Curling of Concrete Slabs

Curling is the distortion of a slab panel into a curved shape by upward or downward bending of the edges and corners at joints or cracks. Curling is primarily due to differences in moisture and/or temperature between the top and bottom surfaces of a concrete slab. The distortion can lift the edges or the middle of the slab from the base, leaving an unsupported portion. The slab section can crack when loads exceeding its capacity are applied. Slab edges might chip off or spall due to traffic when they curl upwards. Curling is typically evident at an early age; however, its occurrence may occur over a period of time. The terms curling and warping are used interchangeably.

Differential changes of top and bottom slab surface dimensions results in curling. These volume changes are most often related to moisture and temperature gradients within the slab. When one surface of the slab changes size relative to the other, the slab will curl at its edges in the direction of relative shortening. The resultant curling is noticeable at the joints, edges, and corners. Drying shrinkage is the primary characteristic of concrete that impacts curling. Factors that increase drying shrinkage of concrete will tend to increase curling.

• Use the lowest practical mixing water content in concrete. Concrete slump has no bearing on curling; mixing water content does. Avoid placement delays that require jobsite adjustments
• Use the largest practical maximum size aggregate and/or the highest practical coarse aggregate content to minimize drying shrinkage
• Shrinkage reducing admixtures can be used in the concrete mixture to reduce shrinkage
• Some aggregates result in higher shrinkage that should be considered in the jointing plan
• Take precautions to avoid excessive bleeding. Place concrete on an absorptive subgrade to reduce bleeding and differential shrinkage. In dry conditions moisten the subgrade to avoid plastic shrinkage cracking
• Concrete placed on a vapor retarder or impermeable subgrade will increase bleeding. Vacuum dewatering techniques on freshly placed concrete can reduce water content to reduce curling
• A low w/cm concrete mixture does not result in lower shrinkage if the paste volume is high. Similarly, high compressive strength does not assure low shrinkage or curling

• Avoid requiring a higher than necessary cementititous materials content. This increases the paste volume and increases shrinkage
• Constructing a thicker slab will reduce curling
• Cure the slab after placement. Moist curing or a well applied high-solids curing compound will reduce rate of moisture loss and reduce the moisture differential
• To minimize curling, joint spacing should not exceed 24 times the thickness of the slab
• For thin toppings, clean the base slab to ensure bond and consider use of studs and wire around the edges and particularly in the slab corners
• Properly designed and placed reinforcement in the upper one-third of the slab perpendicular to the edges will reduce curling. Reinforcement should be placed for 10 feet from the slab edge or construction joint. Load transfer devices that minimize vertical movement should be used across construction joints
• Certain types of breathable sealers or coatings on slabs can work to minimize moisture differential and reduce curling