Practical question for mechanical/Civil engineers

[Claudio L.]

(05-18-2015 05:37 AM)cyrille de brébisson Wrote:  Hello,
I find myself faced with a mechanical problem at home that I am unsure how to resolve and since I am pretty sure that some of you can solve stuff like that in your sleep, decided to post it here. Thanks for your help in advance.

Basically, I am trying to calculate the point of maximum deflection on a board set vertically and used to hold concrete in place while it is being poured in a vertical shaft.

Imagine a 'U' channel, but set vertically, 20cm*20cm by 3meters high. 3 sides are already enclosed in concrete, but the last one is 'open'. My plan is to place a board to 'close' the open face, fix it on top, bottom and brace it, if possible at the point of maximum pressure, which I imagine is NOT going to be the middle!

I would also like to get an idea as to how much pressure will be there.

The lateral pressure of concrete depends on how fluid they make the mix. The worst case is a very fluid mix, in which case it asymptotically approaches the same pressure diagram as water.
So it's safe to say the pressure at any height is no more than 2.4t/m3*h, with h=height of pour above.

Your worst-case pressure for 3 m high pour is then 7200 kgf/m3 or 70.6 kPa, so you got that right.

(05-18-2015 05:37 AM)cyrille de brébisson Wrote:  ps: My thinking was that the point of maximum deflection would be at the point where 1/2 of the maximum pressure is exerted.

...

Am I any close to the solution of just blowing bubbles?

Regarding anchorage: Do NOT anchor it only at 3 points. Use many small concrete screws (like Tapcons, for example) closely spaced along the vertical sides.
If you anchor at 3 points only, your board will also deflect between anchors, and deflection = leaks!. I don't know what kind of board you mean (I think plywood perhaps?), but you need lots of small anchors to make sure the board is tight to the other concrete at all points. With your proposal You'll end up with a mess of concrete on the floor, and your board surface will be "bulged" (despite the fact your anchorage will be sufficient to hold it), and so will the concrete when it hardens. Even small bulging makes it look bad. From an engineering point of view, you could calculate the deflection, of course, but being the owner you want NO deflection.

What I'd recommend you do, is attach to the sides, not top and bottom. That gives you a small span = very small deflections.
Since you are in the US, use 1/2 plywood or OSB with 1/4" Tapcons spaced 6 in. on center, 1 1/2" minimum embedment into the concrete (so you need 2 1/4" long screws). At least in the lower third, then you can increase the spacing to 8 in above, but keep in mind too much spacing means more chance for possible leaks.
If you think Tapcons is too much work, you can use what concrete workers use: an air gun with nails for concrete (I'd go 4 in spacing if using nails) or if you like firearms, use powder actuated fasteners (more fun).

Remember you only have one shot at this: once the concrete is there, if it bulges it will be a mess that's really hard to cleanup, so it pays to put some extra effort in preparations.
If you want a smooth finish (will it be exposed to the view?), use plywood with phenolic finish on one side, and very fluid concrete.

The pour height is important, but not to reduce the pressure (unless you wait several hours between pours). Do it in 2 passes, one right after the other, it won't reduce the pressure, but will help prevent segregation of the concrete and give you an overall better finish. Use a hammer to hit the board (ideally a concrete vibrator) to help the finish.

Good luck with your pour.
Claudio