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I have several 2.3" holes in my concrete walls. I think they were likely used as part of the construction or assembly process. They're maybe an inch to an inch and a half deep, and they look like this:

enter image description here

I'd like to use them to non-destructively mount things to my wall (such as a VESA monitor, let's say). My idea is to put an anchor in them that can brace against the sides of the hole, then mount a rail to it that I can mount my monitor on.

The question is, what kind of anchor can I put in the hole that can non-destructively form a sturdy mount for a rail of some sort to mount a monitor?

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Are you committed to keeping them as is? –  HerrBag Apr 13 '13 at 19:43
    
yes, i'm renting so i want to be able to easily undo whatever i do –  emmby Apr 13 '13 at 20:44
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These appear to be lifting recesses for pre-cast slabs. The crane operator has large steel plugs with eye hooks in one end to fit in the recess. As long as the lifting force in primarily in shear, they stay in place. There are test plugs available for various sized pipes that may fit the recess. They are rubber plugs with plates at either end and a through bolt. Tightening the bolt expands the rubber against the side of the pipe. These are rather expensive, so Michael Karas' wood plug idea is the best IMO. –  bcworkz Apr 13 '13 at 21:42
    
ooo i like the idea of rubber test plugs. Any idea on what I would google or where i might find them? –  emmby Apr 13 '13 at 22:12
    
ah, "test plug" in google images is the right way to go –  emmby Apr 13 '13 at 22:33
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4 Answers

Using these holes for a mount is feasible if you can use two or more of the holes for one rail.

One method is to make round wooden plugs that just fit into the holes. Make it so it can go in as deep as possible whilest not sticking out beyond the surface of the wall.

Next you cut the plug in half as shown below. Place the two halves of the plug into the hole and then pound in some wedges to make the plug fit into the hole as tight as possible. Be careful to use a wedge that does not have too much angle to it so that you can get expansion of the plug as far down in as possible.

enter image description here

After the thing is wedged in place cut off the excess length of the wedge. The plug can now be used as a screw point to mount a flat plank across the wall. It would be to this plank that you mount any additional harware for the monitor mount.

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clever! i admit i was thinking of some sort of expandable mechanical anchor, but the wood might work. –  emmby Apr 13 '13 at 20:43
    
@emmby - Yeah. This technique is basically the same method as used to hold wooden axe handles and hammer handles in place. If you ever want to remove the wedged plugs you can start by drilling several 1/2 inch holes in the plug and then using a wood chisel to split it out. Do also be aware to use more than one of the plugs to hold the plank on the wall. –  Michael Karas Apr 14 '13 at 5:07
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You might achieve a wedge effect (that would be easily removable) with wide, short screws (#14 x 3/4), screwed flush into the cross-wide kerf. Again, I would discuss with landlord any of these methods and permanently filling the hole. –  HerrBag Apr 14 '13 at 13:00
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A variation to the wood plug method is to use two short (or narrow) wedges, one from each end of the plug halves. With one wedge tip pinched between the halves, insert the wedged end into the concrete hole and push in snug by hand. It should be such that the wedge butts up against the back of the hole (assuming there is one) and begins to spread the halves. Then you can pound the plug halves in, forcing the back wedge to further expand the halves tightly. Then pound the second wedge into the front for a better, parallel spread. –  Peter Apr 17 '13 at 13:29
    
Also, I'd recommend putting some kind of tape (duct, gaffers, or better, actual "grip" tape - think hockey sticks) around the wood plug halves. This will give you better non-slip traction on against the concrete. I'd hate to hear that your flat screen TV fell off the wall. –  Peter Apr 17 '13 at 13:32
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Using a 2 1/4 hole saw, cut a plug from some 2x4 lumber. Then use some anchoring epoxy and glue the plug into the hole. Allow 24hr curing. Warm temperatures will speed epoxy curing.

Attach any wood screw to mount monitor bracket.

Heres a no muss/no fuss way to inject the epoxy. You may need to tape across the hole until the epoxy jells. They all mix in the nozzle and harden quite quickly. You use a caulking gun to dispense.

enter image description here enter image description here enter image description here

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Thanks HerrBag! that's pretty good, but i have concerns about removing the epoxy non-destructively in the future... –  emmby Apr 13 '13 at 20:42
    
I would show landlord these solutions and get approval for filling the holes. They would disappear if insulation and wallboard covered it, in usual finishing methods. –  HerrBag Apr 14 '13 at 12:56
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Here's another option based on the answer by @Michael Karas:

Instead of cutting the round plug into halves and using a third, separate thin wedge, cut the plug into 3 pieces: a center block and two side wedges. Pound the side wedges in, and anchor to the center block. As force is drawn on the center block, the wedges will only get tighter. (Kind of a "Chinese finger cuff" idea.) Put some kind of grip enhancement (e.g. soft, thick tape) on the side wedges to help them "stick" to the hole wall. I wouldn't put too much faith in just a plain wood to concrete friction hold.

Split wedge plug anchor

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up vote 1 down vote accepted

Based on bcworkz's comments, I searched google for "test plugs" and found this:

http://www.amazon.com/gp/product/B0085PBM7M

Shaw Plugs 62005 Hex Nut Expandable Neoprene Rubber Plug with Zinc Plated Steel Hardware, 2" x 1-5/16"

Turns out it was exactly what I needed!
A few turns of the nut and I have a solid bolt that I can mount a rail to.

enter image description here

enter image description here

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