Concrete
Why Concrete Cracks (and Which Cracks Matter)
Shrinkage, settlement, and rust, and which cracks are cosmetic vs structural.
Reviewed July 2026
Why concrete cracks comes down to one fact: concrete is strong when squeezed but weak when pulled, and almost everything that happens to a slab, from curing to weather to a shifting subgrade, eventually puts part of it into tension. When that tension exceeds the concrete's modest tensile strength, it cracks. Most cracks are cosmetic and expected; a few signal a real problem. Knowing which is which is the useful part.
Concrete is weak in tension
Concrete's compressive strength is many times its tensile strength. It happily carries crushing loads but splits under relatively little pulling or bending. This is the same reason concrete is reinforced with steel: the rebar carries the tension the concrete cannot. Without help, any force that stretches the concrete, even the material shrinking against itself, can open a crack.
Shrinkage: the most common cause
The everyday reason slabs crack is shrinkage, and it comes in two forms. Plastic shrinkage happens in the first hours, when the surface of fresh concrete dries faster than the body beneath it and pulls itself apart into fine, often random surface cracks; hot, dry, windy conditions make it worse. Drying shrinkage happens over weeks and months as the hardened concrete slowly loses moisture and contracts. Because a slab is restrained by the ground and by its own mass, that contraction creates internal tension, and the concrete relieves it by cracking. Good curing, keeping the surface moist early, reduces both.
The other culprits
Beyond shrinkage, concrete cracks from thermal movement (it expands in heat and contracts in cold, and restrained movement cracks it), from settlement when the soil or base beneath it moves or was poorly compacted, from overload when it carries more weight than it was designed for, and from corrosion of embedded steel. That last one is important: when rebar rusts, the rust occupies more volume than the original steel and pushes outward, cracking and spalling the concrete from within. Too little concrete cover over the rebar, or salt and water reaching it, is what sets that off.
Why control joints exist
Since shrinkage cracking is basically unavoidable, builders manage it instead of fighting it. Control joints, the straight grooves you see cut into sidewalks and slabs, are intentional lines of weakness that give shrinkage cracks a planned place to form, down inside the joint where they are hidden and harmless. Joints are typically spaced a couple of feet apart for every inch of slab thickness. They do not prevent cracks; they decide where the cracks go.
Which cracks to worry about
Most fine, hairline cracks in a slab are cosmetic and stable. Pay attention when a crack is wider than about an eighth of an inch, when one side sits higher than the other, when a crack keeps lengthening or widening over time, or when diagonal cracks appear near openings or in a foundation wall. Those patterns can point to settlement, structural movement, or reinforcement corrosion, and they are worth a look from a professional rather than a patch. When a crack is structural, filling it hides the symptom without fixing the cause.
Frequently asked questions
Is it normal for concrete to crack?
Yes. Almost all concrete develops fine cracks as it shrinks during curing and moves with temperature. Thin, hairline cracks in a slab are usually cosmetic. The concern is not whether it cracks but whether a crack is wide, growing, or offset.
Which concrete cracks are serious?
Watch for cracks wider than about 1/8 inch, cracks where one side is higher than the other (vertical displacement), cracks that keep growing, and diagonal cracks near corners or in foundation walls. Those can signal settlement or structural movement and warrant a professional look.
What are control joints for?
Control joints are the straight grooves cut or tooled into slabs and sidewalks. They are deliberate weak lines that tell shrinkage cracks where to form, hidden in the joint, instead of wandering randomly across the surface. They do not stop cracking; they organize it.
More on concrete
Materials Review is an independent educational resource. It is not affiliated with Pittsburg State University or the former Kansas Polymer Research Center, and it is not a substitute for a licensed engineer. Confirm structural, safety, and code questions with a qualified professional before acting.