Q&A for How to Form Concrete Walls

Sealing concrete can help preserve the color and prevent stains. Unsealed concrete will absorb water, while water will just slide right off of sealed concrete.

First, buy some concrete mix. Then, put on safety glasses, gloves, and a ventilated mask. Pour the concrete mix into a mixer with a ramp or hose. Then, mix in the amount of water specified on the package. Stir the concrete to mix it. Be sure to follow any manufacturer instructions.

Builder's choice, on that one. Depending on the length of the mold and how many external braces are required, I like to stick with whatever will make disassembly easiest without having to pry against your fresh concrete - a trim nailer is usually sufficient, but plain old all-purpose construction screws also work fine. Remember, you wont be leaving the mold on there until it's fully cured - just long enough for the concrete to set up to the point that it won't collapse under its own weight; usually only one to three hours. Remember to coat the inside of your form with fresh motor oil (paintbrush works) for easy release.

Some fresh motor oil (NOT used) applied to the inside of the form with a paintbrush makes an excellent release agent. It doesn't have to be anything fancy - just the cheapest 10W-30 on the shelf. WD40 or other household oils have a tendency to absorb into the plywood, whereas motor oil is thick enough to avoid soaking up. You can usually even find it right at the hardware store when you go to pick up your materials to construct the form.

Pour your footing first, placing re-bar vertical stubbed out of footer, and wait at least a day for the concrete to harden. Then, form your wall on top of footing. Just placing stakes on the form will not hold, instead, you must place kickers on the wall to hold the form. There are many concrete forms available and different ways they work.

That's something that you have to decide for yourself based on the thickness and linear footage of your wall. Part of your planning should be to determine how much concrete you'll need to complete your project - remember, there are 27 cubic feet in a yard and an 80lb bag of concrete mix typically yields just shy of 2/3 of a foot. You also have to consider that most concrete suppliers will generally charge you a light load fee for anything under 7 yards (typically between $100 and $200 on top of the material cost). In the end, it really comes down to how much you're willing to spend and whether or not you're willing to mix and pour by hand. You could also rent a small mixer.

If your footing has rebar in it, 12 inches will be plenty for even a six or eight foot wall.

Yes, you don't want the concrete to crack when the ground shifts under it.

6-8 yards of concrete ought to do it. Make sure you keep it suspended with a sky hook while you're doing the pour.

You can bond new concrete to old concrete, but it usually isn't a good idea if your wall is in bad shape. You can patch damaged areas. If you see a lot of damage, consider replacing the entire wall.

You should use it to close off the area where you need to pour the concrete. Usually, you set it on top of the soil. If your wall is close to a mount of dirt, put it in front of dirt as needed to mold the concrete to the proper width for your wall.

It is possible. Make sure the insulation is thick enough to resist the concrete. You will still need to attach it to some wood boards. Any holes in the insulation will affect the concrete.

Primer does a fairly good job of reducing rough textures, and costs much less than plaster.

You should have, at a minimum, 24" of rebar above grade to your footings every 18". These anchor rebar pieces need to have an L or U shape to adhere the old concrete to new. All rebar needs to be a minimum 2" deep inside the forms in any direction. Retaining walls also need additional support to keep the earth from pushing them over by means of cantilever, buttresses, tapered wall, or a deep base. You can't simply build any wall and call it a retaining wall.