Plywood for concrete forms

[radioman]

since the op is simply trying to save a step of attaching wood to concrete to screw or glue drywall to cover concrete. i would just simply build your forms with 2x4's like most concrete work is done. once it cures, just simply attach furring strips with tapcons and glue. then you can do your drywall like you wanted to.

 

[ovrszd]

I don't think anyone is completely wrong. I think is much miscommunication. I am totally not a chemist. I'm just reading posts as they are written.

Cement absorbs water, not concrete.

Concrete will cure even when submerged under water.

The chemical reactions that cement introduced to water creates is different than the reactions created when water is introduced to concrete.

I think this discussion is simply a difference of position.

I don't believe anyone is completely wrong. If someone is, then it must be me. And I don't even have a dog in this fight.

 

[skipmarcy]

I treated my plywood with some of the Olympic clear wood preservative from Lowe's and let it dry several days before using to form my poured roof on my outside storm room. I've reused the side form pieces many times for different things for several years now since the job. The plywood sheets that formed the ceiling in the room I left in place and later drilled and ran tapcons to keep them in place permanently. The room is approx. 15'x17' and I think I used over 100 landscape timbers & 4x4s along with the 2 laminated beams I built to hold the ceiling up for the pour - don't scrimp on bracing with concrete !! We poured right at 7" deep and it has approx. 2' of dirt over it now and I drive my tractor over it with no worries. Has been done now for about 5 years and no problems.

 

[Streetcar]

treated lumber is relatively cheap insurance for possible water infiltration through the wall.
On hydration cement is the ingredient in concrete that is hydrated

 

[EddieWalker]

I have a need for a poured concrete wall inside an air conditioned and heated building. My plan is to make the form with plywood (and the additional 2x4 bracing etc.) and the plywood will be permanently in place along with anchors (like tapcons) to keep it from separating from the concrete. Then the plywood will be covered with drywall and finished normally. My question is that since the plywood will always be in contact with the concrete does the plywood need to be pressure treated? If it was a sill plate or exposed to moisture, then the answer would be yes, but since it is interior and protected from the weather with pretty consistent environment - does it need to be?

You will be fine doing it this way. You are getting a lot of replies that do not pertain to what you are doing, or what is fairly common practice. Use construction grade 3/4 inch plywood, be sure that your bracing is significant, and you won't have any problems. I've worked on numerous homes built in the 70's with plywood, and framing lumber, left in place and there is no rot at all. It's perfectly preserved on the side in contact with the concrete, but well worn on the exposed side. My guess is the lack of air, or maybe just that no moisture from the air makes contact with it that preserves it so well.

Don't over think this. What moisture that is in the concrete when you pour it will not matter. It's insignificant.

 

[Rock knocker]

The chemical reactions that cement introduced to water creates is different than the reactions created when water is introduced to concrete.

Concrete is the cured product. There are no reactions when cured concrete is introduced to water. The only chemical reaction in the entire process is the hydration of cement when mixed with water, excepting the manufacturing of cement at the factory.

 

[ovrszd]

Concrete is the cured product. There are no reactions when cured concrete is introduced to water. The only chemical reaction in the entire process is the hydration of cement when mixed with water, excepting the manufacturing of cement at the factory.

Yep. I concur. Thought that's what I was saying all along. So we are agreeing that concrete doesn't hydrate??

 

[3Ts]

Wow!! This sure went off track. I apparently did not word my question correctly, but the answer appears to to be that either PT or regular plywood will work for me. So let me try again to describe what I'm doing. I'm putting a storm shelter inside my house. First of all there is already wood framed walls in place. I want to use the studs and nail 3/4" plywood to it as one side of the forms and use the form ties to provide the proper spacing of the plywood. Once the concrete is poured I will only have access to one side of the wall, so the forms on one side will have to remain in place. My concern had more to do with long term rot since the wood will always be in contact with the concrete. Then on the side I do have access to, since the plywood is already in place and the lath is the same thickness, why not just leave the plywood in place as the lath and I can mount anything anywhere on the wall. There's the moisture issue again. This was intended more as a question about after the concrete is poured and now ready to remove the forms, what happens if I don't. Sorry for the misunderstanding.

Sounds like I will be fine using regular 3/4" plywood for the forms and leaving it in place permanently (with the addition of the proper bracing with 2x4s).

 

[ovrszd]

You have nothing to apologize for. This happens on almost every thread that goes more than 20 posts or so.

Hypothetically, if I thought I needed a "tie" every 18", I'd put one every 12". If the means to secure this operation fail you are going to have a disaster.

Also, calculate the weight of this added wall and what it is resting on.

There are several on here that can help you with these things.

 

[Creamer]

Wow!! This sure went off track. I apparently did not word my question correctly, but the answer appears to to be that either PT or regular plywood will work for me. So let me try again to describe what I'm doing. I'm putting a storm shelter inside my house. First of all there is already wood framed walls in place. I want to use the studs and nail 3/4" plywood to it as one side of the forms and use the form ties to provide the proper spacing of the plywood. Once the concrete is poured I will only have access to one side of the wall, so the forms on one side will have to remain in place. My concern had more to do with long term rot since the wood will always be in contact with the concrete. Then on the side I do have access to, since the plywood is already in place and the lath is the same thickness, why not just leave the plywood in place as the lath and I can mount anything anywhere on the wall. There's the moisture issue again. This was intended more as a question about after the concrete is poured and now ready to remove the forms, what happens if I don't. Sorry for the misunderstanding.

Sounds like I will be fine using regular 3/4" plywood for the forms and leaving it in place permanently (with the addition of the proper bracing with 2x4s).

I would not use PT on the interior because of the chemicals. It is made for exterior use.