[TowerTalk] Concrete base spec info requested

[Kevin Normoyle]

Are you in charge of the pour yourself?

I'm not a concrete guy, but I think a conversation with your readymix 
supplier would help.
I think you've got some other issues to ponder.

Looking at the weather for Prescott, AZ, it says rain/snow starting on 
Tues, with lows below freezing (down to 24 on Thurs)
Highs wed through thurs of 43 to 49.  Depends on your point of view, but 
some might say that adds up to a cold-weather pour.

You may want more portland cement just to get an early strength to not 
worry about the concrete freezing before it's cured..(like a 7 sack 
mix?). You don't want to pour on frozen ground. With the chance of 
freezing, you definitely don't want too wet a mix. But a 5-6" slump mix 
is a lot easier to deal with in moving it around the rebar.

If you're using a concrete pumper, then that affects your order. would 
mean: smaller rock (pea gravel), more portland (like at least a 6 sack mix).

Smaller rock is less strong but easier to move around the rebar and 
stuff. Your concrete guy might have a mix that's half 3/4" rock and half 
3/8'" or something like that, which is nice even if you're dumping it 
off the tailgate into your hole.

In short, I'd call up your concrete guy and ask him what's typically 
done for the area, given the weather prediction. If there wasn't the 
weather issue, I'd just order what I just ordered: a 6 sack mix, with a 
mix of rock from 1/2" to 3/4", with 5"-6" slump at the truck.. You might 
ask if they do hot water mixes at this time of year in your area.

Seems a little late to be wondering about the concrete: have you thought 
through all the details of the pour (if you're doing it?) Things happen 
fast and you have to have all your tools and plans for how things should 
happen, all sorted out beforehand.

You might want to insulate the top of the pour (after covering with 
plastic).

Sounds like there's no permit to worry about. You usually don't want to 
show high strength concrete on the plans if you can avoid it, since it 
may trigger the need for a concrete test which is a waste of money. 
(code says anything over 2500 psi should be tested).

A six bag mix will get you 3000-4000psi. a 7 bag mix 3500-5000psi. 
Basically, the compressive strength is not really what you should be 
worrying about.

-kevin
AD6Z


Ray wrote:
> I am pouring two tower bases on Tuesday. I am looking for some general info
> on the concreted needed for the bases. The towers are older, US Tower -
> MA770 and a Tristao - CZ 471. Both engineering details (1999 and 1977) state
> the concrete Fc is 2000 lbs. I think this means the compressive strength of
> the concrete. I thought the typical strength is 2500 lbs, and if I remember
> correctly, on another 70 ft tower I poured 30 years ago, I used 3500 lbs
> strength.
>  
> Can anyone tell me if I should be using 2000 lbs concrete? And are there any
> other additives that should or could be used? 
>  
> Ray, 
> N6VR
>  
> (Near Prescott, AZ)
> _______________________________________________
>