Can steel fibers be added to any mix?
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 plasiticizer
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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