Has anyone designed or worked with "micro-rebar" such as Helix brand? 5

[Ron247]

I am looking into Helix brand micro-rebar. I have just started reviewing their literature and presentation materials. Knowing these are their "sales tools", I was curious if anyone on this site has actually worked with it, designed with it or closely investigated it.

At first I was heavily skeptical at first, but after reviewing some of their information and videos, I am less skeptical but still not sold except for temperature and shrinkage.

 

[GeorgeTheCivilEngineer]

Which codes recognize steel fibers as flexural reinforcement? None in NA, and I have a hard time believing it's buried in Eurocode somewhere.

Correct, not recognised as structural reinforcement in Eurocodes. TR63 in the UK gives guidance as does TR34 and the closest I ever came to using fibres was on an industrial slab - as we had used CMC (non-reinforced piles, basically) for the slab foundation, which was deemed to create (imo theoretical) 'hard spots'... even then the stresses were so low the concrete itself would be fine, but another consultant got nervous. So it was either fibres to enhance flexural strength or a layer of top mesh to give it actual flexural strength! Went for the top mesh as we'd still saved £££ on not using piles.

Other than that I'd only recommend fibres for GBS but I prefer plastic fibres to steel in that case.

 

[Ron]

I don't want to be misunderstood....fiber enhancement in concrete can be a good thing. It just doesn't replace contiguous rebar. Back in the 80's, I was an active member of ACI 544, the committee on fiber in concrete. We debated long and hard about its attributes, and differences in using steel or non-steel fibers. They act differently within the concrete. The non-steel fibers help to mitigate the "first crack" condition, while the steel fibers give more overall strength attributes to the concrete, particularly for the modulus of rupture. This is because fibers are generally dosed in concrete by weight....so one pound of polypropylene fibers results in a much greater number of dispersable fibers that one pound of steel fibers; however, the poly fibers don't add as much strain resistance on a micro-level as compared to steel. Most of the early uses of fiber were in heavy pavement applications such as airfields and industrial pavements. I've used steel fibers in both applications with good results. I've also used ductile steel fibers in hot mix asphalt. That works very well, but never caught on.

My $0.02

 

[rapt]

Tomfh,

Yes, it does say that in a Note. And it also says that redistribution is allowed in another note.

Fortunately Notes are Advisory and not Mandatory in AS codes. Neither of those would have been there if I could have stopped them. If you look at other design methods around the world, redistribution is not allowed. The strain limit mentioned below will also further limit the applicability with moment redistribution (See RILEM method, NZS and others). But unfortunately we were not able to stop this stupid Note being put into the code. But we did succeed in getting the strain limit into the code.

In an area of concrete where the tensile strain is greater than .025, the fibre is assumed to provide 0 tension force. So it is assumed to not exist. In a slab with minimum reinforcement, that would mean that there is no tension capacity from fibre for about 60% (original post 40% was measured from the compression face) of the depth of the section from the tension face. Anything greater than about 15% fibre replacement will not be able to achieve this. So 90% or full fibre replacement is not possible. This is based on the logic that above a strain of about this value, the fibres have completely debonded and provide 0 strength.

Secondly, AS3600 has provided a force/crack width profile that fibres have to comply with. Helix fibre did not comply with this in 2018. If they have not improved their fibre performance since then, it does not comply with AS3600 and the AS3600 rules cannot be used with it.

If designers are designing properly, then they cannot do 90 - 100% replacement.

Also, the Earthquake provisions have separate minimum reinforcement requirements that are not covered by this note which require minimum reinforcement of .004 (All) or .006 (IMRF) if SFRC is used, so basically doubling and tripling the minimum reinforcement requirements in cases where more ductility is required.

 

[rapt]

Tomfh,

Further on this, I just had a response from a member of the AS3600 SFRC sub-committee for ASS3600 and in his opinion it was meant to apply, in his words,

"The intent is that slabs on grade that only rely on flexural tensile strength of the concrete (e.g. ‘pavements’) use a different minimum reinforcement for shrinkage restraint?"

And in my previous post, the 40% was from the compression face, so about 60% of the depth is > .025 strain in tension at minimum reinforcement and any fibre within this 60% of the section depth is ignored.

 

[Agent666]

It really depends on the fibres, fibres with significant ductility do exist. For example dramix 5D fibres. Your typical straight fibres do not stand a chance.

https://engineervsheep.com