How Tight Must Forms Be?

lazerguy

Hi,

Still on that garage foundation...

I'll be pouring the foundation stem wall (no footing) directly on bedrock.
While it is a very solid surface to build on, it is also a PITA to fit forms onto.

Presently, I'm about 20% of building the forms.
I figured the best system to use is to build the form walls with vertical 2x4's, and cut each 2x4's bottom to fit the rock surface as well as possible.
This involves fiddling with angles on the miter saw and even some jigsaw work, and it takes forever.

This is my first time building forms for concrete, and I have no idea HOW TIGHT do the forms have to be???
The deepest part of the pour will be 5 ft.

If I could just square-cut each 2x4 it would really speed up the job, but this would leave 1/4'' to 3/4'' triangular openings between the rock and the bottom of the forms, and I have no idea if this would be acceptable...
How much concrete would escape through these holes?

THANKS A LOT!!!

genecarp

you could leave gaps , but you would have to line the form from the inside to keep concrete from pouring out. 3/4'' is to large to ignore.

__________________

raskolnikov

lazerguy,
That's what they make fiberglass insulation for besides keeping the cold out of the house. Fill your voids with it and stripping becomes that much easier IMHO.

D.

brick4life

lazerguy we fill our voids with spray foam

Chris Johnson

You should have looked into ICF, what you are doing is what I have done many times on the Canadian Shield, with ICF you scribe it and use a cordless sawsall to cut, test it, if it needs a bit more trim again, take off a little too much spray foam it.

__________________
Chris

72chevy4x4

why can't you throw some gravel down on the outside of the forms where the holes are-if you vibrate your concrete, it would seem to me once the concrete meets the gravel, it will stop flowing. Also would depend on how stiff your pour is

neolitic

Mon ami... We now have plywood!
2X4's?

__________________
Put your location in your profile!
(Sorry....it seems there really are dumb questions)

K2

Hand shovel some dry mud in the bottom of the forms, 2 or 3 inch slump, then wet it up to a 4 and you should have no problems except you have never built forms before so you can probably expect some or many blowouts which would make your original question mute. Let's see some pics and maybe we can help. A 5 ft wall can get some real pressuere going. I would figure whatever you think will hold x 8, or maybe 10. I've had many blowouts and always figure x 3 for myself.

K2

If you don't have a tie system or know about ties cancel the mud until you do... And yeah , plywood, but I've done horizontal 2x12 and they work great too.

lazerguy

Thanks guys!

Why didn't I think of it? Spray foam makes it so easy!!!
Now, you're talking!

Neolitic, I use 2x4's because they can be fitted easily to an uneven surface.
With plywood, it would be nearly impossible (seems to me) to fit the rock surface. Too bad I don't have a picture to post. The rock is all jagged edges, peaks and pits.

By the way, 2x4's are way more ridgid than plywood, so you need much less bracing. And they come as cheap as 5/8 plywood by square foot.
Then, I can re-use the pieces to build the roof, which is easier to do then with irregular pieces of ply.

K2

No ties?, No whalers?

Chris Johnson

Don't forget to dowel rebar into the rock as well, cheap insurance

__________________
Chris

lazerguy

Oh yes K2, ties and whalers you bet!

For ties, I use 14GA steel wire doubled-up.
8'' pieces of PVC pipe serve as spacers between the form walls, and are kept in place by the ties that run through them. (spacers also serve to hold the rebar)
The wires are tied from around the outside of the whalers, through the walls.

I figured 2 ties every 4-5 feet horizontally, one tie 8'' from the bottom, one 8'' from the top.

I should really post a picture, but I don't have my camera's download cable here...

What do you think? Will that hold to the pressure?

K2

Ok, just remember most the pressure is on the bottom of the form and that is where the blowouts are 99%of the time and remember a blowout will make your work 10x harder than no blow outs. I'm trying to think how thick 14 guage wire is but I assume it won't break but but it has to be tied well extra well. How far apart are the ties horizontally. I've done my own tie system many times using a mix of fha straps and scrap banding straps from the lumber. On a 5ft. wall i would use ties on 2ft centers and put one at 6 to 8 inches up, one about 20 up and the other about 6 inches down from the top. Extra extra ties picking up/ crossing the corners, (another place likely to blow). Life is wonderful when they all hold, sucks when they don't. ... I've also done snap-ties, Gates, Advanced, Simplex, which are form systems that can use less ties but they use quite a few too. ... I'd rather think i put in 3x too many than be short one. Good luck. Let us know how it comes out.

K2

Oh and you don't need your pipe spreader on every tie, just enought to hold your dimension until your ready. Might get by with one spreader per 5 ties or 10 ties. Once you start the pour the mud will spread your walls plenty, maybe too much. On an 8 inch wall i always set my spreaders at 7-1/2 and alwys end up with a full 8 by the time it is done.

Nac

Like someone else said look into ICF's. I have poured 100,000+ cy of concrete I have never used 2x4" on vertical it is going to be hell to keep everything lined up and strait and you need more ties not just 2 rows I would go a minimum of 3 if I was using a panel system but with job built I would go at least 4 rows. Pour slowly and have an accelerator in the mix. If you are set on using job built forms I would use plywood cut into 2' pieces and get the bottom close enough with spray foam and scrap lumber you can seal the bottom pretty tight and a couple shovel off dirt. Why is the rock so jagged? Was it blasted? Hammered? Make sure all the loose stuff is out.

lazerguy

The rock is jagged naturally, that's how the fracture lines are. I just dug out the gravel and cleaned the rock.
Now I'm 35% of the job so I won't change my system now, but I should definitely look into ICFs the next time.

Thanks to your advice guys, with the spray foam trick I'm moving SO MUCH faster!
Probably building approx. 3ft of forms (both sides, with rebar ties and braces) per hour.

Well, I get it, I'm going to use extra extra ties!!! I didn't think the concrete pressure was so bad.
Maybe use extra whalers too.

As you put it K2, I'll just add plenty more ties especially in the lower half, but I already have enough spacers.

Will certainly post pictures after I strip the forms.
Thanks!!!

concretemasonry

lazerguy -

Why are you so paranoid about some concrete leaking out where the forms do not meet the rock prefectly?

The concept of foam or any tight seal to the rock is rediculous unless you walk around in a different world with a microscope to look at your grinder or sub sandwich.

It is not a quality problem unless it is over a inch or two and not backed up with dirt or gravel because your concrete will bridge, plug and seal any opening that size. Concrete is poured routinely under water with sheet piling that does not meet the rock contours. The amzing thing about concrete is that is diplaces the mud, soil and loose debris because of its weight/density and creates a sound foundation. If you are building a garage you do not have an appreciable load even with your snows. Similar stuctures can be built on a gravel without spread footings and even a wood basement can be built on a 2x6 or 2x8 without a concrete footing - I must be missing something after all my years of being around concrete construction.

As a similar example, even the jagged and fractured ledge in CT is not a problem and that is not much different than your rock.

__________________
Dick

Engineer, designer and consultant recently active domestically and internationally on construction and design in about 35 countries.

BrickieWannaBe

Post some pictures! I would love to see how this is being done.

N