fiberglass rebar in slab-on-grade 3

[MarcusJ]

I have a new job, with a client doing some interesting high-voltage work. They need a fairly elaborate electrical grounding system in their slab-on-grade, and are very concerned about any steel in the slab. They are inquiring about fiber additives in lieu of rebar, but I'm not liking that idea. They'll have a large forklift lifting heavy items and I've only considered fibers for shrinkage cracking. I'd like to suggest fiberglass rebar (or possibly carbon fiber rebar) but I am not familiar with either. Any thoughts, suggestions, lessons learned? Thank you. (Max Loads: 36,000 lb forklift + 28,000 lift capacity = 64 kips.)

MarcusJ

 

[dik]

I would design it as a plain concrete slab. Other than controlling cracking, rebar doesn't really help with normal SOG construction, unless actually designed using rebar. You need a good value for the modulus of subgrade reaction. The distance between sawcut joints has to accommodate plain concrete. and the joints should be filled with a fairly 'hard' joint filler to protect the edges from the forklifts, etc. Sawcuts at the right time. Use as large an aggregate as possible to minimise shrinkage stresses. Good compacted granular backfill, well compacted. Good SOG construction.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[SWComposites]

List of f/g rebar specifications and test methods released by ASTM can be found here:

https://www.astm.org/get-involved/t...ittee-d30/subcommittee-d30/jurisdiction-d3010

 

[DaveAtkins]

My wife and I were at the local Lowe's last weekend, and I noticed "pink rebar" for sale. I picked one up and immediately knew it was fiberglass rebar. Some online research indicates it is Owens Corning, which makes sense, since pink is the color of their insulation. I mention this, because I guess it is becoming more common to use fiberglass rebar.

Anyway, decades ago, at the beginning of my career, we considered fiberglass rebar for a slab supporting an MRI unit. At the time, you could not use steel rebar around an MRI unit, since it would interfere with the imaging. We ultimately decided to go with an unreinforced slab, not because the fiberglass rebar is not strong enough, but because it is not stiff enough. As I recall, the modulus of elasticity of fiberglass rebar is much lower than steel rebar.

I agree with dik - I don't think fiberglass rebar does much for a slab on grade.

DaveAtkins

 

[BridgeSmith]

The specs on the Owens-Corning pink bar - 46.88 GPa to 60.30 GPa. For others like me, who are 'metrically impaired' carbon steel modulus is 200 GPa.

 

[RWW0002]

I attended a seminar recently with Owens-Corning on pink bar. Overall takeaway is that is is not a 1:1 replacement for all cases, but can be a pretty good alternative for many applications (including crack control in slabs).

Contractors seem to love it due to weight difference (although some workflow differences due to bending restrictions etc..)

Price around here can be fairly competitive to rebar depending on quantity.

I recommend contacting Owens Corning. Their field engineer (Jay Pease) is very helpful and knowledgeable about the product and the governing codes and standards that may affect you.

 

[ChipB]

I agree with Dik and Dave.
I had to design a reactor foundation on one of my solar projects.
Fiberglass was considered, but in the end, I just designed it as unreinforced concrete.

ChipB

 

[JoelTXCive]

The fiberglass and carbon fiber rebars are awesome stuff, but they only work in select locations.

The average modulus of elasticity is ~8,000ksi versus steel's 29,000 ksi. To get the stiffness you probably need, then it takes a LOT more fiber bars than steel bars.

Also, the fiber bars do not have a ductile failure mechanism, so you get hit with the .60 phi factor.

 

[bugbus]

On the side of FRP reinforcement, there is less need for crack control in the first place since the stuff won't ever corrode. So I believe the reduced stiffness (and therefore, wider cracks) can be tolerated without having to bump up the quantity of reinforcement.

 

[MarcusJ]

Thanks y'all. I'm reading on unreinforced slab design now.
I might just come around to the idea of using fibers in the mix, just for shrinkage cracking.
Any further comments appreciated, but I think I'm set.

-MarcusJ

 

[bones206]

Look into jointless slabs with shrinkage compensating cement and other admixtures. There are companies out there specializing in this type of slab. I haven't spec'd one of these myself so I'm not sure how the process works. I suspect they are proprietary to some degree and may be a delegated design type situation.

 

[phamENG]

Slabs on grade should never be designed to support weight. The best analogy is that the slab is just paint. You need it to be durable, yes, but the strength comes from the sub grade prep. You need to control cracking and ensure joints are locked together, but that's about it. Most of your attention should be directed at making sure the subgrade is properly prepared and compacted. If it is, your slab will be fine.

Now, if it were a pile supported structural slab or a slab on void forms over expansive clays those would be different...but for a true slab on grade the soil under it should be doing 99.99999% of the work.

 

[RWW0002]

Slabs on grade should never be designed to support weight.

@ PhamENG - I get what you are saying, and subgrade prep is important for sure, but never is a really strong word.. There is definitely a standard approach and standard of practice to the design of slabs on ground. Lots of literature prom PCA and ACI regarding the design (both for reinforced slabs and unreinforced slabs) and lots of information from the geotech world on sub-grade preparation and appropriate modulus of subgrade reaction.

The paint analogy is great for convincing someone not to over-reinforce a 4" slab, but there is 100% a time and place for properly designed slabs on grade. A much better analogy is a structural slab supported by springs - in face that is the exact design approach generally accepted..

 

[phamENG]

RWW00002 - I think you misunderstand my meaning. Loads get applied. Absolutely. And they should absolutely be designed per best industry practices. But those practices are not intended for the slab to carry the weight. My point is...the rebar in the slab is not flexural. It's not there to support the loads on the slab (tension and tie forces notwithstanding). The slab is there to provide a level surface to walk, store, and operate on. If you are trying to rely on rebar to make the slab strong enough to support the loads resting on it, I would say you're doing it wrong.

Use the rebar to resist cracking. Use dowels at joints to prevent relative displacement from concentrated loads. But the most important piece is the subgrade prep.

 

[RWW0002]

@phamENG - I understand what you are saying - However with thicker slabs and higher point loads from forklifts, racking, industrual slabs, heavy equipment etc.. steel is often used as flexural reinforcement at a local or even global level. It is a bit of a oversimplification to say that "Slabs on grade should never be designed to support weight." or that " rebar in the slab is not flexural" Maybe I am in fact "doing it wrong" but as far as I know I follow standard industry practice...

Yes subgrade prep is very important, but good slab design, including flexural steel at times, is also important.

 

[phamENG]

Fair enough. We probably agree more than we disagree, and I'm probably not communicating what I mean very clearly.

Can you point me to some resources for flexural reinforcement in slabs on grade? Everything I use is unreinforced with the exception of crack control or tension ties for PEMBs and similar superstructures, which probably reinforces (sorry) my viewpoint. I'm happy to learn something new, though.

 

[RWW0002]

Sure thing. Part of the confusion is likely slab-on-ground vs mat or raft foundation. Sometimes there is some crossover as slabs get thick and mats get thin..

I am not in front of my references right now but here are a few off the top of my head:

-ACI360 - This lists several design methodologies and would be a good place to drum up additional references
-PCA - There are a couple of design guides for slabs on ground, but the one I see referenced the most is Ringo (1986)
-The department of the Army has a Technical Manual concerning slabs on grade subject to heavy loads. It is a bit dated but still a good resource
-It seems like PNA (the folks who make diamond dowels) have some good resources for design of mat foundations and slabs on grade under hevy loading
-Structural Point (SPMat) has some good information on designing slabs on ground and mat foundations using FEM (obviously biased towards their product, but great information.. Ha!)

I will update the list if I get a chance when I have links/publication on hand.

 

[canwesteng]

Most flexural stresses are resisted by plain concrete in most applications, but you can use the same method from ACI 360 or make a FEA model if you like. Outside of heavy industrial (like haul trucks/excavators) I don't think you should be using bar for flexural stress.

 

[Brad805]

I would not do this in my area where the underlying soils are high plastic clays. The local Wallmart did their floor slab with zero reinf and it didn't even make it thru construction before it looked terrible. They did a terrible job of controlling runoff, so that would have been part of the problem. If you have decent soils, then I agree with others.

 

[phamENG]

Part of the confusion is likely slab-on-ground vs mat or raft foundation.

This is probably it. I don't do raft or mat foundations. Pretty much everything is independent spread footings or wall footings with disconnected, floating slabs on grade.

I haven't done anything with PCA, but I'm familiar with the others regarding slabs on grade. I'll have to look at them a bit closer. Thanks.

One this is for sure: I was too extreme in my language. Pretty sure I told my son to 'never say never' just the other day....(please excuse me while I pull my foot from my mouth)