Why Concrete Cracks — and Which Cracks Actually Matter

Concrete cracks. Not because someone did it wrong, but because of what the material is. The useful question is never whether your slab will crack — it is whether the crack you are looking at is cosmetic, structural, or a symptom of something moving underneath.

That distinction is worth learning, because it decides everything that follows. One of those needs nothing. One needs sealing. One means the slab is telling you about a problem that repairing the surface will not touch.

Why concrete cracks at all

Concrete is strong in compression and weak in tension. Push on it and it holds enormous load. Pull it apart and it gives up early — roughly a tenth as strong, give or take the mix. Everything about how slabs are designed comes back to that one asymmetry.

And concrete shrinks. As it cures and later as it dries, it gets very slightly smaller. A slab is held by friction against the ground and by whatever it is tied to, so it cannot shrink freely. Something has to give, and because tension is where concrete is weak, it cracks.

Add temperature movement — concrete expands in heat and contracts in cold like everything else — and the surprise is not that slabs crack. It is that they hold together as well as they do.

Reading a crack

Three things tell you almost everything: where it runs, whether the two sides are still level with each other, and whether it is changing.

The level test is the one that matters most

A crack you can see is a crack. A crack where one side has dropped below the other is movement, and movement means the ground under the slab is doing something. That is the difference between a surface problem and a base problem, and it is the single most useful thing a homeowner can check without any tools at all — run a hand across it, or set a straightedge over it.

A crack is information. A step in a crack is information about the ground, and the ground is the expensive part.

The causes you actually see in the field

Shrinkage cracking

The most common by far, and mostly benign. Fine cracks that appear in the first weeks as the slab gives up moisture. If joints were cut properly and at the right time, most of this movement goes into the joints and you never notice it.

Plastic shrinkage cracking

Short, random, often parallel cracks that show up within hours of the pour, before the concrete has really hardened. This is a hot, dry, windy day pulling water out of the surface faster than the slab can replace it. It is a finishing and curing problem, not a load problem — and in Redding it is a summer scheduling issue more than anything else.

Settlement cracking

The ground under part of the slab moved and the concrete broke trying to span the gap. This is the one that produces a step, and it is the one that will not be fixed by anything you do to the surface. Common causes: poorly compacted fill, a trench backfilled without proper compaction, organic material left under the slab that later rotted away, or water getting under an edge and washing base material out.

Overload cracking

A slab poured for cars, asked to carry a loaded concrete truck, a dumpster, or a fifth-wheel parked on one spot for two years. Usually shows as a crack radiating from where the load sat, sometimes with the slab visibly dished. Not a defect in the concrete — a mismatch between what it was built for and what it got.

Corrosion cracking

Reinforcement rusts, rust occupies more volume than steel, and the expansion pushes the concrete apart from the inside. Shows up as cracking that runs in a straight line following the bar, often with staining and eventually with pieces spalling off. Usually means the steel was too close to the surface or water has been getting in for a long time.

What to actually do about it

• Hairline and flat: nothing. It is doing no harm. Sealing it is optional and mostly aesthetic.
• Open but flat, in a freeze-exposed area: worth sealing, because water in a crack that freezes will widen it. This matters far more at Mount Shasta elevations than it does in the valley.
• Stepped or widening: find out why before spending money on the surface. Patching a slab that failed because of what is under it moves the problem a few feet and buys a year or two.
• Cracking plus pooling water: treat the drainage first. Water is very often the actual cause rather than an incidental symptom.

What to expect from new concrete

Nobody can honestly promise you a slab that will never crack, and a contractor who does is telling you something they cannot control. What can be controlled is where it cracks and how much it matters: real base preparation, reinforcement placed where the tension is, joints cut at the right spacing and cut on time, and curing that does not get skipped because the crew had somewhere else to be.

Do that, and the slab still cracks — quietly, under the joints, where you will not see it for as long as you own the place.

If you have a crack you are not sure about, a photo usually tells us a lot. Send a couple of picturesand we'll tell you honestly whether it is worth doing anything about, or see how we approach repair and replacement.

Published August 7, 2026 by Shannon Loker, S & L Concrete — California Contractor Lic. #692498.