CONCRETE FORMS
For aesthetic reasons, I would like to use 12″x16″ piers for a porch, and that means no Sono tubes or 6×6 posts. The top of the piers are only going to be 19″ above grade with 42″ below grade. NOTE: Someone else has already dug out the old block footings that were set too shallow, so there is no hope of making nice new holes to help act as part of the form. I know I could use block, but my business is carpentry and I’ve never laid block before. Yeah, I could learn, but this is only for three piers and there isn’t enough money in the budget for me to hire a mason. Also, I thought pouring concrete would be the quickest and easiest way to go when it comes to getting the exact height and positioning that I want.
So my question is this: How should a form be framed for a 5’ pier?
3/4″ plywood and what type of exterior framing?
2x4s or 2x6s? How should they be set and on what schedule?
I’m looking for advise from someone who has build forms in the past.
Thank you in advance.
Replies
concrete forms
I'm a little confused by your question... While you say you need forms for a 5' tall pier, you also say that only 19 inches of the pier is above grade. Unless there is something I don't understand, whatever is below grade doesn't need a form because the earth itself will be your form. At any rate, even a pier of 12 x 16 x 19 inches above grade will exert pressure at the bottom of the form so you want to be careful you've got it under control. The pictures I've attached may be overkill but they will give you the idea and you can go from there. First, a plywood box, screws about every six inches or so. Then the horizontal walers, 2 x 4's on edge, screwed to the plywood. The walers are pinned together at their perpendicular intersections at the corners of the box. Then -- very important -- you install a vertical member to the outside edge of the walers and install kickers off of that vertical member. Pay particular attention to having a kicker at the bottom of your pier because that is where you'll be more likely to have a blow out. All sides of the form need to have kickers. The kickers need to be staked off into the ground solidly or somehow wedged against something that make things completely stable.
Will you have rebar inside? You probably should but that's a whole other discussion.
Don't forget to consolidate your concrete as you place it. A vibrator would be great to have on hand but do some research on how to use it. Don't forget to oil up your forms to make them easier to remove without pulling chunks of concrete away. Don't forget to clean away any concrete at the very bottom outside edge of your form that might trap your form board and make it difficult to remove. Unless there is some really, really good reason to remove the form and finish the concrete the day of the pour, it's a good idea to leave the forms intact for at least a couple of days before dismantaling them.
Concrete Forms
Yersmay1,
Thank you for your input and job site pictures.
As noted in my OP, 16"x16" old block piers have already been dug out by someone else where the new piers are going to go; therefore, the hole is now way too large and "sloppy" to be useful to use as part of the form. Because of that, the form now needs to extend both below and above grade making the accumulated height of the form approximately 5'. As for rebar, in this area a 12" thick basement wall doesn't need rebar, and since the pier is only going to extend about 19" above grade and is going to be back filled on all sides, I didn't think rebar would be necessary at all.
Further thoughts anyone?
Further thoughts
Well... I don't know how 'sloppy' those holes are but they would need to be about two feet beyond your form to get a guy down there to be sure the bottom of the form is kicked off. And a five foot column of wet concrete will blow your forms apart if they aren't kicked off and braced properly. You'd also need to get a guy down there to dissassemble the forms when you're done. This sounds like an awful lot of labor. Have you calculated labor costs and how they would contrast to simply filling your existing holes with concrete and then forming up once you're above grade?
I'm also not so certain about backfilling against these piers all the way down. You'll never match the lateral strength of undisturbed earth really locking those piers into place. That may be something to think about especially because you don't plan to use rebar to give these piers lateral strrength.
I have no idea of what these piers are meant to hold up, but erring on the side of caution is never a bad idea. Is it possible to run all this by an engineer? Two or three hundred bucks for a hour of the guy's time for a look/see can do wonders for a good night's sleep.
Footings?
Your original post does not mention footing under the 16x16" piers.
If the piers are to be load bearing and poured in disturbed soil, you need an adequate footing under each pier. If the old block piers were setting on footing, then use it to set the new pier forms on. In both cases since you already have oversized holes dug, just dig more out to give you adequate room to set the pier forms or to pour a proper footing, then form.
An alternative would be to dramatically increase the hole size to say 32" and pour a monolithic footing/pier to 8 or 10" below grade. Then set your 19" piers forms on top of the monolithic footing/pier.
You need to know the load bearing capacity of your local soil to determin the footing size and rebar reguirements.
Too much concrete
Dave,
I don’t think there is any footing there to speak of at all, and it makes no difference anyway. The old footings, if there is anything left there, are still too shallow for code. By the way, right now it is a caved in muddy hole where the old piers once were. Anything that is still there will have to be removed so that we can go deeper. The up side is that from the bottom of where the old piers sat will be a clean hole in undisturbed soil. As for making a 32" hole ... That would be a lot of concrete given the job size. This is a small job and it makes sense to simply mix the material right there by hand and fill block cells. If I keep it to 12"x16" block then that really isn’t much to mix. If I had a concrete truck coming, then I might go for the overkill pour because it really would not make much of a difference in the cost, but I don't see it in this situation.
Thanks for your input.
Have you calculated how many bags of concrete mix a 12x16x60 inch pier will require? I get 6 and 2/3rds cu ft. Or 22.22 40-pound bags.
Dave, as he indicated originally, the original piers were too shallow and have been dug out. He has to dig deeper to achieve the required footing depth. Unless the porch deck is poured concrete or there's a large "bonus room" above it the 12x16 footprint should be more than adequate for a footing.
Be sure to plan how to get the form apart. You can build the form outside the hole and drop it in, but you won't be able to reverse that to pull it apart. Or plan so that you only need to remove the top part of the form.
If your hole is not already down the 42 inches you don't need to build the form that tall if you can excavate the remaining depth while maintaing a reasonable hole shape. (But of course ANY excavating at that depth is challenging.)
Use CMU block and fill the cells
In this instance I'd recommend using CMU blocks, and just using a construction adhesive instead of grout. (You just need to hold them in place and not have them leak the wet concrete, until the concrete sets.) I'd also recommend that you knock out about half the interior sides of the block to allow the concrete to flow between the cells.
Then drop a piece of number 4 rebar down each cell, and pour them full of concrete.
Back fill and you are done.
To set the final height I'd embed a suitable Simpson pier cap to the height you need, and pack under it after the concrete sets with nonshrink grout.
Jigs is on to something
Pour a base at the bottom of the hole, trying to get a very flat surface "X" number of blocks down from your final height. poke those #4 or #5 rebars in the wet mud and let it set up. Then stack 12x16" CMUs in there, fill the cores and you are done
CMU piers
I hear you Jig. CMU was one of my initials thoughts, but then I thought about using forms to end up with a nice neat and clean looking monolithic concrete pier. The more I churn this all over in my mind with the help of you guys the more I don't see a concrete pier working out as much as I would like it to. The logistics appear too problematical for this situation and greatly outweigh the benefits. The cheap and easy way would be a Sonotube, but again, aesthetically, it isn’t my first choice, and I do care about how the finish product looks. All that said, I am leaning towards the filled CMU method with Jigs suggestion about using adhesive to hold the column in place. If needed, I can incorporate a cap using a form to tweak my elevation. As for the exterior of the CMU, perhaps a smooth sand stucco coat.
I doubt you even need the glue. Once you get the concrete in there they are not going anywhere and you will not have much squeeze out from stacked block. I set a stack of round column block under water and filled them with no problem. Even in the water, washout was minimal.
The glue is cheap insurance
Construction adhesive is inexpensive and will assure that the block stay aligned, even when bumped. The purpose isn't to keep the concrete from bleeding out the joints.
Nothing is worse than having the form blow out about the time you get all the concrete in.
So far as having minimal bleed out under water, you have to realize that the pressure from the water counteracts much of the pressure generated by the concrete. Water weighs 122-lbs per cubic foot, and concrete weighs around 135 to 140-lbs per cubic foot. The water around the form counteracts almost 90% of the pressure generated by the concrete.
We do a lot of concrete in cells and other places. Squeeze out is not really that big a deal if the gap is less than about 1/4". We have to pour the sides of window and door openings solid so I have done a lot of form work for this kind of thiing on renovations where you are adding a door or window. As long as you keep the concrete at a 4 or 5 slump it will stay there. If he starts with a flat pad on the bottom to create a good base, the stack of 12x16 block should be very stable, even before the concrete goes in.
He could also make a hybrid column.Pour up this column of block up to the surface, then build forms above that and pour his solid concrete pier on top with MDF forms to get a slick clean surface. I do think it is easier to just go block all the way and parge it.
The pad
At this point my line of thinking is to set a starter block true and level in a bed of concrete with a short piece of rebar in each cell. Let it set up and build on it.
As for the hybrid pier, it is a good thought but once I’m into the block I might was well just keep with that one method and parage.
My only thoughts now are to build and fill the cells as I stack the block or simply stack all of the block and pour into the cells.
Comments?
With a little bit of rebar you don't need to worry too much about avoiding voids -- a few won't matter.
Do make sure you buy block that can be stacked in the arrangement you want and still keep the cores lined up reasonably well.
I wonder if you couldn't use a large Sonotube for most of the height, and just form up the top the way you want it to appear? It would be a little tricky with regard to frost heave, since unless you use a VERY large Sonotube there will be a lip that the frost can catch, but if you backfill the top portion with pea gravel that should minimize problems.