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Re: Fly Ash

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>Can anyone tell me the pluses and minuses of fly ash?  I have text books
>telling me one thing and technical bulletins telling me the opposite.
>The text books are telling me that fly ash increases concrete shrinkage
>cracks while the technical bulletin is telling me that fly ash reduces
>concrete shrinkage.  Granted, the technical bulletin is from Pozzolanic
>(which is fly ash).  I am trying to use a concrete mix design that will
>limit the amount of shrinkage cracks in the exposed concrete.
>The Concrete Contractor has proposed the use of fly ash and has sent
>literature to back up his claims (25% fly ash compared to the cement
>content).  I do know that by allowing fly ash the cost for the concrete
>will be reduced.  My concern is that the literature may not be telling the
>whole story and he is just trying to save himself some $$.
>I would appreciate anyone's help in this matter.
>Thank you.
>Mike Brown, P.E.

My experience with fly ash mixes has been very good.  The fly ash we tend
to get has a little cementious as well as pozzolanic properties.  We tend
to specify it for mass concrete (thicker than 3 feet).  The advantage is a
slower heat of hydration and a longer period of significant strength gain.
Some design mixes have gained significant strength after 56 days.

I have used cement/fly ash grouts with very high slumps (greater than 11
inches), that had relatively low amounts of shrinkage.  One design mix lost
about a 1/4 inch in 38 inches.  The lower the water/cementious material
ratio the less the shrinkage.

I have also used mix designs which have had as much as 2 lbs of fly ash for
each lb of cement.  Shrinkage and shrinkage cracks tend to increase with
higher water cement ratio's.  Fly ash mixes require more water per cubic
yard for flowability unless water reducers are added with the fly ash.
Therefore, a mix design for fly ash should be tested for strength,
shrinkage, etc.

I have had problems with admixtures not working properly if the LOI (Loss
on ignition) of the fly ash is above 3% or if the LOI varies from the fly
ash used for mix design testing.

If the supplier has good test data or a history for a mix design which uses
fly ash I would have no problem with it.

Scott A. Jensen P.E.                          email:  saj5(--nospam--at)
Lockheed Martin Idaho Technologies           208-526-0544
P.O. Box 1625                                  fax:	208-526-2681
Idaho Falls, ID 83415-3650