Hairline cracks in concrete

A hairline crack is one you can see but barely feel — fine enough that a fingernail catches on the edge rather than dropping into it. They are extremely common, they frequently appear in the first months after a pour, and on their own they are one of the least alarming things concrete does.

That is not the same as saying they never matter. A hairline crack is a starting width, not a permanent one, and the useful question is never "is this crack bad" but "is this crack changing". This page is about telling those apart, and about what to write down so the answer becomes obvious over a few months rather than staying a guess.

What it is

Concrete shrinks slightly as it cures and dries. If the slab is restrained anywhere — by its own reinforcement, by the base beneath it, by a wall or column it was poured against — the shrinkage has to go somewhere, and it goes into fine cracks. This is why hairline cracking is so often described as normal: it is a consequence of the material behaving the way the material behaves.

Visually, shrinkage hairlines tend to be narrow along their whole length, meander rather than run straight, often stop and restart, and frequently show up in a rough pattern rather than as one isolated line. They are usually the same width at both ends.

A hairline crack that is noticeably wider at one end than the other, or that continues in a straight line across a joint it should have stopped at, is behaving differently and is worth treating as a different question.

What seeing this cannot establish

• Width alone establishes almost nothing. A hairline crack over a well-supported base and a hairline crack directly above a developing void look identical on the day you photograph them. What separates them is what happens over the following weeks.
• Nobody can tell from a photograph whether a hairline crack goes all the way through the slab. Depth matters for water movement and for which repairs are even applicable, and it is not visible from above.
• A fine crack in a floor slab and a fine crack in a foundation wall are not the same subject even at the same width, because what sits above them differs. Location changes the significance more than width does.

What this can be associated with

Candidate explanations, not findings. Nobody can establish which applies to your concrete without seeing it.

• Drying shrinkage in the first months after placement, which is the most common association by a wide margin
• Water added to the mix on site to make it easier to place, which can be associated with more shrinkage than the mix was designed for
• Joint spacing wider than the slab's thickness and reinforcement were planned around, which can be associated with shrinkage having nowhere convenient to relieve
• Curing that ended early, which can be associated with the surface drying faster than the body of the slab
• Restraint the slab was not detailed for, such as a pour placed tight against a footing or a column with no isolation

What you can safely check yourself

• Run a fingernail across the crack. If it does not catch and drop in, you are almost certainly in hairline territory.
• Photograph the crack with a coin or a tape measure laid beside it, in daylight, from directly above. Write the date on the photograph itself, not just in the file name.
• Mark the two ends of the crack with a pencil line across it and date the marks. If the crack extends past your pencil mark over the next few months, it is lengthening — which is more informative than width.
• Note whether the crack crosses a control joint. A shrinkage crack usually relieves at a joint; one that runs straight through a joint is doing something else.
• Check whether the crack is damp when the surrounding surface is dry. A crack carrying water is a water-path question as well as a crack question.

When this is frequently cosmetic

• Fine, stable, non-widening cracks in a slab on grade with no elevation difference across them
• Cracks that appeared within the first year, have not moved since, and are not carrying water
• Meandering patterns in an exterior slab where nothing has settled and the surface is otherwise sound

When professional evaluation matters more

• The crack has visibly widened between two dated photographs, or has extended past a mark you made
• One side of the crack has become higher than the other, which changes the subject from cracking to movement
• The crack is in a foundation wall rather than a flatwork slab
• Water appears at the crack after rain, or the crack is damp when everything around it is dry
• The crack has appeared long after the slab was poured with no obvious change in loading

Evidence worth having before you ask for estimates

Every item here is something you can produce without tools or risk, and every one of them makes an estimate more accurate.

• Two dated photographs of the same crack from the same position, ideally at least a season apart
• The approximate age of the slab, if you know it
• A note of whether de-icing salt is used on the surface in winter
• Whether downspouts or a hose discharge anywhere near the cracked area
• A straightedge or level laid across the crack, photographed, to show whether there is any height difference

Questions to ask whoever comes out

• Do you think this is shrinkage, and what specifically makes you think so?
• Would you recommend doing anything at all right now, and what happens if I do nothing for a year?
• If you seal it, what are you sealing against — water, appearance, or both?
• Will sealing it hide the movement I would otherwise be able to see?
• Is there anything about the joint layout here that you would do differently on a new pour?

Where estimates for this commonly differ in scope

These are the places two prices for apparently the same work stop being comparable.

• "Crack repair" may mean a surface-level sealant, a routed and filled joint, or an injection. Those are different jobs at different prices and they are not interchangeable.
• Some estimates price crack treatment per crack and some per linear foot. On a slab with many fine cracks those two produce very different totals.
• An estimate that includes sealing the whole slab is doing two things at once. Ask which part of the price is the crack and which is the seal.

Who tends to do this work

• A concrete repair specialist
• A residential flatwork concrete contractor

ConcreteOwner does not inspect, certify or approve contractors. This describes which trade usually holds this work, not a recommendation of anybody in particular.