Plywood for concrete forms

[3Ts]

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?

 

[ovrszd]

Can't help you with the treated question.

Will offer this. Helped my wife's Grandpa pour a concrete wall against an existing basement wall years ago. Used plywood and braced the snot out of it. Just before we got it full the bracing gave with the rifle shot sounds of breaking bracing. We ended up with a mess. Can't stress enough how much bracing is required to do this!!!!

As the Robot in the old Lost in Space TV show would say, "Danger Will Robinson, Danger"!!!!!

 

[kcflhrc]

The wet concrete will ruin the plywood

 

[747driver]

Why are you leaving the plywood in place? I would install 2x4 or 2x2 treated wood in the new concrete wall form.Then when you take the forms off you will have studs in the concrete wall to nail your new wall to.

 

[Snobdds]

The concrete will never cure right with the plywood always there. The plywood will stay wet and in prevent the concrete from drying.

Look into ICF's for a short little wall. They are forms and walls all in one. Then you can hang drywall off of them.

Yes, brace the **** out of it and tie the old and new concrete together with rebar, epoxied into the old.

 

[/pine]

The concrete will never cure right with the plywood always there. The plywood will stay wet and in prevent the concrete from drying.

Look into ICF's for a short little wall. They are forms and walls all in one. Then you can hang drywall off of them.

Yes, brace the **** out of it and tie the old and new concrete together with rebar, epoxied into the old.

This is inaccurate...the plywood will not prevent the pour from curing...

Also water will stay in the concrete even after it has cured (28 days)...eventually the majority of it will work (evaporate) out but over time and the plywood will absorb the escaping moisture...

If against an existing pour use a bonding agent...

 

[orezok]

The concrete will never cure right with the plywood always there. The plywood will stay wet and in prevent the concrete from drying..

Im sorry, but this is so wrong. Concrete doesn't "dry", it hydrates. The best way to cure concrete is to keep it constantly wet as the plywood will do.

However I agree with embedding studs for future drywall an waterproofing before installation

 

[Snobdds]

Im sorry, but this is so wrong. Concrete doesn't "dry", it hydrates. The best way to cure concrete is to keep it constantly wet as the plywood will do.

However I agree with embedding studs for future drywall an waterproofing before installation

No, it's a chemical reaction between the Portland and the lime. Water just keeps the chemical reaction, Cure, to a slower rate. There is no such thing as hydrating in concrete...

 

[/pine]

No, it's a chemical reaction between the Portland and the lime. Water just keeps the chemical reaction, Cure, to a slower rate. There is no such thing as hydrating in concrete...

Curing: How exactly does it work? << Concrete Countertop Institute


"Portland cement is a hydraulic cement, hence it derives its strength from chemical reactions between the cement and water. The process is known as hydration."

to get technical:
Hydration

 

[Snobdds]

This is inaccurate...the plywood will not prevent the pour from curing...

Also water will stay in the concrete even after it has cured (28 days)...eventually the majority of it will work (evaporate) out but over time and the plywood will absorb the escaping moisture...

If against an existing pour use a bonding agent...

It's pretty standard practice to remove all forms, of the concrete, as soon as the structural integrity is there. Concrete needs to cure and if plywood forms are there indefinitely, they only serves to weaken the concrete.

 

[Snobdds]

Curing: How exactly does it work? << Concrete Countertop Institute

I guess there are using the fancy term of hydrating for water...which we all know concrete has in it.

 

[Snobdds]

I guess there are using the fancy term of hydrating for water...which we all know concrete has in it.

Hydrating is to always put water into something. Concrete "hydrates" once at the initial stage, then there is no more hydrating.

 

[/pine]

Forms are generally removed so they can be reused...the plywood is usually coated with an oil or release agent...

As an example of moisture permeating wood in direct contact with poured concrete...just look at the bearing surfaces of fabricated roof trusses...there will always be a "tin tab" at the bearing points to separate the wood from the conc. even thought the top of the poured beam (lintel) is protected from the elements...

IMO it will take years for moisture to exit through the plywood if left in contact with the pour...a moisture barrier (plastic/rubber sheeting etc.) may be a viable solution...

 

[Rock knocker]

No, it's a chemical reaction between the Portland and the lime. Water just keeps the chemical reaction, Cure, to a slower rate. There is no such thing as hydrating in concrete...

Here is one of the chemical formulas of the hydration of cement during the creation of concrete. You'll see that the water becomes part of the molecular structure of the polymer

2 Ca2SiO4 + 5 H2O---> 3 CaO.2SiO2.4H2O + Ca(OH)2 + 58.6 kJ
Concrete: Scientific Principles


In short, you just need to be quite, cause you don't seem to understand chemistry

 

[ovrszd]

Careful Rock knocker, you have mixed apples with your oranges and may get called out for it.

 

[Snobdds]

Here is one of the chemical formulas of the hydration of cement during the creation of concrete. You'll see that the water becomes part of the molecular structure of the polymer

2 Ca2SiO4 + 5 H2O---> 3 CaO.2SiO2.4H2O + Ca(OH)2 + 58.6 kJ
Concrete: Scientific Principles


In short, you just need to be quite, cause you don't seem to understand chemistry

So yea...portland, lime and water.

What point here are you making?

 

[Rock knocker]

Careful Rock knocker, you have mixed apples with your oranges and may get called out for it.

Explain.

 

[ovrszd]

Explain.

You were the first person to use the word "cement". The nitpickers on this thread may point that out to you in a negative way!!!

 

[Rock knocker]

So yea...portland, lime and water.

What point here are you making?

That the product is CaO.2SiO2.4H2O, which is a hydrous material

 

[mapper]

I remember a wall my dad and I built about fifty years ago, formed with plywood and lots of 2x4 bracing and steel tie rods, we thought. Well the whole thing held together but the plywood bulged out between the bracing so much that when the calculated volume of cement was poured the wall was about 2 feet shorter than planned.
I have noticed that the plywood that the concrete contractors around here use is at least 3/4", and appears to be faced with some sort of hard smooth laminate. It also looks to be void free with a lot more plys than ordinary lumber yard fare. In other words a lot more substantial than what my dad and I were using.