Concrete slabs can develop voids from a variety of reasons, ranging from sliding soil beneath the slab displaced because of a lack of proper compression, to water erosion slowly washing away the soil over time. Voids can lead to a weakness in the slab, causing dips in the surface, tilting or cracks and breaks. Filling the voids is a matter of accessing the void through the slab and then using a pump to place a concrete-based fill material into the open space.
Drill at least four holes through the slab at a point that's centered on the void beneath. Use a drill equipped with a masonry bit to bore 1.5-inch-diameter holes, and arrange the holes to offer even access to the void.
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Connect a hose to a slabjacking pump, then run the hose to the holes leading to the void. A slabjacking pump is specifically made to pump the fill material beneath concrete slabs and can deliver the material quickly.
Mix the fill material in a wheelbarrow, using a spade. Combine one part Portland cement, one part fly ash and two parts clean sand for the fill mix. Add an expander additive to prevent the mixture from shrinking during the curing phase after you pump it into the voids. The amount of additive will be dependent on manufacturer instructions. Add water while mixing until the fill material is the consistency of a thick grout.
Fill the pump with the fill material and set it to a pressure of 10 psi. Pump the material through the hose to fill the void under the slab.
Switch the hose among the drilled holes to fill the void evenly, as you'll want to keep the fill material as level as possible to avoid applying pressure to the slab itself, raising it in the process.
Turn off the pump and remove the hose once the fill material has reached the level of the concrete slab. The void is completely filled in at this point. Mix a small batch of concrete mortar and fill in the holes, leveling the mortar to the surface of the slab using a trowel.
Allow the concrete to cure to completion for three weeks.