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Re: Rebar Grades

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Summarizing several issues and comments.

The seawater intake structure we are working on was built sometime between
1949 and 1951 and was in use until 1995.  We are going to reuse the
structure for an entirely new power plant.  We had a working scale model
made and introduced various improvements for best hydraulic performance.
Our rehabilitation is actually modifications to improve the flow (i.e. add
diversion walls, take out other walls, smooth out some of the corners,
etc.) and the installation of new equipment (i.e. trash racks, traveling
screens, pumps, etc.).

The drawings call out for either 2500 psi or 3000 psi concrete however
there is no mention of the rebar except for the actual call outs.
Interestingly the call outs are in inch diameter or inch square.  That is,
3/4 inch diam. etc. instead of # 6 bar.  Designations must have changed
during the 50's.  We also found a design basis document which described
every structure.  It went into details about the building code (1946 UBC),
concrete strength, and the structural steel grades but again no mention of
rebar.

We had a company, Concrete Science, do a concrete condition survey.  They
hammered every inch, used echo sound, took core samples, etc. and so far
have only found one bad area.  The one bad area is still a mystery to
everyone.  One wall has vertical delaminations  It looks like someone took
a deck of cards and slightly fanned them.  Every 1 1/2 to 2 inches there is
a vertical void.  We suspected corroded rebar however when we exposed the
rebar it looked like the day it was put in.

The core samples are consistently coming in around 5500 psi to 6000 psi.
Not one low break yet.  Concrete cover seems to be inconsistent.  We have
seen several areas with less than one inch cover and others with more.

My best guess as to why the structure has performed so well is that they
used working stress design with low rebar stresses and probably over
designed due to  the hand methods available at the time.  Nowadays, with
high strength concrete/rebar and the use of powerful finite element
software we tend to end up with much thinner sections (and much higher
rebar stresses).  The original structure has 2'-6" to 3' walls whereas some
of my new structures have 18" to 2' walls.  Economics does not allow us to
put in this much fat and I am sure someone will mention that this structure
has significantly exceeded it's original design life (again more dasdurtly
over design).  I also bet that curing was much better.  They probably wet
cured with burlap bags and kept it moist for at least a week or two.

Thomas Hunt, S.E.
Duke/Fluor Daniel

----- Forwarded by Tom Hunt/DFD on 03/17/01 02:13 PM -----
                                                                                                                   
                    "M                                                                                             
                    Hariharan/eng        To:     seaint(--nospam--at)seaint.org                                                 
                    g"                   cc:                                                                       
                    <hariharan.m@        Subject:     Re: Rebar Grades                                             
                    eil.co.in>                                                                                     
                                                                                                                   
                    03/16/01                                                                                       
                    11:55 PM                                                                                       
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Out of curiosity, if the structure is in a good shape, why is
rehabilitation required?

Hariharan


> *** Original Message ***
>
> 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