[TowerTalk] FW: 5000 psi vs 3500 psi concrete?

[David Gilbert]

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 at contesting.com] On Behalf Of Rich
> Hallman - N7TR
> Sent: Monday, September 24, 2012 10:35 PM
> To: towertalk at 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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