Date: Thu, 16 Sep 2010 12:56:56 -0700
From: David Gilbert <xdavid@cis-broadband.com>
Subject: Re: [TowerTalk] Newbee concrete rebar questions
Personally, I think it is highly unlikely that significant corrosion
would travel down the path of a thin wire. It certainly isn't a
moisture issue, since moisture readily penetrates concrete anyway.
## I wouldn't bet on it any time soon. On this last cage I had built, the
re-bar guys installed short 3' long bar's , 2 per corner..right at the top tie,
to stop it from wobbling abt. The ends of the bars were touching the clay
sidewalls.
The PE went nut's when he saw that. I explained to him that the 8 x temp bars
were being removed, when the concrete was 95% to the top.
## We have a condo across town here, that has huge amounts of it's re-bar
literally disintegrated..starting from the very bottom. Dunno if they forgot
to install
dobe blocks under them or not. Some how, vast amounts of water have managed
to get beneath the foundation, and started eating the re-bar. It's now a maze
of hollow tunnels. Those giant concrete support columns and side walls in the
underground parking lot are now seriously compromised. They X-rayed it all,
and also took some of it apart. 1st Time it has ever happened.
## Ever pounded/jack hammered galvanized grnd rods into clay/soil ?
They will disintegrate in 10 years. A buddy [ who was in a rental house]
installed
the bottom section of a 50' galvanized guyed tower, directly into a 2' deep
hole, with no concrete
at all.. then used a house bracket, plus guy wires. When he removed the
bottom section from
the grnd, 11 years later, 65% of it had vanished ! Whole sections of
bracing were gone, plus
the tower legs had huge chunks missing.
## rebar is cheap. Just use some vertical rebar, but use dobe blocks under
them. They are just
small concrete blocks with a slot running down the top side. The rebar sits in
the slot, and won't budge.
There are abt 6 x different ways to tie rebar with the wire. [ on line] Take a
piece of wire, 24" long, and fold it in
1/2.... like a hairpin. That's what's used to tie the bars. The rebar
place also has the blocks..plus loads of wire.
## concrete has loads of compressive strength.. but hardly any tensile [
stretching] strength... hence
the use of vertical rebars. The horizontal grid of bars stops the top from
blowing out...and re-enforces
the legs embedded in the concrete..since they are a distance from the re-bar
ties. [ Horz hoops]
## forget the prime directive. Who ever designed the cage, using 2 x
horizontal grids of bars
in parallel... and no vertical bars was asleep at the switch. It also appears
they did not use
horizontal 'ties' either. The ties are just square hoops, made from one
continuous piece of re-bar,
with an overlap in the corner. Their is usually ties every 6-12", starting at
the top. The ties are
supported by the vertical rebars. The bottom/top grid of bars goes in dead
last. Vertical bars
go inside the ties. It's like they gave you step 3..and missed steps 1+2.
## All the re-bar at the very top should be down at least 3"..and pref 4-5".
The top surface is what's exposed
to the rain. If it's only embedded 1- 1.5" it won't last. You want a min
of 3" on all sides and 2-3" on the bottom.
later... Jim VE7RF
It's
the PH of the concrete that inhibits corrosion of the embedded rebar
even though it ALWAYS gets "wet" unless you live in Antarctica.
Corrosion only travels along a wire or rod if it is large enough that
the PH of the concrete is physically buffered from the iron core. I
don't think that is likely with a thin wire.
If you're worried about it, though, use something like copper wire or
stainless steel wire. There may be some dissimilar metal corrosion
where the wire meets the rebar, but as soon as the junction rots away
the corrosion would stop as well.
Or just use something strong and non-metallic like nylon twine. As I
said, there's no point in worrying about a path for moisture because of
the porosity of the concrete, and you're going to get microcracks in the
concrete anyway.
73,
Dave AB7E
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