About 10 to 15 feet |
| 6 inches | About 1.5 inches | About 12 to 18 feet |
Joints must be cut early. Saw cutting is done after the concrete is hard enough not to ravel but before it has started to crack on its own, which is often within hours of finishing. In hot weather, the window can be as short as 6 to 12 hours. Some crews use early-entry saws that can be used almost immediately after finishing.
Hand-tooled joints are pressed in when the surface is finished. A joint that is cut too late may not control cracking, because the slab has already formed its own crack elsewhere. If you see a random crack running between joints on a new slab, late or shallow cutting is one possible reason.
Control joints are not the same as expansion or isolation joints. Isolation joints, often filled with compressible material, separate a slab from something that moves differently, such as a house wall, a column, a garage slab or a stoop. Expansion joints in outdoor slabs are used less than people think, but are common where a driveway meets a garage slab or a street.
If a slab touches the foundation, a garage floor or a sidewalk, ask where isolation joints will go. Missing ones can cause cracking at the connection.
Walk around a finished driveway or patio and the joint layout becomes obvious. Straight, narrow lines dividing the slab into panels are control joints. Lines against the house, the garage or a step are usually isolation joints. Thin, irregular lines that run across a panel are cracks, which means the joints did not catch the stress there.
Saw-cut joints are narrow and crisp, while tooled joints have a rounded, slightly wider edge. Either type can work when done at the right time and depth. What matters is whether the panels are sensible shapes: close to square, free of narrow slivers and without sharp interior corners. A panel that is long and thin or has a notch cut out of it is likely to crack somewhere.
If your slab has no joints at all, or has random cracks in between widely spaced ones, the cracks themselves become the joints. Sealing them keeps water out, and in cold climates that is worth doing before winter.
On decorative work, joint placement is a design choice as well as a technical one. Contractors often align joints with pattern lines, borders or color changes so they are less noticeable. Stamped patterns may include tooled grout lines that double as control joints.
Colored concrete shows cracks and joint lines differently, and saw cuts can leave a lighter line against integral color. Ask how the contractor plans to handle that, and whether joints will be filled or sealed. A good layout starts on paper and gets reviewed with you before concrete arrives.
Ask for a joint layout before the pour. A good contractor can sketch where joints will go, how deep they will be cut and when. In a stamped or colored patio, ask whether joints will be hidden in the pattern. In a driveway, ask about the joint at the garage and at the street.
Joints also affect the look. Some homeowners prefer fewer, deeper-cut joints, while others want more panels for a tidier crack pattern. Our pages on concrete driveways and patios and stamped concrete describe how we plan layouts, and you can send us a project for a quote.
No. They give shrinkage cracks a planned place to form. Cracks from settlement, overloading or poor curing can still occur outside the joints.
A saw can cut joints into an existing slab, but doing so will not undo cracks already there. It is mostly useful for new sections and for managing future cracking in a repaired area.
Many are left open on driveways, while some are filled with a flexible sealant to keep out water and debris. Ask your contractor which approach suits your slab.
The joint may have been too shallow, cut too late or spaced too far apart. A crack nearby is often still just a shrinkage crack, but a wide or uneven one is worth asking about.
Describe the project and the surface you have now, and we will reply with what we need to know.