Fibre Reinforced Concrete

[hannis]

Hi,

In the design of reinforced concrete water retaining structures somewhere in the region of 30%(depending on wall thickness, loading etc) of the reinforcement is there to restrict cracking to 0.2mm. Has anyone used or considered replacing this anti-crack reinforcement with steel fibres, to create a steel fibre/loose bar composite.

Regards

Hannis

 

[JedClampett]

I'm guessing that someone has tried everything once. If steel fiber reinforcement was preferable to plain reinforcing, it would be recommended by the PCA, ACI and other code agencies.

 

[Focht3]

As I recall, the big problems with steel fiber reinforcement (initially) were: 1) uneven distribution, 2) protrusions from the concrete surface, and 3) corrosion over time, leaving an uneven appearance. I don't know if these have all been addressed by the fiber manufacturers -

 

[JedClampett]

Many, many years ago, there was a big push to use fiber (metal and plastic) reinforcing. From what I see in the technical literature, it's still being heavily promoted. Besides the reasons that Focht3 cited, I remember that the batch plants didn't like them because the metal fibers wore out their mixers prematurely.

 

[Focht3]

The polypropylene fibers make for fuzzy slabs, too!

 

[Ron]

hannis...fiber reinforcement has been used many times for the application you describe. Don't let the manufacturer's sales pitch override common sense though.

The fiber is there to "enhance the properties of the CONCRETE", not to provide system reinforcement. In short, it will help the concrete but you still need structural reinforcement. The property enhancement of the concrete will come in both increased compressive strength and increased tensile/flexural strength. This will help reduce cracks in an otherwise good design.

I prefer steel fibers for these applications. Polypropylene fibers are not as good, in my humble opinion.

The steel will corrode at the surface, but it doesn't extend into the concrete very far and the fibers are discontinuous so there is not much chance of corrosion continuity. Besides, the alkaline concrete passivates the embedded steel, helping to resist corrosion of the embedded pieces. You can also get stainless steel fibers, but more expensive.

Production problems still exist. If the fibers are not introduced slowly into the mix, balling can still occur (the manufacturers want you to believe otherwise). This is a minor nuisance and can be easily overcome. Make sure the concrete slump is increased by the addition of a plasticizer, as the workability of fiber concrete is less than conventional concrete.

 

[wsheppard]

You might take a look at PVA (polyvinylalchohol) fibers. This has very good pull out resistance, won't make the slab 'fuzzy' because it doesn't float and lends itself to better mixing/distribution. At 12mm a 1.6 pound per yard mix is a good starting point, up to around 2 pounds.

In larger quantities, it CAN be used to increase reinforcement. At 44 pounds per yard it replaces steel. Very cool stuff, but not well known in the US.

Some great work has been done at UofMich. You might also like to see this -

http://www.kuraray.co.jp/pvaf/english/