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Rebar Spacers in PC beams

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In a message dated 1/28/2005 11:39:12 AM Eastern Standard Time, 
hemstad.ml(--nospam--at)tkda.com writes:
This is fairly standard.  And yes, it may initiate some cracks, but with
that much steel in the bottom of a small (at least before they put the
slab on) beam, you're going to get significant shrinkage cracking before
any load gets on it. 


Maybe I am missing something,  but what is the connection between the
amount 
of steel and the shrinkage cracking?

Gail Kelley


I'm picking up old messages that I didn't have a chance to respond to at
the time.  Gail, my reasoning was that in a precast beam such as this,
there is no surrounding structure to restrain shrinkage.  The only
restraint comes from the reinforcing.  With light reinforcing, the
concrete may actually be able to shrink without too much restraint from
the reinforcing; between bond slip and compression shortening of the
bars, the concrete can shorten enough that it won't crack as much as
we'd expect.

With a lot of steel, both of these relieving mechanisms are lessened.
The rebar will restrain the concrete, causing it to crack more, albeit
in the form of a lot of small, well distributed cracks.  When we put
increased rebar in concrete to "control shrinkage" (over that required
for strength), we may actually increase the total amount of cracking
(number of cracks times average crack width) by increasing the restraint
offered to the shrinking concrete; but this is countered by the cracks
being kept smaller by bond acting on the greater surface area of the
larger amount of rebar.

This is explained better than I have done here in an old (really old)
PCA document called "Circular Concrete Tanks Without Prestressing."  I
made my photocopy in 1985, but I don't know how old it was then.  I've
always considered it to be a good discussion of the idea of adding rebar
to decrease cracking.

Mike Hemstad
TKDA
St. Paul, Minnesota

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