How to Extend Concrete Walls

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Many homes and buildings are built in phases. The first phase provides for an initial complete building, and later phases are designed to expand from this existing construction. Once the foundation is in place for your expansion, you may need to know how to extend the concrete walls from your existing building into your new concrete walls. This process is subject to strict inspection guidelines, so it is important that you follow all the steps closely to get it right.

Things You'll Need

  • Tape measure
  • Lumber crayon
  • Hammer drill
  • Drinking straw
  • Protective eyewear
  • Concrete epoxy
  • Epoxy gun
  • Rebar dowels (3' min)
  • Rebar (as per design for wall)
  • 2x4s
  • Hammer
  • Concrete nails
  • Concrete forms
  • Concrete

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Measure and mark where you will need to drill to dowel out from your existing wall. Doweling out means to epoxy rebar into the existing wall to provide a means of tying the old and new walls together. There are a few guidelines for deciding where your dowels need to be placed. If you have a blueprint, the specific measurements are marked on it. If you don't have a blueprint, follow these guidelines: walls under 6" thick only require one dowel per horizontal tie-in, while all other thicknesses require two; you must have the same number of dowels as there are horizontal bars in the existing wall (if you don't know how many, place a dowels every 12" for the height of the wall); and lastly, your dowels must match the size of the rebar to be placed in the new wall.

Drill 6" in depth for each hole unless there is a different depth measurement called for in your blueprint or local building code. Make sure your hammer drill has a bit designed for concrete, not masonry. A masonry bit will not be up to the task.

Blow out all the dust from each hole using a drinking straw. Put one end in your mouth and the other into the hole and exhale forcefully. Make sure you are wearing protective eyewear when you do this.

Load your concrete epoxy into your epoxy gun following the instructions provided with the system. Insert the tip of the epoxy gun into one hole and pump the trigger. Once the hole is completely filled with epoxy, put the end of one of your rebar dowels in it. Push the dowel in as far as it can go, and then move on to the next hole. Do this until all the dowels are in place.

Build in place or set the rebar wall for your new concrete wall. Follow your blueprints exactly.

Nail a 2x4 to each face of the existing concrete wall at the edge using a hammer and concrete nails. Attach the beginning of your wall form system to these 2x4s with nails. You can now construct the rest of your wall forms for your new wall following the instructions and procedures for your specific wall system. Once you have completed that and the wall is adequately braced (as per your system), pour the concrete, and finish the wall as you would any other.

Tips & Warnings

  • To get the best epoxy bond on your rebar dowels, push the rod in and out of the hole several times (with the epoxy in place) before fully inserting it, to get rid of any air bubbles that may be hidden in the epoxy.
  • The strength of the tie-in with the epoxy dowels you use to extend your concrete wall is what will prevent your wall from separating, cracking or even falling over in the future. Do not skip any part of the steps to properly prepare the holes for epoxy, and make sure that they are inspected by someone qualified, to lessen the potential for later liability and repair.
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