where: [[lambda].sub.1] is the expected pull-out length ratio; [[lambda].sub.2] is the fibre orientation factor; [[lambda].sub.3] is the group reduction factor associated with the number of fibres pulling out per unit area; [tau] is the
average bond stress of a single fibre embedded in the concrete; [V.sub.f] is the fibre volume percentage; [beta] is the bond factor, accordingly to Campione (2008), can be assumed being equal to 0.5 for round fibres, 0.75--for crimped fibres, 1.0--for hooked fibres; [l.sub.f] and [b.sub.f] are the length and the diameter of fibre, respectively.