How to Fix a Sinking Slab Concrete Floor

Bore a series of 1- to 2-inch-wide holes through the surface of the sunken concrete slab using a core drill. Space the holes 3 to 8 feet apart, depending on the size of the slab, and 1 inch from the edge. Ensure the holes are spaced equally apart.

Use a rock hammer to drill a 1-foot-deep hole through the core holes along the sunken concrete slab.

Place a hydraulic concrete pump near the concrete slab. The pump includes a pressure gauge that allows you to monitor the rate at which the repair mix is added to the holes. Insert the hose from the pump into a hole in the slab.

Prepare the mix for the slab by mixing 1 part fly ash and 1 part Portland cement to 2 parts sand in a wheelbarrow. Add to the mix shovelfuls of expander additive to prevent shrinkage. Pour water into the wheelbarrow and mix thoroughly until it achieves the consistency of thick mortar. Add this prepared mix to the pump.

Pump the mix through the hose in the core-drill hole at 50 pounds per square inch of pressure. The mix slowly raises the slab as it flows underneath.

Continue to pump the mix into the holes in the slab until you jack it to the required height. Stop frequently to check the level of the sunken concrete slab as it is being raised to ensure you pump an equal amount of mix into the holes. If one side of the slab concrete floor is higher than another, insert the hose into a hole along the lower side and pump it with the mix until it is level.

Remove the hose from the hole and set the pump aside. Prepare a batch of quick-dry mortar to pour into the holes, until flush with the surrounding concrete surface. Wipe excess mortar from the slab with a flat trowel.