Concrete leveling or slabjacking is the action of bringing sunken or sagging concrete level again. Typically accomplished by drilling a hole or a series of holes in the sunken concrete, concrete leveling involves large amounts of a liquid mud-like substance or a closed-cell polymer foam being pumped under the concrete through the drilled holes. The pressure of the injected mud not only pushes the concrete up, it also remains in place to give support to the concrete as well as discourage the future sinking of the concrete. Once the concrete leveling is finished, the mud is smoothed where it was pumped through the holes and the latter virtually disappear.
Concrete is typically poured on top of a compacted sand base. Often, the sand is not compacted and settled enough prior to the concrete being poured. This can result in the erosion of the sand by rain fall and natural settling. When this happens, the concrete will sag or settle, causing it to be much lower than originally intended. This can lead to water collection, cracked and broken concrete or damage to any structure built on the concrete. If plausible, concrete leveling is a much more cost-efficient method of bringing the concrete back to the correct height as compared to breaking out the old concrete and re-pouring the slab.
The most common substance used in concrete leveling is a mixture of sand, cement and soil that fills in the void beneath the sunken concrete. In some applications, a crushed limestone is mixed into the concrete leveling mixture. This added stone provides a more solid structure beneath the concrete and aids in preventing future deterioration of the substrate. Once a slab has been leveled, it must not be walked on or used for several days in order to allow sufficient time for the concrete leveling mixture to harden and cure. Failure to observe this curing time could result in the concrete sinking once again.
When possible, a closed cell polymer foam is used to level the slab. Using the foam for concrete leveling provides a substrate that will not break down and is also watertight. The power of the expanding foam is used to lift the concrete instead of the pressure of the injected material. The expanding foam will seek out all voids in the sub-surface and expand to fill them equally. The foam remains in its expanded form underneath the raised concrete and will not erode or change shape, preventing the concrete from sagging once again.