FAQ - Mixing steel fibers 

All you need to know about Steel Fiber Reinforced Concrete

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Yes. Introduce steel fibers after all other ingredients are already in the truck. Set the truck mixer on charging speed and add the fibers slowly into the mixer. Mix for about 5 minutes at charging speed.
The steel fibers can also be added to the aggregate batch belt, if there is safe access.

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Bekaert recommends continuing mixing at the highest drum speed for about 4 to 5 minutes after all steel fibers are added to the truck.

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Yes, also adding fibers on site to the truck mixer is feasible. Gradually dose fibers in the mix, this is typically done via a conveyor belt.

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Steel fibers can be used in concrete, mortar and grout. Generally harsh mixtures, containing very few amount of fines and/or an unsteady sieve curve at a higher fiber volume can create mixing and dispersion problems. Simply mixing steel fibers into any concrete will most probably not utilize all the positive effects fibers can provide to concrete. Depending on the type and amount of fibers adjustments to the concrete mix may need to be made.
For example:

  • Increasing the content of fines
  • Adjusting the grading curve
  • Adjusting the amount of plasticizer

For concrete strength up to an actual strength of around 8000 psi typical fibers with normal strength wire are sufficient (majority of applications). For higher concrete strength´s than middle strength or high strength, fibers can be required to avoid a brittle behaviour. If special cements, aggregates or admixtures are used (seldom the case), a preliminary mix/pump test is recommended.

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A properly designed concrete mix is essential for avoiding fiber balling. In order to avoid the potential for fiber balling related to fibers with a high l/d (aspect) ratio (meaning high performing fibers), glued fiber technology has been developed. Glued fiber bundles will spread the glued bundles evenly on “macro level” and during mixing the bundles separate into individual fibers. In essence the glued bundle temporarily lowers the aspect (l/d) ratio of the fibers for easy mixing. That´s how balling can effectively be avoided and a homogenously mix of high performing steel fiber reinforced concrete can be achieved.

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Steel fiber mix designs are similar to those commonly used for plain concrete mixes. Recommended aggregate gradations and mix proportions are provided in local standards. Using the largest practical top size aggregate and a well-graded combined aggregate blend as opposed to a gap-graded blend can minimize shrinkage. Steel fibers may cause a reduction in slump due to their stiffness. This does not necessarily equal a reduction in workability. Depending on ambient temperatures and placement method, mid-range water reducers are commonly used to enhance workability for mixes with more than 30 to 40 pounds per cubic yards of steel fibers.

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Yes, the addition of steel fibers at typical dosage rates of 25 to 65 lb/yd3 will reduce the apparent slump by 1” to 3”. However, this does not necessarily equal a reduction in workability. Use of vibratory consolidation, restores the workability to the SFRC.

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