Concrete shrinks as it sets and dries, and when that shrinking is held back, the slab cracks. Joints are planned cracks: lines cut or tooled in so the slab cracks where you want it to. Guidance spaces control joints at 24 to 36 times the slab thickness, with a maximum of 15 feet, so about 10 feet for a 4 inch slab. Isolation joints go where a slab meets something fixed, like a wall or step.
Concrete sidewalk expansion joint spacing is something you see on every walk and rarely think about. The lines across the slab are there for one reason: concrete will crack, and joints tell it where. Here is what they do, how far apart they go, and what we see when they are missing.
What are the lines for?
Concrete shrinks as it sets and dries. If that shrinkage is restrained, the stress exceeds what the concrete can take and it cracks. Joints are "pre-planned cracks" that control where the cracking occurs (NRMCA). A line cut across the slab makes a weak spot, so the crack forms in the line, where it is straight and hard to see, instead of wandering across the middle of the panel.
Two kinds matter on a walk. Control joints (also called contraction joints) are the cut lines across the slab. Isolation joints go where a slab meets something fixed, such as a wall, a footing, a step or a light pole (NRMCA). People call them all "expansion joints," and the isolation joint is the one that lets the slab move against its neighbor.
How far apart should control joints be?
NRMCA guidance for control joints:
- Spacing of 24 to 36 times the slab thickness, with a maximum of 15 feet. For a 4 inch slab that is about 10 feet.
- Panels square, or nearly so, with the length no more than 1.5 times the width.
- Joint depth at least a quarter of the slab thickness, and not less than 1 inch. Another NRMCA sheet gives about a quarter to a third.
- Saw cuts typically 4 to 12 hours after finishing (NRMCA).
That 4 inch slab is NRMCA's example, not a Chicago sidewalk rule. If you are working in the public way, the sidewalk needs a CDOT permit (City of Chicago), so ask what spec applies to the joint layout. For thickness, see how thick should a sidewalk be.
What happens without joints?
Most cracking comes from construction practice, and missing joints are on that list, along with poor subgrade preparation, watered-down concrete, poor finishing, fast moisture loss and inadequate curing (NRMCA). A long slab with no joints, or with joints too far apart, tends to crack on its own line.
Cracks are not automatically a failure. No concrete is completely crack free, and most early random cracks rarely affect structural integrity or service life (NRMCA). Reinforcing mesh does not prevent them either. Properly placed steel limits crack width, and mesh should be cut or stopped across control joints (NRMCA). To tell a harmless crack from one to worry about, see concrete cracks: which ones to worry about.
On older Chicago walks we often find joints that were spaced far apart, or a long run with a single joint near the middle, and a crack running across the panel next to it. That does not mean the slab is failing. It does tell you the layout was not planned around where the concrete would crack.
What to ask before a pour
Ask your contractor where the joints will go, how deep they will be cut, and when. If the answer is "wherever," that is the answer. When you are deciding what to do with a cracked old walk, see fix, replace or wait. Akats pours Chicago sidewalks, and our concrete team will walk you through the joint layout before the pour. The concrete guide covers the rest.
Common questions
NRMCA guidance is 24 to 36 times the slab thickness, with a maximum of 15 feet. For the 4 inch example in that guidance, that is about 10 feet. For a public sidewalk, ask CDOT what specification applies.
People use the terms loosely. A control joint is a cut or tooled line that guides where the slab cracks. An isolation joint, often called an expansion joint, separates a slab from a wall, footing, step or pole so it can move independently.
Sources
- National Ready Mixed Concrete Association: Concrete in Practice 4: Cracking Concrete Surfaces (PDF)
- National Ready Mixed Concrete Association: Concrete in Practice 6: Joints in Concrete Slabs on Grade (PDF)
- City of Chicago: Guide to Building Permits: What Types of Work Do Not Require a Building Permit?
Facts last checked: October 7, 2026.
This article explains general rules and common situations in Chicago. It is not legal advice, and City rules change. For a specific property, check with the Chicago Department of Buildings or a licensed professional.