On 9/17/2010 8:52 AM, Jim Thomson wrote:
> 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.
I think it's more the PH than water.
> ## 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.
I've seen and read about that happening with substandard construction
and concrete. I think the rebar must be at least 3" from the edge of
the concrete. I've also seen both photos and in person where the road
salt penetrated the concrete and caused corrosion in the rebar. The
corrosion causes expansion and deterioration of the concrete. The
expansion actually causes chunks of the concrete to "pop off" exposing
the rebar.
> 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.
That sounds like salt, poor construction, or a combination of both and
is not uncommon in both underground and above ground parking lots.
> They X-rayed it all,
> and also took some of it apart. 1st Time it has ever happened.
Maybe there. It's happened other places to parking lots, over passes,
bridges, and is a major problem where ever salt may be present. The PH
of the ground water is also important.
> ## 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.
I've used "dirt bases" in sand, clay, and sand clay mixtures with no
failures. I just removed a commercial 5 footer (with tubular legs) that
had been in the ground well over 20 years with only slight surface rust
where the air and water can get together near the surface. The inside of
the legs appears to be fine with the galvanizing still intact. I
removed one dirt base that was just welded up steel and then painted.
That one had the zinc rich Rustoleum(TM) undercoat and a metallic over
coat (from spray cans). Minor urface rust was evident from the color of
the dirt adhering to the base
OTOH I have taken down towers "with dirt bases" where the base had
almost disintegrated after just a few years. I've also run into tower
bases set in concrete where the tower legs at the top of the concrete
had almost disintegrated "from the inside". The corrosion was in just a
narrow band. I've also run into bases set in concrete that did not have
proper drainage and although they looked good, most of the legs were
gone from the concrete up to the hinged part. I have no ideas as to how
they still held up the tower. This paragraph is a very good reason for
using a pier pin base, or a hinged base using solid legs and not a tower
section set in concrete.
As an added note, we, in general think this kind of corrosion would be
easy to detect. A few years back I disassembled a typical ham tower of
the 25G size. The legs and base looked and felt good. So I took the
thing down, section by section. When I got ready to just tip the bottom
two sections over I discovered "one leg hinge just swinging in the
breeze" The other two along with the guys had been holding it fine and
apparently I had missed or didn't hit the leg hard enough with a hammer
just below the hinge when I checked it. It's things like that that add
a few gray hairs. Actually I could use a few more gray ones, most have
fallen out.
In many areas, here included, you can put up to 60 or 70' of tower of
the 25G type on a dirt base. OTOH in many areas it's not advisable to
even use a base set in concrete, but rather use solid legs/J-bolts set
completely in concrete for a pier pin base.
> ## 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.
the wiring method is not important, as long as it works.
> ## 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.
And rain now days tends to be a bit on the acidic side particularly down
here in the northern part of the lower 48.
> 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.
PH of the surrounding soil can be important. It may be advisable to mix
in a bit of lime (maybe a lot of it) if the surrounding soil is highly
acidic as in swampy.
> 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.
I used high fiber/high strength concrete so the stuff is full of little
"things". I doubt a few more small wires are going to hurt anything.
73
Roger (K8RI)
> 73,
> Dave AB7E
>
>
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