Concrete cracks — plan for it
Concrete shrinks as it cures (up to 1/4" per 100 feet) and cracks where tension wins. Control joints are the planned weak spots that invite the crack to happen where you chose — every 8–12 feet each way on a 4" slab. Slabs without control joints still crack; they just crack arbitrarily.
The cracks that matter
Hairline shrinkage cracks inside control joints: cosmetic, sealed with concrete caulk if desired. Wide (>1/4"), offset-height, or growing cracks: structural questions — settlement, subgrade failure — that deserve a professional look. The difference is regularity: planned cracks are straight and contained; unplanned ones wander and step.
The prevention stack
Compacted subgrade (settlement is the root of most bad cracks), proper thickness for the load, reinforcement centered in the slab, control joints cut within 24 hours, and curing kept moist for a week. Five boring steps, all cheap at pour time, all expensive to reverse — the whole truth of concrete is that the prep is the product.