All concrete cracks. Period. Some cracks you can see and some are so small they’re invisible to the naked eye. What most homeowners don’t know is that concrete is supposed to crack, and it’s not necessarily a harbinger of structural disaster.
Concrete is at its greatest volume the moment it hits the forms. From that time on it loses water, both to hydration (the chemical reaction between cement and water) and to evaporation.
Similar to a drying sponge, as the water leaves, the concrete shrinks. That shrinking causes internal tension stresses. Concrete, while excellent at handling compression, is not very good at resisting tension. The result: cracks. The trick is to plan for cracks so that when they occur they’re not ugly or structurally compromising.
Of course, sometimes cracked concrete does indicate a structural problem. In such cases the concrete on each side of the crack can wind up out of plane, causing a tripping hazard in a slab on grade. And when a crack opens up 1/8 inch or more there may be underlying trouble. If in doubt, call an engineer to help diagnose the problem.