Slab curling is the shrinking of the slab surface that makes the slab try to curl upward, cracking in the process.
Slab curling takes place during the first few days of curing. The slab surface cures faster, and therefore shrinks more quickly, than the slab core. This difference in shrinkage rates results in slab curling. In most cases, the slab doesn't actually curl. When the slab surface shrinks and pulls inward, the slab tries to curl upward. As the slab tries to curl, it usually cracks instead.
The vapor barrier
The vapor barrier, which is laid out before the concrete is poured, provides water protection. However, the vapor barrier has one drawback; it contributes to slab curling. It prevents the ground from absorbing the water in the fresh concrete. The vapor barrier slows the curing process in the slab core, increasing the difference in shrinkage rates between the slab core and the slab surface.
The curing process
In order to understand how to prevent slab curling, you have to understand curing. Concrete is a composite material, consisting of coarse aggregate, sand, cement, and water. Curing is a process in which the cement and water in the concrete bond and harden through a chemical reaction called hydration. As they bond, the cement and water bind the aggregate and sand to form a strong foundation structure.
Cause 1:
Inadequate concrete mix
Often, when weather conditions are hot and dry, contractors add excess water to the mix to make it easier to work with and to extend the working time. The excess water increases the likelihood of slab curling.
Solution
Reduce the amount of water in the mix. One effective way to reduce the water content while maintaining workability is to add a plasticizer to the mix. Plasticizers increase the strength, fluidity, and workability of the concrete mix without adding water.
Cause 2:
Incorrect curing methods
Hot and dry conditions make water evaporate more quickly from the slab surface, causing slab curling. Builders who don't keep the surface of the slab moist throughout the curing process are often left with cracked slabs. It's critical to provide enough time for hydration to take place. Inadequate hydration reduces the slab foundation's strength, durability, impermeability, resistance to abrasion, and resistance to freeze-thaw cycles.
Solution
Refrain from repeatedly trowelling the slab surface. Troweling repeatedly could cause the cement paste to rise, moving the aggregate downward in the slab; this causes the slab surface to shrink more quickly. Slowing down the slab surface shrinkage helps prevent slab curling.
Keep the slab surface moist during the curing process, using one of these methods:
- Keep the slab surface moist by misting water on it for a minimum of three days.
- Keep the slab surface moist by applying curing blankets to the slab for a minimum of three days. Curing blankets are burlap sheets that cover the slab. Periodically wet the sheets down to keep the concrete surface moist. Don't apply curing blankets until the concrete has cured enough to resist surface damage.
- Keep the slab surface moist by covering the surface with polyethylene sheeting for a minimum of three days. Don't apply polyethylene sheeting until the concrete has cured enough to resist surface damage. Secure all edges of the sheeting to prevent wind from drying the surface.
- Keep the slab surface moist by applying spray-on curing compounds. Although this method is the most inefficient one, it doesn’t require maintenance once it's been applied to the surface. This method is also the least expensive. Curing compounds should never be used on surfaces that will be exposed to bright daylight.
