In our industry, the terms “curling” and “warping” are used interchangeably. ACI 116 defines “curling” as “the distortion of an essentially linear or planar member into a curved shape, such as the warping of a slab, due to differences in temperature or moisture content in the zones adjacent to its opposite faces.”
The finite volume of hardened concrete is a function of its temperature and moisture content. As concrete loses moisture, the concrete will lose volume and cause it to shrink. As the moisture content is increased, the concrete will increase in volume, causing it to expand. As the temperature decreases, the volume will decrease causing the concrete to shrink. As the temperature rises, the volume will increase causing the concrete to expand.
Other types of volume change, such as autogenous shrinkage caused by the hydration of cement and expansion caused by the formation of ettringite, do not affect curling. These types of volume changes occur somewhat more uniformly throughout the mass of the concrete.
Curling normally occurs at or near a joint. The bottom of the concrete is moist while the top of the concrete has dried out. This causes the top to lose volume and shrink and “curl up.”
If detected early enough, wetting the top surface of the concrete can make the curled surface return to its original shape. In most cases, however, the curled surface will have to be ground flat, and the bottom surface void that was formed by the curling action will have to be filled with a mortar or concrete.
An ounce of prevention...
The best fix for curling is prevention. Control excessive air movement across the surface of the concrete. If the slab is outside and exposed to the elements, erect wind screens. If the slab is indoors, close the overhead doors and other large openings. Do not allow a heater to blow hot air across the surface of freshly placed concrete. During placement, use E-Con, an evaporation reducer by L&M, to slow the moisture loss at the surface. The benefits of a proper curing protocol cannot be overstated and are critical.
It has been found that when concrete is placed over a well compacted granular sub-base that has been moistened with water well before the placement of concrete, curling is greatly reduced.
The bottom line
The bottom line is this: Surface deformation, whether you call it curling or warping, does not have to happen if proper steps are taken to protect the top surface from drying out faster than the bottom part of the concrete.
There is a section in ACI 302.1R that discusses the causes and cures of curling in greater detail than I can here. It can be found online at www.concrete.org and a copy can be purchased for a nominal amount. Also, please refer to the Fall 2004 issue of Concrete News for an informative article by CTL's Scott Tarr, entitled: “Interior Floors don't curl, but they do warp, and joints suffer.” It can be found at www.lmcc.com. Look for it in the Concrete News archives.
© 2008 L&M Construction Chemicals, Inc. | ConcreteNews September 2008.