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I have a new safe that I want to put out in the garage. I'd like for it to be in a specific corner, but that corner has a few quirks.

First, the slab is pretensioned. No cutting, drilling, coring, etc. All around the garage the concrete comes up about 6 inches, and drywall starts above that. In this particular corner the concrete juts out, making a sort of "L" shaped curb that sticks out about 4 inches. Here's a super-crude ascii-art isomorphic picture of what I mean. Yeah, it's a weird angle, but I can't describe it any better.

             |
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             /\
            /  \
           /    \
          /  /\  \
         /_ //\\  \
        |__|/  \\  \
        /       \\  \
       /         \\  \
      /           \\  \

What I was thinking of doing is pouring more concrete with a form to make an 18" x 24" x 6" pad that my safe can go on, but I know nothing about pouring concrete. Anything I need to know?

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Since we aim to be the source for home building and repair information, asking for another source of information is counter productive. If you want to know how to do something (home improvement related), ask us. Please don't ask us to tell you where to learn something (home improvement related), because the answer should be here. –  Tester101 Dec 20 '12 at 12:17

1 Answer 1

up vote 10 down vote accepted

This was a project I did earlier in 2012. We wanted to install central air conditioning for our new furnace and there was no better place to put the outside unit than on this side of the house. Unfortunately, the pad there was a wedge shape. So I made a form and turned it into a rectangle. Couple hours of work including a trip to Home Depot for concrete.

Determine the depth of your pad - For mine, I used 2x6's to build my form. You should use a material that will determine your depth, ie: a 2x4 for a depth of 3.5 inches

Use screws to connect the wood together - this will make disassembly of the form easier when you're finished.

Form

Mix the concrete per the recommendations on the bag.

Materials

If you're concerned about strength, you can use rebar rods or wire mesh inside the pour. Pour half the depth, lay the mesh or rebar, pour the rest.

Make sure the concrete is level or sloped if need be. A 2x4 can be used to do this or a wooden level. Make sure to wash the level when you're done.

Make sure to tamp the concrete down into the corners of the form and into the field of the form. You don't want any air pockets. Use a trowel to help move the concrete around. Use the edge of the trowel to help remove air pockets.

You want a nice flat surface. Use the level or 2x4 that spans from edge to edge to "screed" the surface flat. Use a flat trowel to raise a bit of water to the surface to make it nice and clean.

Pour'd

Let the concrete set per the instructions on the bag - if the weather is colder, give it another day or 2.

Once everything is dry, remove the forms.

Installation

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Good answer, but when pouring concrete that must bond to existing concrete it is a good idea to get some liquid bonding agent and mix it in per the instructions with the water. It helps new concrete better adhere to the surface of older concrete. –  maple_shaft Dec 20 '12 at 13:28
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Point taken and understood - but neither of these 2 installations require much engineering. Neither pad will experience much movement on the surface and neither are bearing much weight (compared to a house or car being on top of it). Should I have used a bonding agent? I could have; but knowing how much the condensor weights and what the pad would be used for, it wasn't a big deal IMO. –  lsiunsuex Dec 20 '12 at 14:22
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If it is outside and exposed to weather, I would have been worried about a large crack forming down the seam there eventually. Structurally though to support an AC unit, you are fine. –  maple_shaft Dec 20 '12 at 14:42
    
Awesome. Remarkably thorough. I appreciate it and totally have the confidence to do this now. Also, thanks for the side comments. I'll get some bonder and I don't have to worry about water/ice as this is indoors. –  Hounshell Dec 20 '12 at 17:27
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Only comment is - raising water to the surface actually weakens the surface of the slab. Screed and float and then WAIT until the concrete has started to set - THEN use a steel trowel to give it a clean surface. –  Paul Dec 21 '12 at 2:27

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