Date: Tue, 25 Sep 2012 03:34:34 +0000
From: Rich Hallman - N7TR <firstname.lastname@example.org>
To: "email@example.com" <firstname.lastname@example.org>
Subject: [TowerTalk] 5000 psi vs 3500 psi concrete?
Im about to pour 16 yards of concrete for an HDX-689. Are there any real
benefits to using 5000 psi vs 3500 psi concrete?
ex KI3V, N3AMK, WB3JOV
Telnet: dxc.n7tr.com N7TR DXCluster
## sure, 3500 psi in a lot of cases will no longer meet local building codes
for some applications.
Here in BC, the standard used to be 20 mpa ( 2900 psi). It’s now 30 mpa
Stuff like basement walls, floors etc, when built with stronger concrete, wont
absorb moisture near
as much. Way less cracks too. Stronger concrete doesn’t cost much more..and a
tower base is a one shot deal.
I used 17 cubic yards of 35 mpa ( 5075 psi) for my own HDX-689 base. We
emptied TWO cement trucks
into the pumper truck’s hopper. This was a line pump, with a 4” hose run along
the ground, to the 9’ deep hole.
Then just pump it in. I rented a vibrator with a 15’ cable on it. The
vibrator worked slick. Get the one that
is aprx 12” long x 1.5” to 2.0” diam. They are dirt cheap to rent. When using
the 4” diam pump hose, a special
mix Is used, with aggregate no bigger than .375” . Drop the vibrator in
there..and the mix just flattens out asap, instantly.
There is a technique to using the vibrator though.... like raking it across
E-W and also N-S. You can also put it
right up against the rebar cage in some places.... and vibrate the cage in
that area. Don’t over vibrate though. Just hit
it for a sec to flatten it. If u over vibrate, the aggregate will start to
sink. So u gotta be quick , and have one person
running the vibrator hose... while a 2nd person handles the on-off switch for
it. The fellow who runs the pumper truck will
be the one running the 4” hose..right at the tower base, directing and moving
the 4” hose about.
He used a remote on-off switch on the end of a 75’ cable in my case.
Meanwhile, the cement truck guy is stuffing concrete
into the pumper trucks hopper at a steady rate. The hopper only holds so much
concrete at a time, so it can only be so full.
And if the pumper guy, who is 60’ away at the base of the tower shuts off the
hose, so he can move it, the cement truck
guy has to be on top of this..and not overfill the hopper.
In my case, the whole thing ran smooth. When all done, and trowelled, etc, we
covered it all up with some old bed sheets, then
used a real fine spay from the garden hose every 35 mins to mist it real good,
always keep it wet. The fresh concrete will just suck up
water like crazy for hrs on end, so you gotta be constantly be spraying water
on the sheets for hrs. I spayed it for
several days after that. It was a hot day that day, which is part of the
issue. I had my brother and a friend help me.
Then 2 x cement trucks and one pump, so 6 of us in total.
I have ZERO cracks on my base, none. Not even hairline cracks. That’s 2
years now. Here in BC,they are now on about
seismic rated everything. Another reason the stronger concrete has to be
used. That, and more and stronger rebar. 60 ksi
weldable rebar is the standard. In eastern canada, they sell 5000 and 6000 psi
cement.... pick one.
Considering the huge $$$ you will have invested in the HDX-689 Plus ants,
cement, cranes, etc, the difference in price for
the stronger cement is really miniscule. The cost of the pumper truck is the
same regardless of cement used.
Don’t be in any rush to torquing up leveling nuts on the anchor rods either.
Although the concrete may appear hard on the
exposed portions above ground, its gonna be goopy down below. You don’t want
any anchor rods rotating on their axis.
Use loads of never seize, marine grade, made by loctite, on all the anchor rod
theads, and also threads of the 24 x nuts.
Then u will have an easy time of it when it comes to fine tuning the leveling.
I double nutted on top of the big plate..with
way less TQ on the 2nd pair of 12 nuts. (36 nuts in total, 12 below the
plate, 24 on top).
Later........ Jim VE7RF
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