The properties of fibres for use in concrete are covered by BS
EN 14889, Fibres for concrete,
Part 1, Steel fibres – Definition, specifications and conformity,
Part 2, Polymer fibres – Definition, specifications and conformity.

One requirement is for the supplier to declare the unit volume of fibres in kg/m³ to achieve a residual flexural strength of 1.5 MPa at 0.47mm central deflection and/or a residual flexural strength of 1MPa at 3.0mm central deflection.  The standard beam test is covered by BS EN 14845, Test methods for concrete, Part 2, Effect on strength. Reference concretes for the manufacture of specimens are covered in Part 1 of this Standard.