[TowerTalk] pre-rebared concrete (long)

[WA9ALS - John]

> Can someone in the "concrete business" enlighten us?

My neighbor (non-ham) is in the concrete business.  His response to this
discussion is below.  If you're interested read on - else hit the big
"D".

> I would like to preface this reply to your question about concrete
> reinforcing by saying that it has been 8 years since I have been in
the
> Ready Mix concrete part of the business.  At the time I was running
our
> concrete plants my knowledge of this kind of reinforcement was as
follows.
>
> In poured concrete structures there are generally two kinds of
> reinforcement, primary and secondary.  Primary reinforcement usually
> consists of the reinforcing bars that you see being used in buildings
and
> bridges.  This kind of reinforcement works by using the tensile
strength of
> the bars.  When you use more than one layer of bars, for the concrete
around
> them to deform, it has to pull on the bars thus using their tensile
> strength.  This greatly adds to the strength of the structure.  The
other
> kind of reinforcing is secondary reinforcing which you may recognize
as wire
> mesh or a wire fence like product more commonly seen being used in
> driveways.  This is used to control cracking of a slab.  It is not
meant to
> keep the concrete from cracking but to merely hold the sections
together
> when a crack does occur.  Flat slabs will almost always crack after
they are
> placed due to the fact that the top of the slab dries faster than the
bottom
> and therefore shrinks faster than the bottom.  This shrinking causes
the
> slab to curl up and as it does it reaches the point where it can not
support
> its own weight and then cracks.  It is common to place control joints
in the
> concrete slabs that let the slab relieve these stresses in a
controlled
> manner.  The secondary reinforcing, when in the correct location in
the slab
> will hold the crack together.
>
> To the best of my knowledge, the reinforcement that you refer to is a
type
> of fiber mesh reinforcement.  There are several brands made of
different
> types of materials that can be used.  Some are made of nylon fibers
commonly
> called Nycon fibers, some are made of polypropylene fibers and are
commonly
> called Fiber-mesh and yes I believe there are also fibers made of a
very
> small wire mesh.  These mesh type of fibers are commonly the small
diameter
> bundles of fibers that you described and may somewhat resemble
fiberglass
> strands in the concrete.  This class of reinforcement was always
considered
> part of the secondary reinforcement that is meant to control cracking
and
> not to add structural strength to the concrete.  This is, in my
opinion, a
> much better alternative for secondary reinforcing because it is mixed
> throughout the concrete and eliminates the possibility of getting the
> reinforcing in the wrong place as is possible with the wire mesh
(fence
> type) product.  There is also the belief that the fibers can actually
> intercept the formation of cracks when they are in their microscopic
stage
> and keep them from forming.
>
> I always believed that if someone could come up with a fiber mesh type
of
> reinforcing that could be mixed in the concrete and which would
provide
> primary or structural reinforcing it would be a wonderful product that
would
> greatly reduce the cost of building structures such as your tower
bases.  To
> my knowledge, this can not yet be done.
>
> You mentioned that the steel mini-rebars sometimes rust.  With any of
these
> products, you will see some of these fibers at the surface.  The
> polypropylene and the nylon types usually break down in the presence
of
> sunlight and go away at the surface.  At one time there was a concern
that
> the alkali content of the concrete would attack the nylon fibers and
make
> them ineffective and this is why I used to sell the polypropylene
type.
> However, based on the amount of the nylon fibers in use, I assume that
this
> issue has been taken care of.  I never had any experience with the
steel
> type of fiber mesh so I don't have first hand knowledge of their
properties.



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