A slab on grade loses moisture from the top while its underside stays damp, so the top shrinks more and the panel edges lift like a shallow dish. Curled joints rock under forklift wheels, crack across corners and leave voids beneath the slab edges. High water content, a vapor retarder directly underneath, thin slabs and short curing all make it worse, and grinding plus slab stabilization grout are the usual repairs.
Joints in aisle six have curling bad enough that the forklifts clunk at every one, grind them and inject underneath.
Slabs placed straight on a vapor retarder curl more, which is why some engineers specify a granular blotter layer on top of the plastic.
Also heard as
- curl
- warping
- dishing
Related terms
Drying shrinkage
The volume loss of hardened concrete as it gives up moisture over months and years.
Vapor retarder
A plastic sheet under a slab on grade that slows ground moisture from rising into the concrete.
Control joint
A planned groove or sawcut that makes a slab crack in a straight line exactly where the crew wants it.
F-numbers
FF and FL, the paired numbers that measure how flat and how level a concrete floor is.