1. Concrete is porous as all get out and water moisture will get to
your rebar no matter how integral it is. It's the alkalinity in
concrete that keeps the rebar from rusting. Typically a piece of
concrete like a portion of a CMU block is used to support the rebar off
the bottom of the hole so that the coefficients of expansion are similar.
2. Yes, suspending the rebar with steel wire works. Just make sure
it's strong enough to withstand the force of the pour, and string the
wires in a trapezoidal fashion so that the rebar can't swing side to side.
3. Yes .. bond the rebar to the tower base and its ground system. In
most jurisdictions that is required by code.
4. Best to stay with the Rohn recommendations, but I'd go with six
inches clearance from ground just to make sure there was never any
standing water around my tower base.
5. Makes sense, but I don't see any downside to it so why not do it on
general principle?
73,
Dave AB7E
On 6/11/2012 9:31 AM, Missouri Guy wrote:
> Hi Gents,
>
> I'm planning to erect a new 68' self-supporting BX tower and I'm
> going over Rhon's specifications for the concrete foundation.
> (I have NO desire to deviate from those specs.)
>
> A few questions:
>
> 1. There's a grid of 3/8" rebar about 3" up from the bottom
> and is about 4ft square and flat. What's the best
> way to support that off the bottom of the hole while
> the concrete is being poured around it? And will
> the supports allow water to enter and corrode the rebar?
>
> 2. There's also a grid of 3/8" rebar about 3" below the
> TOP surface of the foundation. The rebar is placed at 12" centers.
> I'm *thinking* that the grid can be suspended from the wooden
> jig I'll build to hold the anchor bolts firmly in place while the
> concrete is being poured. From your experience, would
> that be the best/easiest way?
>
> 3. I've read/heard somewhere that rebar in concrete
> tower foundations should be connected/bonded to the tower
> ground system. Seems logical, but is that true?
>
> 4. The Rohn spec sheet shows the TOP of the fountation
> 6" above grade. Why the 6"?...to allow for "growth" of the
> soil around it? Why not, say 3" and dig the foundation
> hole a little deeper?
>
> 5. The 1-inch anchor bolts have a nut and washer above
> and below the U-brackets that support the tower. Is it
> customary to place some anti-sieze compound on the
> threads to easily allow minor adjustments due to any settling
> of the foundation...say in 5-10 years or so?
>
> 73 and thanks for your input,
>
> Charlie, N0TT
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