Fiberglass reinforcing bars in flat plates 1

[cliff234]

One of our flat plate projects is out for bid. The reinforcing steel in the slab consists primarily of #5 bars. One of the concrete bidders wants to use fiberglass reinforcing bars instead of A615 Grade 60 bars. (Just the straight bars.) This is a new one for me. They claim the limited availability of reinforcing steel, and skyrocketing cost of Grade 60 bars is making the use of fiberglass reinforcing a more economical choice. I am very skeptical of this claim.

Does anyone have any experience designing reinforced concrete with fiberglass reinforcing bars? If so, what has been your experience? What are the things we need to watch out for? Are there any good articles online that anyone knows of?

 

[Settingsun]

Very different material. Look closely at deflection as the frp bars are probably less stiff. There's also a change in philosophy from under-reinforced design to over-reinforced due to the brittleness of frp - you want to crush the concrete as it has at least some ductility.

Not sure about punching.

In short, not a simple substitution to match As*Fy.

 

[hokie66]

steveh49 is correct. The stress-strain curves are a lot different, with the glass fibres having about half the tensile modulus in the normal reinforced concrete working range.

 

[dik]

and due to bond issues, don't terminate bars at the same location... stagger the terminations.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[Toby43]

Not dealt with GFRP before, but a couple further items that I would consider needed attention-
1. FRP suffers from creep, does it not? (happy to be corrected). If it does then common RC theories would need appropriate adjustment.
2. GFRP is anisotropic I believe - Shear strength of the bar itself? If appreciably lower than steel, then "integrity" reinforcement at columns may pose a problem.

 

[bugbus]

ACI 440.1R-15 is a great source of info for this

Although I've never used FRP reinforcement, I did do a bit of research into it when we were investigating various options for reinforcing some concrete in marine environments.

From what I have gathered:
(1) The greatest drawback is the brittle failure mode, which makes it unsuitable for any application where you are relying on ductility or moment redistribution. I would include a flat plate floor in this category.
(2) The bars are weak in shear as Toby43 said, therefore unsuitable for dowel action.
(3) The anisotropy mentioned by Toby43 also affects the compressive strength of the bars, which is about 50% of that in tension (the failure mode being splitting / delamination of the bar).
(4) Strength under sustained loading (i.e. creep rupture) can be as low as ~30% compared to the instantaneous strength. Therefore, large strength reduction factors need to be applied.
(5) Elastic modulus may be roughly 1/3 or 1/4 compared to steel. Considering (4) and (5), you would probably need a greater quantity of FRP bars compared to steel.
(6) Notwithstanding (5), you are able to tolerate much wider cracks than for a steel reinforced structure because the stuff obviously doesn't corrode. Various codes (JSCE, CAN/CSA, ACI) allow up to 0.5-0.7 mm.
(7) The bars do indeed creep, but the creep strains are typically small, and your working stress will likely be quite small considering all the above points.
(8) There are other factors such as ambient temperature that affect the strength and behaviour of the bars also.
(9) Any bends in the reinforcement need to be pre-fabricated. The presence of any bend reduces the strength of that part of the bar by about 50%. Again, the bars tend to split / delaminate as they try to straighten. Also I believe the minimum bend radius is quite a bit bigger than for steel, which makes it less suitable for fitments.

 

[hokie66]

I have never used fibreglass bars, but would be concerned about gusmurr's point (1) combined with (6). Cracks would be where the bars in tension would break.

 

[HouseBoy]

I have a ICF wall that the owner/builder wants to use FRP rebar. It's a one story, single family residence.
Seismic class A so... that's not a big issue.
One "problem" though is that the ICF "design tables" all use grade 60 steel (naturally).
It's a very big house with A LOT of windows so there are many places with 2 or 3 or 4 ft long wall sections between windows
Walls are 6" concrete core (flat plate style not waffle style) and 10 ft tall. Roof truss span is 30 ft +/-
There are many 3 and 4 ft headers (not too worried about those). The 9 to 15 + ft opening s will be designed using LVL or steel headers.

Some questions:
Is this a circumstance where creep and ductility are "less" of a concern (since we are dealing with walls having relatively light loads? (still got to design for wind loads but I think I handle that.

Everyone is so enamored with the higher strength that they think they can actually get away with LESS area of rebar!
Does the "strength deduction factor" (=.55 I read somewhere) roughly equate to "about" 2x area of rebar?
I am trying to give the client a rough idea of what to expect if I have to design it according to ACI.

The owner/builder does have a 15 ft tall (accessory) building 60 ft x 75 ft that the he wants to build with 6" core walls. I said I will not touch that one. He says that a pole building of that size will stand up just fine so how can I say that a concrete building would not NOT?!

Any "design guides" available for this? (preferably low cost versions...)

Thoughts?

 

[ALK2415]

to know the difference just check these numbers

GFRP TYPE = {Cheap relatively} (Glass fiber reinforced polymer)
Ef = 48 GPa [Ffu = 950 MPa]
Es = 210 GPa [Fu = 420 MPa]

CFRP = {expensive ones} (Carbon fiber reinforced polymer)
there advantages ;
Ef = 209 GPa [Ffu = 3250 MPa]
Es = 210 GPa [Fu = 420 MPa]
two major companies exist in USA & CANADA
ASLAN & V-Rod
but they quite demonstrated especially in Canada bridges
the other two types are less common [A-FRP & B-FRP]

the Design Code [strengthening] Link
the Design Code [Reinforcement]Link

 

[Tomfh]

Is there any ductility? Or is it just *BANG*?

 

[ALK2415]

@ Tomfh
CFRP is the best type since its behavior (deforms like steel bars <--- cz both have almost same elastic modulus )
add to that Japanese Co. produced [since 1990] a higher Modulus [480 GPa]
Many university & research lab adopted this type in industry
Code of design also progressed greatly
the only draw back is the Cost/meter !
so they tend to be used in expensive project and not residential ones
Ductility achieved through higher concrete compressive like HPC & UHPC [Mostly in bridges projects]

 

[HouseBoy]

THANK YOU adn26 !!

I'm sure they will be using GFRP.
Again, they are looking at it from the standpoint that it's STRONGER so you need LESS.
Besides, this is what they have been doing for years now and they've "never had a problem..."

 

[ALK2415]

@ HouseBoy
just make sure the deflection submit to ACI serviceability limitations [increase slab depth if possible]
focus on this phrase "FRP will not attend its full tensile capacity unless it reach certain/considerable deflection/elongation"
which hard to reach due to ACI serviceability limitations
therefore try to use deeper sections as much as you could
also take notice the stirrups manufacturing/fabricate are not onsite [take special process at factory] which will delay your project
as a supplement we did use steel-rebars [as stirrups] since they are stronger and easily fabricate on construction site
this the site of Pultrall company Link

if you are in US so you would contact the ASLAN company Link

ONE LAST THING to clear the main difference you should is the Elastic Modulus {the higher you get the best design you achieve}
Japan has its own technology in field [even France import their products]

 

[MotorCity]

I have used them a few times. They cannot be bent on site. They can be cut, but you are more or less stuck with their shape when they arrive on site. They also cannot be fabricated to the same shapes (bends, radius,etc.) as conventional rebar. You are forced to lap splice sticks together to get the shape you want.

 

[ggcdn]

We've been getting asked this on a few recent projects. The GRFP sales reps must be out in force. The easy answer was no. The more nuanced answer came with a hefty extra services fee. Contractors decided A615 was available after all, once receiving that.

-JA
try [link calcs.app]Calcs.app[/url] and let me know what you think

 

[ALK2415]

Just as confirmation that these G-FRP Bars are well established in construction industry of Canada
this Engineer periodically publish [on LinkedIn] his projects selling shipments
Link