The enhancement of performance-related properties of bituminous binders by means of fibers

In this paper the authors present the results of an extensive laboratory investigation carried out in order to understand the rheological effects of fibers on bituminous binders. In particular, specific homopolymer polyacrylonitrile fibers have been selected as appropriate for use and have therefore been thoroughly analyzed. It is shown that these products do not have simply a filling effect in the binder but on the contrary are responsible of specific changes in its rheological behaviour, decreasing the stiffness at low temperatures, increasing the overall hardness, decreasing the width of the relaxation spectrum and decreasing the temperature susceptibility. Moreover, they have a direct effect on the elasticity of the binder which is revealed by a consistent decrease of the phase angle. The experimental data clearly suggest that the observed rheological effects quantitatively depend upon the dosage and length of the fibers. In this context some hypotheses are made regarding the mechanisms by which the fibers cause such an enhancement of the properties of the binder. The results of the viscoelastic modelling of the rheological data obtained from the use of a dynamic shear rheometer are also presented. Master curves of the complex modulus and of the phase angle, expressed as a function of fiber dosage, have been derived and have been subsequently used for the calculation of rheological parameters that may be linked to predicted field performance. Based on the obtained results a number of conclusions can be drawn that may guide the pavement engineer in the proper use of these innovative products.