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Author Topic: Concrete form, or just pour it in the hole??  (Read 6368 times)
VE4TTH
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Posts: 12




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« on: September 23, 2012, 04:41:27 PM »

Putting up a tower, and I was wondering if I should build a form for the concrete or just pour into the hole?

The hole is 5 feet deep and 5 feet square. The tower is a free standing 64 ft item made by LB (a company based in Russel Manitoba Canada) that calls for a minimum of a 4 X 4 X 4 base, and has a rated capacity of 25 square ft wind loading and 250 pound limit.

My thinking is that pouring into the hole will create a better mechanical connection between the concrete and the ground, (undisturbed clay and gravel under 12 inches of top soil) and using a fourm would require backfilling the loose dirt afterwards, creating a less than solid hold on the ground?

Am I right, or am I in error??
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AC5UP
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« Reply #1 on: September 23, 2012, 04:52:37 PM »

Pour it straight into the hole and don't forget to poke the wet concrete with a steel rod or pipe to release the air pockets...
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WV4L
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Posts: 136


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« Reply #2 on: September 23, 2012, 05:30:33 PM »

Yeppers, what 5UP said. Sure it will take some more concrete but that's OK too. That's how I did mine. Here's a link to some pics.  http://home-and-garden.webshots.com/album/581985535UIVslI

Good Luck with the project!

73

Wayne C.
WC4L
« Last Edit: September 23, 2012, 05:50:29 PM by WV4L » Logged
WB4SPT
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« Reply #3 on: September 23, 2012, 05:33:56 PM »

use a 2x6 form above grade. 
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WB2WIK
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« Reply #4 on: September 23, 2012, 06:18:27 PM »

Yes, any form used should be "above grade" (above ground surface level), never beneath the ground.  The concrete should be poured into a hole dug in undisturbed soil with no below ground form.

The "above grade" form adds nothing except cosmetic attractiveness, to make it look neat.  Not really required, but most of us do this just for looks.
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KD8GEH
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« Reply #5 on: September 23, 2012, 06:29:40 PM »

My 2 cents:

I do the hole, use rebar cross hatch and into the ground. I always form above ground 2-6 inches as the other op mentioned and slope the top for water runoff. I would suggest never covering the bottom of the tower with dirt or allowing water to lay the the tower base. The issue with this is the tower can eventually rust, weaken and fail. Makes for an interesting ride to the ground from the top! There's a lot more to it than cosmetics hihi.  Shocked 

Theres a reason I know this, but shall never admit it in an open forum... or have I already ¿  Embarrassed

73 de  Dave  KD8GEH Dayton
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G3RZP
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« Reply #6 on: September 24, 2012, 12:38:41 AM »

Advice I followed was to rent a concrete vibrator - basically a piece of rod rotating inside a sealed tube and driven by a motor. This really gets the air pockets out.
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N4NYY
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« Reply #7 on: September 24, 2012, 05:26:52 AM »

I did a 3'x3' and poured it straight into the hole (for mast). I had them include those reinforced fiberglass strands into mix, as I did not use rebar. I made the hole so perfectly cubed, that my friends still make fun of me today. I used a 2x4 for the form above the dirt. It was a couple inches wider than the buried portion. I also used standard concrete. I did not use that high PSI type.
« Last Edit: September 24, 2012, 05:31:55 AM by N4NYY » Logged
KB3HG
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Posts: 404




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« Reply #8 on: September 24, 2012, 06:57:15 AM »

The hole works for me, a level form to hold the base structure or bottom section of tower attached to a welded rebar cage, like what was previously mentioned grade the top for water runoff. Plan for ground connections to the base. In the event of a direct hit the concrete wont' explode.  In the past I have even under cut the soil for a flat topped piramyd as to present more surface area, a bit of over kill but concrete was cheap and the tower never fell or bent.

Tom Kb3hg
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K8AXW
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« Reply #9 on: September 24, 2012, 08:51:46 AM »

You pretty well got the total package of advice here.... all of it right on.  I have one more suggestion though. 

When I installed my 50' Rohn tower, I used the 4' short section in concrete on which to mount the tower.  I was reluctant to put this short piece of galvanized tower into the concrete directly because of the cohesiveness of concrete with metal. 

I "painted" the 4' short section with roofing tar a couple days before pouring the concrete.  I also painted the first 18" of tower with the tar because of my dog at the time wanted to "salute" various points of the yard, which included my tower!

I didn't use any rebar, just poured the concrete in the hole.  And, as one mentioned, use a pipe, stick or 2X4 to release the trapped air pockets.

Just something else to consider, for whatever it's worth.
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KD4VVC
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« Reply #10 on: September 26, 2012, 08:41:53 AM »

Air pockets - you can rent a concrete vibrator for very little, and they do an amazing job - a little dab will do you, you don't wnat to over do it, or the aggregate will migrate to the bottom.

Using the Rohn engineered specs, you want the concrete to be about 6 inches above grade, so form there, but the rest is in contact with ground.  Make sure to but a 6 inch layer of gravel in the whole, tamp it down.  The tower base should go into the gravel about 3 inches, this way any water that gets in the legs from above can get out.  It may be overkill, but they spec the use of rebar, rebar gives the the concrete addtional strength (remember concrete really only has compressive strength) - again, may be overkill for a tower, but you are only going to do it once.  Probably wouldn't hurt to paint the legs, but when you do that the reaction the zinc has with the curing concrete, cannot occur, so you will not have the layer of protection formed by the products of the reaction.  Which protects better, I don't know.

I will say one thing, Rohn spec'd #7 rebar for  a free-standing base = that is 3/4 in rebar - seems way overboard to me, but I had a source for some at no cost, so I fave a grid at the top and bottom of the stuff.

Also, if you use rebar, keep at least 3 inches of concrete minimum around it.  Corrosion of the rebar will cause cracking and spawling.  But the rebar helps prevent crackign otherwise............
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K9YLI
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« Reply #11 on: September 26, 2012, 08:44:04 AM »

I didn't see anyone mention  this.
You live in  freezing  country..
If you are putting in a  hollowleg  tower.   be sure the legs sit on the dirt at the bottom
of the hole. The legs have sweating moisture build up and
will fill and freeze.
Tv towers (rhon 25) at  this house  must not be all the way through the concrete..  
one leg has verticle split 4 feet above ground.
I drilled  1/8 inch  holes in  all three legs at the concrete surface to drain them.
a lot of water came out of the  split leg.


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K8AXW
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Posts: 3685




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« Reply #12 on: September 26, 2012, 08:47:10 PM »

Quote
Using the Rohn engineered specs, you want the concrete to be about 6 inches above grade, so form there, but the rest is in contact with ground.  Make sure to but a 6 inch layer of gravel in the whole, tamp it down.  The tower base should go into the gravel about 3 inches, this way any water that gets in the legs from above can get out.

'Nuff said about that....... but will add, since the tower base will be below the frost line, freezing water shouldn't be a problem.  At least it hasn't been here for over 15 years.
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AA4HA
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Posts: 1378




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« Reply #13 on: September 28, 2012, 05:53:32 PM »

I didn't see anyone mention  this.
You live in  freezing  country..
If you are putting in a  hollowleg  tower.   be sure the legs sit on the dirt at the bottom
of the hole. The legs have sweating moisture build up and
will fill and freeze.
Tv towers (rhon 25) at  this house  must not be all the way through the concrete..  
one leg has verticle split 4 feet above ground.
I drilled  1/8 inch  holes in  all three legs at the concrete surface to drain them.
a lot of water came out of the  split leg.

For all of the tower jobs I spec out they are always bottomed with 6" of gravel to allow drainage from inside the tower legs.. I do that with wood poles too as it extends the life expectancy of the wood, even when it is compacted dirt around the pole. Rebar reinforcement does not have to be overly complicated a neat trick is to stand up the tower section that is going to be embedded in the concrete and get a half dozen of those ten foot long galvanized steel ground rods and drive them down into the hole at all sorts of crazy angles until everything is below-grade. Then take 5-10' of #6 bare copper and a dozen acorn nuts and sort of use the copper wire and acorn nuts to make this spider web where the rods and tower base are all connected together (have some fun with it). That is also significantly improving your structure grounding as each linear foot of metal that is in contact with the concrete and the earth increases the surface area of the ground system.

Make a wooden form that extends several inches underground and several inches above grade. When you do the pour you can make your tiny adjustments on the base section so it is true. Then do the shake rattle and roll to get the air bubbles out and "float" the concrete with a 2" x 4", if you can find a warped 2" x 4" you can actually get the center of the concrete to "crown" so water does not accumulate there. Usually I will write in the concrete the dimensions of the foundation, how many cubic yards (the driver can tell you, ask) and the date of the pour. If someone later wants to make that tower taller they run into "complications" from a tower manufacturer or P.E. about the foundation dimensions. It is nice to see it there engraved onto the concrete.

I like the fiberglass reinforcement too, another one I liked was this epoxy resin reinforced concrete for a project I did in Louisiana (96' Trylon free standing that has gone through several direct hurricane damage paths and still looks great).

The "exploding concrete" thing is interesting. Concrete really takes about 150 YEARS to completely cure as there is always moisture present. Since concrete is alkali it is also partially conductive. What may have been happening is "spalling", that happens when concrete is exposed to heat and the steam can explode out of a small spot. One of the leading causes of structure failure on 9/11 with the WTC was the floor decking being heated from underneath and the concrete was actually exploding off the floor above. I saw a NIST video test of that floor system, that concrete was throwing chunks off like a popcorn machine due to the heat. Not giving lightning current "a happy path" to ground, well it will just decide to make it's own. In the post 9/11 WTC 1 building they are including a plastic-like reinforcement like the fiberglass. Under high heating the plastic actually melts and makes tiny steam channels so the concrete does not fail due to spalling.

Oh, in one of the little el-cheapo constructed communications buildings at the base of one of the sites I was working at I saw the exploded concrete thing. They had equipment racks just standing on the bare concrete floor, feedlines running in at the top and down to the transceivers, then this wimpy little green 14 gauge ground wire that was attached to the screw on an outlet plate to ground the entire rack. All around the base of the rack there were hand palm sized spalled off areas from the floor. The lighting current was getting to the rack and jumping the distance to the concrete foundation rebar because nothing was bonded. Somebody was paying a few hundred dollars a month to stash their $30,000 police trunking gear in this building. No wonder I kept getting calls to go back over and over again to fix things.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
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