Non metallic rebar

[trashcanman]

Just curious. Do any of you use non-metallic rebar for reinforced concrete? Seems like a good idea to avoid corrosion and failure. What kind and does it work? Thanks.

 

[jayrod12]

There's been lots of research put into CFRP and GFRP deformed bar reinforcing but it is expensive and requires a different set of design rules(in a sense). It also has no shearing strength (or at least no appreciable shearing strength) therefore it is only usable in certain applications.

I'm not sure if there are other types of accepted non-metallic reinforcing available.

 

[TehMightyEngineer]

I've used GFRP before on two projects now. In my experience, I'd look into using GFRP if the cost of major repairs down the road greatly outweigh the cost of the GFRP (or you feel those repairs wont be performed), or if it's a highly corrosive environment, non-metallic reinforcement is required (MRI machine rooms for example), or it's a critical component where failure could be highly costly or dangerous.

It's not too hard to design with but definitely takes a little more work than rebar and there is still some research needed to fill the gaps. Finding contractors with experience with it will also be tricky, though mostly it's not going to be too much different for them to work with. Biggest lesson learned for us is GFRP must be pre-bent when formed at the manufacturer. Thus, if the contractor mis-orders some bars the wrong shape or size then it's a multi-week lead time to get new bars made.

Fire and high temperature will cause serviceability failures (sagging and cracking) but structural performance can be maintained similar to regular rebar in terms of life-safety fire ratings. Essentially, fire = damaged structure even for small fires.

Working with the bar is annoying for the contractor. Stuff itches like crazy and they'll probably add a lot of cost due to the complexity, risk, non-standard construction, and generally larger amount of work required for GFRP.

Don't expect to save much on rebar area, even with the higher strengths. Various penalties always seem to kick out these gains when you run the numbers. Add in the dowel shear and compression issues and you might even need more rebar than with just the plain black bar.

All that said and all things equal I still prefer GFRP to other options like MMFX, epoxy coated, and stainless steel when you consider all the pros and cons. I'd use it more but it's hard to find customers who want the added durability or need something more than a 75 year structure.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[TehMightyEngineer]

Another option other than GFRP is plain concrete with polypropylene fibers for crack control. This is pretty much limited to compression only structures, though as all the research I've seen shows that only steel fibers have any effect on the tensile properties of concrete.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[KootK]

You've also got to be a bit more careful about deflection with GFRP. Not as stiff as steel.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

It also has no shearing strength (or at least no appreciable shearing strength) therefore it is only usable in certain applications.

Can you elaborate? I'd never considered this. What are the no go applications? Dowel action in shear friction scenarios?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TehMightyEngineer]

Dowel action in shear friction scenarios?

I believe this is the majority of the concern. Essentially my understanding is shearing and compression of the bars is not fully understood. It has some strength in both but the limit states are not fully understood last I checked.

You've also got to be a bit more careful about deflection with GFRP. Not as stiff as steel.

Not just deflection but also crack control and anything else involving the modulus of elasticity. We had a project recently involving prestressed bridge panels where the bursting stress resistant reinforcement was replaced with GFRP. The design engineer (not me) just did a 1:1 replacement using the AASHTO provisions and we had some panels develop large cracks along the strands due to the bursting stress opening up the concrete before the GFRP got effectively engaged.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[jayrod12]

Yes. No dowel action allowed (At least that's how I remember it when I was taught about it). It essentially has zero strength perpendicular to the strands. All the strength of the material is tensile along the fibres. So in the shear friction scenario, you'd need to orient your dowels at an angle to ensure they would be engaged in tension across the shear plane.

This is also why the FRP used in reinforcing scenarios has multiple layers with the strands in different orientations depending on the application.

I don't actually have further information directly available to me. I'd have to dig up some info that was provided to me by a prof that was one of the leading researchers with the stuff.

 

[jayrod12]

Although to be fair, I just googled FRP dowels and there seemed to be a plethora of articles on using them in pavement joints. Which would counteract my concerns.

 

[TehMightyEngineer]

Well, that goes along with my understanding that it does have some strength in the resin binder and the fibers, but further research was required. Perhaps that research has now been done (it's been about two years since I dove through the state of the art information for GFRP rebar).

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[jayrod12]

I always felt that there must've been some capacity in dowel action but the way it was explained to me (which to be fair was almost 10 years ago) was no dowel action was allowed.

 

[TehMightyEngineer]

no dowel action was allowed.

If I remember correctly (again, 2 years since I last looked) this is what ACI 440 concluded and probably where the recommendation came from.

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[canwesteng]

Do you normally account for dowel action with rebar? I never have, but it's there anyway I guess.

 

[KootK]

Lakehead...but after me. Soon I shall know your true identity Batman. Soon.

I trained under an even holier FRP deity: Bank.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[KootK]

Do you normally account for dowel action with rebar? I never have, but it's there anyway I guess.

Your doing this implicitly in some measure, I believe, in both of these scenarios.

1) Vc diagonal tension shear calculation and;

2) Shear friction with smooth surfaces.

#2 is the one that would make me nervous.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[structSU10]

I don't have experience using it, but there are basalt rebar as well.

 

[TehMightyEngineer]

Aramid fiber (same stuff as kevlar) rebar as well. I eagerly await carbon nanotube rebar!

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com

 

[jayrod12]

Lakehead...but after me. Soon I shall know your true identity Batman. Soon.

Not Lakehead. I did have 2 friends that went that route however. I took the long route and fought it out with the local Uni.

Sincerely,

Bruce Wayne

 

[jayrod12]

And the prof I had it was this wonderful lady.

http://rrc.academia.edu/RobinHutchinson

 

[TehMightyEngineer]

Shouldn't that be Bruce Wayne, P.Eng?

Professional and Structural Engineer (ME, NH, MA)
American Concrete Industries

http://www.americanconcrete.com