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-   -   How do you keep your forms from sticking to the concrete? (http://www.diychatroom.com/f105/how-do-you-keep-your-forms-sticking-concrete-159888/)

tibberous 10-12-2012 09:57 PM

How do you keep your forms from sticking to the concrete?
 
Is there any prep you need to do to plywood forms, or do they just pry off when your done? I plan on coating the concrete with a sealer after, so I don't want to use anything that would mess up the adhesion.

gregzoll 10-12-2012 10:01 PM

Spray with a solution of old motor oil & kerosene fuel, to release.

concretemasonry 10-12-2012 10:20 PM

If you are trying to form a box, the interior forms must be designed and built tp be collapsible and avoid pressure on the walls when they are stripped. - Similar to the methods used in the precast concrete industry where forms are steel and need little to prevent the the concrete from adhering to them like what occurs with wood forms. For boxes, a taper to the inside surfaces can allow total stripping upward with some force (usually hydraulic).

For large, flat form surfaces, some sort of adhesion/bond breaker (usually a light oil coating) works if the forms are not too absorptive.

Dick

Fairview 10-13-2012 01:24 AM

Petroleum products, including paraffin, tend to pock the finished product while vegetable oils don't. You may find an aerosol product handy.

notmrjohn 10-13-2012 10:19 AM

Lightly coat inside wooden forms with a vegetable oil or commercial release agent, let it soak into wood an hour or so. Don't want form to be wet. Wrap form with heavy kraft paper or rosin paper. Pull paper smooth and wrinkle free staple to forms along outside if needed. Paper will prevent release agent from interfering with sealant later. After concrete has set a few days, remove any paper with jet of water and stiff bristle brush.
Heavy mill plastic can also be wrapped smoothly around forms.
Any large wrinkles in paper or plastic can get trapped inside concrete, exposed edges plastic will eventually break down but couls interfere with sealant.. Wrinkles could cause pockets that will cause small chip outs.

Canarywood1 10-13-2012 10:28 AM

Form oil is what's normally used but i doubt you will find any in a small quantity,you could use light weight motor oil cut 50% with kerosene or a paint thinner will work also,why do you want to seal the walls with a sealer???

jomama45 10-13-2012 06:56 PM

I hope you're placing "water-stop" between the two pours, or using a one-piece membrane over the entire thing, rather than relying on sealer on the exterior to keep the water in.......................:no:

shinyhalo 10-15-2012 09:19 AM

Staple cheap plastic drop cover on it?

Literally one week ago I used a piece of plywood with nothing on it. I left it in contact with the concrete (fast setting from home depot) for two days and it fell right off when I moved it.

notmrjohn 10-15-2012 10:25 AM

If wood form is new and smooth it often will "fall right off" when concrete contracts. If there is any old concrete on them, or if there are rough knots or other projections or indentations though they may take chunks out of concrete when the do .

tibberous, there has been mention of "boxes" and questions of why you want to seal the concrete. Tell us more about the project, exactly what are you making?

tibberous 10-15-2012 10:58 AM

Quote:

Originally Posted by notmrjohn (Post 1031192)
Tell us more about the project, exactly what are you making?

A 450 gallon, in ground, concrete water tank.

Hole is dug, concrete, gravel, rebar, tie-wire and cement mix are here (and assembled). I'm hoping today to:

- Add the gravel and tamp it down (14 bags of quickcrete all-purpose gravel)
- Form the box out of the rebar mesh and tire / bend it in place
- Pour and vibrate the floor (20-25 bags to be 4-5 inches)

Then I'll let it set up for a couple days while I build in inner form (there is no outer form, just the dirt) Then I'll place the inner form and pour the walls.

Then some more days of curing.

Then waterproofing with this pool stuff I got that hopefully works good. Not 100% sure the best way to waterproof it yet, even thought about using fiberglass resin, since it seems to work really good on wood.

tibberous 10-15-2012 11:04 AM

Btw, if your wondering why I didn't just get a plastic tank and bury it, the answer is simply that I couldn't get it in the house. 125 gallons was the biggest tank I could find that would fit through a door.

I thought about replacing a door with a sliding glass one, and taking advantage of the hole to get the tank in, but decided this would be easier than cutting through walls / floor.

notmrjohn 10-15-2012 12:19 PM

Hokay... This is a water storage tank? Are we being invaded by aliens from desert planet? I hope you have thought this whole concrete tank idea through.
What are dimensions and how thick are walls?

You could have used four of the 125 tanks. Or a flexible bag, similar to water bed. A plastic liner for tank may be an idea similar to above ground swimming pool or pond liner may work.

I think gunite would be a better choice than poured concrete, that's how swimming pools are done.

Be sure and key the walls to floor, walls should overlap onto floor as well as go down outer edges of floor. Use long re-bar across floor bent up into walls. Possibly double layer of mesh, a few inches apart and tied to spacers and rebar.

And take a look at this, http://www.backwoodshome.com/article...sworth101.html

tibberous 10-15-2012 01:42 PM

Quote:

Originally Posted by notmrjohn (Post 1031265)
Hokay... This is a water storage tank? Are we being invaded by aliens from desert planet? I hope you have thought this whole concrete tank idea through.
What are dimensions and how thick are walls?

You could have used four of the 125 tanks. Or a flexible bag, similar to water bed. A plastic liner for tank may be an idea similar to above ground swimming pool or pond liner may work.

I think gunite would be a better choice than poured concrete, that's how swimming pools are done.

Be sure and key the walls to floor, walls should overlap onto floor as well as go down outer edges of floor. Use long re-bar across floor bent up into walls. Possibly double layer of mesh, a few inches apart and tied to spacers and rebar.

And take a look at this, http://www.backwoodshome.com/article...sworth101.html

Shooting for 4-5 inches thick, although the back wall will probably thicker. My plan for the rebar is to shape the parameter, then cut the bottom and fold it in -- that way I'll have the perfect height and a bottom to wall tie. I was just planning on using the thick mesh and ties, no rebar bars.

tibberous 10-15-2012 01:49 PM

Quote:

Originally Posted by notmrjohn (Post 1031265)
You could have used four of the 125 tanks

Thought about that, but you run into a lot of additional problems. If you just connect them with BH valves you end up with a setup thats hard to clean and hard to pump all the water out of (you can only attach a BH fitting so close to the bottom)

You also end up having to dig a weirder shape (plastic tanks and generally bulbous, as rectangular tanks would need far more plastic to not deform)

jomama45 10-15-2012 03:08 PM

Quote:

Originally Posted by tibberous (Post 1031210)

Not 100% sure the best way to waterproof it yet, even thought about using fiberglass resin, since it seems to work really good on wood.


I told you a number of times two different ways to do it, and I'm not simply guessing at it either. I've built a number of waterproof concrete assemblies through the years, including many septic tanks, and can tell you that the joint between the two pours is almost guaranteed to leak..............


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