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Re: Admixtures for Concrete Clarifier Tank - further thoughts

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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