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RE: High Fly-Ash in PT slab

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Joe,
 
Glenn suggests a very good reference on concrete mix design.  There are others in the US Army Corps of Engineers references.  You can get any of them through the Whole Building Design web site. 
 
The problems that you will encounter are:
1.  Early strength - Fly ash tends to serve as a retarder to a degree which will effect early strength gain.  This may delay tensioning operations which will cost money.  This can be offset, by the addition of some condensed silica fume which will give you more LEED points.  And silica fume will really light up the early strength gain.  Be careful in that all ash is not the same.  F ash will behave differently than C ash.  All ashes will not have the same pozolonic characteristics.  Be sure to ask for 3, 7, 28 and 90 day breaks in the submittal.  Have the lab plot the strength vs. time curves.  Ambient temperature will effect the strength gain curve.  You will gain some benefit if you have reactive aggregate. 
 
2.  Workability - The flat work guys will complain.  It is stickier.  They will have to use magnesium trowels and take added precautions.  Curing is actually a bit more forgiving.  It will not bleed the same way to which they are accustomed.  It will behave differently in the methods used to convey the plastic concrete.  Pumping is the least desirable.  A lot of regions are now using mini-conveyors which are great because you don't have to accommodate the pumpability issue. 
 
Polycarboxolate super plasticizers will help to minimize the water and get you the early strength that you need and will reduce the setting time.  But not all polycarboxolate supers are not the same.  Even the same suppliers will have different supers that will behave differently.  Getting a good testing lab that works closely with the concrete suppliers is a big help. 

Regards, Harold Sprague


 

From: ggator1256(--nospam--at)cox.net
To: seaint(--nospam--at)seaint.org
Subject: RE: High Fly-Ash in PT slab
Date: Fri, 7 Aug 2009 16:34:29 -0400

Joe,

 

Federal Publication EM 1110-2-2000 is at http://140.194.76.129/publications/eng-manuals/em1110-2-2000/toc.htm.

It is Concrete for Civil Works and has discussions of fly ash and alkali-silica reactions.  It also helps with strength and permeability (decrease). VDOT loves it for the alkali-silica reaction which causes those fine numerous cracks.

 

A Structural Engineer, P.C.

Glenn C. Otto, P.E.

Virginia Beach, VA 23454

 

 

-----Original Message-----
From: Joe Goldbronn [mailto:jgoldbronn(--nospam--at)barrish.com]
Sent: Friday, August 07, 2009 12:48 PM
To: seaint(--nospam--at)seaint.org
Subject: High Fly-Ash in PT slab

 

I have a client that is trying to maximize their “Build-it-Green” points by using high fly-ash in the concrete mix.  They are requesting between 20%-30% fly-ash.  Per the soils engineer we also need to use a slab-on-grade post-tensioned slab.  I have never tried to use high fly-ash on a post-tensioned slab before and am having a hard time finding anyone or any reference that has done so.  Does anyone have any experience with this or know of any good references?   

 

Joe Goldbronn



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