How to Pour Concrete Under Old Foundation

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Old foundations can become weakened by air or water getting underneath them and over time eroding either the soil supporting the foundation or the concrete itself. You may be able to see such a pocket at the base of the foundation, or if you tap on the foundation or slab you may hear a hollow sound. You will need to pour concrete under the old foundation in order to rebuild its structural integrity. This is a process requiring care.

Things You'll Need

  • Hammer
  • Shovel
  • Survey stakes
  • Form boards (if needed)
  • Hammer drill (if needed)
  • Ramset gun (if needed)
  • Concrete pump hose
  • Concrete pump
  • Water hose
  • Concrete
  • Concrete vibrator
  • Magnesium float
  • Locate the areas with gaps underneath the old foundation. This may be as simple as finding where, along the sides, you can see the bottom of the foundation or see underneath it. You may have to use a hammer to tap the top surface of the foundation to locate hollow points. Flag areas that are close to the edge of the foundation with survey stakes.

  • Dig underneath the foundation until you locate the open pocket. If your hollow pocket was found by tapping on the foundation's surface, use a hammer drill to drill out the concrete until you have accessed the space.

  • Build side forms for gaps that exist beneath the foundation and are accessible from the side. Do this by using a ramset gun to attach form boards to the existing foundation with the bottom of the board extending at least four inches into a trench you have dug at the base. Use dirt to help support the board by banking it against the outside of the board.

  • Create a gap near the top edge of your form board that will fit a concrete pump hose. Concrete pump hoses come in all sizes, so make sure you pick one appropriate for the size of the space you need to fill; larger hoses will deliver concrete at a faster rate and are good for deep gaps but can overwhelm a small space. If you will be pouring from above and into the foundation, make sure you can fit your concrete pump hose into the space.

  • Fill the gap with water; this is harder to do with a side form, but get enough water into the gap to at least cover the bottom. Doing this will provide you with a means of knowing if the concrete has filled even the parts you cannot see, as the concrete will displace the water as it fills the hole.

  • Begin to pump concrete beneath the old foundation. Work at a slow steady pace with a slightly dry but also slightly higher aggregrate mix than usual. You do not want a high cement-only mix, or the presence of the water will weaken the final process. Every so often, pull out the concrete pump hose and insert a concrete vibrator. Move the vibrator around several times, withdraw it and continue pumping.

  • Pump enough concrete that it goes beyond the surface of the opening you created in your forms or foundation. Use a magnesium float to level the excess to the surface of the old foundation.

Tips & Warnings

  • When vibrating the concrete, think of a clock face and make sure you push the vibrator to each hour on the face to make sure that you have done an adequate job of allowing the concrete to go everywhere needed.
  • Do not add water to the concrete in the truck to make it more of a slurry to fill the hole; use the vibrator to make it fill the space. Adding water in the mixing truck will severely lessen the structural integrity of the cured concrete, and your structure could fail.

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