Short answer
Concrete shrinks as it dries and is weak in tension, so it cracks. NRMCA says it is not possible to make completely crack-free concrete, but joints, good subgrade, a moderate-slump mix, correct finishing and curing control it. Most random early cracks rarely affect structural integrity or service life.
Key facts
- It is not possible to produce completely crack-free concrete (NRMCA CIP 4)
- Joint spacing: 24 to 36 times slab thickness, about 10 feet for a 4-inch slab, 15 feet maximum (NRMCA CIP 6)
- Panels square or nearly so, length no more than 1.5 times width, no L-shapes (NRMCA CIP 6)
- Wire mesh will not prevent cracking; it tends to keep cracks from opening (NRMCA CIP 6)
- Keep new concrete moist for at least 3 days (NRMCA CIP 4)
Why it cracks
NRMCA explains that concrete expands and shrinks with moisture and temperature, and its overall tendency is to shrink. Because it is weak in tension, a slab that is held back from shrinking cracks. Early cracks come mostly from temperature change and setting. Later, drying adds more cracks or widens old ones.
NRMCA lists the usual causes of cracking: missing or poorly placed joints, poor subgrade preparation, high-slump concrete or water added on the job, improper finishing, fast drying, and little or no curing.
What joints do
NRMCA calls joints “simply pre-planned cracks.” Contraction joints create weak lines so cracks form there. Isolation joints separate a slab from walls, footings and columns, and separate driveways and patios from sidewalks, garage slabs and stairs. Construction joints are where two separate pours meet.
Its spacing rule is 24 to 36 times the slab thickness, with 15 feet as a maximum, and grooves at least a quarter of the slab thickness deep. Conventional saw cuts go in within 4 to 12 hours after finishing.
Which cracks matter
NRMCA says most random cracks that appear early, while unsightly, rarely affect structural integrity or service life. It names two exceptions: D-cracks from freeze-thaw damage in some porous aggregates, and cracking from alkali aggregate reaction.
Plastic shrinkage cracks, shallow parallel cracks that form while the concrete is still soft, come from the surface drying faster than bleed water can replace it. Crazing, a web of fine surface cracks, does not affect structural integrity.
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