Billy NY or any concrete expert

I have two questions concerning the pour. The footing is not perfectly flat so the bottom of the plywood forms must have some tapering gaps---worst case should be about 1/4". My plan is to hire someone with a small concrete pump which requires a six sack mix and pea gravel. Should I be concerned about the amount of concrete that will get out under the forms? Cleanup after the pour is not a problem, but I don't want to have to deal with a gusher. If this sounds like it is going to be a problem, what should I do about it?

The second question concerns the use of one of those vibrator/compactors that you stick into the concrete as it is being poured. Somebody once told me that you could blow out a form by overusing one of those, which strikes me as ridiculous (like compaction is going to make the concrete weigh more per unit volume than its maximum possible weight?) Nevertheless, I thought I'd better ask if there's anything I need to know about using one.

Thanks in advance, and feel free to address anything else you think it would be good for me to know.

Stan

I feel a lot more comfortable now about how the pour will go, as well as reassured and much more satisfied about how I've put this together.

Many thanks, Stan

HTH

Areo

Vibration's important according to the American Concrete Institute, the Portland Cement Association, and me. Blow-outs come from insufficient use of snap ties/walers, or whatever you're using to hold the forms together. We'll assume you have the wall braced adequately.

I well-remember my first pour as a novice. The readymix salesman took one look at my new vibrator and cautioned me not to use it. "One touch will break a tie." He was completely wrong, fortunately I went with what I'd read. According to the trade organizations, over-vibrating is extremely uncommon, under-vibrating normal.

My roofs go from 200 tons up (300 psf and up). Makes you very careful about the walls/footings.

We don't count any gap smaller than 3/4", but then I don't use pea gravel and pour a stiffer mix than you can pump. With your short wall, not much pressure even at the bottom.

You didn't mention drainage. You don't want water running between your footing and wall, hard on the rebar. You can still insert a bentonite waterproofing rope inside your form if you're concerned. Glued or tacked to the footing to keep it in place.

We have cold joints all the time, not an issue with rebar going between (other than a place to leak). Your wall's so short that tipping isn't much of an issue unless you have unstable fill, like expansive clay. You apparently believe that you have enough footing/rebar.

BTW, I once poured almost 200 lin ft of 4' retaining wall for a client who wanted a board fence look. He got it. From more than 8' away, it was weathered boards. He'd chosen to pay for colored concrete. You get texture from whatever you have in the form.

Good luck with your pour.

When I worked at the lumber yard years ago, there was a special plywood just for concrete forming, was 3/4" thick and I forget the name of it now, was expensive then, seemed to have some oil impregnated coating to it. The veneers had the least plugs (football shaped plugs). I have delivered full units of it on occasion back then but not very often, the days of lumber forms were long gone then. Good to spray form release agent on the forms for easier stripping, can't say I remember what the finish looks like on the concrete if using aspenite or similar pressed board, it will mirror itself if it has any notable texture to it, and can stick to the concrete. When I don't have or use form release, I just soak the forms and keep em wet as best can be done.

Tapping the forms does help the finish, even with the vibrator, you can still have voids, we do it on the edges of slabs where they are exposed and walls. The worst thing is an interruption in the pour or running short on material, that is when you will see the cold joint and voids in a wall. Vibrating the material as it's placed is a requirement called for by most 03300 specifications if I recall, even when doing a curb or similar shaped components.

Concrete air entrainment is going to vary no matter what, I think that is partly why the mixing barrels on the trucks have counters on them, the max rotations called for by American Concrete Institute (ACI) was 300 revolutions, after that an inspector can reject the material based on that criteria either to much air entrainment or the aggregate starts to settle out, lot of obscure characteristics about this material. ACI has some intense reference material, really an engineers kind of reading, but nonetheless interesting to learn from.

One thing is in Stan's favor is that it's just under 3'-0" tall, should be an easy pour, especially with a pump so you can place it where you need it without interruption. Looks like about 5.5 yards,(round to 6 or a little more) half a truck load, should go quickly.

Another thought comes to mind is drainage behind the wall, did you place sloped weeps, short sections of PVC through the form would suffice, backside to have perforated pipe, tied in with "tees" bed in clean crushed stone and cover with filter fabric, backfill with gravel. Use a hardware cloth type screen or other device to keep critters out.

As far as a pump, make sure the concrete supplier can provide you with a pump mix design for a pump truck, your pump guy ought to have the specifics, on the big jobs, a pump mix design is always submitted for approval, along with the other conventional mix designs, so your supplier should have no problem there. If you have any doubts on the quality of the mix, get 4 test cylinders like what the testing labs use, pack each in 3 lifts (1/3 of the cylinder at a time), rod each lift 25 times, screed the top off on the last lift. You can have a test lab break the cylinders to confirm the compressive strength, but this is a little on the overkill side, if your supplier is reputable, there should be no worry about strength, but if you want to be absolutely sure, make sure the supplier knows the mix will be tested, keeps em honest, but like I said if you ask for a specific mix, it should be fine from a reputable supplier.

If the bottom of the form has gaps, can you run a toe board on top of the footing at the base of the form ? You could shoot it in with a powder actuated fastener or just masonry nails ( wear good eye protection with both !) 1/4" should really not be a problem, make sure the water/cement ratio of the mix design is right, meaning you don't want a real loose mix, but now thinking that it is a pump mix, it may have to be that way, the aggregate is smaller in sieve size for it to flow in a pump and not clog, the mix design has to allow for it to work in the pump correctly. When they do a slump test of the material they place it in a mold on a flat surface, pull the mold off, and measure how far it settles hence "slump" it will sag more the looser it is, meaning water/cement ratio, more water in the mix. The water cement ratio also determines it's ultimate strength, if too much water is added on site to the mix, it will substatially weaken the compressive strength, it's not good to deviate too much from the design mix. A 1/4" gap, ought to clog up no problem. It may leak some fluid but not enough to concern me, the finish may reflect those gaps but at 1/4", and the condition of the existing being uneven, you have done the best you can do to narrow it to 1/4".

The above is just some general information about concrete, if your supplier has a proven (used before) pump mix design, and your pump guy is experienced, it should be as simple as pouring it into the hopper and the gaps should not be an issue, try to seal them with toe boards if you can.

What I'd be more concerned about for a blow out is the vertical and diagonal bracing you have in place if using lumber forms, not a form system, there is no overkill when it comes to bracing, 12" to 16" on center and each should be diagonally braced and staked, however you form it, bracing is very important each must be strong and properly secured.

A concrete vibrator can and will blow out a form, especially if it is not braced properly. An example I can think of: I did a 6' to 8' high retaining wall pour at a friend of mines house, and he did not know much about these vibrators, I had extensive knowledge of them, but I kind of kept quiet about it, let him call the shots, he does lots of concrete slabs in his pool repair business, I was just there to help, he's a great friend, very lucky to have good friends like him. I brought the concrete vibrator, well it ended up that they filled the form to high to use it and being lumber forms, when I hit it with the vibrator, all the concrete settled at once, Yep I looked like a real idiot, it blew, I should have done what I knew to do, will explain that next. Well all was not lost, but because it was used when the form was getting full, the vibrator caused too much settling at once and it bulged and blew out. Well with everyone scratching there heads, I said well don't just stand there ( then I was a little p#ssed but knew we had to fix it ) get shovels on that blown out area, get that 1010 dozer over here and shove that form back, we corrected it and it was to be faced with stone anyway so it really was not a big deal.

The vibrator will settle the concrete uniformly and insure a better finish against the form, but do it in lifts, just run the tip in there and yank it out quickly, you'll see how it works, take some material and put it in a bucket loosely, hit it with the vibrator, watch how fast it settles, just hit it once quickly and keep it moving, you don't want to leave it in one spot while it's on, as you will settle the aggregate to the bottom. fill the form evenly by say 1/3 or so, place the tip quickly into the material and pull it out, move right along, place more material, follow the chute as it's poured, repeat until you get to the top of the form, if you wait to use one of these when the form is full, it will all settle at once, and it will put a tremendous amount of pressure against the form. Even with big pours on construction sites, they do about the same thing, settle it as you place it. When the drop is over 8'-0" they use a trunk to reduce the fall, and they settle it with a vibrator, but as they go along, not all at once.

Other things, make sure any electrical for lighting, drainage or other items you may want has been considered, put your weeps in, conduit or what have you, make sure no braces extend above the form, makes it easier to finish, and soak the forms before you pour.

Sounds like you are ready, double check that bracing and add more if need be, tell us how it comes out, glad to be of any help !

Vibrator- Seems strange (until you do it), but you can easily blow up the forms with indiscriminate use of vibrator. I don't know the physics, but you can break the plywood form eventually, as the concrete compacts. I don't think you'd even need one on a 34" wall- just use a broom handle up and down a couple time per running foot. If you do use one, just lower it once to the bottom, and right back out- then move down a foot or 2, repeat. All you're doing is getting the air bubbles and voids out, and a little vibration goes a long way.