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Why Use Glass Fibers to Reinforce Concrete?

07/23/2015 11:26 PM

I keep seeing articles about this - about how to do it, but nothing about why to do it. I assumed for a while that glass had a higher modulus of elasticity than steel, and so would do its reinforcing job with less cracking of the concrete, but this turns out to be wrong - mild steel has a higher modulus by a factor of about three.

So why struggle with chemical compatibility issues to use glass?

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#1

Re: Why use glass fibers to reinforce concrete?

07/23/2015 11:29 PM
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#2
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Re: Why use glass fibers to reinforce concrete?

07/23/2015 11:33 PM

Great article. Thanks for pointing it out.

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#3
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Re: Why use glass fibers to reinforce concrete?

07/24/2015 12:03 AM

Sure.

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#4

Re: Why use glass fibers to reinforce concrete?

07/24/2015 12:04 AM

For the same reasons that they add glass fibers to plastic.

It makes an inherently weak material stronger.

Glass mat, glass cloth could be used as well as other reinforcements.

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#9
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Re: Why use glass fibers to reinforce concrete?

07/24/2015 10:43 AM

Of course, but steel is traditionally used for that purpose.

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#16
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Re: Why use glass fibers to reinforce concrete?

07/26/2015 12:43 AM

Yes, and steel still is required for resisting moments and other tensile loads.

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#5

Re: Why use glass fibers to reinforce concrete?

07/24/2015 1:52 AM

See also about fly ash and pozzolan.

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#7
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Re: Why use glass fibers to reinforce concrete?

07/24/2015 7:46 AM

isnt ash more of a filler?

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#10
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Re: Why use glass fibers to reinforce concrete?

07/24/2015 10:44 AM

It also reacts with the alkaline products of curing concrete to strengthen it somewhat...and makes it more chemically compatible with alkali-sensitive substances like glass!

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#6

Re: Why use glass fibers to reinforce concrete?

07/24/2015 2:19 AM

Steel rusts. Glas fiber does not.

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#11
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Re: Why use glass fibers to reinforce concrete?

07/24/2015 10:45 AM

True, but neither does properly covered steel.

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#8

Re: Why Use Glass Fibers to Reinforce Concrete?

07/24/2015 9:34 AM

Keep in mind that while the fiberglass does improve concrete strength there is no way to totally stop the concrete from cracking.

The objective is to control where the cracks occur, minimize the number of cracks that can occur, and minimize the negative affects of the crack(s) to structural integrity.

Fiberglass reinforcement is an additive measure and in no way displaces the need for steel rebar in the concrete except in very small areas that are not subjected to vibration or soil movement.

For instance;

If sidewalks, floors, and curbing are installed using only fiberglass reinforcement the concrete will crack and if there is not any rebar to maintain surface level continuity the surfaces on each side of the crack(s) will not stay in the same plane.

This will result in uneven, broken concrete and complete loss of structural integrity.

If there is any vibration stress to the concrete it will deteriorate even more rapidly.

I have observed numerous instances wherein the principal engineering group has made the fatal mistake of specifying pouring concrete with fiberglass reinforcement and without rebar.

The incidence of concrete structural failure was/is greater than 90% with only very small surface areas less than 144 square feet able to maintain structural integrity and only if not subjected to any vibration and/or soil movement.

The cost incurred from lost time in project completion schedules, removing the failed concrete and/or mounted equipment, and for redoing the concrete is overwhelmingly high.

I would not consider using fiberglass reinforcement exclusively in concrete except for temporary structures, walkways, and parking blocks.

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#12
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Re: Why Use Glass Fibers to Reinforce Concrete?

07/24/2015 10:47 AM

Very good to know. Thank you.

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#13

Re: Why Use Glass Fibers to Reinforce Concrete?

07/24/2015 12:24 PM

From Post #1 by jack of all trades:

http://www.concretenetwork.com/glass-fiber-reinforced-concrete/benefits.html

Lighter weight: With GFRC, concrete can be cast in thinner sections and is therefore as much as 75% lighter than similar pieces cast with traditional concrete.

According to Jeff Girard's blog post titled, "The Benefits of Using a GFRC Mix for Countertops", a concrete countertop can be 1-inch thick with GFRC rather than 2 inches thick when using conventional steel reinforcement .

An artificial rock made with GFRC will weigh a small fraction of what a real rock of similar proportions would weigh, allowing for lighter foundations and reduced shipping cost.

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#14

Re: Why Use Glass Fibers to Reinforce Concrete?

07/24/2015 8:04 PM

One reason (amongst others) why fibers are used in concrete mixes is to reduce micro-cracking of concrete during the concrete curing process.

Please be advise that there as several different synthetic fiber material types in addition to steel and glass fibers.

Here's a very basic explanation of why fibers are used in concrete mixes:

http://www.nrmca.org/aboutconcrete/cips/24p.pdf

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#15
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Re: Why Use Glass Fibers to Reinforce Concrete?

07/26/2015 12:40 AM

Good link Moosie.

OP should have a look at the application guidlines at the end of that.

As I suspected, the glass fibres are secondary reinforcement only, seems very suited to decorative uses but not a Replacement of any moment-resisting or structural steel reinforcement

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#17
In reply to #15

Re: Why Use Glass Fibers to Reinforce Concrete?

07/26/2015 9:19 AM

Bingo Wal!!!!! Yes, only use as a secondary reinforcement, mainly for shrinkage and micro-crack reduction. There's no way in hell it's use can lend itself in providing a proper Transformed Composite Section, as well as providing an adequate Resisting Moment due to Bending, and Shear Resistance.

One very important thing to remember here, standard reinforcement steel (Rebar), when properly applied in accordance with ACI 318, is to provide ductility to the reinforced concrete structural member particularly in the high Tensile Stress zone to prevent a sudden catastrophic structural failure......it gives you some advanced warning that the member is overstressed. Furthermore, Glass fibers are brittle and offer NO ductility whatsoever.

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