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January 26, 2021

Concrete Reinforcement – You Want Rebar or Steel Fibres?

It’s actually a trick question.

Why? Because rebar does “one thing”, and steel fibres do “another thing”. You don’t get to choose. You either want the thing that rebar does, or the thing steel fibres do. You might want both. Then you use the product, or products, your concrete needs.

To find out what “the thing” and “the other thing” is, read on.

Concrete Reinforcement

Generally speaking, concrete used in structures needs reinforcement. This is because concrete is strong in compression (e.g. 25-50 MPa) and only a few MPa in tension. In a reinforced concrete member, the concrete takes care of the compression forces, and the steel reinforcement takes care of the tensile forces.

Rebar

Rebar is reinforcing bar. It’s either in single lengths, or welded together into a mesh grid called “mesh” or “fabric”. Both forms are the main reinforcement used in concrete structures. Sizes range from 6 mm diameter to 36 mm diameter, and are continuously deformed (they have knobbly bits on them to bond with the concrete and resist pull-out).

Rebar

Rebar Steel

Steel Fibres

Steel fibres are made from hard-drawn high-tensile steel wire. They’re continuously deformed and manufactured in various lengths and shapes, typically 40 -60 mm long.

Steel Fibre

Steel Fibres Close-up

So steel fibres and rebar have some things in common. Both are made of steel. Both are used to reinforce concrete, and both can be found in floor slabs. Because of those common features, it is tempting to think of the two products as fundamentally alike, differing only in size. But that’s wrong.

Steel fibres and rebar work differently and accomplish different goals. Sometimes you need steel fibres, and sometimes you need rebar, and only in a few limited situations can one effectively replace the other.

To put it simply, steel fibres prevent cracks, while rebar limits the width of cracks.

How Steel Fibres Prevent Cracks

Let’s start with some definitions. For our purposes here, a crack is a break big enough to see with the naked eye. Cracks often penetrate a slab from top to bottom and may extend all the way across a slab’s width. A micro-crack is a break visible only with a microscope. Micro-cracks typically do not go all the way through a slab, nor all the way across its width.

Microcrack

Microcrack 

All concrete has micro-cracks, and some of them appear at an early age. When concrete is subject to tensile stress, which can come from drying shrinkage, thermal contraction, or applied loading, some micro-cracks grow wider and longer and become cracks you can see. Not every micro-crack grows up to be a crack; most of them stay small. However, every crack started life as a micro-crack.

Neither steel fibres nor rebar can eliminate micro-cracks. But any piece of embedded steel, in the right place, has the potential to stop a micro-crack in its tracks. This is true of steel fibres, rebar, or even mesh.

But here’s the trick. The embedded steel has to be very close to the source of the micro-crack to stop its growth. Once a crack has developed enough momentum, nothing can stop it from ripping all the way across a slab. This is where steel fibres prove their worth, because only fibres can provide the dense distribution of steel needed to catch every micro-crack.

Steel Fibre

Steel fibres mixed with concrete

If you make concrete with steel fibres at 24 kg/m^3, you get, on average, 730 fibres in each 100 x 100 x 100 mm cube of concrete. No micro-crack can go more than a few millimetres without running into a fibre. In contrast, rebar designs always space the bars at least 100 mm apart in slabs, giving micro-cracks plenty of room in which to grow.

The Role of Rebar

Since steel fibres do such a good job preventing cracks, why would we ever reinforce concrete with anything else? Well, it turns out that fibres can’t prevent every crack in every situation. With enough tensile stress driving it, a crack can rip past the fibres and grow wide. A suspended concrete slab will bend, placing the rebar in tension.

A concrete beam under load bends, placing the concrete in compression and tension

No amount of steel fibres will hold onto that tension. But ground slabs can be engineered to keep tensile stresses low, and then steel fibres do fine as the sole reinforcement. The value of rebar lies in its ability to resist tensile stress after the concrete has cracked, and also in its length, which allows for continuity over long distances.

The longest steel fibres are about 60 mm long, and the fibres best suited to preventing cracks are shorter than that. In contrast, rebar comes in 12-18 m lengths, and with the right laps, it can be extended without limit. Only rebar can provide continuous reinforcement over the full span of a suspended slab or beam.

No Equivalence

Engineers wishing to switch a design from rebar to steel fibres sometimes use formulas or tables to determine the fibre dosage. While such formulas and tables may be mathematically correct and can be useful, they lead to a fallacy: the idea that steel fibres and rebar are equivalent and that one can readily substitute for the other.

In truth, steel fibres and rebar can never be equivalent because they work differently and achieve different goals. If a design really needs one, no amount of the other will accomplish the same result.

Steel Fibres and Rebar Together

Since steel fibres and rebar achieve different goals, it often makes good sense to use both. A ground slab in a bunded process area is an example of a highly restrained slab. There are pump bases and other thickenings, kerbs, etc all preventing an otherwise free and slippery slab to expand and contract on a plastic sheet.

The restraints prevent the movement and so high tensile stresses develop in the concrete which require decent amounts of rebar or mesh to control the cracking. Note I said control, not eliminate. Cracks promote moisture penetration, and ultimately degrade the slab.

Utilising steel fibres in conjunction with rebar could further reduce the extent and width of the cracks and improve durability.

Rebar & steel fibre concrete reinforcement

Conclusion

Next time someone asks “steel fibres or rebar?” remember that it’s not really a choice. If you want to stop micro-cracks from growing into visible cracks, you need steel fibres. If you want to limit the width of cracks that will occur anyway, you need rebar. And if you want to achieve both goals, maybe you need both products.

If you require further information, get in touch. Perhaps you have a slab that could benefit from steel fibres now that I’ve got you thinking!

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