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Re: Admixtures for Concrete Clarifier Tank - further thoughts

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----- Original Message ----- From: "Harold Sprague" <spraguehope(--nospam--at)>
To: <seaint(--nospam--at)>
Sent: Thursday, April 27, 2006 4:19 PM
Subject: Re: Admixtures for Concrete Clarifier Tank - further thoughts


I find it incredible that a concrete plant would opt to increase both cement and water to avoid super P. That is like shooting ones self in the foot and at 6" spacing up the ankle, leg, wallet, etc.

This is what I thought as well. I am glad that I am not alone on this. Maybe this is why he doesn't work at that concrete plant anymore. He told me that it was cheaper for them to increase both than add the chemicals. I had never thought about it until he made this statement and I figured the w/c alone would address my concerns and they would naturally add the super-p.

I would think that in lieu of using a slump limit (which is a very poor indicator of w/c ratio and workability) I would opt to put in a limit on the water. This would by default limit the w/c ratio. You are still stuck with a slump for the target indicator of workability.

For a 35MPa mix used for sewage/water treatment (or even a flat slab or slab on grade to minimize shrinkage) how would you spec the mix to minimize shrinkage assuming there are no shrinkage compensating cements available. I have used the larger aggregates for floor slabs and that seemed to work well. With the more heavily reinforced items I haven't been straying from the 3/4" aggregate.

If I or one of my staff have authority to approve the mix design submittal, I don't have a problem in using F fly ash. It can provide some benefits regarding density, lower heat of hydration, reactivity, reduced propensity for cracking, pumpability, etc. But I limit the maximum amount of fly ash to 15% of the total cementitious materials and use a substitution rate of 1.5 pounds of fly ash for 1 pound of cement. I know that that is old school, but I am a bit of a chicken.

So has this produced a concrete strength consistently above your min specs? (and conversely not too high above your specs). When you say this is old old school is this? Do you know what the current recommendations are regarding this?

Fly ash also gives me a LEED tree hugger atta-boy, but that is not high on my list of motivators.

My motivation is always that I don't want any problems..on this particular job that soured us the slumps were up and down and the mobile concrete technicians (truck drivers) were adding water to the mix on site after being repeatedly told that it was heading back to the plant if it didn't get to site properly batched.

Harold Sprague

Dave Handy

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