Underpinning - mesh fibres in concrete

[Norcon]

Meanwhile what actual benefit would these fibres have to an underpinning strip foundation, that makes them a necessary or even desirable addition?

Added strength without any addition of cement.

A cube test with fibers and non fibers.
Which one sustains the highest force before failure?

 

[^woody^]

So by that logic, we should reduce all cement content of concrete and add fibres to all mixes instead?

The OP could also remove the sand and have an no-fines mix too

There is still no valid reason and no benefit of fibres in this situation

 

[pred]

[/quote]



What happens when you add cement to a concrete mix?
It increases the strength!
[/quote]



Rubbish! I think you'll find.

I think you'll find it becomes very hard and brittle like porcelain.

The reason I believe this, some stupid git had rendered a wall using a high ratio of cement, it was solid like china, unfortunately it didn't stick to the wall, I gave it one gentle tap with my LUMP hammer and the whole lot shattered like glass.

 

[freddiemercurystwin]

There is no evidence to suggest that bananas are useless in a foundation either

Actually I beg to differ, I once specified bananas for a foundation and I couldn't believe my eyes when the building subsided.

 

[^woody^]

Peel them first, stupid

 

[Norcon]

So by that logic, we should reduce all cement content of concrete and add fibres to all mixes instead?

Yes. 30 newton with fibers is similar to 35 newton.

The OP could also remove the sand and have an no-fines mix too

No keep the fines.

There is still no valid reason and no benefit of fibres in this situation
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There is. It increases the strength.
Like I said. Spend a day on the breaker and you'll know why. I get the felling you know jack sh@it about what you are talking about. Another office boy who never gets his hands dirty.
The very fact alone that fibers reduce cracking is an indication of their strength inducing properties.

 

[tony1851]

Fibres can be useful for slabs to reduce shrinkage cracking, and in some cases remove the need for a designed movement joint.
But in the case like the OPs, involving foundation concrete, is it really necessary? Shrinkage is not an issue in that situation, neither is enhanced strength.
Would the fibres be more expensive than the reduced amount of cement?

 

[^woody^]

I get the felling you know jack sh@it about what you are talking about.

That's odd, because I got the exact same feeling about your knowledge of design and specifying ..... which is not just about throwing a load of stuff together because "it might be stronger"

You stick to barrowing concrete, I'll stick to designing it. Thank you

 

[Norcon]

Your not qualified to design concrete. Only experience in the concrete world gives you that.
Field experience son. Just like that link earlier explained.

 

[rocky_robin]

Wow, I never realised that my topic could be such a discussion point.

I contacted the technical dept in one of the uk's largest manufacturer of fibres and said that I'd been advised to use fibres in my underpinning.
The chap had to discuss with his technical director along with others in the company and they came back saying that they were not willing to suggest anything as they had never trialed it nor had any request to use fibres for underpinning.

I talked to the structural engineer (again) and he gave the reason for using 'long' non-steel fibres was that I could not access the area with rebar. Also that the rebar would be required to join onto the next rebar section but as I'd have to wait until the concrete was set before excavating the next section. (obvious safety reasons)

I talked to my local ready mix company and they were not surprised that the structural engineer was non-committal, however they would supply the bags so I could mix on site.

I found this procedure online, but it only suggests using a 23N mix (structural engineer suggested a C30 strength) Hmmmmm

http://www.e-lindsey.gov.uk/NR/rdon...2-73DF8253DF44/0/022traditionunderpinning.pdf

 

[^woody^]

I talked to the structural engineer (again) and he gave the reason for using 'long' non-steel fibres was that I could not access the area with rebar. Also that the rebar would be required to join onto the next rebar section

That makes him even more of an idiot (or perhaps another farmer who has worked in the field)

Rebar is used to join one bay to the adjacent bay so that once set, the new foundation acts as one.

Fibres will not do this at all

 

[rocky_robin]

My thoughts too. I'm a mechanical engineer so it did not make sense to me. Would u install a rebar into on. Excavated slot and push through into the soil next to it? Cement that area and excavate carefully around the rebat? Once that is cemented in, they could be linked together?

Its difficult checking out peoples underpinning designs on the internet.

 

[^woody^]

You would tend to bang a couple of ø20mm or so x 500mm bars into the earth on the centre of adjacent bay to give a 250mm tie

 

[Norcon]

I talked to the structural engineer (again) and he gave the reason for using 'long' non-steel fibres was that I could not access the area with rebar. Also that the rebar would be required to join onto the next rebar section

That makes him even more of an idiot (or perhaps another farmer who has worked in the field)

Rebar is used to join one bay to the adjacent bay so that once set, the new foundation acts as one.

Fibres will not do this at all

Have a look at what he said above in bold.
He's advocating using re-bar but also fibers to add strength in the non re-barred section.
He's certainly no idiot.

Maybe he's just a big farm boy eh?

But obviously you could add rebar across the entire bay and overlap the ends. With the fibers also adding further strength.
You want to use a vibrator also. No concrete is ever finished properly unless its vibrated.

Also when you start re-barring you want to move up to 35 newton.
Stick with me and I'll keep you right woody.

 

[rocky_robin]

Norcon, I don't want to burst your bubble, but I may not have written the post clear enough.

The structural engineer said not to put any rebar, as access would be difficult. He said fibres only............

I think I will stick in rebar regardless and would rather it be stronger than fibre alone. I don't trust an 'engineer' when they can't give me a nice confident feeling.........

Robin