Concrete Control Joints

Why concrete cracks and what joints do

As concrete cures and dries it shrinks. The slab is held back by friction against the base and by anything it is attached to, so tension builds up until it cracks. A crack will form somewhere. A control joint is a groove cut or tooled into the slab that reduces its thickness at that line, so the crack happens there, straight and tidy, rather than at random across the surface.

Joints do not prevent cracking. They control it. A slab with well-placed joints will still crack, but the cracks will be in the joints where they are hidden and hardly matter.

Spacing rule of thumb

NRMCA’s CIP 6 gives a common rule: space joints at 24 to 36 times the slab thickness, with a maximum of about 15 ft, and keep panels as close to square as possible. ACI 302.1R gives the same range for slabs on grade. For a 4 in slab that is 8 to 12 ft in each direction.

Use the closer end of the range with high shrinkage mixes, hot weather, unreinforced slabs and where the slab is restrained. Use the wider end where the mix is low-shrinkage and well cured. Sidewalks are typically jointed at a spacing equal to their width, such as every 4 or 5 ft.

Depth and timing

The groove should be at least one-quarter of the slab thickness deep, so a 4 in slab has a joint at least 1 in deep, a 6 in slab 1.5 in. If the joint is too shallow the crack may wander around it. Joints are either tooled into the fresh surface with a groover or cut with a saw. Saw cutting must be done early, as soon as the concrete is hard enough to cut without ravelling the edges, typically within 4 to 12 hours of placing depending on the temperature and mix. Waiting too long gives the slab time to crack before it is cut, so plan the saw before you pour.

Layout rules

• Keep panels square, or at most about 1.5 times as long as wide.
• Avoid L-shaped and odd panels; cracks run from re-entrant corners.
• Line joints up with columns, wall corners and any fixed objects, such as drains and manholes.
• Extend joints through to the edges and forms.
• Keep joints consistent in both directions to form a grid.
• In a long narrow slab like a sidewalk, cut across the width at regular intervals.

Isolation and construction joints

Control joints handle shrinkage. Isolation joints separate the slab from things that move differently: foundation walls, columns, steps and drains. They use a strip of compressible material, such as expansion joint filler, and allow the slab to move without cracking. Construction joints occur where one pour stops and another begins, and are used when a day’s work ends. They often have dowels or keys to carry load across.

Do I need joints in a small patio?

A 10 × 10 ft patio at 4 in could be left without any joints inside the 8 to 12 ft range, but a single cut across the middle is cheap insurance. Anywhere the slab is longer than the spacing or the shape has a re-entrant corner, add joints. For a driveway, cut joints in a grid and also at the junction with the garage slab and the street.

Sealing joints

Joints can be left open on walks and patios, or filled with a flexible sealant to keep out water and debris. On exterior slabs in freezing climates, sealing reduces water entry. Choose a sealant designed for concrete joints and follow its instructions for depth and backing.

Plan the layout on paper

1. Sketch the slab and any fixed objects.
2. Pick a spacing within the range and divide each dimension into equal panels.
3. Mark the joints on the forms or the base.
4. Arrange the saw or the tools before the pour.
5. Cut within the recommended window.

Sawing compared with tooling

Layouts for common shapes

• Sidewalk: a joint across the width every 4 to 5 ft, or the width of the walk.
• Patio: a grid with panels of 8 to 12 ft for a 4 in slab.
• Driveway: joints every 10 to 12 ft in each direction, plus an isolation joint at the garage and street.
• Garage floor: a grid aligned with columns and door openings.
• Narrow slabs: put a lengthwise joint down the middle if the width is more than 12 ft.

Problems and fixes

• Random cracks beside the joint: it was too shallow or cut too late.
• Spalling at the joint edge: sawn too early, or traffic too soon.
• Curling at joints: the slab dries faster at the top; improve curing.
• Cracks across a panel: the panel is too long. Add a joint next time.
• Cracks in a re-entrant corner: add a joint from the corner.

Example: joint plan for a 20 × 20 ft slab at 5 in

At 5 in the 24 to 36 × thickness rule gives 10 to 15 ft spacing. A single joint in each direction at 10 ft creates four 10 × 10 ft panels, which is near the lower end of the range and gives good crack control. Cut the joints to at least 1.25 in deep (one quarter of 5 in) as soon as the concrete can be sawn without ravelling, and make sure the saw cuts reach the edges of the slab. Isolate the slab from the foundation wall with a strip of joint filler. If the slab is long and narrow, add joints across the narrow dimension.

Frequently asked questions