What Causes Cracks in Concrete Slabs?
Almost all concrete slabs crack — the question is whether the cracks are normal shrinkage or a structural problem. Plastic shrinkage, drying shrinkage, and restraint cause the hairline cracks seen in nearly every slab. Structural cracks come from settlement, overloading, or subgrade failure and show vertical offset, widening over time, or patterns that follow load paths. Learn to tell them apart before paying for repairs.
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Normal shrinkage cracking
Concrete shrinks as it cures — 1/16 to 1/8 inch per 10 feet is typical. Control joints are saw-cut to force the cracks to happen in straight lines at planned locations; when joints are cut too late, too shallow, or too far apart, the slab cracks wherever it wants. Hairline, stable, no vertical offset: normal. These cracks are cosmetic and do not affect structural performance.
Cracks that indicate real problems
Worry about cracks wider than 1/8 inch that keep growing, cracks with one side higher than the other (vertical displacement), cracks radiating from column bases or load points, and map-pattern cracking over large areas (which can indicate alkali-silica reaction or sulfate attack). Water coming up through slab cracks suggests a failed vapor barrier or high water table — a separate problem from the crack itself.
Prevention and repair
Prevention: proper subgrade compaction, vapor barrier, adequate thickness and reinforcement for the loads, timely control joints, and proper curing. Repair depends on diagnosis: epoxy injection for structural cracks that need load transfer, polyurethane for waterproofing, and slab jacking or replacement when the subgrade has failed. Never inject a crack without knowing why it formed — sealing a settlement crack without fixing the settlement just moves the problem.
The bottom line
Hairline stable cracks are normal shrinkage; growing, offset, or load-pattern cracks are structural. Diagnose the cause (subgrade, loading, water) before choosing between epoxy, waterproofing, or replacement.
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