Surface Crazing: The Classic Spider Web
Crazing is a network of fine, shallow cracks that form on the surface of fresh concrete as it dries. The pattern typically resembles random irregular hexagons, with each segment two to four inches across. Individual cracks are usually hairline width and shallow, often visible only when the surface is wet or in raking light.
Crazing happens because the top layer of concrete dries and shrinks faster than the body of the concrete below it. The surface layer wants to contract, but it's constrained by the still wet mass underneath. That tension has to relieve somewhere, and it relieves as a network of fine cracks distributed across the surface. The pattern is the visual signature of differential shrinkage.
What Causes It
Anything that accelerates surface drying during the cure window can cause crazing. The most common culprits are pouring concrete on a hot day without proper curing protection, finishing the surface while bleed water is still rising, troweling with excess water on the surface, or letting wind dry the surface before the concrete has set. Building practices have improved on this over the decades, but a lot of older basement walls and slabs show some degree of crazing from when proper curing wasn't routine.
The American Concrete Institute publishes guidance on curing procedures that effectively eliminate crazing risk. Modern poured walls done by competent contractors rarely show significant crazing because curing compounds and surface protection have become standard practice.
What It Looks Like Up Close
If you press a fingernail across a crazing pattern, you'll feel the surface as essentially smooth. The cracks are too shallow to catch the nail. Width is typically below 0.005 inches, which is roughly the thickness of a sheet of paper. The cracks don't extend through the concrete body. They're confined to the top 1/16 to 1/8 inch of surface paste.
Map Cracking: The One to Watch
Map cracking looks superficially similar to crazing but operates at a different scale. The pattern is larger, with polygonal segments typically four to twelve inches across rather than two to four. The cracks are wider, often visible at 1/32 to 1/16 inch, and they extend deeper into the concrete body rather than staying at the surface.
Map cracking is associated with internal deterioration mechanisms, most often alkali-silica reaction (ASR) or freeze-thaw damage. ASR is a chemical reaction between certain aggregates and the alkalis in cement that creates an expansive gel inside the concrete. The gel pushes outward over time and cracks the concrete in a polygonal pattern. Freeze-thaw damage creates a similar pattern when water inside the concrete expands during repeated freezing cycles.
How to Tell It Apart From Crazing
The fastest tests are crack width and crack depth. Crazing stays under 0.005 inches and stays at the surface. Map cracking typically exceeds that and extends into the concrete. Run a paper clip along a suspected crack. If it catches, it's deeper than crazing and probably warrants a closer look.
Pattern scale is the second test. Crazing creates a fine, tight network. Map cracking creates larger polygons with cleaner segment boundaries. If you can see the pattern clearly from across the room, it's probably map cracking rather than crazing. Crazing usually needs you to look closely.
Related Patterns That Get Confused
Several other crack patterns sometimes get described as spider web but mean different things. Knowing them helps you avoid misdiagnosis.
Plastic Shrinkage Cracking
Plastic shrinkage cracks form when concrete is still curing and the surface loses water faster than bleed water can replace it. They tend to be longer, straighter, and more parallel than crazing. Often you see them on slabs poured on hot or windy days. They can be deeper than crazing but usually don't compromise the structural body of the concrete.
Drying Shrinkage Network
As concrete continues to dry over months and years after the pour, it can develop a broader network of shrinkage cracks that look weblike from a distance. These tend to follow restraint patterns from rebar, control joints, or wall corners. They're typically more directional than true crazing and may extend through more of the wall thickness.
Pattern Cracking From Settlement
Differential settlement can create a network of cracks that looks irregular and weblike, especially when multiple movement events have occurred over time. The giveaway is that settlement crack networks usually radiate from a central point or follow lines of stress, while crazing is more uniformly distributed. Settlement cracks also tend to have visible offset between the crack faces.
When a Spider Web Pattern Actually Needs Attention
Most of the time a spider web pattern is purely cosmetic and the right response is to do nothing. There are a few exceptions worth flagging.
Look at the cracks for staining. Rust colored deposits coming out of the crack pattern suggest the cracks have extended deep enough to reach the rebar and oxygen and moisture are reaching steel reinforcement. White crystalline deposits, called efflorescence, mean water is moving through the cracks and depositing dissolved minerals at the surface. Both indicate the pattern is deeper than surface crazing should be and worth a closer look.
Check the wall for any associated bulging, deflection, or larger cracks. If a spider web pattern appears in conjunction with other movement signs, the spider web itself may not be the issue but it's part of a larger picture worth evaluating. A structural engineer can sort this out in a routine evaluation.
Finally, if the pattern is on a structural element exposed to weather, like an exterior foundation wall above grade, deeper cracks can accelerate freeze-thaw deterioration over time. Sealing the surface with a concrete penetrating sealer is a reasonable preventive step even if the cracks themselves are not currently structural.
Quick Reference Decision Path
If you're looking at a spider web pattern right now and trying to decide what to do, here's the approximate decision tree. Measure crack width. If under 0.005 inches, it's cosmetic crazing and you can ignore it. If between 0.005 and 0.020 inches, look for staining or efflorescence and consider monitoring for changes. If wider than 0.020 inches or extending visibly into the concrete body, get a structural opinion. The cost of an independent engineer evaluation is typically $300 to $500 and provides certainty either way.
Documentation is cheap insurance. Take dated photographs from consistent positions. If you're not sure whether a pattern has changed over time, you need a baseline. Monthly photos for six months will tell you whether anything is moving or whether what you're looking at has been stable for years.