Curling refers to the distortion of a concrete slab into a curved shape, and it occurs due to the differences in either temperature or moisture between the top and bottom sides of the slab. This distortion lifts the middle or edges of the slab right from the base, leaving some portion of slab unsupported. The unsupported section cracks when heavy loads beyond the capacity it can handle are applied. When the slab section warps upwards at its edges, they are more likely to chip off. Read on to learn more about curling and how to minimise it.
Why Do Concrete Slabs Curl?
Moisture and temperature gradients in the slab cause changes in slab dimensions. When one surface of the slab changes relative to the other, the slab warps at its edges. The curling is affected by the ability of hardened concrete to contract, which is known as drying shrinkage. Anything that influences drying shrinkage of concrete will increase curling.
The highest incidence of upward curling of the edges of a slab is when the top surface of the slab shrinks relative to the bottom. When a slab curls soon after replacement, the curling is a result of rapid surface drying and poor curing. For the top surface, excessive bleeding due to water sprayed on the surface or high water content in the slab and lack of surface moisture increases surface drying shrinkage in relation to the bottom surface. Bleeding occurs when slabs are placed on a vapor retarder or when topping mixtures are placed on slabs. In such cases, the shrinkage disparities from top to bottom are larger than for the slabs that are on an absorptive subgrade.
Long joint spacing and thin slabs increases curling. Thus, thin unbounded toppings should have a close joint spacing. In industrial construction services, the idea of close joint spacing is undesirable due to a large number of joints and related joint maintenance problems. This must then be balanced against the probability of curling at the joints and random cracks.
Temperature differences between the top and bottom surfaces of the slab cause curling. When the top part is exposed to the sun relative to the bottom, downward curling of the edges occurs. During the cold nights, the top surface is exposed to cold and will contract in relative with the bottom. The curling due to temperature differences adds to the upward curling that comes about as a result of moisture differences.
How to Minimize Slab Curling
The following steps will help minimize the curling potential in concrete slabs.
1. Use the minimum water quantity requirement in the concrete.
2. Use aggregate content of the highest coarseness to reduce drying shrinkage.
3. Take measures to stop extreme bleeding. In dry conditions, place the slab on a damp and absorptive subgrade. This prevents the forcing of bleed water to the top of the slab.
4. Avoid using vapor retarders made of polyethylene unless the slab is covered with at least four inches of a granular fill.
5. Avoid cement content that is higher than is necessary. You can use slag or pozzolan in place of high cement content.
6. Cure the concrete joints and edges thoroughly. If you use membrane-curing compounds, apply twice the recommended rate at right angles to each other.
7. Although it is good to minimize curing, use a joint spacing that is less than 24 times the slab thickness.
8. When using thin toppings, thoroughly wash the base slab to facilitate the bond. Make use of wires and studs around the edges and slab corners.
9. Use thicker slabs or increase the slab thickness at the edges.
10. The use of properly designed and placed slab reinforcement may reduce curling. Use load transfer devices that reduce the vertical movement across construction joints.
12. Some brands of breathable sealers minimize moisture differentials on slabs and reduce curling.
When curling in a concrete slab cannot be tolerated, there are alternative options such as the use of shrinkage-compensating concrete, vacuum dewatering, shrinkage reducing admixtures and post-tensioned slabs. These alternative options should be decided before construction since they can increase the cost of the project. Other remedies for slab curling include pounding the slab and sawing contraction joints, injecting a grout to fill voids under the slab and grinding slab joints.