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.

 

[Tomfh]

We've come across it on two projects. What's interesting is it acts as flexural reinforcement.

On one job it replaced 95% of the reinforcement on a suspended slab on grade, suppported on screw piles. A few regular bars over the piles, the remainder was helix.

The long slabs still cracked like any other long slab. Some cracks were 0.5-1mm wide (one or two of them on a 50m long slab), which was a bit disappointing, but otherwise it seemed to work ok.

On another job it replaced the cages in footings.

In our experience the design is by specialist consultants who work with Helifix, similar to how PT is designed in some areas. I wouldn't want to design it.

The concrete finishers hated it, due to the sharp fibres, and the fibres looked ugly at off-form concrete edges. It took some prettying up...

 

[Ron247]

The appearance was something that seemed unreal in their presentation. They said it finished the same and that it did not stick out of the concrete when the forms were removed. I found that hard to believe.

 

[Tomfh]

There were definitely stray fibres visible when they stripped. Some of them protruded which is what the tradesmen didnt like. They are stiff sharp fibres that can spike you pretty easily.

 

[fel3]

Tomfh...

Helix's website has a video link (

https://www.youtube.com/embed/ORrawHcaA0M?rel=0).

At about 2:09 I see a fiber sticking up out of the slab.

============
"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[Ron]

Helix is just a steel fiber. The micro-rebar crap is a marketing tool. Steel fiber is a very good concrete mix enhancer, but it should not be substituted for proper reinforcement. It slightly increases the compressive strength of an otherwise normal concrete and increases the modulus of rupture, which obviously helps in flexure.

Be careful. Under failure loads, the mode of failure will become brittle, not ductile as you want with concrete.

 

[Tomfh]

but it should not be substituted for proper reinforcement

This seems to the the point of contention.

Helix says you can use it in lieu of conventional reinforcement, and they will provide certification of flexural and shear strength. The codes recognise it and provide design methods.

And yet many engineers say to hell with that.

Which is the right way?

 

[Ron247]

The brittle/ductile failure mode is one of my main concerns. I would prefer testing that compared a traditional concrete member to a Helix member. More of a direct comparison. As an example, a 5' wide elevated slab that is 12' long made of each type of reinforcing. Then test to failure.

Right now, I am looking at slab on grade and strip footings on grade, but I foresee the elevated stuff coming up next as a question.

 

[bones206]

I just read over the ICC-ESR for Helix. Seems like its scope is limited to T&S for plain concrete and slabs-on-ground.

 

[canwesteng]

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

 

[Tomfh]

Here in Australia it’s used as structural reinforcement. It is used as flexural/shear reinforcement, not just temp/shrinkage.

AS3600-2018 recognises fibres as structural reinforcement.

 

[Agent666]

NZ code has provisions for steel fibres as structural reinforcement. Our current code requires some conventional reinforcement in addition to the fibres. But Eurocode approaches can be used to get rid of most of this provided you provide a dosing of fibres that meets some minimum residual stress from memory. Typically still used for slabs on grade or in residential raft type foundations to replace shear reinforcement for houses where liquefaction might result in a loss of support.

The helix type of fibres aren't in our local market as far as I'm aware. Mostly seems to be dramix and their 3D, 4D and 5D fibres which are quite different and are capable of anchoring the fibres to achieve better performance (even yielding the fibres. These helix things don't have any anchorage apart from the twist which anecdotally just doesn't seem as good. Google dramix 5D fibre to see a picture and you'll understand about the anchorages.

https://engineervsheep.com

 

[rapt]

Tomfh,

In AS3600/5100, it can only be used as a partial replacement for normal flexural reinforcement, varying amounts but not more than about 10%. And it cannot replace minimum reinforcement at all. In fact, minimum flexural reinforcement requiremnets probably increase when steel fibre is used.

The problem you get is that fibre takes tension up to a crack width of about .3mm but at reducing amount from about .05mm. So as the crack widens, the tension force it can carry reduces. Eventually it gets to the point where it releases comnpletely (assumed about .002 tension strain) and all of the tension force is then carried by normal reinforcement. So if you have reduced your normal reinforcement by 10% and the crack becomes wide enough, then you are 10% under-reinforced at ultimate strength. If you reduce normal reinforcement by 90%, you are stuffed. Unless it is a slab on ground so it cannot fall down!

On top of that, all steel fibres are not the same. As I understand it, Helix in its current form does not conform with the AS3600/5100 rules and reduces strength more quickly than some fibres and than the default available tension force/crack width option in AS3600.

If you have a .5mm crack, the fibre is probably completely failed and providing no tension force across the crack.

Further to that, it should be assumed to have no flexural capacity in members required to be designed for Seismic action and it is also very doubtful in fire resistance or any member that requires plastic redistribution of actions, as they all require high strains and wide crack widths.

 

[TLHS]

I wouldn't want to be the test subject for this kind of stuff, which tends to be the issue. Are you going to save so much money that you want to go out on a limb.

That being said, the ICC report saying that it's used in the design of structural plain concrete doesn't mean it isn't a rebar alternative, just that it is designed under those clauses. It would make sense, as it's more of a homogeneous material with a consistent linear response than the very non-homogeneous response of traditionally reinforced concrete. Traditional rebar design has your tension lumped at the rebar points. This has it spread through the cross section, and presumably has less ductility. So it's a triangular stress distribution, or something similar.

That being said, I have no idea what their documentation says with regards to the tensile stress it gets you to or whether than makes the plain concere methodologies comparable to more traditional strengths.

 

[rapt]

TLHS,

There are 2 problems with this,

- its tension resistance reduces with increasing strain. So as it receives more load, it can carry even less than it did previously, so as its received more strain, it loses capacity.

- until it can carry none.

Neither of those is a consistent linear response! It has a reducing linear response leading to an abrupt failure at a relatively low strain for flexural concrete members.

 

[Tomfh]

In AS3600/5100, it can only be used as a partial replacement for normal flexural reinforcement, varying amounts but not more than about 10%. And it cannot replace minimum reinforcement at allAll

Where I’ve seen it is ground floor slab on piles. So it avoids the minimum reo requirement. Almost all the reinforcement was replaced with helix.

 

[Ron247]

I am still digesting the UES (Uniform Evaluation Services) report ER-0279. I sounds like it is advocating flexural steel.

 

[rapt]

Tomfh.

I would consider that to be outside the code if the slab is supported on piles as it is flexural and the AS3600 rules apply.

If it does not need piles and the ground conditions are adequate to support the slab then it is a slab on ground and SOG logic applies. But the slab should be separated from the piles.

Partial pile support is flexural.

 

[TLHS]

Doing some quick math on the ICC report, I think a 12" slab with maximum dosage gets you 35kN*m per meter length or 95kip*in / ft. That's a few times less than a reasonably reinforced slab, but it's also not nothing. Given that the actual material costs of concrete without the labour costs for bar are pretty low, that might be useful in some situations.

I still probably wouldn't use it.

 

[Tomfh]

AS3600 might want to clarify things then, as there are many Helix slabs on piles that exploit the code clauses that say minimum reo isn’t required if the slab is partially or fully supported on soil.