Basalt fibre reinforcement bars - working with them?

[kxi]

We've got a largish area of concrete slab to do - some farm roadway, some barn floor, some house ground floor slab. All had been SE designed for 200mm concrete with two layers of A393 steel mesh top and bottom, with 400mm edge thickenings where appropriate. This I felt was likely overdoing it, but that seems to be par for the course. Ground is clay and hoggin, then some crushed concrete over that, then 50mm concrete blinding. 

 

There was going to be a large steel reinforcement cost for material + labour, and I've a general concern about the longevity of external concrete with steel reinforcement. Over a 60 year lifespan it will inevitably crack (if not immediately), water will get in and the steel start to blow it up. (Discussed at length in previous post).

 

So I'd been keen on using basalt fibre reinforcement (that's bars - not the little fibres) instead, aka BFRP. These:

- Are actually a bit cheaper than the A393 steel for an equivalent tensile strength (6mm bars)

- Are MUCH lighter and easier to cut, in theory giving a big labour saving. (Each A393 sheet being about 70kg vs the basalt fibre sheet I'd guess 10kg-ish)

- Never corrode

 

The SE has given his tentative blessing, but groundworkers fairly unsure about how they would work with this bendy stuff, as they need to be able to walk on it, and run concrete pump pipe over it.

 

So I made up some example sheets and laid in the proposed build-up - about a 100mm spacing between top and bottom sheets. 

 

 

 

(This was a test - for real would use the ready-made sheets)

 

Then tried to walk on it:

https://photos.app.goo.gl/PwYGDXyaBew13oR76

 

As you can see, even at light foot pressure it bends all the way to the floor, then springs back up. Meaning to walk on it you've either got to step on it all the way down OR only walk on fully supported parts, perhaps using boards over the top.

 

Has anyone ever worked with such bendy stuff e.g. GFRP reinforcement bars?

Any suggestions of the best way to handle the requirement to be able to walk on it?

 

N.b. BFRP supplied by Chris H at Orlitech https://orlitech.co.uk/

 

 

Edited April 26, 2021 by kxi

[gravelrash]

Remember that even steel mesh requires several supports per m2.  I note you have the mesh on a roll as well, Would be nice to here how you get on with it.

Have you spoken with Chris at Orlitech? Im sure he will have knowledge of the product usage.

[kxi]

@gravelrash Current situation is:

- Groundworkers happy to go with this, just questions on how to keep the 3mm roll mesh fastened down, and to handle walking on the 6mm mesh - whether to put loads of supports, or less and then let it spring back up

- SE happy with the 6mm basalt mesh for external slabs and barn floor slabs, but wants steel for the house slab and any of the thickenings that support walls (though these are single storey and non-supporting). Says the basalt is too new for him to be comfortable with it in the more regulated areas, as there's very little precedent for it in the UK. Given he's open to all kinds of other innovative approaches and he's even used CFRP in the past, you can see why BFRP is going to have an uphill struggle to gain traction here.

 

In a couple of weeks, we shall see what's it like to work with.

[gravelrash]

I had the same issues from my SE over eps foundation design...but asked him to show me calcs and he came back saying ok.

I used Orlitech for reinforcing bar which I have chopped up for wall ties...a substantial saving on Teplo ties (1900%) but hassle from BC until I could prove its  very similar material and spec. 

[George]

I've got nothing to add, but just wanted to say this is a really interesting technology and fair play for using it. When they say basalt I assume it's essentially glass fibre but using mineral sources instead of glass. The basalt is just a marketing spin (quick google - yes this is the case).

 

However, I'm afraid to say that your concrete will still crack somewhere after 50 years. But that's OK... nothing lasts forever. And there's only two types of concrete... concrete that has cracked and concrete that is going to crack.

 

 

 

 

 

[Gus Potter]

Hello all.

 

As George says it's an interesting material, the price is coming down due to volume etc so can be an attractive option for the self builder. I can see why SE's are cautious.. for a reason.

 

As an overview we can see that say the Orlitech bars have a significantly higher tensile strength than steel rebars. ~ 2-4 times say so you can see how they are "stronger".

 

But they are much more stretchy! This is reflected in what is called the modulus of elasticity.. Youngs modulus. Ordinary steel rebar has a modulus of elasticity of say 205 GPa while these types of glass/ basalt/silica based bars have a Young's modulus of say 60 - 90 GPa. So they are say 2 -4  times more stretchy.. elastic.

 

This elasticity is important for concrete design as the concrete has to move more before the bars take up the same load as a similar sized steel bar. Thus you have to be aware that you could either get bigger cracks.. or more of them. If you are doing a basement that needs to keep water out then crack control can be essential. If you are say designing a beam then the beam will deflect more before the bars take up the load. Again this needs some thought. In the main you can increase the diameter of the bar (or use more of them) to mitigate the deflection.. and this impacts on the economics.

 

@kxi Here Kxi is doing an external slab say. Often you may use say a A142 mesh with sawn joints say every 4 -6 m depending on the geometry for a hard standing area.  The larger the area of the slab between the joints often the heavier mesh. These sawn joints encourage the shinkage to take place at the joint where you don't see it and this leaves the slab looking good.. if you have taken care laying it, the sub base and curing it properly. If you use a more stretchy mesh, Basalt say, then you may get more cracking where you don't want it.

 

In simplistic theory you could argue that if you have a ground bearing slab resting on a very very slippy DPM then it should behave a bit like a steel reinforced ground bearing slab when curing and shrinking.. but it will be more flexible when you come to run a few waggons over it. Thus it may not last as long?

 

Kxi.. by all means use it for your external slabs or the drive, it will be a bit of an experiment to some extent. The key to getting this to work is careful and exact preparation of the sub base, get it evenly compated.. as level as you can, use a plastic DPM so as the concrete shrinks it can easily slip about so that the movement takes place at the edges or at the joints.

 

 

 

 

[saveasteading]

Interesting product, and seems relatively easy to handle.

It would be so handy to be able to pick up a roll when needed in a hurry.

Thanks for showing this.

 

I am thinking that it is suitable for crack control more than for structural strength, so in a ground bearing floor slab.

 

Where is it obtained, and can you give guidance on the cost?

[SteamyTea]

11 hours ago, Gus Potter said:

This elasticity is important for concrete design as the concrete has to move more before the bars take up the same load as a similar sized steel bar.

This, without doing the calculations, would be my concern.

Not sure if there is any real advantage, except in lightweight, prestressed concrete construction.

[kxi]

To clarify, my main reasons for using BFRP are:

 

1 It will never corrode

This is mainly a concern for external concrete. As @George points out, concrete will likely eventually crack. With steel reinforcement (especially on a ground slab) this means water gets in, it will then very likely spall and the whole structure need repair or replacement. This might take 50 years to play out, but I'd prefer things to last much longer than this.

 

There is a concern from some that this current age of steel reinforced concrete will result in vast amounts of crumbling structures, at great financial and environmental cost.

 

https://theconversation.com/the-problem-with-reinforced-concrete-56078

Perhaps why China is putting so much effort into non-steel reinforcement alternatives. This mega-project using GFRP for similar reasons due to harsh conditions: http://galencomposite.com/news/saudi-aramco-runs-a-project-using-galen-rebars/

 

Basalt looked to me like the best of the alternatives:

 

As it happens the concrete slabs that we had to break up in the farm roadway were unreinforced, 60 years old, and had no cracks (why we had to remove them is another story), testament to being well installed I suppose. The steel reinforced concrete portal frame barn is now spalling and will need repair.

 

My steel frame supplier (who puts up a LOT of industrial and agricultural buildings) is similarly negative about steel reinforcement in ground slabs - concrete will last a very long time on its own. Adding steel builds in an inherent flaw.

 

2 Bit cheaper

Surprisingly, it looks like using the basalt mesh might work out cheaper to use than the equivalent steel (mainly labour savings). Unless it all breaks after 5 years of course.

 

3 Ease of install

This is what Orlitech seems to be selling it on https://orlitech.co.uk/composite-mesh-orlitech/ , and I suspect the difference is marginal in some cases. But in our case we were going to hand place 100 x 70kg sheets of A393 mesh, which is a lot of heavy lifting. The equivalent basalt mesh sheets are easily carried and placed by one person who could do it all day.

 

 

Both basalt and glass fibre mesh seem to be much more used in the US than the UK, but when I last checked, there was still caution over basalt as it is new - so limited to slab on grade work. As in the linked post above, basalt seems to be superior to glass fibre due to better tolerance of the alkaline environment in concrete.

 

Felt like it was worth an experiment. BC had no problem provided the SE was happy (which he partly was).

 

 

[kxi]

17 hours ago, saveasteading said:

Where is it obtained, and can you give guidance on the cost?

 

https://orlitech.co.uk/ (I'm using)

there's also https://basalt.tech/products.html who I've not spoken to

 

If you want a LOT of it you can buy direct from Galen

https://www.alibaba.com/product-detail/Basalt-fiber-wire-mesh-ROCKMESH-buy_1600071677485.html?spm=a2700.galleryofferlist.normal_offer.d_title.56906dc1BNT3Jx

 

For our use, the material cost is slightly cheaper than the equivalent steel (though as above one might argue about what is equivalent).

[saveasteading]

I don't think these concerns about reinforcing steel are generally justified. Farms buildings are about the worst environment. Very wet  nearly all the time, and slurry is not going to help. The workmanship tends to be 'agricultural' so water leaks through. I can't remember the chemistry, but steel and concrete coexist very well unless there is another problem.

Specifications are low too (and there is no factor of safety on barns).

 

Likewise for steel barns: agricultural sheds are designed down to a price. I once suggested to a farmer that he should use galvanised steel, and a better cladding coating to extend the life by decades, and he just said it would be his son's problem.

 

On a house, it should never be a problem.

 

Still interested in these glass mesh sheets though. For a ground floor slab once they have helped the crack control they don't have much structural role.

[George]

4 hours ago, kxi said:

There is a concern from some that this current age of steel reinforced concrete will result in vast amounts of crumbling structures, at great financial and environmental cost

 

 

 

I think their concerns are misplaced.

 

As long as buildings can pay for themselves and perform well within their intended design life, you're best off stopping there, else you risk over specifying and using resources better placed elsewhere. There's a cost (financial and environmental) involved if you end up over designing.

 

There are as many reasons to demolish, dismantle or otherwise mess around with buildings as there are putting them up in the first place!

 

Still, there are plenty of good reasons to switch steel for something else.

 

 

[Gus Potter]

Hello all.

 

By all means explore using this material, I'm all for using new technology.. it's not that new. There are recognised design codes that cover this material.

 

For the self builder.. a practical point. Using say FRP (Basalt say) rebar for an external "non structural slab" may well be cost effective if just laid in flat sheets. To get the best out of it you need to control the other parameters.. compaction, selection of the concrete type and strength, preparation of the sub base, the extra support to maintain the concrete cover etc..it does not corrode but if the cover varies too much you get uneven stresses in the slab which will promote cracking. This may put your local builder off or they may add a bit to the price.

 

If you are using it say in a structural concrete beam or a basement then you'll often need some bent bars. FRP bends need to be formed in the factory.. you have no wriggle room. Thus if you don't get all your setting out absolutely spot on you have trouble ahead. Steel rebar can be "tweeked" a bit on site.. FRP?

 

You may say.. well.. that is up to the builder to get it right.. you can but this will come at a cost..

 

For the technically curious. When you design say a reinforced concrete beam subject to downards loading you often aim to design the rebar as the "weakest link". Steel (in the tension zone of a beam) will stretch (yield) quite a lot before it fails. On the top of the beam the concrete is in compression. When concrete fails in compression it is a much more sudden (explosive) failure.

 

In summary one thing you do is to make sure that if a reinforced concrete beam / slab is over loaded then the occupants of a building get some warning.. big cracks appearing etc. You achieve this by taking advantage of the ductility property of the steel once it reaches it's yield point. Ductility and elasticity are two different properties.

 

But FRP (Basalt say) does not have quite the same forgiving nature to some extent as common structural steel / rebar. Once steel starts to yield it can "stretch" quite a bit before it fails. In fact with common structural steel you get a little extra out of it before really bad things start to happen. With FRP it is appreciated that you may get some yielding but often not as much as say with steel. Once you take this safety aspect into account (which you have to do to comply with the building regs in terms of structural design for domestic structures say) and look at things in the round then the difference between the two materials becomes less marked.

 

If you are considering using this for your build then it's worth a bit of research to check that while the big easy areas look good price wise you are not making a rod for your back later.. say with corners / beams or reducing the number of contractors you can go to to get a good price.

 

 

 

 

 

 

 

 

 

 

[SteamyTea]

7 hours ago, Gus Potter said:

For the technically curious. When you design say a reinforced concrete beam subject to downards loading you often aim to design the rebar as the "weakest link". Steel (in the tension zone of a beam) will stretch (yield) quite a lot before it fails. On the top of the beam the concrete is in compression. When concrete fails in compression it is a much more sudden (explosive) failure.

This was the case with carbon fibre chassis in the early days.  To get around the problem they incorporated some Kevlar.  Not for adding strength, but for when the inevitable failure happened, it failed less catastrophically.

Since the early 1980's better weaves and resins have been developed and the knowledge base is now huge and, you only have to watch Romain Grosjean's crash through the barrier at Bahrain to know how things have moved on.