[TowerTalk] Using old concrete in new pour

[Al Kozakiewicz]

For every complaint I hear about how the stereotype of hams being cheap is unfair, there are 30 stories like this.

I've thrown chunks of old concrete and big stones into steps and slabs that are not structural in nature - they only need to support themselves and the occasional human walking across them.  Probably OK for a guyed tower where the base need only have some mass and a modest amount of compressive strength.

A tower base for a self-supporting tower is an engineered structure.  If you deviate from the plans, then the engineering is no longer valid and the specification of the resulting structure may be insufficient for the intended purpose.  Cement adhesion (is there a better term?) to the aggregate and other materials like old concrete tossed into the mix is not the same as the bond created in the cement itself on a particle level as concrete cures.  In that sense, "concrete is not concrete".

More worrying is the "soupy" adjective.  For a given concrete mix there is but one optimum amount of water to add.  Too much or too little and the concrete will be weaker than specifications.  You don't alter the flow characteristics of concrete by varying the amount of water added but with additives specifically designed for the purpose that do not affect the ultimate strength.  It strikes me that someone willing to save a bucks on concrete by reusing scrap knows nothing about plasticizers and would be giving doubly bad advice by suggesting you just add more water to the mix.

I would definitely ignore your friends suggestion.

Al
AB2ZY

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From: TowerTalk <towertalk-bounces at contesting.com> on behalf of David Gallatin via TowerTalk <towertalk at contesting.com>
Sent: Tuesday, October 21, 2014 7:53 PM
To: towertalk at contesting.com
Subject: [TowerTalk] Using old concrete in new pour

Hello everyone,
A ham friend today suggested using concrete from an old pad that's broken into small (maybe fist size or a bit smaller) pieces and using it in
a new pour, specifically for my new 30 foot aluminum tower. He said you make the new mix "a little soupy" and toss the old chunks in as you go, the idea being less new concrete is used (and paid for) and when it dries you can't tell a difference "concrete being concrete". He did not specify what kind of ratio of old to new is used but he does have two pads of his own he has done this with that hold 60' steel self supporting towers.
I have tried to research this and come up with nothing. I did find reference to concrete being recycled (crushed) into aggregate size and used as such in new pours but what he is talking about does not seem to be the same thing.

 Obviously I am a tad concerned about doing this. Has anyone heard of this technique before?

73,
David, AA9G

ex W5DCG and KC9EEV
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