I suspect that the use of 5,000 psi versus the more typical 3,500 psi
concrete wouldn't have a huge effect on the reliability of the final
installation for such a large foundation, but it's a risk not to follow
the design specs. The biggest jeopardy might be if your building permit
includes the original specs and the building inspector decides to get
picky enough to ask to see the invoice for the concrete. I'm not sure
there is enough difference in cost to offset the potential hassle with
the inspection ... there is indeed a higher percentage of Portland
cement in the higher mix but I believe that some of the higher strength
comes from simply having less water in the mix (and that doesn't cost
anything extra in terms of materials).
On the other hand, I fully agree with Matt that 5,000 psi is going to be
a stiffer mix and you'd want to make sure it is thoroughly rodded to
make sure it flows everywhere and minimizes voids that would counter the
purpose of using stronger concrete in the first place. Even with an
electrical vibrator it's tough to keep ahead of the pour when you're
dumping close to two truck loads of concrete into the hole.
73 es good luck,
Dave AB7E
On 9/24/2012 9:13 PM, KM5VI wrote:
Hans is right, if the design calls for 5000 psi then it should be adhered
to. Crank-up tower foundations function more as ballast (to prevent
overturning) than as structural columns. Because of the massive size of the
foundations involved, the stresses developed in the foundation generally
aren't that great except in the area of the anchor bolts and to some degree
the shear loading at the upper exposed slab edges. Therefore, I'm not sure
you get much real benefit from the added compressive strength as long as you
develop the strength required for the anchor bolt design. US Towers usually
includes the anchor design numbers with the foundation calculations as well
as the overturning moment - it is sometimes insightful to know the
assumptions used in these calculations.
I don't know your climate but in South Texas we will sometimes (particularly
in the summer) see retardants and/or ice used in the high strength concrete
mixes because the high percentage of Portland creates a lot of heat in that
massive slab that builds up faster than it can dissipate. This sometimes
results in thermal stress cracking during the cure period. You may want to
consult with your batch plant about the logistics of deliver with respect to
placement and cure time. Some installers will also water cool their slabs
for the first few days - but this won't really do much for a tower base
because of the limited surface area. The slump (water content) of the mix
also affects the developed strength of the mix. Less water = more strength
but less workability. Also be aware that batch plants sometimes put some
rather generous safety factors into their mixes to mitigate their risk
exposure to rework back-charges from coming up short on strength (wrecking
out slabs is expensive). We have seen some 3000 psi mixes break at over
5000 psi.
Hope this information is of use and I'm sure you will get a lot of good
input from others on the reflector.
73 & Hope you project goes well!
Matt
KM5VI
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of Rich
Hallman - N7TR
Sent: Monday, September 24, 2012 10:35 PM
To: towertalk@contesting.com
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?
Thanks...
Rich N7TR
ex KI3V, N3AMK, WB3JOV
www.n7tr.com<http://www.n7tr.com>
Telnet: dxc.n7tr.com N7TR DXCluster
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