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Re: Low Concrete Quality

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The other thing about silica fume is that it is very fine (particles about
1/10th the size of cement grains IIRC), but it tends to clump together.  The
primary benefit in using it over fly ash is that when properly dispersed, it
will fill in voids between the cement grains, leading to a much denser matrix.
But if its not properly mixed, it clumps together and does very little for the
mix.  I would guess that if it was actually used, that is what happened.   And
it is VERY expensive.  As I remember, it can account for about half the cost of
a high strength mix at 5%-10% replacement.

Chris Willcox, P.E.

syed faiz ahmad wrote:

> Gentlemen
>
> Silica Fume is a very fine grained pozzolanic material. like fly ash its
> also an industrial bye-product. You are right. It is principally used to
> ensure a durable concrete & now a days its almost an essential ingredient
> for a high strength concrete, mostly because it also acts as a water
> reducer. So you are able to obtain a fairly workable concrete with a low
> water content. And, as you know strength is inversely proportional to w/c
> ratio; lower the w/c  ratio value, higher the strength. It is also used for
> all other reasons for which a pozzolanic material like fly ash is used. But
> mind you, its a very expensive material. And, its use to obtain a meagure
> 2000 or 2500 psi concrete (which can ordinarily be achieved) will not be an
> economically viable proposition. Regards,
>
> SYED FAIZ AHMAD; MENGG, Mem ASCE
> SENIOR STRUCTURAL ENGINEER
> SAUDI OGER LTD
> RIYADH, SAUDI ARABIA.
>
> >From: Michael Lo <Mlo(--nospam--at)EYPae.com>
> >Reply-To: seaint(--nospam--at)seaint.org
> >To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
> >Subject: RE: Low Concrete Quality
> >Date: Mon, 18 Jun 2001 10:54:28 -0400
> >
> >Just a question about the use of silica fume. Isn't silica fume an
> >admixture
> >that increases durability and provides significant strength gains in
> >concrete?  I thought it was one of the key ingredients in high strength
> >concrete and would be a bit skeptical of the contractor's claim of use of
> >this admixture since you're getting concrete in the range of 2000-2500 psi.
> >
> >
> >
> >
> >
> >Michael Lo, P.E.
> >Associate
> >Structural Department
> >Einhorn Yaffee Prescott - NYC
> >440 Park Avenue South
> >New York, NY 10016
> >Tel:(917) 981-6024
> >Fax:(917) 981-6100
> >
> >
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