Why Concrete Cracks, and What Control Joints Do About It

By The Grayline Team · 2/24/2026

If a contractor promises concrete that never cracks, be careful. Concrete is strong in compression and weak in tension, and as it cures and the ground moves it will always want to pull apart somewhere.

What a control joint does

A control joint is a deliberate line of weakness cut into the slab, so the inevitable crack forms hidden at the bottom of a straight groove instead of wandering across the surface.

Reinforcement and base

Reinforcement keeps those cracks tight, and a compacted, well-drained base removes the soft spots that cause the worst random cracking.