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Re: concrete driveway: thickness ; mesh or not ?

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Re: concrete driveway: thickness ; mesh or not ? Joe 05-01-2007
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Posted by Robert Allison on May 8, 2007, 9:48 pm
Bob Morrison wrote:
> In a previous post Goedjn wrote...
>
>>I thought the point of the fiberglass was to limit spalling.
>>Does it really increase the tensile strength of concrete?
>>
>
>
> Some manufacturers claim it does. As I said earlier, the jury is still
> out on whether or not fibermesh helps slabs resist random cracking.
>
> I am of the opinion that it may help a little, but not enough to warrant
> the cost. I prefer to increase the strength of the concrete the old
> fashioned way: add cement and reduce water content. And, saw cut crack
> control joints as soon the concrete is hard enough to walk on without
> leaving marks (4-12 hours).
>

When I first came into contact with fibermesh (about 20 years
ago), it was limited to a couple of uses. We added it to
concrete for apartment balcony slabs and sometimes to
sidewalks. In the case of the balconies, we would pour these
at a thickness of 2" to 3". Not really enough room for any
other kind of reinforcement and chicken wire was way too
difficult to work with. The sidewalks had WWM, but the
fibermesh was added to help eliminate surface cracks.

In no way was it ever represented as a substitute for steel
reinforcement. Every time I hear that, I just have to laugh.

It may be ok in some areas, but here we have expansive soils,
rock, etc. that makes reinforcing steel absolutely necessary.

Someone mentioned that highways were constructed with
fibermesh only. They are building numerous highways here and
I have examined several of the projects under construction.
Here they use #5 rebars 1' OC with a 7" thickness, 5,000 PSI
concrete.

I will ask why they don't just use fibermesh. They could
probably use a good laugh.

--
Robert Allison        
Rimshot, Inc.
Georgetown, TX

Posted by Chuck on May 9, 2007, 4:12 am

> In a previous post Goedjn wrote...
>> I thought the point of the fiberglass was to limit spalling.
>> Does it really increase the tensile strength of concrete?
>>
>
> Some manufacturers claim it does. As I said earlier, the jury is still
> out on whether or not fibermesh helps slabs resist random cracking.
>
> I am of the opinion that it may help a little, but not enough to warrant
> the cost. I prefer to increase the strength of the concrete the old
> fashioned way: add cement and reduce water content. And, saw cut crack
> control joints as soon the concrete is hard enough to walk on without
> leaving marks (4-12 hours).
>
> --
> Bob Morrison, PE, SE
> R L Morrison Engineering Co
> Structural & Civil Engineering
> Poulsbo WA
> bob at rlmorrisonengr dot com

Bob;

I worked on the revision project at the Seattle Center fountain back when I
worked with TRA. The fibermesh additive was used in the slab on grade
around the fountain. I heard that it has worked quite well. Now my
interest is up, haven't visited there in quite a while guess I will have to
see if cracks are not seen or abundant. :-)).

Chuck...

____________________________________________________________
Charles I. Dinsmore, PE SE RA, M.ASCE ~ ci.dinsmore@gmail.com



Posted by Chris Lewis on May 8, 2007, 12:18 pm

> I thought the point of the fiberglass was to limit spalling.
> Does it really increase the tensile strength of concrete?

Air entrainment (and being careful with making sure you have
proper mixing/good quality materials) is the primary cure for
spalling. Fibermesh is supposed to prevent cracking like
rebar does. However, some suppliers suggest/say that fibermesh
helps limit spalling, early cracking, resistance to chipping
and improves surface durability too.

Obviously the suppliers have an incentive to tout the benefits,
but the existance of industry (eg: ASTM) tests to show the
actual improvement shows there must be some truth in it.

I have no doubt that steel mesh or full rebar (eg: stress concrete)
is better than fibermesh in terms of brute strength, but
fibermesh is very cheap compared to rebar, and is a good choice
in situations where you want a little more reassurance with
situation that doesn't really need steel.

I'll be doing something similar when I experiment with making
some "hypertufa". The recipe calls for 2 parts portland cement,
three parts sifted peat and three parts perlite. And a "handful"
of concrete fibermesh (a cup loosely packed for 5 gal of dry
ingrediants).

[Making some synthetic rocks to stick on top of cedar log
fence posts.]
--
Chris Lewis,

Age and Treachery will Triumph over Youth and Skill
It's not just anyone who gets a Starship Cruiser class named after them.

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