Why Does Concrete Crack?

Concrete cracks mainly because it shrinks as it dries and cures, and concrete is strong in compression but weak in tension — so as it shrinks, it pulls itself apart. Some hairline cracking is normal and expected; the goal is not zero cracks but controlling where they happen and keeping them tight.

The trade saying is “there are two kinds of concrete: concrete that has cracked, and concrete that is going to crack.” Good practice — a solid base, the right water content, control joints, and proper curing — is what separates a hairline you never notice from a structural problem.

Drying shrinkage (the usual cause)

As the water in fresh concrete leaves, the slab shrinks — about 1/16 inch for every 10 feet. If the slab is restrained (by the subgrade, rebar, or its own corners) that shrinkage shows up as cracks. Too much water in the mix makes it worse, because there is more water to leave. This is why a low water-to-cement ratio and control joints matter.

Settlement and a weak base

If the ground under the slab is soft, unevenly compacted, or washes out, the slab loses support and cracks under its own weight or the first load. A well-compacted gravel base is the single best defense — most “random” cracks trace back to subgrade problems.

Temperature and freeze-thaw

Concrete expands in heat and contracts in cold; without expansion joints, large slabs can crack at the edges. In cold climates, water that soaks into concrete and freezes expands and flakes the surface (spalling) — air-entrained concrete and a good sealer help.

How to prevent cracks

You cannot stop concrete from shrinking, but you can control it: compact a good base, keep the water-to-cement ratio low, add rebar or mesh on structural slabs, cut control joints at the right spacing, and cure the slab wet for several days so the surface does not dry too fast. See the control-joints guide for spacing.

Common questions