Hairline crack
A very narrow surface crack, typically no wider than a credit card edge. Most hairline cracking in slabs is shrinkage-related and cosmetic.
What does hairline crack mean?
Width alone is not a complete diagnosis, but narrow, stable, non-offsetting cracks in a slab on grade are usually benign. What matters is whether they widen, offset, leak, or propagate over time. In exterior concrete even hairline cracks eventually admit water, which in freezing climates can widen them season by season.
In practice
Homeowners frequently panic over hairline cracks in a new driveway that are entirely normal and will never become a problem.