It shows up within days or a few weeks of the pour — either a fine, web-like pattern of hairline cracks across the surface, or a straighter, more defined crack running across part or all of the slab. Both are common enough that they alarm people out of proportion to the actual risk in many cases, but they're not the same thing and shouldn't be treated the same way.
The pattern of the crack tells you a great deal before anyone even touches the slab. Fine, shallow, randomly patterned cracking is a different situation to a single, straight, wider crack that runs the full width of the slab.
Telling surface crazing from a structural crack
Fine map cracking — a network of shallow, hairline cracks with no clear direction — is typically a surface phenomenon, often from the top of the slab drying and shrinking faster than the concrete underneath during curing. It's unsightly but usually shallow, not affecting the slab's strength.
A single straight or slightly curved crack, especially one that runs the full width or depth of the slab, is a different matter and needs treating as potentially structural until proven otherwise, particularly if it aligns with a re-entrant corner such as an internal angle around a step or opening.
The likely causes
- Plastic shrinkage from the surface drying out too quickly during curing, especially in wind or direct sun with no protection
- Insufficient reinforcement, or reinforcement positioned too low or too high in the slab rather than at the correct depth
- Movement joints omitted from a large pour, meaning the whole slab has nowhere to relieve shrinkage stress except by cracking
- Sub-base that wasn't compacted evenly, leading to differential settlement under parts of the slab
How to check which applies
Look at whether the cracking is shallow map crazing across the surface, or a full-depth crack you can see continue through the slab edge if it's exposed anywhere. Check whether cracks line up with re-entrant corners or changes in slab shape — that pattern points strongly to missing movement joints rather than a material or curing fault.
Ask, or recall, how the slab was cured after pouring — if it was left uncovered in hot, windy or sunny conditions without being kept damp or protected, that supports plastic shrinkage as the explanation for surface crazing specifically.
Before covering the slab with a floor finish
Do not lay an expensive floor finish over a slab with unexplained full-depth cracking without investigating first. A structural engineer's assessment is worth getting where cracks are more than surface-deep, span a significant portion of the slab, or where you're unsure of the sub-base preparation and reinforcement used.
Common questions
Is fine map cracking on a new slab something to worry about?+
Generally no — it's a common curing-related surface effect and doesn't usually affect the slab's structural performance. It's worth filling and sealing for appearance and to stop water ingress, but it's low risk in most cases.
How soon after pouring does cracking usually appear?+
Plastic shrinkage cracking can appear within hours to days of the pour while the concrete is still curing. Cracks appearing weeks later are more likely related to sub-base settlement or missing movement joints than to the initial cure.
Can I fill the crack myself while I decide what to do?+
Filling shallow surface crazing for appearance is low risk, but filling a full-depth crack cosmetically before it's investigated can hide movement that's still ongoing. Get it assessed first if there's any doubt about its depth or cause.
Get your slab crack assessed before you build on it
We inspect the crack depth and pattern on site and bring in structural engineering input where it's warranted, before any floor finish goes down.