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How much rebar in my 16" 12' concrete pier?

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#1 Guest_**DONOTDELETE**_*

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Posted 11 August 2004 - 08:29 AM

I'm picking up a 16" Sonotube today that is 12' long. The pier itself will be exactly 12' long as it is going up through a deck. The actual height above the deck will be 29", capped with Dan's Deluxe Pier and then a Milburn Deluxe wedge. The foundation will be a 4' cube.

I plan to use 1/2" rebar and was wondering how many 20' pieces I should get that will go up through the column? I was thinking something like 10 pieces. Each piece would start at the bottom of the 4' cube (spread out) and then extend up through the column.

And then I was thinking about having 3/8" rebar loops every 12" or so. This would then all be welded together. At the base, I would cut a 16" diameter hole in 3/4 plywood and cover the 4x4 hole and then attach the sonotube to it. This will allow me to do a single pour, vs doing the base first and then the column. I do realize that I'll have to pace my pouring so that gravity doesn't make the cement ooze out from under the sheet of plywood.

Do you guys see any problems with this approach?

Here is where I got my inspiration from:

http://www.noomoon.c...mainastroOH.htm

#2 Guest_**DONOTDELETE**_*

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Posted 11 August 2004 - 09:10 AM

That should work fine. I work as an eng. inspector and have installed lots of piers over the years to hold up many various things. Sounds like you are going to have a very nice setup when finished. Is this for your 10"?

#3 Guest_**DONOTDELETE**_*

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Posted 11 August 2004 - 09:46 AM

Yep, this will be for my 10" for now. Down the road, I might trade up to a 14" with a 100-120mm refractor piggy backed. According to Ken Milburn, my wedge should handle that just fine. He did recommend drilling a 4th hole in the tilt plate since the 14's have a 1/2-13 hole in the center of the drive base. He also suggested changing the plate pivot bolts from 3/8" to 1/2".

But all that aside, my main objective right now is to have the 10" on a permanent polar mount for astrophotography that I can control from inside my house. I plan on building a roll-away shed around the pier just like in the link I sent.

I feel that by having the scope permanently mounted in a convenient location, the scope will see much more use. While I had no troubles setting the scope on the tripod in alt/az mode, I find it to be a real pain to mount it onto the wedge in polar mode. Plus, it is not as stable in polar mode on a tripod sitting on a wooden deck.

#4 michaeloconnell

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Posted 11 August 2004 - 10:38 AM

That's plenty rebar. Keep it towards the outside edges rather than close to the centre. Try to space it regularly around the outer periphery of the sonotube but at the same time keep the outer edgs of the rebar a couple of inches from the edge (does that make sense?) If it's too close to the outer edge it could lead to problems also. Too far in to the centre and it'd be of limited use.

If you simply pour it down the tube, you'll have huge voids in the concrete at the foundation and in the pier.
Personally, I'd strongly recommend pouring the base first and then poring what's called a "kicker". Basically, it's a pier except it's only a few inches high.
First, position all your rebar.
Then pour the base and make sure it's well compacted and there's no air voids.
Cut say six inches off the end of the tube.
After you have compacted your concrete, position your plywood with the hole in the centre. In this hole place the
short bit of sonotube. Now pour the concrete to the top of this. This will create what's called a kicker.
Allow the concrete to cure for a few days.
Before pouring the pier, "scabble" the top of the kicker. The purpose of this is to roughen the top surface to allow proper bonding of old to new concrete. This could be done by lightly hammering a chisel lightly into the top surface of the kicker so as to create a slightly roughened surface. Take out small pieces, not huge chunks and don't go right to the very edge. Shouldn't take too long to do.

When that's done, and after you've pulled off the short bit of sonotube surrounding the kicker, fit the remainder of the sonotube over the rebar and down onto the kicker.
Then pour your pier.

I don't mean to know your plans or anything, but for a pier of that size, as an civil engineer I *strongly* recommend pouring the base first and then the pier. It'll make a *huge* diffrence to the overall strength of the pier.

#5 Guest_**DONOTDELETE**_*

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Posted 11 August 2004 - 10:46 AM

If you simply pour it down the tube, you'll have huge voids in the concrete at the foundation and in the pier.


Not if he uses a concrete vibrator. I would pour it monolithic. JMHO

#6 michaeloconnell

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Posted 11 August 2004 - 11:09 AM

One that would compact the entire foundation to a depth of 4' with plywood covering it?
The pier wouldn't be too bad as you could run the vibrator down the centre of the tube and you could vibrate the tube itself (assuming he's gonna use one of these). It's getting into the corners of the foundation from the top of a 20' high tube that could be difficult.

There is also one other major problem however. As you'd pour the pier, there'd be a hugh pressure build-up at the base. This could lift both the tube and the plywood. Would the plywood be able to withold the pressure of 20' of concrete trying to burst up? It'd have to be a very stiff mix and a secure piece of plywood for it to work without any problems.
Don't get me wrong, I respect your view Khrome. I'm just not convinced that one pour is the way to go.

#7 Guest_**DONOTDELETE**_*

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Posted 11 August 2004 - 11:20 AM

I was assuming the 4x4 base is in the ground and that the "slump" of the concrete would be extremely dry say 2" slump to minimize it trying to run out. But definitely gonna have to have some beef to hold the forms.

#8 Guest_**DONOTDELETE**_*

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Posted 11 August 2004 - 11:56 AM

Thanks for the feedback. The top of the 4x4 base will be flush with the ground, which is very level.

My plan is to pour the base first (the plywood would be held up from the ground to allow this), then drop the plywood down into its final position, and then slide the tube down from above over the rebar. Next I would add a couple of feet of conrete to the tube. Wait a couple of hours until that starts to set, and then pour the remaining 10'.

Also, I was planning on sitting something heavy on top of the plywood to keep it from popping up from the pressure, although a slow pour should minimize the pressure I would think.

Since I plan to pour the base directly, I think I can minimize/eliminate air gaps, even without a vibrator. And yes, I was going to tab on the tube itself to avoid air gaps in it.

I have no first hand experience with pouring a pier, so I appreciate the feedback. I'd hate to screw this up and have to go rent a jack hammer and compressor and start all over again!

#9 John Jarosz

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Posted 11 August 2004 - 01:25 PM

You need a really good form structure to contain the concrete if is poured in one piece. I vote for two pours. If you notice the guy with the first pier did his in two pours.

If the concrete forms don't hold, you have an unbelieveable mess to deal with.

The other thing to consider is that if you are going to have voids, they will probably occur in the base. If you pour the base first you can verify it before pouring the pier.
I assume if you do it in one pour, you'll pour everything thru the top of the tube.

How much concrete will this use? Are you using readymix (delivered) or are you mixing this yourself? If you haven't calculated the volume of concrete yet, you may be suprised.
If it's readymix, the guy is going to want to deliver it and leave, not do an itty bitty pour wait and pour again. (Unless you have a friend doing the delivery)

If you do it in one pour, that is a lot of concrete to get to the top of the tube.

#10 Guest_**DONOTDELETE**_*

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Posted 11 August 2004 - 02:07 PM

Thanks for the feedback. The top of the 4x4 base will be flush with the ground, which is very level.

My plan is to pour the base first (the plywood would be held up from the ground to allow this), then drop the plywood down into its final position, and then slide the tube down from above over the rebar. Next I would add a couple of feet of conrete to the tube. Wait a couple of hours until that starts to set, and then pour the remaining 10'.

Also, I was planning on sitting something heavy on top of the plywood to keep it from popping up from the pressure, although a slow pour should minimize the pressure I would think.

Since I plan to pour the base directly, I think I can minimize/eliminate air gaps, even without a vibrator. And yes, I was going to tab on the tube itself to avoid air gaps in it.

I have no first hand experience with pouring a pier, so I appreciate the feedback. I'd hate to screw this up and have to go rent a jack hammer and compressor and start all over again!


Just as I suspected, I think you have it all under control :waytogo:

#11 Guest_**DONOTDELETE**_*

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Posted 11 August 2004 - 04:05 PM

I hope so! I think the reason that other guy didn't do it monolithic was because the ground wasn't level in his case.

I just calculated how much concrete I need. Looks like the base will take 112 80# bags and the column 28. That's a total of 140 bags, or 11,200 lbs!

I think I'm going to rent a mixer for this job...

#12 Guest_**DONOTDELETE**_*

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Posted 11 August 2004 - 04:21 PM

The column = 3.14x.67x.67.12= 16.9 cu. ft.
The base = 4x4x4= 64 cu. ft.

about 81 cu. ft./27 = 3 cu. yds.

here 3 yds. = $70 per yd. delivered = $210 well worth NOT having to go to Lowes and load up all those sacks of concrete and NOT having to mix it :grin:

which brings me to a question...how are you getting the concrete up on the deck to drop into the tube? :question:

We usually use a pumper truck :lol:

#13 John Jarosz

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Posted 11 August 2004 - 05:41 PM

I think 1 yd of concrete = approx 1800 lbs. So it's prolly about 5400 lbs. (Tthat's the dry weight of cured concrete, wet is heavier - I think)

If you're going to mix it yourself, absolutely rent a mixer, and con some friends to come over and help, because it will be a lot of heavy lifting. Have the sand and aggregate delivered, then you just deal with the bags of cement.

#14 Guest_**DONOTDELETE**_*

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Posted 11 August 2004 - 06:26 PM

It runs a little higher than that here. 2500 is $74 and 3500 is $78. So that would work out to $234 for the 3500.

The 80# bags are $2.77 at Lowes, so that would be to ~$390 for 140. Renting a 2 bag mixer runs $45/day.

I wasn't aware that I needed sand or aggregate? I was planning on just getting those Quikrete bags from Lowes where you just add water and they are good for 4000 PSI. Would that not be the way to go for the cemet?

But yes, mixing it yourself is about twice the cost and a lot more labor. But, in my case, I don't really have a way of getting a 9 yard 40,000+ lbs truck to the pour site. It would have to back up over a recently landscaped back yard, and then contend with a 15-20% sideway slope.

I do have a 12k flatbed trailer and a '02 duramax, so picking up 3 pallets and change from Lowes would not be a big deal. I also have a front end loader on my tractor for unloading the bags and taking them to their destination (not going through the back yard). I can raise my bucket high enough to drop off the 28 bags onto the deck. Once the base is done, the plan is to roll the mixer up on the deck for the final mixing.

The pumper truck idea is interesting though, but I bet that would add a lot more than the $250 saved by buying in bulk. I wonder how far (horizontal distance) they can pump? I'm also not sure they are interested in sticking around until the base has cured a little before the final pour as that would not be good for the truck or pump I would think.

My gut feeling is that by mixing by hand (with a mixer) my fill rate will be just about right and that having a guy dump 3 yd in 45 minutes would be rather risky.

How should I attach the tube to the plywood base? I was thinking that I would use construction adhesive to join the two ahead of time and augment with screws directly through the tube into the sheet. This would mean the hole would have to be cut very neat so that the tube will slip through and be flush with the underside of the plywood. But I think this will be the strongest.

#15 EdZ

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Posted 11 August 2004 - 06:28 PM

Concrete weighs 140-145# per cubic foot. A yard of concrete weighs nearly 2 tons.

Using a 2 bag mixer at 10 minutes a batch for loading and mixing and pouring, would take about 12 hours (without breaks for lunch) to mix and place that much concrete. By the time you are done you will have moved 6 tons of material 3 times, once into the mixer, once from the mixer to the pour and finally into the pour. You are planning on moving 18 tons of material by hand. You and what army???

The quality and strength of your concrete is most dependant on the amount of water added to the mix. You need to take strict control over the amount of water stone and sand added for each bag of concrete, or you end up with weak concrete. All the rebar in the world won't do you any good if you don't control the mix.

If you intend to use quickcrete, you just need to strictly control the water. No need to mess with sand and stone. I would not set a pallet of concrete bags up on the deck unless you plan to distribute the load over a wide area. Assuming you deck can hold 150#/SF, you would need to distribute the load of 28 bags over 15SF.

get this stuff truck delivered, even if you can't set it up so the truck can chute it into the tube. You can use a 2 Cu Ft wheel barrow, ramp it up to the tube and maybe get it all in in several hours. I don't think this job can be done by hand.

You will end up with nearly 1800 PSF pressure at the bottom of the tube. If you don't have the bottom of that tube tied down with shackles, you are going to get a blowout. The pressure cannot be reduced by slowing the pour. The concrete acts as a liquid until it's set. Build a 2x8 frame around the bottom of the tube. Spike thru the inside of the tube and out into the wood frame. securely lag the frame to the base as if you were framing a house to never fall down.

edz

#16 Guest_**DONOTDELETE**_*

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Posted 11 August 2004 - 07:02 PM

When we pour manholes the forms are completely open at the bottom. I have poured them as much as 25' tall in a single pour. If you bring it dry enough (1"-2" slump like I stated earlier) you should be able to pour it. I have much more concrete stacked up in a 6' diameter manhole 25' tall with an open bottom than what he will have here.

#17 EdZ

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Posted 11 August 2004 - 07:08 PM

Open to what, the bottom of the hole. So if the concrete comes out from under the bottom of the forms it goes into a hole 20' deep in the ground.

just my opinion.

ever see a blowout. there is no stopping it. If it happens, it's start all over.

He's got far greater problems than how that form is attached.

Maybe you want to truck deliver and wheel barrow the concrete into the foundation and let it set. then pick another day to hand mix and place the much smaller pier. 64 CF = 80% of all the concrete is in that 4' cube. That 30 to 40 wheel barrow loads. Lay plywood down across the entire traffic area or you may not get those wheel barrows back to the pit. Those wheel barrow loads would be 200-300# each, gauranteed to rut a yard.

edz

#18 Guest_**DONOTDELETE**_*

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Posted 11 August 2004 - 07:27 PM

I should be able to drum up 4 very capable guys to help out. I was hoping to get the job done in 8 hours. I was planning on setting the mixer up so that all I had to do was tilt it to drop the load directly into the pour. Like you mention, the tube itself is only going to be the last 28 bags, the vast majory goes into the base.

Having said that, I might see if I can get a concrete guy out here to **** the possibility of a truck delivery and the associated cost.

Your point about 1800 PSF at the base is well taken, I'll follow your advice about shackling down the tube for sure.

#19 John Jarosz

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Posted 11 August 2004 - 07:29 PM

II can raise my bucket high enough to drop off the 28 bags onto the deck. Once the base is done, the plan is to roll the mixer up on the deck for the final mixing.


If you have a front loader with a bucket, can you use the bucket to transport the liquid concrete from the cement truck to the pour? Moving the concrete is unbelieveably hard.

Can your deck support that load? I would be worried about this approach.

Mixing the concrete up on the deck will destroy the aesthetics of the deck, IMHO. Mixing concrete is very dirty, messy (did I mention heavy?) work. The grit gets into everything and any liquid residue will get into the pores of the wood in the deck.


The forms need to be incredibly strong. Adhesive is not appropriate for anything to do with concrete forms. The design of the forms should be such that ALL of the load is taken up by the strength of the form material. The fasteners should only hold the forms in position and they should not try to absorb any of the load from the concrete. (This last point is difficult to achieve in practice, but you should always think about the forces trying to cause a rupture of the forms)

In spite of all this, if you still feel you should do this with mix-it-yourself, then seriously consider doing this in two pours, allowing the base to cure before doing the column. You could even put some threaded fasteners in the base to support the sonotube in place for the second pour. Doing it this way means you don't have to take all that weight up to the deck either.

I'm not being negative, I'm just trying to save you some grief.

John

#20 Guest_**DONOTDELETE**_*

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Posted 11 August 2004 - 08:00 PM

John, excellent points. I didn't think about using the front loader, I even have a old junk bucket I can put on it. The loader is rated at 1800 lbs carrying capacity and 3800 lbs "breakout". So at 145# a cu. ft., I should be able to haul 12 cu. ft. on each run. So that would be 6 runs to the base where I could drop the load directly in the hole, and two runs to the deck (the last 75' would be wheeling it across level ground directly to the tube.)

I should be able to get the 6 runs to the base done in 5-10 minutes each, depending on how liquidy it will be in the bucket (don't want to spill any). And I'm not sure how much 12 cu. ft. will take up in the bucket, probably a lot (bucket is 6' wide) so additional runs may be necessary.

How long can a cemet truck "hold its load" before it needs to dump the remainder and rinse?

#21 John Jarosz

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Posted 11 August 2004 - 08:06 PM

After re-reading your original post, I think I understand your approach better. Correct me if I'm wrong:
It sounds like you're using the earth as the perimeter form for the cube.

If this is true, then you are certainly deep enough to be safe from any frost heave. But since you are using structural rebar, I think you could get by with less mass in the center. What you need is a trench footing (maybe 12" wide at the surface belled out to 14-16" wide at the frost line. The center could be earth. The top of the pad (4'x4') could be 18"deep (12" is prolly OK) with the rebar for the pier imbedded halfway into the top of the pad.
The important thing is to make sure you're below the frost line with the bottom of the base. The shape I'm describing is an upside down cup or bowl. Doing the design this way can save you a lot of concrete pouring. For the trench foundation, you can rent a Ditch Witch. Just some additional thoughts.

John

#22 Guest_**DONOTDELETE**_*

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Posted 11 August 2004 - 08:22 PM

You are correct John, the base perimeter will be a hole in the ground. The surface is 100% level. I was planning to use a 24" auger that will go down 5' to get the hole started, then cut out the rest with a showel.

Since I am indeed on level ground, perhaps just drilling a 24" hole to a depth of 5' would be sufficient? The ground is very hard (clay) and full of rocks.

#23 Rusty

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Posted 11 August 2004 - 08:40 PM

For a column that high, I'd suggest using the base. IMO, two pours would be better (do the math, and the pressure at the base with a monolithic pour will be scary)- place rebar in the base, and leave some projecting for additional "tooth" for the second pour. Maybe some CEs here can comment, but I don't think welding the rebar is necessary; risers and rings can be wired together.

And I'd go along with the idea of the pumper - the mixing is done at the site, and can even be adjusted as you go further up the column.

And you need to decide on the strength of the concrete mix - I had the slabs/footers for my house addition done with 3500 psi instead of the usual 2500. My only regret is that I didn't use fiberglas-reinforced for certain parts....

#24 serious_sam

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Posted 11 August 2004 - 09:00 PM

just a few points:

1. NEVER weld rebars together. unless designed by a professional engineer and inspected by licensed rebar inspectors, you are going to create a metallurgical notch and weaken the bars. i NEVER design anything that would require bar-to-bar welding. use wires to tie the bars together. they will suffice.

2. use a few pours. what rusty suggested as the projection is called "doweling". typically the length of doweling required would be 40 x bar diameter. for a #4 (1/2") bar it would be about 20" long. find out what bars you need at the column (total steel area should be about 1% of the concrete area), then bend the same number of #4 bars and embed them in the footing, with 20" sticking up. you can leave a shear key on the top of footing by embedding a 2x4 on top of the wet concrete. upon concrete curing, remove the 2x4 and you'll have a tooth on the footing.

3. as for the ratios of mixing, i've checked thousands of design mix reports but can never remember the ratios they use.

4. 3000psi stone concrete should work in most cases. no need to get fancy here

5. 16" diam. pier is huge. i would make do with 12.

#25 John Jarosz

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Posted 11 August 2004 - 09:00 PM


Since I am indeed on level ground, perhaps just drilling a 24" hole to a depth of 5' would be sufficient?


If you mean to eliminate the base, I'm not sure. But a 36" diameter base down to the frost line should be fine. In this case for what you are using this for, deeper is more important that how wide the base is. A square base doesn't buy you much additional help. Remember you need rebar in the base as well.

I agree that the rebar doesn't have to be welded. Tied with wire is routine. Just make sure the structural rebar overlaps a good distance where ends meet.

Using the top of the base after it's cured to shut off the pier is exactly what I would do. Good thought Rusty.

John


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