Concrete answers
What are control joints in concrete for?
The short answer
Control joints are the straight grooves cut or tooled into concrete. They create planned weak lines, so when the slab shrinks and moves, it cracks along the joint, out of sight, instead of randomly across the surface.
Why concrete needs them
Concrete shrinks as it cures and moves with temperature and the ground. It will crack somewhere. Joints decide where.
Spacing and depth
A common rule of thumb is to space joints, in feet, at about two to three times the slab thickness in inches. A 4-inch slab would have joints roughly 8 to 12 feet apart. Joints are usually cut about a quarter of the slab's depth, and panels are kept close to square.
Timing
Saw-cut joints have to be cut soon after the concrete can be cut cleanly, before it starts cracking on its own. Tooled joints are formed while the concrete is still plastic.
Cracks in joints are normal
A crack running along the bottom of a joint is the joint doing its job.
This page is general information. Every slab and every yard is different, so the right answer for your concrete comes from looking at it in person.
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