Steel Fiber Reinforced Concrete (SFRC) – Properties, Mix Design, Advantages and Disadvantages

Steel fiber reinforced concrete

Steel fiber reinforced concrete is a composite material having fibers as the additional ingredients, dispersed uniformly at random in small percentages by volume in plain concrete. Steel fiber reinforced concrete is manufactured by adding steel fibers to the ingredients of concrete in the mixer and by transferring the concrete into molds.

Properties of steel fiber reinforced concrete

Shrinkage

The volume change shown by concrete bodies due to the loss of water is known as shrinkage. Steel fibers lower the plastic shrinkage crack widths to a certain extent when compared to plain concrete.

Creep

Creep is the long-term distortion that a material undergoes when subjected to a constant load. The flow of water out of the cement paste and the prorogation of micro cracks are two factors that cause the concrete to creep. It can be seen that the steel fibers have a negligible effect when low fiber content is added while a significant improvement is gained with a larger amount of steel fibers. It should also be remembered that for ground slabs, flexural creep is more critical than compression creep.

Durability

Due to their effects on the alkali-acid reaction, leaching properties, resistance to chloride or sulfate attack, reinforcement corrosion, and freezing and thawing characteristics, porosity and permeability are important elements determining the concrete’s durability. Because of the greater W/C utilized to promote workability, SFRC mixes had significant porosity and permeability at first. Reductions in the W/C ratio have recently been possible, resulting in relatively low porosities and permeability. Tests revealed that the SFRC has permeability values similar to ordinary concrete, implying that, except for steel fiber corrosion, the SFRC has the same durability as plain concrete.

Also Read: What is Lightweight Concrete?

Mechanical Properties of Steel Fiber-Reinforced Concrete

Toughness

It is the total energy absorbed before the complete separation of the specimen. It is calculated as the area under the load-deflection curve plotted for the beam specimen used in a flexure test. Steel fibers significantly improve concrete toughness.

Flexural Strength

The addition of steel fibers into plain concrete will increase the low flexural strength of plain concrete. Flexural strength for plain concrete is the stress capacity determined through a third-point loading test, which strives to find the stress at a maximum load that can be sustained by a prismatic beam.

Compressive Strength

The inclusion of steel fiber in concrete increases its compressive strength value relative to the fiber content.

Shear Strength

Steel fibers increase the shear capacity of concrete significantly. Shear strength capacity is important for pavements. Corner and edge break-off might occur as the result of exceeding the shear capacity of concrete; storage racking or raised storage legs can also punch on the floor.

Also Read: What is Shotcrete 

Steel fiber reinforced concrete mix design

The main objective of designing a steel fiber reinforced concrete mix design is to produce adequate workability, ease of placing, and efficient use of fibers as crack arrestors, besides the other objectives desired in any normal concrete.

The addition of steel fiber to the concrete mix should be accompanied by the addition of cement paste to maintain the degree of workability and provide an adequate bond of the fibers to the concrete mix. The amount of added cement paste depends on three principal factors the number of fibers, the Shape and surface characteristics of the fibers, and the Flow characteristics of the cement paste. Steel fiber blends are designed using the coupling concept. To put it another way, standard concrete mix proportioning criteria can be used to create trail mix, and the workability can then be changed when steel fibers are added. The Portland Cement Association created the mechanistic mix proportioning design approach in 1977, and it was based on three principles:

• The addition of steel fibers should be accompanied by the addition of an amount of cement paste sufficient to coat the fibers and to ensure their bond in the concrete mix.
• The added fibers and cement paste should be treated as a replacement for an equivalent volume of the plain concrete mix
• The water ratio in both plain and SFRC mixes remains unchanged.

Guidelines to serve the purpose of SFRC mix design:

1. Coarse aggregates should be limited to 55% of the total aggregate.
2. Water cement ratio of the concrete should be kept below 0.55 (0.35 is recommended).
3. The SFRC should have a minimum cement content of320 kg/m³.
4. Reasonable sand content of 750 – 850 kg/m³ is recommended.
5. The workability could be improved by increasing the cement paste, which is possible by the addition of slag to replace the cement.
6. The maximum size of aggregate used in the concrete should be 19 mm.

Advantages of steel fiber reinforced concrete

• Increases the ductility, toughness, strength, fatigue endurance, deformation characteristics which are the major reasons for saving time, cost, and materials while using the SFRC

Disadvantages of steel fiber reinforced concrete

• Loose fibers at the hardened surface might be blown onto aircraft engines or tires which leads to unsafe operation
• Personnel is scraped or sliced by an exposed fiber while working on the concrete surface.
• At the aggressive exposure conditions, corrosion of the surface could take place eventually influencing the appearance of the surface.

Application of steel fiber reinforced concrete

This steel fiber reinforced concrete is commonly used in pavement, slab casting, slab repairs, silica fume, and runway.

How do you make fiber-reinforced concrete?

1. Prepare the concrete mix
2. Add fibers last to the mixer
3. Add the fibers gradually at a maximum dosage of 40kg/min
4. Mix for at least five minutes at the highest drum speed
5. You can pour the concrete either directly or by pump
6. The fibers are glued in bundles that are gradually released during mixing.
7. Pour the mix into a dry environment
8. On a well-prepared underground and within an hour of mixing
9. Vibrate the concrete immediately and thoroughly after pouring to avoid fibers at the surface
10. Apply hardener to ensure hardness by applying a dry-shake hardener and troweling until you get a mirror-like finish
11. Finish off with curing compound to avoid drying shrinkage cracking

Also Read: How to calculate cement sand and aggregate quantity in concrete?

Fiber-reinforced concrete vs steel-reinforced concrete

There are the following differences between Fiber-reinforced concrete and steel-reinforced concrete

Fiber-reinforced concrete	Steel-reinforced concrete
Reinforced with short linear fibers	Reinforced by either metal reinforcement bar, wire mesh, or some other kind of metal
A composite mix of concrete and fibers,	Traditional steel mesh concrete that can withstand heavy loads
Have high modulus of elasticity and flexural strength	Increases durability and reliability
Eg: Natural fiber reinforced concrete, carbon fibers, asbestos fibers

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