What are Glued/ collated steel fibers?
Glued/ Collated steel fibers are hard drawn, low carbon, high tensile steel fibers of specific length and diameter, glued together using water soluble glue. Steel fibers are added to concrete and can replace partially or completely the conventional reinforcement. Concrete made using Steel fibers are called “Steel Fiber Reinforced Concrete “- SFRC. Typical dosage of steel fibers in concrete varies from 15 to 45 Kgs/m3. In structural applications, higher dosages can be used to provide very high endurance properties. Glued steel fibers provide crack resistance, shear strength, fatigue resistance, impact resistance, flexural strength and post crack ductility.

How are collated steel fibers different from Synthetic fibers?
Synthetic fibers are short fibers of polypropylene. Typical dosages are 600 to 900 gms/ m3. Unlike steel fibers, Synthetic fibers provide no reinforcement by way of post cracking resistance. Synthetic fibers are mainly added for water retention in concrete, which helps in plastic shrinkage. After curing, the concrete with synthetic fibers remains as brittle as conventional concrete. Steel fibers not only control shrinkage but also enhance the physical properties of concrete.

How does steel fibre work?
The hooked ends of the glued steel fibers anchor itself firmly in the concrete matrix. These help in absorbing more deformations, providing strength and stability to concrete. The glued steel fibers are more or less evenly distributed in the concrete matrix. Typical spacing between two fibers is 10 to 25 mm. In the event of minor crack initiation, the steel fibers block them at the initiation stage itself and prevent it from developing in to major cracks.

What is the difference between loose and collated/ glued steel fibers?
Loose steel fibers are supplied loose. They may not have uniform size and can be inconsistent in tensile strength. The major disadvantage of loose fibers is its tendency to clump together forming fibre balls. Loose fibers also tend to occupy more volume when packed. Glued steel fibers are cold drawn and stuck together by water soluble glue. Glued steel fibers are compact and tend to disperse readily and integrate in to concrete, without forming balls. Since glued steel fibers are more compact they occupy less space when packed.

Can steel fibers replace conventional / traditional reinforcement?
Steel fibers can replace traditional reinforcement for non structural applications viz. floorings, highways, airstrips etc. For structural applications, complete ductility is required to avoid collapse after cracking i.e. the structure should have a skeleton to support it; hence rebars are mandatory. In case of elements with low to medium bending movements, like slabs it is possible to eliminate wire mesh substituting it with glued steel fibers. However, beams & beam column joints can be additionally reinforced with fibers providing enhanced resistance against cyclic loads and higher shear strength.

Can I reduce slab thickness using steel fibers?
When compared to conventional concrete slab, slab thickness can be reduced up to 25% using steel fibers with dosages as low as 15 Kg/m3. Detailed design manual has to be referred and software needs to be adopted.

Does the fibre corrode?
Glued steel fibers come with a small sectional area and the concrete envelops it by forming an alkaline membrane, which prevents corroding. Since the fibers inhibit any major crack formation, the exposure of fibers is only limited to the concrete surface. Hence the chances of corrosion are very negligible.

Will the fibers show on surface of flooring? What should be done to avoid this?
The steel fibers tend to lay flat without protruding on the surface. Usage of screeds and trowels ensure that the fibers are laid evenly. Incorporation of sufficient fines in the matrix will further ensure that fibers are not visible on surface.

What is ductility and post cracking behavior?
Ductility is measured by plate tests. Concrete by nature is a brittle material. It exhibits elastic behavior till cracking and then collapses completely. Rebars and fibers are added to support and overcome this complete collapse. These reinforcements ensure that load bearing capacity is not totally lost after cracking. Full ductility means the cracked portion exhibits higher resistance than un-cracked section; while partial ductility means the cracked portion continues to offer resistance to load. Post cracking ductility can be full or partial depending on the fibre dosage.

How do I ensure adequate distribution of steel fibers in concrete matrix?
The main parameter is mixing time. 1 minute per meter3 of concrete is sufficient to disperse the glued steel fibers. Give minimum mixing time of 5 minutes at full rotational speed. Never add glued steel fiber as the 1st ingredient. It may be added during or towards the end of mixing cycle.

Do glued steel fibers influence process ability of concrete?
By the addition of glued steel fibers, the wet concrete stiffens a bit. In technical terms, there is a reduction in slump. It is advisable to add admixtures or super plasticizers to overcome this slump. Correct dosage of super plasticizers can be ascertained by trials.

Can SFRC be pumped?
Concrete with even high dosages of steel fibers 100Kg/ m3 can be pumped. Please maintain minimum hose diameter of 5 inches / 125 mm. There is a reduction in slump, which can be corrected using super plasticizer.

What is balling, how to prevent this?
Balling is the phenomenon by which, the fibers come together forming a bunch during its integration in concrete. Glued steel fibers by its typical geometry and size do not form balls. It is advisable to allow sufficient mixing time, neither too low nor too high.

What is the recommended dosage of steel fibers?
Recommended dosages are 15 to 60 Kg/m3 of concrete. This depends on the criticality of application and the ductility required. Structural Engineer is the best judge to decide the optimum dosage. For structural applications, dosage could be as high as 100 Kg/ m3. Minimum steel fibre dosage can be determined using McKee formula: Minimum Dosage = 67,658/ (Aspect Ratio)2 Kg/cm3.

What is the minimum fibre dosage required in SFRC?
In concrete matrix, when the fibers are faced too far apart a crack may pass between the fibers. Hence the fibers have to close enough, known as average spacing, which is determined the no. of fibers in a particular volume. Hence a minimum quantity of fibre is required in the matrix, to be effective. Minimum average spacing is 0.45 X Fiber length. Minimum Volume Fraction = Vol of 1 fibre (Min Avg Spacing) 3.

How do I determine the dosage of steel fibers?
The dosage is best determined when the concrete if fresh and before it is laid. This allows corrections to be made in concrete mix. As a practice, 10 lts of the concrete mix need to be sampled out for analysis. The aggregates can be washed out and the steel fibers can be extracted with magnet, dried and weighed. After the concrete has hardened, the samples can be drawn only by drilling. This process is not only cumbersome, but the results are also not accurate as the fibers tend to get crushed. Other instrumental methods can be employed but these are costly.

How and when do I add the steel fibers?
Steel fibers can be added during the process or after batching of concrete. However, steel fibers should never be dosed as the 1st ingredient. These can be added using conveyor, blasting machine or manually in to the mixer. Adequate safety precautions need to be taken while handling the product.

Does steel fibre help in construction of earthquake/seismic resistant structures?
Yes it does. Steel fibre reinforced High Performance Concrete can undergo large displacements without developing wider cracks. It imparts high ductility, which is one of the essential properties of beam column joints. Addition of fibers decreases the rate of stiffness degradation appreciably when compared to structures without fibers. Load carrying capacity and resistance to cyclic loads are enhanced remarkably using steel fibers.

What is the cost impact of glued steel fibers?
Typical fibre content is 0.5 to 2.5% by volume of concrete. For typical slab application there could be around 42% cost savings. The cost impact will also depend upon the glued steel fiber dosage recommended by the structural engineer. Since glued steel fibers can wholly or partly replace rebars in non structural applications, there is significant cost savings on material alone. Moreover, glued steel fibers can save time and labor. There are immense benefits by way of improved fatigue resistance, crack resistance, anti-seepage, ductility and impact resistance.

What are the standards specified for structural SFRC (Steel Fiber Reinforced Concrete) applications?
Steel fibers are in use for more than 30 years in structural & other applications worldwide. Unfortunately, there are no standards available for structural SFRC in India. The situation is no better in other countries. The only countries that have standards for structural SFRC are Austria & now Germany. ASTM A 820-04 [1] and EN 14889-1:2006 [4] categorizes steel fibers based on the manufacturing process and their base materials.