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Question DetailsAsked on 5/13/2016

how to repair a 3" X 3" hole in a exterior concrete wall

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That size hole, assuming a load-bearing foundation wall (which almost all exterior walls would be), the cut out reinforcing should be replaced first, then the hole formed up and new concrete poured into the void, ensuring that during vibration all air is removed by having a vent tube coming out from the top of the hole. Normally the forming would be built so there is a "hopper" on it extending above the hole elevation so the wet concrete actually extends above the top of the hole on the outside of the form, so as it is vibrated and settles into place there is concrete there to replace the air and avoid a gap at the top of the repair.


The rebar replacement is done by drilling holes alongside the existing rebar that was cut off and into the concrete for the development length of the rebar (depends on diameter of bar). With larger rebar, commonly the number of bars would be doubled to void having to drill 2-3 feet into the concrete - commonly a 1 foot embedment is was is needed. The holes the rebar goes into are filled with mixed neat cement or epoxy grout (no aggregate - just cement) then the rebar shoved in to provide the bond.


By code in most areas the repair requires a structural engineer's design - and certainly should have if a structural wall that carries bending forces as opposed to just vertical wall loads. Most common place where that would occur is with elevated (not on-grade) concrete floors or where there is a structural concrete (as opposed to mass concrete) porch, deck, or balcony cantilevering off or supported off the wall, where continuity of the rebar that was cut to make the hole has to be reestablished and may require 2-4 foot holes for the new rebar.

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Oh heck - darn it all - I misread that as a 3x3 foot hole - oh well, I will leave that response in case someone can use it in the future.


For a 3 INCH hole, just use a non-shrink patch mix concrete mix (available in 5# and sometimes smaller bags or boxes) - you will need to provide some sort of facing like duct taped on plywood scrap over the hole so the mix does not slump out, and rod it down in or use vibratory force on the form (tapping with a hammer, vibratory sander, etc) to be sure it settles down and makes good contact with the cleaned concrete surface all around the hole surface. Depending on how good a job you do with the compaction, you may have to go back and make an additional small batch to shove into a void with a pointer or trowel along the top edge of the repair as a second round - you can avoid that by doing the hopper thing above on the form on the "bad" side of the hole for better appearance - can be a hopper scabbed onto the form or a piece of large diameter PVC pipe poking through that you keep filled while vibrating the form wood so it has material to settle down and fill the void completely. After the repair, if you used a pipe pull it out and tape over the hole. If hopper scabbed onto the "form", after grout has set enough to "green set" - firm and won't sag but not yet hard - remove the hopper and form and trowel repair the surface to desired finish condition. Typically 15-30 minutes with epoxy cement grout, more like 2-3 hours if regular cement based grout. For smooth finish you can hand or power sand the surface smooth while green-set if desired.


For full cure be sure to keep the patch wet for specified curing time if epoxy grout, for at least a full day if cement grout - you can just tape a large rag over it and keep that wet in most cases.


Before grouting - read instructions on package, but normally - and certainly with all portland cement grouts - you want to have the existing cleaned concrete surface fully wetted for 5-10 mintues beforehand so it does not soak up the mix water out of the repair grout, then wipe off free water to a damp but not puddled surface before applying the grout.


If this hole has dirt on the outside so can't form up both sides (like below-ground foundation wall) then clean it out well full depth of the wall and just let the dirt be the back form - will leave an ugly plug of grout on the back side but if below ground who cares.

Answered 2 years ago by LCD




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