Fibre reinforce concrete foundations 1

[benholland82]

Hi all,

This sounds like a simple engineering problem but I'm stuck!!

Basically I need a foundation for a high mast light column but I don't want to use 'traditional' reinforcement due to the amount and location of services. I've approached a fibre reinforcement manufacture and I have been told that fibres can be used to resist the bending. Regarding shear however there is very little enhancement and should use the shear strength of mass / plain concrete.

What they couldn't help me with is the issue with the minimum area of reinforcement required by EC2 and how to deal with Early Thermal Cracking with fibres.

So have anyone got any ideas how I could check the use of fibres for ETC and EC2's requirement of minimum reinforcement?

Or alternatively, could I design these as mass/plain concrete foundation (approximate size 2 (wide) x 1m (deep)), but if I do how do I deal with ETC.

Thanks in advance,

Ben

 

[hokie66]

Fibres are not reinforcement. At best, they enhance some properties of the concrete, e.g. impact resistance. They won't help in a light pole footing.

 

[benholland82]

Thanks for your reply, so fibres can't control early thermal cracking? I thought fibre reinforced concrete was suppose to be a wonder material!!

Any ideas on a mass concrete foundation. If I make the distance from the edge of the column base plate to edge of foundation less than the depth of the foundation, spreading the load to the extremities of the foundation so to make sure the foundation is acting in compression only??

I'm finding very little design guidance for mass concrete.

 

[hokie66]

I am not sure what you mean by early thermal cracking. I have heard this term before, used by Europeans, to mean essentially what most of us call drying shrinkage cracking. If that is correct, shrinkage cracking is probably not a big concern in a footing of that size. However, I would never cast a piece of concrete that big without reinforcing it, and see no reason why a light pole footing should not be reinforced. If there is room for the services, there is room for the reinforcement.

 

[Ron]

Double ditto on hokie66

 

[jayrod12]

Small fibres in the concrete mix are not reinforcement if you ask me, yes they will actually help quite a bit with shrinkage/drying cracking but there isn't enough research yet to tell you how much.

If we're talking FRP rods then yes they provide great bending resistance, at a price of course. It's a whole new design philosophy when using FRP rods or sheets as they are a strictly linear material until very volatile failure and so any codes on the design limit the allowable stress considerably. And if frp sheets can be used for shear strengthening of bridge piers and beams why wouldn't it provide some shear resistance in a footing.

 

[ACtrafficengr]

I saw a presentation on steel fiber reinforcement some time back. They claimed that, unlike poly fibers, their product can be a substitute for rebar. I have no experience, personal or vicarious, with the product.

http://www.bekaert.com/en/Product C... steel fibres for concrete reinforcement.aspx

 

[benholland82]

Thank you all for your comments.

Just the clarify: What we (the UK) call early thermal cracking (ETC) occurs when the tensile strain, arising from either restrained thermal contraction or a temperature differential, exceeds the tensile strain capacity of the concrete. Whereas, dry shrinkage cracks are formed by restraint to volume changes caused by loss of excess water. Both require reinforcement of one type or another.

I've spoke to a few manufactures / suppliers and the consensus seems to be that there is not enough research to tell us if fibres would be able to resist the forces in such a foundation and further testing would be required, which is apparently being undertaken but results wouldn’t be expected for 18 – 24 months. Although, one supplier said their fibres would resist the bending and the shear capacity should be determined based on plain / mass concrete but when I asked him about ETC and if there was a way for checking compliance with EC2 minimum reinforcement (0.0013bd) he couldn't answer me, which didn't give me much reinsurance.

It is this minimum reinforcement which is giving me a headache as the deeper I make the foundation the more reinforcement I required. I am looking to have 25 mm dia. at 125 mm crs for a 1.5 x 1.5 x 1.8 m (deep) foundation. This seems a little excessive to me!

 

[Ron]

ACtrafficengr....manufacturers have been claiming this for years, but it only works for slabs on grade where conventional reinforcement doesn't do much anyway. As hokie66 noted, fibers only enhance the properties of the mix...not the system. Fiber increases the flexural strength of concrete (steel fibers moreso than polymer fibers) but offers little increase in the compressive strength. Early shrinkage and total shrinkage are reduced; however, control joints should still be used.

Fiber enhancement DOES NOT SUBSTITUTE FOR REBAR!!!

 

[hokie66]

benholland82,
I am not familiar with your codes, but the minimum reinforcement provisions of most codes have exceptions for footings and for thick elements. These exceptions sometimes allow only the outer part of the structural element to be considered when determining minimum reinforcement area.