Curling, or warping, of concrete slabs is a phenomenon that occurs with differences in moisture or temperature between the top and bottom of a slab. Anytime there is an increase in temperature or moisture, there is a corresponding expansion in concrete slabs, whereas a decrease in temperature or moisture results in a shrinkage in the concrete slab dimensions. Curling develops when one surface of a slab is moist and/or warm while the other surface is dry and/or cool. The larger the gradient between the top and bottom of the slab, the greater the tendency to curl.
Curling typically occurs within about 30 days after construction when the excess water provided for workability evaporates, resulting in concrete shrinkage. This shrinkage takes place at the exposed surface where drying winds and a low humidity remove moisture. A wet subgrade would cause a large difference in moisture conditions between surfaces and further aggravate the result, which is upward curling. When upward curling occurs, corners and edges are left unsupported and tend to break when loaded.
Downward curling also exists, occurring when the top surface is exposed to a warm, wet climate while the bottom surface is still cool and dry. For example, highway slabs actually curl down slightly during the day when the sun heats the top of the slab.
Indoor slabs that are not exposed to varying climate conditions are also subjected to curling and warping. In most cases, it is more permanent and more severe than that experienced by outdoor slabs. Curling of indoor slabs generally occurs early in the life of the slab and is usually caused by differentials between surface moisture contents.
If curling occurs early in the life of a concrete slab, it may possibly be remedied. If the curling is due to a moisture differential in which the surface is dry and the bottom is wet, it may be corrected by thoroughly wetting the surface through ponding or soaking. Controlling the temperature of the water used could also be effective in overcoming any possible temperature differentials in the warped slab. When the slab flattens, usually after a considerable period of soaking, it should be sawed into smaller panels or a seal should be applied to the surface to maintain the moisture balance.
The best ways to avoid curling are to reduce moisture differentials between the top and bottom surfaces, reduce the shrinkage potential of the concrete mix used, and construct the slab to combat curling, each of which is described below.
Measures to Reduce Moisture Differentials
- Thoroughly cure the slab, especially during the early stages.Curing with curing compounds rather than water may reduce the water content in the concrete and help reduce moisture differentials.
- Reduce moisture loss from the surface by applying coatings, sealers, and waxes.
- Use crushed rock or gravel fills under slabs instead of moisture barriers. If you have to pour on an impermeable membrane, place 3 inches of sand on top of the impermeable membrane.
Measures to Reduce Shrinkage Potential
- Reduce the total water added to the mix. The less water added, the less there is to evaporate.
- Use a lower cement content. High cement contents may cause surface shrinkage. The mix design should still use the lowest water-to-cement ratio possible and contain enough cement for proper strength development and durability. The Portland Cement Association recommends a minimum cement content of 564 pounds per cubic yard and a maximum water-to-cement ratio of 0.45 for concrete to resist severe exposures.
- Avoid aggregates known to have high shrinkage potential.
- Avoid using admixtures or constituents that may increase drying shrinkage potential.
Construction Measures to Reduce Curling
- Design thicker slabs. Thinner slabs are more likely to curl than thick ones.
- Use shorter joint spacing. Slabs with long distances between control joints curl more than those with shorter joint spacing.
- Place dowels across control joints when slabs are more than 6 inches thick. This prevents vertical movement.
- Reinforce with #4 bars in both directions within panels, but not across control joints.
- Design square slabs. Square slabs curl less than rectangular.