The AM Forum

I finally got my tower foundation installed just in time before the cold weather sets in here in Idaho. It will still will be a month before  the concrete is fully cured. The ground was sandy so the soil had to be wet down to prevent a cave in. It took 3 1/2 yards of concrete to fill the hole. The tower that I have is a Tristao 54' crank-up/ tilt over with a mosley  CL-33 tribander for now. I still need to build a tilt over fixture for the tower but for now just want to get in the air.
Joe

Problem with concrete these days is that they charge like it is made from gold...

... congrats on ur base. I've been needing a base put in for longer than I would be willing to ever own up to. The sites available have "issues".

The CL33 will play FB...

More pix!! Wide shots too!! :D

Hi Joe,

Nice project.

A few suggestions:  

The rebar should be tied with wire, not welded. Welding creates undue stress and potential concrete cracking, whereas tied wire rebar can move as the concrete sets.  Notice all the big buildings use ties.  The simple hand tool and ties are available at any building supply outlet.

It's hard to tell from the pic, but is there a dried layer of concrete already poured on the bottom with a new layer yet to be poured?   If so, it is better to pour it all at one time for more strength. Two layers are like a big crack in the overall structure, strength-wise.  Layers, if needed, should all be poured together within 30 minutes at most. Wet concrete bonds poorly to dried, already-set concrete.

Another point  - I cannot remember the exact code, and maybe you are within spec, but the rebar ends need to be at least 2-3 inches from the end of the concrete block. IE, if a bar's end is too close to the outside face of the concrete block it may break through and water will rust through the steel bar. This may cause a big hole and failure, cracking.  


The problem is that a self supporter and tilt-over like yours puts a tremendous stress on the base and should be bullet proof.. Whereas, a guyed, stationary tower can almost stand with no base at all - very little shear stress, just vertical weight.


I realize I'm nit-picking and the tower will probably be fine as-is,  but thought I'd mention these extra details for other guys doing a similar project in the future.

Have fun with the new Mosley beam, OM!

T

Hi Joe,

Tristao made made my tower as well before the company was sold to US Towers.  The equivalent model number of mine is the MB-550.  I have the tilt base and the tower has held up very well for the last 15 years or so.  The loading your putting on it will not stress it at all so you will be in good shape.  Just remember to keep the lifting and tilting cables in good shape since if they fail its not so good.   My hole is similar and it is shaped like a pear where the bottom flares out.  I did mine larger than they required since I wanted to use it on a bigger tower later and that time has come since I just got a Heights 82' crank up tilt over one that's almost ready to go up.   

You should be all set for some good 20M AM activity!

73,
Joe, GMS       

Hi to all,
Tom you absolutely correct about using wire to tie the rebar. In this case there   are tack welds at the top of the cage but the rest were wire wrapped. The rebar cage is supported at the bottom with four concrete deck blocks. It was a single pour, I will post more photos.
Joe   

Few more photos

another photo

Looks and sounds like you covered all bases, Joe.  Good job!


I was also wondering about how we could increase the strength of the tower itself....  I know that standard self supporting towers should not use guy wires.  But would it make sense to run a set of guys at the first 20' level?... as a strapping safety measure in case of high winds?  They could stay intact when the tower was cranked down.  They would be released when the tower is tilted over, of course.


T

Of course, there's a mechanical need to have the rebar move; I get that.

However, I'm curious if there's an need to worry about the rebar becoming corroded at the tie points: that might lead to rectification of RF.

W1AC

Hmmm... never thought of that.   And it sure would be hard to fix... :-)

I know that the commercial cellular towers use the ties.  Big Al/ K1JCL, who used to do installations, mentored me when I put up my pair of self supporting 190'ers.  I lost 25 lbs that summer installing the intricate rebar matrix and preparing the 30 cubic yard bases in the hot sun...

I suppose rectification would show up when transmitting and causing RFI into other radios in the shack or even neighbors.  I've not seen it in my own system here.

Anyone else have practical experience with this potential rebar rectification issue?


T

I guess a lot depends on how the self-supporting tower is mounted. My EZ-Way self-supporting/tilt-over 75 foot tower has been standing for 36 years. I don't use guy wires for the first section. That was the point for getting the self-supporting tower (no guy wires). If you look carefully at the picture below, you can see that the lower tower section doesn't even come in contact with the concrete. The 10 foot high, stationary pole, roughly 6 or 7 inches in diameter,  is what rests on the concrete. There is a robust hinge at the top of the pole; the other half of the hinge is welded to the tower. When the tower is plumb, there is a steel rod that goes through a welded bracket at the lower part of the pole to lock the bottom of the tower in place. There are 6 plus yards of concrete, rebar, a frog(he jumped in the hole just before we started to pour the concrete), and a bottom layer of rock in the base.

(http://amfone.net/Amforum/gallery/25_03_07_09_11_28_45.JPG)

it seems to me that the tower is supposed to be grounded at the bottom, so the reybar carries nothing, no voltage, no current. No rectification.

The concrete in the main is an insulator.

                     _-_-

Concrete has the property being slightly alkaline which causes steel to become passive with regard to corrosion when steel is embedded in concrete. Concrete is also definitely conductive. Concrete that is closer to the earth and in direct contact with the earth will have a much higher moisture content and therefore, be more conductive.

I think rectification of RF would come from a broadcast station pumping 5,000 watts or more.
Ham radio is just a little tickle........1.5kw PEP!???
Fred

Remember to always keep the ground rods at the base of the tower out of the concrete.  They should be place around the tower and bonded accordingly.   To many cases of blow to bits concrete if a strike occurs.  You need to direct that energy away from the base and the lightning will track the path of the lowest ground impedance. 

Joe, GMS

I have worked on quite a few commercial tower installations in Alaska.....starting around 1999 or so, the specs required all rebar to be powder coated for corrosion resistance....However, I have seen rebar that has been in the concrete underground for 30 years that hardly had any rust on it....so conditions are variable I guess...

In recent years rebar with epoxy and other coatings have shown up but that would be overkill for ham tower work.  The only common cases of rebar failure are poorly maintained roadways, particularly in the north, where winter salt solution and freezing action gradually crack the concrete and take out the rebar.  There was a company in the 80s who sold a lot of precast concrete products and they used an additive that supposedly decreased cure time without materially impacting ultimate strength which seemed to be true but it turned out to be highly corrosive to rebar and caused very early expensive failure.  One of the southwestern states had to replace a lot of stuff in their roadside rest areas because of it.

For my towers I paint the area where the leg enters the concrete with a little bit of rubber sealant. It probably isn't necessary but it avoids water pooling if the interface point between the leg and upper concrete surface has minor cracking.

Concrete is definitely quite conductive as anyone who contacts something with higher voltage while standing barefoot on concrete can attest.  If you have bare concrete basement floors then a non-conductive rubber mat around the bench area is a must just in case you forget.

Before I guyed any sort of self-supporting tower I would at least drop a note to the maker to see if they have any advice since it may have points not designed to support the heavier down force created when the shear force is converted to a compression force by the guy wires.  I did lightly guy my Hy Gain 18HT to prevent a second occurrence of the base insulators shearing but this is a VERY simple structure unlike crank up/fold over towers used to support large arrays.

I suspect that the conductivity of concrete is a function of the water content at any given time and the salt inherent in the concrete itself. It may be a conductor but not a good one, which is why the ground rods are put outside the concrete and in the ground... but whatever.

I think the Rebar used in roadways is "Parkerized" - the green stuff on the bars.

Concrete is a good conductor, and, I suspect in the event of a direct lightning strike to the tower, the ground rods away from the concrete base would bleed off some of the energy. You really don't want all the energy propagating into the concrete and heating up the rebar causing an explosion or cracking of the base.

Of the materials involved, concrete is the least conductive.

Copper - pure         392.90   W/m C
Aluminum 2024-T3      190.40   W/m C
Aluminum 6061-T6      155.80   W/m C
Aluminum 7079-T6      121.10   W/m C
Steel AISI C1020      46.73   W/m C
Steel AISI 304         16.27   W/m C
Concrete (sand & gravel)   1.8000   W/m C

You can do everything by the book where lightening protection is concerned and when it hits it'll do as it pleases!