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FW: Low Concrete Quality

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First, you should not use silica fume unless you have a specific reason for
doing so.  Silica Fume is typically used either to increase durability for
a severe sulfate or chloride issue or it is used to produce very high
strength concrete.  For typical durability issues, silica fume is used at a
5 to 10 percent cement replacement.  Note that this is a cement replacement
to achieve your original design concrete strength.  The industry standard
in the U.S. is normally 7.5 percent.  For very high strength concrete,
silica fume is added in addition to the cement.  For these applications the
silica fume content can be 15 to 20 percent.  Note however that this a
very, very special application and extensive testing in the mix design and
placing is required.  If my memory is correct, I believe they have poured
several high rise buildings in the Chicago area with 14,000 psi concrete
and are experimenting with 20,000 psi.

As for your immediate problem, you are on the right track.  My gut feeling
is that the silica fume is not the problem.  Either too much water was
added along the way or you were shorted some cement.  Hopefully your
petrographic test will indicate cement, water, and silica fume content.

By the way, we are using a 10 percent silica fume mix and the cost adder
from the batch plant is about 20 percent.

Thomas Hunt, S.E.
Duke/Fluor Daniel

----- Forwarded by Tom Hunt/DFD on 06/19/01 12:17 PM -----
                    Zapata               To:     seaint(--nospam--at)                                                 
                    <Lzapata@otep        cc:                                                                       
          >            Subject:     FW: Low Concrete Quality                                     
                    11:42 AM                                                                                       
                    respond to                                                                                     

Thank you for your suggestions, and I take advantage to make another

> How can we be sure about what percentage of Silica Fume should be used in
> order to achieve all these wonderful benefits? ACI mentions ranges up to
> 20 % but in this case contractor used between 8-10 %. Is it good enough?
> In addition to this, we met with the contractor and they accepted to give
> a warranty to the client for the life of the building (25 years) but we
> request some tests to support this document. A load test and a
> "petrography" were suggested. In this last test 2 core samples will be
> taken, one from a "bad" pedestal and one from a "good" location. These
> samples will be analyzed by an independent laboratory to identify and
> compare mix compositions (including Silica Fume %) and other properties
> (permeability, porosity, water absorption and Abrasion-Erosion
> resistance).
Thks again.

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