· Michael Blair
Concrete shrinks as it cures and moves with temperature for the rest of its life. Cracking is not a defect in itself. What matters is width, whether the two sides have moved relative to each other, and whether it is still changing.
The three things to look at
Width first. Hairline cracks you can barely catch a fingernail in are usually shrinkage and cosmetic. Anything approaching a quarter inch deserves attention.
Then displacement: if one side sits higher than the other, something underneath has moved and the crack is a symptom rather than the problem. Then movement over time — mark the ends with a pencil and a date.
Shrinkage and control joints
Concrete loses volume as it cures and will crack somewhere. Control joints are cut to decide where, and a slab cracking neatly along a joint is the joint doing its job.
Joints cut too late or too shallow do not control anything, and the slab cracks randomly beside them. Joints should be roughly a quarter of the slab depth and cut early.
What Middle Tennessee soil adds
Clay expands when wet and shrinks when dry, and our seasons deliver both extremes. A slab on poorly prepared clay moves with it, which produces displacement rather than simple shrinkage.
That is why base preparation matters more here than the concrete mix. Compacted gravel over properly prepared subgrade is what keeps a slab flat.
- Hairline, no displacement: cosmetic
- Along a control joint: working as designed
- Wide with one side higher: subgrade movement
- Growing over months: active, investigate
- Cracks radiating from a corner: often a re-entrant corner without proper reinforcement
Repair or replace
Sealing a crack keeps water out, which matters because water freezing in a crack widens it. Polyurethane or epoxy injection suits narrow, stable cracks.
Replacement is the honest answer where a slab has multiple displaced cracks, where it has heaved, or where the subgrade has failed. Patching a slab whose base has gone is temporary by definition.