dear sir,  

plz  go through the following link

https://www.hindawi.com/journals/ace/2014/217473/ 

Thank you Mr. Siva Kishore.

The aspect ratio is depend on the type of fiber.

Thank you Mr. Carlos for these information.

I hope that you give me the specification that mention the relation between fiber length and maximum aggregate size.

I think the aspect ratio (L/D) " Length / Diameter" is the more important variable in the using of steel fibers  than its length only.

Dear Dr. Ahmed El-barbary

ACI Code put limitations on maximum aggregate size in relation with the smallest dimension of concrete member. So, is the any limitations on the length of steel fiber in relation with the smallest dimension of concrete member ?

Desta Al-Sherrawi, you could consider the tradicional recommendation for fiber reinforced shotcrete what sugests that the fiber length should be less than one third of the nozzle diameter in order to prevent clogging. So, an important aspect is workability: you have to verify if the fiber length is not too long that cause problems (take a look at this paper:  http://www.scielo.br/scielo.php?script=sci_arttext&pid=S1516-14392015000601284). On the other hand, if the fiber length is close to the smaller dizer of the element, it could provide a wall effect orienting the fibers and improving the fiber reinforcement. 

Thank you Dr.Antonio Figueiredo for this information.

Dear Dr. Mohannad

the max aggregate size in concrete  times 2 to 3 is the length of the fiber which is recommended 

For laboratory tests, the maxim length of the fibers should be equal or less than one third of minimum dimension of the mold. 

Dear Mr. Carlos Méndez-Esteban… you are so helpful. Thanks a lot

Thank you Dr. Amjad...

Thank you Mr. Amir, I'll be glad if you have a specification mention that.

Yes. in the ASTM C1609 in the section 7.1.3 you can see the requirements as: 

7.1.3 The width and depth of test specimens shall be at least

three times the maximum fiber length.

Dear Mr. Amir.

This specification was Withdrawn 2006, can I depend on it ?

Thanks Dr. Ravi Ganesh...

Yes, you can. Because by any update you see some minor modifications. even you can find new version of that standard

Dear Mr. Amir.

I think this specification was canceled in 2006 not update.

There is certainly a relation between fiber aspect ratio and grade of concrete. It matters for bond strength up to certain limit. 

Dear Mr. Uday

My question was about the relation between the steel fiber length and the smallest dimension of the concrete member not about aspect ratio.

Thanks

I think there is no relation between those parameters.

Dear Mr. Ali

I think the steel fiber orientation is important in casting and in strength.

Dear Dr. Mohannad,

Take a look on this paper

https://www.researchgate.net/publication/304347176_EVALUATION_OF_MECHANICAL_PROPERTIES_OF_HIGH_PERFORMANCE_SELF-CONSOLIDATED_CONCRETE_ENHANCED_BY_DISCRETE_STEEL_AND_POLYPROPYLENE_FIBERS

Article EVALUATION OF MECHANICAL PROPERTIES OF HIGH PERFORMANCE SELF...

Thanks Mr. Ali for this article.

The concrete was self-consolidated.

Yes it was Dr.Mohannad,

Suppose you had to choose between two products, sight unseen. One is said to be long and strong; the other, shorter and not as strong. No further information is provided.

Given that choice, you might be tempted to pick the first product. Long and strong — that has a certain ring to it.

But if the products in question are steel fibers, those attractive adjectives can steer you wrong. Though fiber length is a complicated subject, for most purposes shorter fibers work better than longer ones. And strength turns out not to matter at all.

In this photo the steel fibers range in length from 1 to 2 inches. Though it may look less imposing, the 1-inch fiber in the middle is the best of the three for concrete workability, even distribution, lack of exposed fibers at the floor surface, and crack prevention.

FIBER LENGTH

The steel fibers sold for use in floors range in length from 1 to 2-1/2 inches. The 2-1/2-inch dimension seems close to the practical upper limit. Beyond it, tangling becomes a big problem. In contrast, the 1-inch dimension is nowhere near the lower limit. Fibers less than 1-inch long are usable and may even offer advantages — a subject we will get to later. But for now, the market offers steel fibers from 1 to 2-1/2 inches long.

Within that range, fiber length affects concrete floors in at least five ways:

Concrete workability

Fiber distribution

Exposure of fibers at floor surface

Crack prevention

Residual strength or flexural toughness

Four of those ways favor shorter fibers. Only the last, residual strength, goes the other way and supports an argument for longer fibers.

FIBER-RELATED EFFECTS THAT FAVOR SHORTER FIBERS

So short fibers outperform long ones when it comes to concrete workability, fiber distribution, exposure of fibers at the floor surface, and crack prevention. The American Concrete Institute defines workability as “that property of freshly mixed concrete…which determines the ease and homogeneity with which it can be mixed, placed, compacted, and finished.” Fibers make concrete less workable — a statement that holds for every kind of fiber, and for every length. But the effect on workability varies hugely with fiber type, dosage, and length. Shorter fibers reduce workability less than longer fibers, and the difference can be dramatic. When you hear concrete finishers complain that steel fibers make their jobs hard, you almost always find that their bad experiences occurred with fibers at least 2 inches long.

The main reasone we use fiber is to prevent early cracks (first 2 hour the concrete has no tensile strength) . Later we want them be durable against tire load induced stress.

Thinner but longer (5 cm) is better. Thick ones are hard to compact in RCC pavements. Also, thick ones makes more gap between aggregate in fresh PCC concrete since they have good ressitance agisnt laying down the material.