New Slab Curls Only At Doorway Edge Decision Tree

Why this matters

Curling at a doorway is a moisture and temperature gradient lifting the slab edge, and the doorway concentrates the cause: that edge dries from the top and from the open air faster than the interior, builds a steeper differential, and has a free or weakly restrained perimeter to lift into. Mistaking curling for settlement (it is the opposite problem, the edge goes up, not down) or for a structural crack sends you down the wrong repair path. ACI 360R treats curling as a predictable consequence of differential drying shrinkage and thermal gradients across the slab thickness, governed by slab thickness, joint spacing, restraint, and the moisture differential between top and bottom.

Symptom presentation

The slab edge at a doorway lifts above plane, creating a perceptible lip you can feel underfoot or catch a straightedge under. It often shows at a free edge, at a sawn joint near the door, or where the slab meets a threshold. Curling typically develops over the first weeks to months as the top surface dries and shrinks faster than the bottom, which stays moist against the subgrade. The lifted edge may rock under load and the adjacent joint may spall from the rocking.

Quick checks

• Straightedge across the edge. A consistent upward bow toward the free edge or joint, highest at the corner, is curling. A downward dish is settlement; a vertical step with no bow is a crack.
• Check whether the edge moves. If the lifted edge deflects or rocks under foot or a tire and returns, it is an unsupported curled edge, not a fixed defect.
• Note the moisture story. A doorway with a wet exterior side or a vapor-open subgrade and a drying interior builds the steepest gradient.
• Check joint spacing near the door. Wide joint spacing and large panels curl more at their edges.
• Confirm slab thickness at the edge. Thin edges and thin slabs curl far more than thick ones for the same drying.

Isolation tree

Start with the straightedge profile.

Upward bow toward a free edge or joint, highest at the corner, classic curl: confirmed differential drying shrinkage curling. The top of the slab has shrunk more than the bottom and pulled the edge up. Now branch on whether it is stabilizing or progressing. Re-measure the lift over two to four weeks. Curling usually peaks as the slab reaches drying equilibrium and then holds. A still-increasing lift means the gradient is still steepening (interior still wet, surface still drying).

Upward bow but the slab sits over a vapor-open subgrade with a wet exterior threshold: moisture-driven curl that may persist because the bottom never dries to equilibrium against a chronically damp base. This is the doorway-specific aggravator. The gradient stays alive.

No bow, edge stepped down: this is settlement, not curling. Wrong tree. Check subgrade support and edge bearing.

No bow, vertical crack with no lift: shrinkage or restraint crack. Wrong tree.

Bow present but only at one corner where two free edges meet: corner curling, the most common and most severe form because two drying faces meet. Confirms the gradient mechanism; corners curl first and most.

Confirming diagnosis

Confirm curling by mapping elevations with a laser or string line across the panel: a smooth rise from interior low point to a high edge or corner, symmetric to the panel geometry, is the curl signature. Document the magnitude of lift and whether the edge rocks. To confirm the moisture gradient as the driver, take relative humidity readings in the slab per ASTM F2170 at shallow and deep probe depths; a wetter bottom than top confirms the differential that curls the edge.

Distinguish from thermal curling by timing and conditions: thermal curl tracks the surface being hotter or colder than the base (sun-exposed daytime, or a cold floor over warm ground) and reverses with the temperature cycle, while drying-shrinkage curl is largely permanent. A doorway edge that is consistently up regardless of time of day is drying-shrinkage dominant.

Remediation

If the curl has stabilized and the lip is modest and not load-critical: grind the lifted edge flush to restore plane, then seal the adjacent joint so the now-flush edge does not ravel. Grinding is the standard cure once the slab has reached drying equilibrium and stopped moving; grinding too early just exposes you to a second grind as the curl keeps progressing.

If the curled edge rocks and carries load (doorway traffic, a tire, a threshold): the void under the lifted edge must be filled so the edge bears again. Pressure-grout (polyurethane or cementitious) to fill the void under the curl, then grind the top flush. Filling the void stops the rocking that spalls the joint.

If the bottom stays chronically wet against a damp threshold: address the moisture source (drainage, threshold flashing, a vapor retarder problem) so the gradient can equilibrate, then grind. Grinding over a slab whose bottom never dries can let the curl partially re-form.

Prevent it on the next pour: reduce panel size and tighten joint spacing per ACI 360R guidance so each panel curls less at its edges, use a low-shrinkage mix (lower water content, larger top-size aggregate, well-graded), cure properly to slow surface drying, and place a continuous vapor retarder where the subgrade moisture differential is the concern. Thicker edge sections at doorways resist curl.

References

• ACI 360R, Guide to Design of Slabs-on-Ground, curling mechanics, joint spacing, and mitigation.
• ACI 302.1R, Guide to Concrete Floor and Slab Construction, curling causes and finishing/curing controls.
• ASTM F2170, Test Method for Determining Relative Humidity in Concrete Floor Slabs Using in situ Probes.
• PCA, Concrete Slab Surface Defects: Causes, Prevention, Repair (IS177), curling identification and repair.