Curling is the phenomenon when the surface of the concrete shrinks greater than the middle and bottom of the slab. Mud drying does the same thing, see picture.
The bowl like affect it causes within the jointed areas creates raised areas along those joints. In warehouse applications it creates speedbumps and wear for forklifts and wheeled vehicles. It makes flooring a real challenge.
A few things to drastically decrease curling:
Use a well graded mix. All Duke City mixes use combined gradation technology.
So called GAP grading requires more water to fill the voids left by large aggregate next to small aggregate. Intermediate aggregates established by combined gradations fill that void.
DO NOT ADD ACCESS WATER TO THE MIX- the more water in the mix the more it shrinks.
Use water reducers to increase slump for workability
Since the cause is uneven evaporation, top drying out faster than the bottom; create similar conditions for the surface as well as the subgrade.
Don’t over wet the subgrade unless you can drown the surface
VERY IMPORTANT—Keep the surface wet with curing methods
Wet burlap and keep wet
Dike and flood surface area
Cover in plastic to seal in moisture
Hot-Windy days are worse conditions than rainy days
Curling can be reduced using the right tools and techniques.