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RE: Wire reinforcement in salbs on ground

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For a typical office SOG, I've specified fibers quite a bit.  I don't
use steel fibers in this situation... I use fibrillated polypropylene
fibers at 1.5 pounds / cubic yard.  I specify steel fibers for heavy
floors... warehouse, etc.  We always give an alternate for WWF for
lightly loaded floors (offices) and rebar (typically #4's @ 18" oc)  for
heavily loaded floors (warehouses).  We typical space control joints at
3 - 4 times the slab thickness.

>From the test results and the few floors I've seen with fibers, they are
great at crack control.  They can't be used for strength... you must
have WWF or rebar for a structural slab.  But for an office SOG, fibers
are a good option.

But, I'll admit that I don't have a lot of data on my own projects with
fibers.  Most contractors around here use our WWF or rebar alternate
simply due to lack of experience with fibers.  This is the case even
though the final installed cost is less with fibers than with rebar.  We
also work with national corporations which require stained slabs and
embeds stamped in slabs.  We don't use fibers in these cases because
they can affect the finished appearance.

One thing I like about fibers is that it is more difficult to use them
incorrectly than rebar or WWF.  It is rare to find the reinforcing mat
in the correct location.  Of course, fibers can be used incorrectly...
for one thing, they can "ball up" in the concrete mix.

Just one opinion.

 
BDH
 
-----Original Message-----
From: Gary Hodgson & Associates [mailto:ghodgson(--nospam--at)bellnet.ca] 
Sent: Thursday, February 23, 2006 7:01 AM
To: seaint(--nospam--at)seaint.org
Subject: Re: Wire reinforcement in salbs on ground

Kevin,
I have gone through a few jobs with floor problems,
caused (thankfully) by others.  As Bill P said steel
fibres are expensive.  The wire mesh is next to useless
unless you stand over the workers.  It is impossible to
lift the mesh when the workers are standing on it and
then you don't know where it is in the slab.  One of
those afore-mentioned jobs had to be ripped out in part
--guess where the mesh was--right, still lying on the
sub-base.  Now I only specify heavy guage welded wire
fabric in mats or rebar and that the reinforcing has to
be blocked up and I send someone to watch them.  I also
prefer rebar at 14 to 16" o.c. so the workers can step
between the bars.  The w.w.fabric has to have planks put
down for access to the work. I also specify saw-cuts at
30 x the slab thickness in inches (from some conc
publication) with saw-cutting to 1/4 depth in approx 10-
12 hours after placing.  My 2 cents worth :-).
Gary


On 22 Feb 2006 at 23:00, Kevin Below wrote:

> I was having a discussion with a client about using wire fabric
> reinforcement in a slab on grade.  His "other" engineers are becoming
> very gung-ho about using steel fibres.
> But the present project is an office building, with mostly light loads
> on the ground floor (except for the library), not a highly loaded
> distribution or warehouse building with racking and fork lifts.
> I recommended a 100 mm slab with wire fabric and control joints at 4
> metres.
> It's about the same price as 125 mm without wire fabric and control
> joints at 4 metres.  The "other" engineers prefer the latter.
> I prefer the former, since it's a system that has been used in this
area
> for at least 28 years, no, that's a joke.
> Conventional wisdom here is that wire mesh is no good since it's
always
> at the bottom, never in the slab.  But I remember reading an article
> once in Concrete Construction arguing that the bottom is the best
place
> for the reinforcement, since that is where the sub-grade friction is,
> and this is what causes the cracking.
>  
> We all know that the mesh ends up on the bottom, since we have seen it
> there when we demolish old slabs.  But if the mesh is pulled up into
the
> slab occasionally, like say 1 metre c/c, I would say that there will
be
> some sort of composite action between the slab and the mesh, and that
> the mesh will be at least partially effective in resisting cracking.
>  
> I have used a 100 mm slab without mesh, for a lightly-loaded slab with
> no floor covering, where the occasional crack is not a huge problem.
> Using a Soff-cut saw for the joints worked wonderfully to control
> cracking, at least to date - 1 year old.
> 
> But I am reluctant to forego the mesh on the present job, because the
> floor covering is linoleum, which will reflect cracks.
> 
>  
> 
> Any thoughts on this - what do you prefer - a 4" slab with mesh, or a
5"
> without ?
> 
>  
> 
> Kevin
> 
> 
> -- 
> No virus found in this outgoing message.
> Checked by AVG Free Edition.
> Version: 7.1.375 / Virus Database: 268.0.0/266 - Release Date:
> 2006-02-21
>  
> 



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