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RE: 1950 Rebar

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Yes.  Limiting rebar stress is one indirect approach to limiting concrete
crack size, and thus limiting chloride ingress, and thus maintaining the
concrete alkalinity and rebar corrosion passivation.  We still can limit
crack sizes by making the proper calculations for durability.  Sadly some
engineers don't make the appropriate calculations required for durability.

Some of the older structures did perform better for durability than others.
But there are new structures that will perform better or equal to older
structures if they are designed correctly.  Designing a durable structure
correctly goes well beyond what the code REQUIRES.  The ACI code looks at
minimums.  Durability is much more of a design effort.  As for concrete in
sea water, durability design requires an understanding of materials behavior
in a sulfate and chloride wet environment.  Once the appropriate cover is
selected, the rebar originally selected for strength should be re-evaluated
to minimize crack size.  The various components of the concrete mix should
also be selected with regard to durability.  Aggregate type and gradation
should be studied, cement should be of the appropriate type with extra care
of the quality of the lot.  Admixtures should be selected based on placement
method and intended performance.  Test placements should be required, a
preconstruction check list developed, and inspections should be rigorous.
Proper durable design does not end when the specs and drawings go out the
door.

I agree that there has been too much of a focus on miracle cures for
corrosion.  I have seen many of them.  A salesman can call and claim his
product is the ultimate cure for corrosion, and after a few years the
miracle falls short of the mark.  There are new products that can help, but
many of the old proven methods for corrosion mitigation are still the best.
I don't think that we need to go back to ASD in concrete, but it is a simple
and easy sanity check.  ACI 350 hung on to the ASD the longest, but they
eventually came over to the dark side (as some would call limit state).

Regards,
Harold O. Sprague


> -----Original Message-----
> From:	M Hariharan/engg [SMTP:hariharan.m(--nospam--at)eil.co.in]
> Sent:	Saturday, March 17, 2001 12:08 AM
> To:	seaint(--nospam--at)seaint.org
> Subject:	Re: 1950 Rebar
> 
> This leads me to wonder if the limitation on maximum stresses in the rebar
> is
> the basic reason for the durability! Less cracks, less chloride ingress
> and less
> corrosion. I have heard some friends who had inspected some old marine
> structures state that even in places where the concrete had spalled due to
> other
> reasons, round (not deformed) bars had shown remarkably few signs of
> corrosion!
> It has led me to wonder if, for marine structures, the ASD approach with
> reduced
> steel limiting stress would be a better choice compared to limit state
> design
> with exotic specifications and coatings for reinforcement. Any feedbacks
> would
> be welcome. TIA
> 
> M. Hariharan
> Engineers India Limited
> 
> 
> Sprague, Harold O. wrote:
> 
> > Ah yes an opportunity to blow off a little dust from my books and from
> > between my ears.
> >
> > The ASTM Designation for this steel is A15.  Bar was provided as plain
> or
> > deformed.  Either were broken down into 3 grades of steel with
> properties
> > listed below:
> >
> > Grade:                            Structural      Intermediate      Hard
> > Tensile strength (ksi):         55 to 75         70 to 90           80
> > Yield point (ksi):                    33                  40
> > 50
> > Allowable str. (ksi):                16                  18
> 18
> >
> > They could also be Rail Steel A16 which would have the same mechanical
> > requirements as the A15 Hard.
> >
> > Regards,
> > Harold O. Sprague
> >
> > > -----Original Message-----
> > > From: Tom.Hunt(--nospam--at)d-fd.com [SMTP:Tom.Hunt(--nospam--at)d-fd.com]
> > > Sent: Thursday, March 15, 2001 5:11 PM
> > > To:   seaint(--nospam--at)seaint.org
> > > Subject:      1950 Rebar
> > >
> > > I am working on rehabilitating a 1950 concrete seawater intake
> structure
> > > that was designed per the 1946 Uniform Building Code.  The drawings
> call
> > > out for either 2500 psi or 3000 psi concrete and the rebar is called
> out
> > > as
> > > either square bar for deformed round bars.  Does any know what rebar
> > > grades
> > > were used in this time period?  I know 40 ksi rebar has been around
> for a
> > > long time but as grey as my hair is this is before my time.  I might
> add
> > > that this structure is in amazingly good shape with little
> deterioration.
> > > Goodos for our fathers and grandfathers.
> > >
> > > Thomas Hunt, S.E.
> > > Duke/Fluor Daniel
> > >
> >
> 
> 
> 
>