How to fill a void under a concrete slab

Written by l.p. klages
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Concrete slabs float on the surface of the ground, on top of mud and other material that's often softer than the concrete itself. Over time, water can sweep away the soft ground that once supported the concrete slab, leaving a void underneath. Voids under concrete can lead to serious problems like sinkholes and cracks in the slab. It's important to keep the concrete fully supported by the ground below. If there is a void under your slab, you need to fill it using a process called mudjacking.

Skill level:

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Things you need

  • Concrete core drill
  • Grout hose
  • Clamps
  • Grout nozzle
  • Mudjack pump
  • "Mud" type concrete material
  • Concrete
  • Trowel

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  1. 1

    Drill a hole through the concrete slab with the core drill, right above the void to be filled.

  2. 2

    Place the grout nozzle into the hole, with the grout hose attached to it. The hose should be attached to the mudjacking pump on the other end. Use clamps to secure the hose on either end.

  3. 3

    Fill the pump with the special mud material by pouring it into the top, much like a concrete mixer. Begin pumping the material slowly into the hole by turning on the pump.

  4. 4

    Stop pumping when the void is filled. The pressure should be visible by a slight bulge on the upper surface of the slab. If there are any cracks, you'll see the mud squeezing through those cracks. That means the void is completely filled.

  5. 5

    Remove any excess mud from the surface of the slab if any came through the cracks using a rag.

  6. 6

    Remove the nozzle and hose from the hole in the slab.

  7. 7

    Slide the cylinder of concrete from the core saw back into the hole. There will be mud under it, so it will not fall below the surface of the slab. Spread a small amount of concrete or concrete epoxy across the surface of cylinder before using the trowel. This should seal the surface and minimise the appearance of the hole.

Tips and warnings

  • A normal drill will create a hole by destroying and removing whatever material is in the way of the drill bit. A core drill is different because it keeps that material in tact.
  • Mudjacking equipment is very specialised, so it may be wise to hire a mudjacking company to do this work instead of attempting it on your own.

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