[TowerTalk] Concrete "Slump"
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
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.
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?
Dino...k6rix at arrl.net
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