Saravanan Umakanthan

The computation of modulus of bituminous mixes from a beam bending test conforming to an undamaged yet steady state is not straightforward. The most widely used practice of choosing the 50th or 100th cycle, for modulus computation, does not ensure that the material response has reached a steady state, crossing the initial transient stage of drastic modulus reduction. In the current study, it is analytically shown that the material retardation time governs the attainment of a steady state response. The study proposes a methodology for post-processing the four-point beam bending test data so as to identify the cycles that could result in a sensible computation of material modulus. The experimental investigations involve four-point beam bending (4-PB) tests carried out on four bituminous mixes, prepared using an unmodified and three modified binders. The 4-PB tests are carried out at 20 oC, 10 Hz frequency at strain levels of 200, 400, 600 and 800 micro-strains. The harmonics of the input/output waveform has been used to identify the cycles that can be used for the computation of a material modulus, and the modulus is computed from the Lissajous plots (stress–strain plots) of the cycles prior to damage initiation. Based on harmonic analysis, the current study specifies a range of modulus for each mix at various strain levels, and the values are further compared with the modulus computed using the conventional post-processing procedure. It is observed that the conventional procedure of using the stiffness modulus at 100th cycle results in higher values of modulus than the ones computed using the proposed post-processing method, as a consequence of the transient response of the material before attaining a steady state.

The current binder testing protocols in the oscillatory domain use peak stress-strain data for material characterisation. The viscoelastic linearity limits are also based on such data. For a rigorous characterisation of the viscoelastic response of the binder, it is necessary that one records the complete waveform of the material response during oscillatory testing. This paper reports the waveform recorded for unmodified, crumb rubber modified and Styrelf modified bitumen during oscillatory loading. The waveform was collected for strain amplitudes of 1% and 5% at 30°C, 40°C and 50°C temperature. The linear and nonlinear behaviour of the material was studied using the geometrical symmetry of Lissajous plots. It was found that the material response was nonlinear. An appropriate frame invariant nonlinear constitutive model was used to predict the waveform response of all the binders tested. © 2013 Copyright Taylor and Francis Group, LLC.

The mixing temperature for binders is normally chosen by the pavement engineer based on a specific 'viscosity' required during hot mix asphalt production. Majority of the unmodified binders exhibit Newtonian behaviour at the mixing temperature and hence the determination of the same is straight-forward. However, when modified binders are used, experiments using a rotational viscometer indicate that the binder exhibits viscoelastic non-Newtonian fluid characteristic even at very high temperature. Consequently, the 'viscosity' varies with time and the location where it is measured, and hence is not a unique property of the material. In this work, a thermodynamically consistent, frame-invariant viscoelastic non-Newtonian fluid model was developed to characterise the rheological properties of the binders tested in a rotational viscometer. In the investigation reported here, two types of modified binders, polymer and crumb rubber, and one unmodified binder were used. These binders were subjected to steady and variable shear rate experiments in a rotational viscometer. The viscoelastic non-Newtonian model developed was able to predict reasonably the response of binders subjected to various protocols. In addition, bituminous mixtures were fabricated at different mixing and compaction temperatures using these binders, and the evolution of volumetric properties was investigated. The experimental investigation on mixtures showed that for identical aggregate gradation, the apparent viscosity of the binders played a critical role on the final volumetric properties obtained.