A control joint is a groove cut or tooled into a concrete slab that creates a deliberate weak line, so natural shrinkage cracking happens along the joint instead of randomly across the surface.
In depth
All concrete shrinks slightly as it cures, and shrinkage means cracking. The only question is where. Control joints answer that question by giving the slab a straight, clean place to crack below the surface, keeping the visible slab looking intact.
Control joints also divide flatwork into panels, and those panels can settle independently. When one panel drops and its neighbor does not, the control joint becomes a trip hazard, one of the most common problems we are called to fix on sidewalks and driveways.
How Acme applies it. We lift settled panels back level at the joints, then recommend sealing joints so water cannot keep flowing beneath the slab and re-eroding the soil. Joint sealing after raising is one of the simplest ways to protect the repair.
Related terms: Expansion Joint, Joint Sealing, Change in Level, Trip Hazard
Frequently asked questions
What is the difference between a control joint and an expansion joint?
A control joint manages shrinkage cracking within a slab. An expansion joint separates a slab from another structure so both can move without pressing on each other.
Why do slabs settle unevenly at control joints?
Each panel sits on its own patch of soil. If washout or poor compaction affects one panel more than the next, the panels move differently and a step forms at the joint.