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Re: Rebar Grades

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Tom,

When I took concrete design in 1958, all reinforcing was round and, except 
for rebars with diameters greater than 1", they were expressed in the number 
of eighths of inches in their nominal diameter.  Rebars greater than 1" 
diameter were designated #9, #10, #11, and #14, which were manufactured using 
the areas of previous square bars, 1", 1-1/8", 1-1/4" and 1-1/2" square, 
respectively.

A15 reinforcing, IIRC, just had the bar number rolled into the deformation 
pattern and there was no different identification for structural, 
intermediate, or hard grade.

A16 reinforcing (rail grade), (again, IIRC) had a rail symbol rolled into the 
deformation pattern.

The 1946 ACI code may even have been a liberalized code.  At the beginning of 
WW2, codes were greatly liberalized, "temporarily" increasing allowable 
stresses 15 to 20 percent to require less material to produce more in support 
of the War Effort.  Some of the liberalize values were carried over into post 
war codes.

Reinforcing then, as today, was manufactured from junked cars (and possibly 
scrapped WW2 ships) and there was very little new billet steel mixed in with 
the car metal.

Just because concrete sections were larger than what would be designed today 
does not mean that they were over designed.  It might mean that they complied 
with the criteria of the time.  In 50 years from now, will someone say that 
we are over designing if what we are now designing does not fail?

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Tom Hunt wrote:

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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

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