For reinforcing of forming materials of the cold setting type such as concrete or various cross linking plastic materials, solid or foamed, is provided a thermoplastic fibre material, the fibres of which have a surface layer of mineral particles which are partly and only partly embedded in the thermoplastic fibre material, to which the particles are firmly weld bonded, such that the exposed outer particle surfaces are firmly bondable to the forming material. Hereby the mineral particles will form a rigid connection between the forming material and a central fibre core or layer of high tensile strength, whereby the thermoplastic fibres are effectively surface bonded to the forming material so as to show a high reinforcing effect.