Mike, Be careful. Don't let anyone add excessive water to make the mix
easier to work with or self leveling or whatever. That lowers the strength
irrespective of what the strength would have been (how many bag mix.)
You can have the delivery guy add a water reducer plasticizer shortly before
pouring (but do allow time for it to thoroughly mix in) as the additive does
not have a long pot life. This additive makes the mix act as if you had
added water. It makes the mix easier to work, easier to get the voids filled
for good reinforcing steel to concrete contact. This additive does not
reduce the strength of the mix. Allows you to use the optimum water for
strength while being easy to flow easy to work. The additive is not
expensive.
Good luck to you, I get a little pain in my back just thinking about the job
ahead of you.
Patrick AF5CK
-----Original Message-----
From: Mike Reublin NF4L
Sent: Sunday, December 01, 2013 4:01 AM
To: Grant Saviers
Cc: towertalk reflector
Subject: Re: [TowerTalk] More crank-up questions
Thanks Grant.
I'm buying the base/cage from the mfg. I think it's all fastened together.
Hy-Gain calls for 2000 psi/28 days and a max of 7 1/2 gls. of water per
sack. I'll talk to the concrete guy about a stronger mix.
I'll be using a raising fixture and tilt plate for the antenna, so no
climbing required.
Mike NF4L
On Nov 30, 2013, at 8:10 PM, Grant Saviers <grants2@pacbell.net> wrote:
Contrary to other advice, do ground the tower anchor bolts to the rebar.
Then you have a great Ufer ground, considering the area of the concrete in
contact with the earth. The rebar should be tied per code, with
sufficient overlaps and inside the concrete envelope per code. Depending
on your site and storm patterns, additional ground rods may be
appropriate.
For my two HDX589's we mounted the anchor bolts tightly to the base plate
and tack welded rebar between the six bolts to make a solid sub frame so
that the bolts wouldn't move when the concrete was placed and vibrated.
That way the concrete can be placed and finished without the interference
from the base plate. This sub frame was wire tied to the main rebar cage.
After the concrete hardened the base frame was installed and leveled. You
can order stronger concrete (4000psi or higher) than the UST spec (2500)
for a very slight up-charge. The limiting factor in concrete for towers
is tensile strength, not compression, considering the tensile/compressive
strength ratio. A free standing tower has opposite forces in the legs, 1
or 2 in tension and the others in compression when the wind blows hard.
Proper water content and curing is important. You can get a slump test
and post cure strength report from an independent testing outfit. Code
required this for my towers and I think it cost about $250 per tower, as
they were poured on different days.
The 589 is positive pull down, but it doesn't matter vs the HD70 since for
either design the tower weight is always on a cable, unless down and
blocked for climbing. Better to avoid that anyway and use a ladder or
rent a boom lift.
The NF7P coax standoffs work well for me - the loop types not the "holds
coax off the ground" type.
Grant KZ1W
snip..
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