Isothermal hardening in bitumen and asphalt mix

It is well established that the mechanical properties of bitumen can change isothermally at moderate and low temperatures, because of physical changes occurring in the binder.

It is well established that the mechanical properties of bitumen can change isothermally at moderate and low temperatures, because of physical changes occurring in the binder. However, evidence of an impact of isothermal hardening on the mechanical properties of asphalt mixtures is very limited. The main focus of this paper is to investigate the importance of isothermal hardening on the characteristics of an asphalt mix, in particular on its low temperature fracture resistance. The objective was to investigate whether the phenomenon of isothermal hardening should be taken into consideration when evaluating binders for their low temperature performance.

In a first part, binder properties were investigated, using a bending beam (BBR) and a dynamic shear rheometer (DSR). A study of hardening effects at different temperatures (from –25°C to +25°C) was performed including the reversibility of this phenomenon and the influence of high strain levels applied during isothermal storage. In a second part, hardening effects on the corresponding asphalt mixes were evaluated using two type of tests: uni-axial tension/compression and tensile stress restrained specimen tests (TSRST) to investigate mix stiffness and fracture properties respectively.

For the five binders investigated, the isothermal hardening became more pronounced at low testing frequencies (below 1Hz) or at long loading times. Differences were seen between the binders, in the temperature dependency of the hardening as well as in the degree of hardening. Isothermal hardening could be “reversed” by heating the binder 20- 25°C above the isothermal temperature. By applying high strain oscillations during isothermal storage, the extent of hardening could be reduced. In modulus testing on the asphalt mix an isothermal hardening was also observed although to a lower extent. In mix fracture tests, the duration of isothermal storage had no influence in the case of unstressed samples. If samples were stored constrained, isothermal storage times were important because of stress relaxation, but there was no influence of isothermal hardening. Therefore, these limited results indicate that a low temperature cracking related performance test on binders does not need to consider the effects of isothermal hardening. But of course, and as is already known, these effects must be taken into consideration for the precision and reproducibility of stiffness and elasticity measurements.

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