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Re: Admixtures for Concrete Clarifier Tank

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Looks pretty complete.  I would push the aggregate size up as much as practical to help minimize cracking.  If possible use 1 1/2 or at least 1 inch nominal maximum size.  Fly ash is not a bad idea and a lot depends on availability in your area.  Fly ash is normally used in the 20 to 25 percent cement replacement range.  You may want to stay with Type II cement since it has a lower heat of hydration rate which will also help minimize cracking.  Your w/c ratio of 0.45 is probably acceptable however we normally use 0.40.  Not shown but you will also want to tighten up on your rebar spacing, i.e. use smaller bars with closer spacing for good crack control.

Thomas Hunt, S.E.

"Joe Grill" <jgrill(--nospam--at)>
04/25/2006 04:06 PM
Please respond to seaint
"seaint" <seaint(--nospam--at)>
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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