[TowerTalk] Concrete "Slump"

tombaugh tombaugh at discoverynet.com
Thu May 29 14:54:14 EDT 2003


Most of these comments are good however.
A vibrator in the hands of an overzealous HAM may actually be bad.
(All inuendo aside) Over vibrating concrete causes the heavy materials
(ROCK) to settle and the light material (AIR) to rise.
Air in the concrete is not a bad thing, in fact many times air is
intentionally introduced using admixtures. Rock on the other hand is
important in the strength of the mix. Rock provides the largest surface area
for Portand Cement (the GLUE in concrete) to adhere to. If all the rock is
at the bottom the strength of the top area of the pad will be relatively

The cost of 3500 pound or even 4000 Pound mix is not significant in the
large scheme (guessing $1 / yd). The pour is a relatively small and the
price will be proportionately small. We pour 4000 pound concrete almost
exclusively. YES it is overkill! However, the tradeoff is in workability.
Stronger concrete has more of the expensive stuff.. the glue.. the Portland
Cement and that is what makes the mixure "creamy" and easy to work. Too much
water can "appear" to give these properties but good concrete is good
concrete. Add water and increase slump (good description below). but good
concrete (4000 pound mix) will have a good slump (lower number) because it
has more GLUE.

Good Luck..

Tom Baugh
----- Original Message -----
From: "Dino Darling" <k6rix at arrl.net>
To: <towertalk at contesting.com>
Sent: Thursday, May 29, 2003 1:04 PM
Subject: Re: [TowerTalk] Concrete "Slump"

> Yes on the 3000# concrete.  I would think that the owners manual would
> 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
> 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
> 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
> (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
> 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
> 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
> >to advise on my site for a 5' x 5' x 6' deep base pour. He asked if I
> >"3000 Lb. concrete". And he asked if I wanted a "5 inch slump".  I told
> >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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