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Re: Fracture in concrete

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Christopher,

I don't know much about the use of FEA or fracture mechanics in concrete,
but can comment on some general things of concrete that you may not be
aware of since you don't seem to do much with concrete design.

The first thing to realize is that the "strength" of a concrete member
depends first on whether you are considering it reinforced or not.  The
VAST majority of structural concrete is considered and designed as
reinforced, but some is considered/designed as unreinforced (or "plain" by
some termonoligies).

In reinforced concrete (not talking prestressed at the moment), concrete
is ALWAYS considered cracked for strength design.  There can be some
debate between considered as cracked or uncracked for servicability issues
(i.e. deflections, etc).  In reinforced concrete design, the concrete is
assumed to do nothing in the tension zone except to "connect" the
reinforcement to the rest of the member that does the compression "work".
Thus, the reinforcement does all the tensile work which includes limiting
the width of the crack.  The reinforcement won't necessarily arrest the
crack as much as limit how wide the crack will become.  The length of the
crack will be "arrested" more by the limit of the tensile zone and thus
changing to a compression zone.

As a result of this, servicabliliity issues become more of an issue of
"when" the check in the service life is occuring when determining whether
to consider cracked or uncracked.  If you want to determine
deflections/drifts with very minimal loads (i.e. maybe during construction
before full loading occurs), then an uncracked section might be used.  But
in most cases, you would still use a cracked section for deflections/drift
since the cracks will occur with fairly low loadings.  In otherwords, the
base assumption of reinforced concrete design is that it HAS cracked.

If you do design the member as an unreinforced member, then the obviously
the section will have to be designed as uncracked.  This is due to the
faction that there will be no reinforcement to prevent the crack in the
tensile zone to become wide in an unstable manner that would allow the
crack to propigate through the full cross-section.  So, in unreinforce
concrete design the member will be designed to stay below the level where
the concrete would crack.  This is typically considered by using the
modulus of rupture.  This is a value that is determined by testing that
represent the stress at which concrete would fail in tension (i.e. crack).
Obviously, when designing there would also be a factor of safety included
in the process.

Again, when dealing with things like fatigue and shrinkage, reinforcement
would like be used in such cases, unless the loads (or shrinkage) are very
small.  In the case of shrinkage, in theory, if the something like a
slab-on-grade is properly cured, then reinforcement for shrinkage would
not be necessary.  And in fact, there would be many that shrinkage (at
least due to curing/setting of the concrete) is not really affected by
reinforcement...in otherwords, that it is much better to provide a good
curing for hte concrete than supplying reinforcement.

Now, prestressed concrete is a slightly different animal.  Again, the
basic assumption is that prestressed concrete will potentially crack, but
not to the level of normal reinforced concrete.  In prestressed concrete,
it is certainly possible to have to member stay under compressive loads
for the entire cross-section under all loading conditions.  The whole idea
behind the prestressing is to "counter-act" the tensile stresses with some
imposed compressive stresses rather than just "holding" the section
together as in reinforced concrete.  I am sure someone who deals more in
prestressed concrete for a living may chose to chime in and
clarify/correct any of my general comments.

I can say that there are those in material research part of civil
engineering that do a lot of fracture mechanics with concrete.  This is
typically more of the material type's area than the structural type's
area, so I am not any where knowledgable about it.

HTH,

Scott
Ypsilanti, MI


On Mon, 25 Nov 2002, Christopher Wright wrote:

> This is a curiosity question for any concrete guru with a few minutes to
> kill. . I don't do structural concrete and I'm enough ignorant on the
> topic so I still use 'pour' for 'place' and 'cement mixer' for 'concrete
> hauler.'
>
> There are inquiries from time to time on a FEA list I frequent from
> people looking to do cracked sections with FEA, and I got curious whether
> these guys are just student newbies in a FEA course or whether there's
> active research in fracture mechanics of concrete.
>
> What's the current practice on figuring the strength and ultimate
> capacity of cracked beams?
>
>  Is fracture mechanics used? (I was taught to use the uncracked section
> in my only exposure to design in concrete, but that doesn't square with
> what I know (a fair amount) about fracture propagation and critical crack
> length.)
>
> Is reinforcing considered to arrest cracks? (Pretensioning probably will,
> but what happens to cracks in non pretensioned structures.)
>
> How is fatigue cracking or propagation of shrinkage cracks considered?
>
> Christopher Wright P.E.    |"They couldn't hit an elephant at
> chrisw(--nospam--at)skypoint.com        | this distance"   (last words of Gen.
> ___________________________| John Sedgwick, Spotsylvania 1864)
> http://www.skypoint.com/~chrisw
>
>
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