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Re: Admixtures for Concrete Clarifier Tank - further thoughts

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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.

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.

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.

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

Harold Sprague

From: "Dave Handy" <dhandy(--nospam--at)>
Reply-To: <seaint(--nospam--at)>
To: <seaint(--nospam--at)>
Subject: Re: Admixtures for Concrete Clarifier Tank - further thoughts
Date: Thu, 27 Apr 2006 11:09:24 -0400

In addition to Harold's comments about using the plasticizers/water reducers, we have noted that the mix shall be designed for a slump of 1.5" and then super-p added to increase the slump to 4"+/-1". I used to think that the w/c ratio would cover it to reduce the water added and then found they would increase both the water and the cement content to increase the slump instead of using super-p. The w/c ratio would still be ok but you would then have more water and cement in the mix that would lead to more shrinkage. Any thoughts on this procedure?

We have stayed away from fly ash because we had a bad experience on a big job with quality control. The strength and slumps were all over the map. This was probably an isolated case but it has soured us on using it.

Dave Handy, P.Eng.

----- Original Message -----
  From: Joe Grill
  To: seaint
  Sent: Tuesday, April 25, 2006 7:06 PM
  Subject: Admixtures for Concrete Clarifier Tank

I'm trying to finish the design and detailing for a circular concrete clarifier tank for wastewater treatment.

I guess I would like to get a little verification for a couple of items. The tank is in an area where there is no frost of freezing concerns, therefore I am considering the following:

  No calcium chloride admixtures

  Maximum water/cement ratio .45 (ACI 350 table 4.2.2)

  Minimum f'c= 4000psi (ACI 350 table 4.2.2)

Type II cement ( will ask that "owner to provide soils evaluation of sulfate exposure")

  ¾" maximum aggregate (ACI 350 table 4.2.1 even though no frost problems)

  5% air content (ACI 350 table 4.2.1 even though no frost problems)

  4" maximum slump

  Tank is not covered, therefore no protective coatings

Are there any advantages/disadvantages to the use of fly ash? I was not going to spec. it, but if it is advantageous, what kind and how much? Hopefully, that isn't asking for too much information. I need to do a little more reading about fly ash. ACI 350 seems to suggest the use (and provides tables for % of weight of cementitious materials) of fly ash where there is exposure to deicing chemicals, but there shouldn't be any of that in use here (Southwest desert).

  I appreciate any help or other suggestions that may be given here.

  Joe Grill

  Joseph R. Grill, P.E. (Structural)

  Shephard - Wesnitzer, Inc.

  Civil Engineering and Surveying

  P.O. Box 3924

  Sedona, AZ  86340

  PHONE (928) 282-1061

  FAX (928) 282-2058


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