Cracks in concrete are inevitable — but not all cracks are equal. Some are harmless and cosmetic, while others signal serious structural issues. Before attempting any repair, you need to correctly identify what kind of crack you’re dealing with. This guide walks you through the most common types of cracks in concrete, what causes them, and how to tell them apart.
These are thin, random cracks — often 0.1–0.3 mm wide — that appear shortly after concrete is poured and begins curing. Caused by rapid moisture loss during setting, they’re mostly cosmetic and found on slabs or walls with poor curing or hot, dry conditions. They usually don’t penetrate far and don’t move, making them candidates for sealing or cosmetic repair only.
Crazing looks like a web of fine, shallow cracks — often barely visible unless the slab is wet. It’s caused by drying shrinkage or surface paste issues. These don’t affect structural integrity and usually appear in trowel-finished or power floated surfaces. They don’t require structural repair but may need densifiers or coatings to prevent moisture ingress and dusting.
If cracks are wider than 2–3 mm, or show signs of displacement (one side higher than the other), you may be dealing with drying shrinkage cracks, thermal cracks, or structural settlement. Check whether cracks follow straight lines or form “V” shapes — diagonal or stair-step cracks often point to foundation movement. Use a crack gauge or pencil line to monitor widening over time.
Cracks forming from slab corners, joints, or around columns often relate to poor joint spacing or restrained movement. These cracks usually reflect stress relief and are common in slabs without proper control joints. If located at slab intersections or at saw-cut lines, they can often be repaired with flexible joint fillers or polymer-modified mortar without full removal.
On walls and foundations, vertical or diagonal cracks may result from settling, hydrostatic pressure, or shrinkage. Measure width and check if water leaks through. Vertical cracks under 3 mm that aren’t growing are usually shrinkage-related. Diagonal cracks near corners or windows may signal foundation rotation or subsidence. Mark and monitor any that show widening or moisture.
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