Concrete surface crazing

Crazing is a network of very fine, shallow cracks in the top layer of a slab, usually forming irregular cells a few centimetres across. It looks like the glaze on old pottery, and it is one of the few things in this library we can describe as generally cosmetic without much hedging.

It is worth its own page precisely because it looks alarming and is usually not. Homeowners find a web of cracks across a whole slab, conclude the concrete is disintegrating, and get quoted for work the condition does not call for.

What it is

Crazing forms in the cement paste at the very surface, typically within a fraction of a millimetre of the top. The layer of paste at the surface dries and shrinks faster than the concrete beneath it, and the resulting tension relieves as a fine network.

The cells are characteristically irregular polygons, and the whole pattern tends to cover a broad area rather than being one crack going somewhere. Crazing does not run in a line, does not concentrate at corners, and does not produce any height difference.

It is often much more visible when the surface is damp or drying — water sits briefly in the cracks and outlines the pattern. That is why homeowners frequently notice it for the first time after rain on a slab they have walked over for years.

What seeing this cannot establish

• Crazing tells you about the surface paste and essentially nothing about the slab beneath. It is not evidence about support, thickness, reinforcement or structural condition.
• It is not the same as scaling, flaking or spalling, though it can precede surface loss in a climate that cycles. Crazing is cracking; those are material coming away.
• The pattern does not indicate whether the slab will deteriorate. Many crazed slabs stay exactly as they are for their whole life.

What this can be associated with

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

• Rapid surface drying during finishing or curing — sun, wind, or low humidity on fresh concrete
• Overworking the surface during finishing, which can be associated with drawing excess fines and water to the top
• Water sprinkled on the surface during finishing to make troweling easier
• Curing that ended early or was never applied, so the surface lost moisture faster than the body
• Finishing while bleed water was still present, working it back into the surface

What you can safely check yourself

• Look at the pattern shape. Irregular cells covering a broad area is crazing; lines that travel somewhere are not.
• Run a fingernail across it. Crazing cracks are typically too fine to catch a nail.
• Check for any height difference across the cracks with a straightedge. Crazing produces none.
• Wet the surface and photograph it — the pattern shows far more clearly, which makes for better documentation.
• Check whether any material is actually coming loose. Run a hand across and see whether you pick up grit. That is a different condition.

When this is frequently cosmetic

• Nearly always, where the pattern is a fine irregular web with no material loss and no height difference
• Interior floors where the crazing is visible but the surface is otherwise sound and not shedding
• Exterior slabs in mild climates with no salt exposure

When professional evaluation matters more

• Material is coming away from the surface, not merely cracking — that is scaling or spalling and a different question
• The surface produces grit or dust underfoot, which points at a weak surface layer rather than crazing alone
• The slab is new and you want the pattern documented while any workmanship conversation is still live
• The crazing is on a slab you plan to coat, because surface preparation and coating adhesion are affected

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.

• A photograph of the wetted surface, which shows the pattern most clearly
• A close-up with scale showing the cell size
• A straightedge photograph confirming no height difference
• The slab's approximate age, and the season it was poured in if you know
• A note of whether the surface sheds grit when rubbed

Questions to ask whoever comes out

• Is this crazing, or is the surface actually losing material? What are you looking at to tell the difference?
• Does this need anything done to it at all?
• If I want it to look better, what are the options and which of them are cosmetic only?
• If I seal or coat this, does the crazing affect how well the product bonds?
• Is there any reason this would affect the slab's durability in this climate?

Where estimates for this commonly differ in scope

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

• Crazing is sometimes used to justify resurfacing an entire slab. That may be a legitimate appearance choice, but it should be presented as an appearance job rather than a repair.
• An estimate for "surface repair" on a crazed slab may include grinding, may include a coating, or may include an overlay. Those differ hugely in cost and appearance.
• If a coating is proposed, the preparation is the part that determines whether it lasts, and it is the part most often described vaguely.

Who tends to do this work

• A concrete repair specialist
• A decorative or specialty concrete finisher

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