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. 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
On 9/16/2010 12:27 PM, Dick Dievendorff wrote:
> I have never personally installed rebar.
>
>
>
> My tower and guy anchor foundation drawings specify a "mat" of #5 (5/8")
> rebar , 5 rods each way, top& bottom, just below the top surface and just
> above the bottom surface of the concrete block.
>
>
>
> I know to suspend the bottom mat at least 3" above the bottom of the hole
> with little concrete blocks so that the bottom rebar mat ends up 3" above
> the bottom of the concrete so that it is adequately covered.
>
>
>
> I also need to suspend a top 5 x 5 mat about 1-1/2" to 2" below the top
> surface of the concrete block. If it were going to be later protected from
> the elements, I'd be able to suspend it with wires from the wooden form and
> cut the wires flush with the foundation after the concrete sets. But since
> this tower foundation is going to be exposed to the elements, I'm worried
> that even if I cut the wire off flush, the wires themselves will become a
> conduit for corrosion to get to the embedded rebar.
>
>
>
> What techniques are used to support the top "mat" so that it ends up about
> 1-1/2" below the top finish surface?
>
>
>
> I found "high support chairs" in some literature, but the top mat is about
> 28 inches from the bottom of the hole, and the "chairs" don't seem to be
> that high.
>
>
>
> Maybe I can find some vertical plastic rods to tie the top mat to the bottom
> mat and the plastic rods could stick out the bottom.
>
>
>
> I'm sure this is a well-understood fairly basic technique by those who do
> this. But I need to be educated.
>
>
>
> 73 de Dick, K6KR
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