[TowerTalk] Concrete "Slump"

Dino Darling k6rix at arrl.net
Thu May 29 12:04:56 EDT 2003

Yes on the 3000# concrete.  I would think that the owners manual would have 
a recommendation.


The slump number is the amount of "drop" the concrete makes out of a 12" 
mold.  They use a 12" cone (mold) and fill it from the bottom.  Then they 
invert the cone (pointy end up) and lift this cone off the concrete (ever 
make sand castles?).  The "cone of concrete" will drop from the original 
12" to...say...7" (measured from the ground up).  12 - 7 = 5  Now you have 
a 5" slump!  How is that for the technical explanation?  :-)  Go with the 
5" slump.  Too much water is not good!  It won't "hurt" the concrete, but 
man will it take forever to cure (concrete cures and does not dry!).


Yes, a vibrator will move concrete into the hole, but that's not why you 
use it...kinda.  By using a vibrator, you perculate the air bubbles (VOIDS) 
out of the concrete!  These trapped air bubbles causes voids and will 
result in a weaker base/foundation/wall/etc.  You usually cut joints in 
concrete to give it a place to crack!  Concrete shrinks as it CURES and it 
WILL crack!  These voids are perfect "weak spots" for the concrete to crack.

Shrinking concrete!

The slower the concrete cures, the stronger the concrete will be and the 
less cracks you will have.  The top is exposed to air and the water can dry 
(evaporate) too fast.  Its a good idea to keep it wet with the hose (not 
too much water).  As it gets harder, you can even place wet towels over the 
exposed concrete.  Also, if you have real dry ground, it will SUCK the 
water right out of your base!  Just before the concrete goes in, spray the 
inside of the hole and let the ground suck this water up!  Again, not too 
much water!  Just get the dirt wet.  Any standing water in the bottom of 
the hole will be displaced, but you shouldn't have any standing water if 
you didn't use too much in the first place.

Ground Rod at the bottom of the hole?  (This is MY question)

I've often wondered if this is a good idea.  I'm told that NOTHING can 
escape the concrete block!  No rebar, etc.  My thinking is that if I drove 
a 3/4", 10' ground rod in the bottom of the hole and tie it to the rebar 
cage, I'd have a real good ground (or one in each corner tie to the 
cage!).  I'm told that this would allow ground water to seep in and could 
ruin the rebar.  I'm not convinced and could use an education from the 
group.  So, be sure to support the cage so concrete gets all the way around 
and use a VIBRATOR!  They are cheap to rent and easy to clean!

At 12:08 PM 5/29/03 -0400, you wrote:
>Following advice received here (thanks!), I had the concrete guy here today
>to advise on my site for a 5' x 5' x 6' deep base pour. He asked if I wanted
>"3000 Lb. concrete". And he asked if I wanted a "5 inch slump".  I told him
>I'd get back to him.  What do these terms mean? What should I want?
>Jerry K3BZ

Dino...k6rix at arrl.net 

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