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An expansion joint is a full-depth gap between two pieces of concrete, filled with a soft, compressible strip, so each piece can move without cracking the other. You'll find them where a driveway meets the garage slab, a walk meets the driveway, or a slab meets a wall or step. A control joint is different: it's a groove cut partway into one slab so the slab cracks in a straight line at the bottom of it.
What are expansion joints in concrete?
Expansion joints in concrete are gaps that run through the full thickness of the slab and separate it from whatever is next to it. The American Concrete Institute describes an expansion joint as a gap that lets sections grow and shrink (opens in a new tab), and says that in slabs on the ground the gap is filled with a compressible material.
The concrete industry usually calls them isolation joints. The NRMCA says these joints separate a slab from walls, footings, columns and other slabs so each one has independent vertical and horizontal movement (opens in a new tab). Homeowners and most stores call them expansion joints, and this site uses that name.
Concrete moves for two reasons. It expands and shrinks with changes in temperature and moisture, and the NRMCA notes that its overall tendency is to shrink (opens in a new tab). An expansion joint gives that movement somewhere to go.
Where you’ll find them at home:
- Where the driveway meets the garage floor or the street
- Where a sidewalk or patio meets the driveway
- Around the edge of a garage or basement floor, against the foundation wall
- Around posts, steps, light poles and other fixed objects set in the slab
The NRMCA lists the same spots, from driveways and patios meeting sidewalks, garage slabs and stairs (opens in a new tab) to light poles and other points of restraint.
Control joint vs expansion joint: what’s the difference?
A control joint is a groove that goes partway into one slab. An expansion joint is a gap that goes all the way through and separates two pieces of concrete. One tells a slab where to crack; the other keeps two slabs from pushing on each other.
The ACI describes a control (contraction) joint as a formed, sawed or tooled groove (opens in a new tab) that sets where cracks will form. The NRMCA says to cut the groove one quarter of the slab’s thickness (opens in a new tab), and at least 1 inch deep. The slab cracks along the thin spot under the groove, out of sight.
| Feature | Control joint | Expansion joint |
|---|---|---|
| Other names | Contraction joint | Isolation joint |
| Depth | A groove, 1/4 of the slab's thickness | The full thickness of the slab |
| What it does | Makes the slab crack in a straight line | Keeps two pieces of concrete from pushing on each other |
| Where | Across the middle of a driveway, walk or floor | Where a slab meets another slab, a wall, a step or a post |
| Made by | Sawing or tooling a groove after the pour | Setting a filler strip before the pour |
| What's in it | Nothing, or a joint filler added later | Fiber, foam or old wood strip, often sealed on top |
| Normal to see | A crack at the bottom of the groove | A strip that has shrunk, rotted or pushed up |
Definitions from the ACI and NRMCA, linked above.
Not sure what you’re looking at? The crack or joint guide shows how to tell a joint from a crack.
What about construction joints and cold joints?
A construction joint is where one pour of concrete stopped and the next one started, planned ahead of time. A cold joint is the unplanned version, left when a pour was delayed and the first batch began to set. The ACI defines both in its joint FAQ (opens in a new tab).
You’ll see a cold joint most often where a basement floor meets the wall, or partway up a poured wall. It’s a seam, not a crack, and it gets treated as a joint.
Do driveways and patios need expansion joints?
Yes, where they meet something else. The NRMCA’s slab rules call for isolation joints at junctions of driveways with walks, curbs or other obstructions (opens in a new tab). Within one driveway or patio, control joints do the work.
Control joints need to be close enough together to catch the cracks. The NRMCA puts the spacing at 24 to 36 times the slab thickness, about 10 feet for a 4 inch slab (opens in a new tab), and no more than 15 feet. InterNACHI uses a slightly tighter rule: no more than 30 times the thickness and not over 15 feet (opens in a new tab).
If your driveway has big panels and random cracks between the joints, the joints were probably too far apart. Those are cracks, and the slab crack repair steps cover filling them.
What goes in an expansion joint?
A compressible strip, set before the concrete is poured. The NRMCA lists asphalt-impregnated fiber sheeting and compressible foam strips (opens in a new tab) as the usual materials. Older homes often have wood boards instead.
The strip sits below the surface over time, and the top of the gap fills with dirt, sand and weeds. That’s when it needs work. A sealed top keeps out water and the grit that the Federal Highway Administration says can cause spalling (opens in a new tab) at the joint edges.
How you refill one depends on its width and what’s left in it:
- Rotted wood or missing filler: pull out the old strip and refill the gap. The expansion joint repair guide walks through it.
- Wide or deep gap: set backer rod to the right depth, then seal over it with a flexible joint sealant.
- Narrow control joint: fill the groove with a joint filler made for concrete joints.
Should you fill control joints too?
You don’t have to, but filling them keeps water, sand and weeds out. Control joints in driveways, walks and patios are often left open, and they work fine that way. Fill them if weeds are a nuisance or water is getting under the slab.
The NRMCA says joints in floors that take heavy traffic should be filled with a material capable of supporting joint edges (opens in a new tab) so wheels don’t chip them. A garage floor is the home version of that.
Use a joint filler, not a crack filler or caulk. The polyurea joint kit below is made for control and expansion joints, and its data sheet says it meets the floor joint filler guidelines in ACI 302 and ACI 360. Its instructions cover joints from 1/8 to 1/4 inch wide; wider joints are covered in the repair guide.
Comparing it with other fillers? The product chooser sorts them by job.
When is a gap at a joint a bigger problem?
When the concrete on either side has moved. A joint that has opened wider, a slab that has dropped so one edge sits higher, or a slab that now tips toward the house points to soil or drainage trouble under it. Filling the joint won’t fix that.
Watch the joint between the driveway or patio and the house. If that slab has sunk and water runs toward the foundation, fix the slope first, then seal the joint. If cracks in the foundation wall line up with the moving slab, have a structural engineer look at it before you fill anything.
An edge that sticks up is also a trip hazard. Lifting, grinding or replacing the slab is a job for a concrete contractor, not a joint filler.
Which joint guide do you need?
Start with what you see. Each guide covers one job:
- Not sure if it’s a crack or a joint: crack or joint
- Old wood strips or an expansion joint that needs refilling: expansion joint repair
- Sizing and setting foam rod in a joint before sealing: backer rod guide
- Random cracks in the slab, not at a joint: concrete crack repair
Every guide in this section
Is it a problem?
How to fix it
Questions
Are the wood strips in my driveway expansion joints?
Yes. Older driveways and patios often used wood boards as the expansion joint filler, and wood rots, holds water and grows weeds. The expansion joint repair guide covers pulling them out and refilling the gap.
Does a basement floor have an expansion joint?
Usually, yes, around the edge. InterNACHI notes that builders lay fibrous material around the perimeter (opens in a new tab) before pouring a garage or basement floor so the slab can expand without cracking the foundation wall. That's the thin strip or gap you see where the floor meets the wall.
Can I fill an expansion joint with mortar or concrete?
No. A rigid fill locks the two pieces together, which defeats the joint. The next time the slabs expand, the pressure goes into the concrete beside the joint and cracks or chips it.
Is a crack in the bottom of a control joint a problem?
No. That crack is the reason the joint is there. The NRMCA calls joints simply pre-planned cracks (opens in a new tab), and a crack at the bottom of the groove means the joint did its job.