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Re: Increase of f'c over time

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What? The cost of obtaining ae least one core for testing is minimal, and at
the same time you could sample the soil immediately below the slab. The time
required (when you have the equipment) is less than it has taken to discuss
this by email, perhaps I am missing something.
Chris Lillback PE
PS
----- Original Message -----
From: "Barclay, George" <GBarclay(--nospam--at)lgt.lg.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Friday, December 21, 2001 5:48 AM
Subject: RE: Increase of f'c over time


> Yes, you are right about not relying on "reasonable assumptions of
> strength".  I am not doing that.  What I did not explain in my original
> email was that there is another aspect of the client's job in which a
> portion of the concrete structure is being demolished.  During this
> demolition, the client will take cores and test them.  This is the
procedure
> that I have outlined the assumptions portion of my report to him.
However,
> this will not be accomplished before he has to somewhat "commit" to his
> superiors that the placement of the new equipment onto the existing
concrete
> slab can take place.  I have told him that I will not "commit" to the
> concrete strength being what I have used in the calculations without
backup
> test data.  Granted, this is a dicey situation for him, so he would like
to
> have some kind of backup study showing this increase of strength over a
long
> period of time does occur.
>
> F.Y.I.  Something else that I did not explain was that the "member" that I
> am investigating is an unreinforced, 10" thick slab on grade, so I have
also
> had to make assumptions on the modulus of subgrade reaction.  Therefore, a
> "failure" of the slab will not be a catestrophic failure.  However, the
new
> equipment the client is placing is VERY deflection sensitive.
>
> Thank you for your reply Bill.
>
> > -----Original Message-----
> > From: Bill Polhemus [SMTP:bill(--nospam--at)polhemus.cc]
> > Sent: Thursday, December 20, 2001 9:02 PM
> > To: seaint(--nospam--at)seaint.org
> > Subject: RE: Increase of f'c over time
> >
> > Never, ever, ever rely on "reasonable assumptions of strength" when
> > evaluating an existing structure. If you check the model building codes,
> > for
> > example, you will find that existing structures MUST be evaluated using
> > empirical measurements. That's why, whenever I'm doing a
> > renovation/retrofit
> > job, and the contractor/owner tells me "oh, and I have the drawings
right
> > here," I tell him "that's nice, they'll serve as a good reference. But
> > they
> > will NOT be relied upon as the final word. I will take measurements
> > including if need be assessments of the actual strength of the in situ
> > materials.
> >
> > What you did at the outset was fine, for a first approximation. But
you're
> > going to need material testing to back up your assumptions if you want
the
> > existing structure to sustain an appreciable increase in service
loading.
> >
> > This is where this game gets dicey, because contractors/owners don't
want
> > to
> > hear that; they think we ought to know these things, I suppose by
osmosis.
> >
> >
> >
> > William L. Polhemus, Jr. P.E.
> > Polhemus Engineering Company
> > Katy, TX, USA
> > Phone (281) 492-2251
> > FAX (281) 492-8203
> > email bill(--nospam--at)polhemus.cc
> >
> > -----Original Message-----
> > From: Barclay, George [mailto:GBarclay(--nospam--at)lgt.lg.com]
> > Sent: Thursday, December 20, 2001 5:35 PM
> > To: 'seaint(--nospam--at)seaint.org'
> > Subject: Increase of f'c over time
> > I am performing a job that involves the capacity of structural plain
> > concrete.  The new (heavy equipment) demand exceeds the flexural
capacity
> > of
> > the member using the original concrete strength given in the 1960
> > construction documents.  The client obviously wants to avoid
modification
> > of
> > the structure.  He has good reasons which I will not go into here, for
> > length of the discussion.  I know that concrete strength increases over
> > time.  I spoke with one of my grad school professors from whom I
> > originally
> > heard of this increase.  He said that he wouldn't be surprised if the
> > strength of the concrete had as much as doubled over that amount of
time.
> >
> >
> >
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