Basics

Why Control Joints Open Up

The lines across your driveway are not decoration and they are not weaknesses that failed. They are the part of the design doing its job, and an open one is usually good news.

Updated October 2026 · Burlington Concrete

Joints generate more unnecessary worry than anything else in residential concrete. People notice them widening, notice the filler cracking, and reasonably conclude that something is going wrong. In almost every case the opposite is true: what they are looking at is the slab behaving exactly as it was designed to.

What a joint actually is

A control joint is a deliberate line of weakness. Not a decorative groove, not a division between separate pours. A weakness, put there on purpose.

The reasoning is worth following because everything else falls out of it. Concrete shrinks as it cures and continues to move with temperature for the rest of its life. It is also gripped by the ground beneath it, so it cannot simply get smaller uniformly. Something has to give.

Given no help, it gives wherever the stress happens to peak, which is typically a long irregular line across the middle of the most visible area. Given a joint, it gives there instead, below the surface, along something straight that a person chose.

So the joints are not places the slab might crack. They are places it is intended to, and an open joint is the evidence that it worked.

Why they are where they are

Spacing is not arbitrary and it is not aesthetic. It follows the depth of the slab, because a thicker slab can carry stress further before relieving it. Multiply the thickness by something in the region of twenty-four to thirty-six and you have the working spacing, with fifteen feet as a ceiling no matter what the arithmetic suggests.

Depth of cut follows the same logic from the other end. Reach roughly a quarter of the slab's thickness and the line is weak enough to attract the crack, while the slab as a structure is still carrying what it always carried.

Timing is the third variable and the one that actually goes wrong on site. Leave the cut too long and the slab has already relieved itself elsewhere, so the joint arrives at a line the concrete stopped caring about. That is the usual explanation for a driveway carrying both a neat set of joints and a wandering crack of its own.

What an open joint is telling you

Look along it rather than at it. Two things matter.

Is the gap even along the line? A joint that has opened consistently is shrinkage and thermal movement, which is ordinary. A gap that is wide at one end and closed at the other suggests something is rotating rather than simply contracting.

Are the two sides still level? Put a straight edge across it. Level means the ground below is holding and the joint is doing its job. A step between the two sides is a different matter entirely, because that is the base moving rather than the slab shrinking, and no joint was ever going to prevent it.

That second check is the whole diagnosis, and it takes about four seconds.

Sealant is a consumable, and nobody says so

The flexible material in an exterior joint is doing an unreasonable job. It has to stay elastic across a temperature range of well over a hundred degrees, while the gap it lives in gets narrower every summer and wider every winter, with grit ground into it by tyres and shovels throughout.

It does not do that forever. Sealant that has gone hard, cracked or pulled away from one face after several years has not failed prematurely; it has reached the end of what it was. Renewing it periodically is maintenance, and it belongs on the same mental list as clearing gutters rather than on the list of things a contractor got wrong.

It is worth doing, and the reason is drainage rather than tidiness. An open joint is a direct route from the surface into the base, and the base is the only thing holding the slab where it is. Keeping water and grit out of that route is cheap insurance on an expensive surface.

Isolation joints, which are a different thing entirely

Worth separating, because the two get muddled and they are not the same.

A control joint is a line within one slab, telling it where to crack. An isolation joint sits between the slab and something else: a garage wall, a step, a post base, a drain. Its job is to stop two structures that move independently from being rigidly attached to each other.

Concrete poured hard against a foundation wall will eventually crack, because the two are moving to different rhythms and something has to give. An isolation joint gives them permission to move separately, and it is a strip of compressible material rather than a saw cut.

If you are looking at a crack running parallel to a wall a few inches out from it, that is very often a missing isolation joint rather than a jointing-spacing problem, and it is worth identifying correctly because the two have different answers.

The one thing not to do

Do not fill a control joint with anything rigid. Mortar, sand-cement, a hard epoxy: all of them defeat the point. The joint exists so that two pieces of slab can move relative to each other, and gluing them together simply relocates the movement to a place nobody chose.

If a joint looks untidy, the answer is a flexible filler intended for the job, not a permanent one. And if a joint has stepped rather than opened, filling it at all is cosmetic work over a base problem, which is covered in why water decides how long a slab lasts.

Where several joints have stepped rather than opened, that is the point at which concrete repair becomes the conversation, and when a driveway actually needs replacing sets out where the threshold sits.