A. Padmarekha

Viscoelastic Transition of Unaged and Aged Asphalt

AbstractThis investigation reports the viscoelastic solid to fluid transition of asphalt as the temperature is varied. Air blown, blended asphalt and pitch obtained from Arab mix crude were tested under unaged, short-term aged, and long-term aged conditions. All the materials were subjected to two experimental protocols in the 20–50°C temperature region. In the first experimental protocol, the material was subjected to frequency sweep in small amplitude oscillatory shear and in the second protocol the material was subjected to large deformation stress relaxation. From the analysis of the experimental results, it was seen that asphalt exhibited a viscoelastic solid regime, a viscoelastic fluid regime, and a transition regime in the temperature range tested. In the small amplitude oscillatory shear, the transition regime was captured using a frequency-dependent storage modulus (G′)—loss modulus (G″) crossover. In the stress relaxation experiments, the material exhibited three distinct slopes and the transit...

Large amplitude oscillatory shear of unmodified and modified bitumen

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.

Influence of warm mix additive and loading rate on rutting of warm mix asphalt pavement

Two major types of warm mix asphalt (WMA) technologies exists and they are wax-based and chemical-based. The wax-based additive melt completely in the binder at the mixing and compaction temperatures and hence enable construction at lower temperature. The chemical-based additives facilitate similar results during construction, however, by reducing the friction between the aggregates and binders. As it is well known, WMA technologies exhibit tendencies for increased rutting due to reduced ageing during construction, however, the wax-based additives exhibit better performance in laboratory trials since at pavement service temperatures, the wax crystallise and provide increased stiffness to the mastic. The presence of the wax and the associated temperature sensitive properties (crystallisation and melting) influence the viscoelastic response of the WMA at pavement service temperatures. This investigation focuses on such issues for one wax-based additive and one chemical-based additive along with a control hot mix asphalt (HMA) binder. This paper presents three important results. In the first, the rheological response of the binders are characterised and in the second the rutting and the mechanical properties of mixtures are quantified. It was seen that bituminous mixture with wax-based additives exhibited better rutting resistance characteristic at temperature below 40 C, identical response in the temperature range of 40–50 C and more prone to rutting at the temperature above 50 C when compared to HMA mixture. It was also seen that WMA mixture with chemical additives consistently exhibited higher rutting. Taking into account the temperature sensitive response of WMA, the final result relates to simulation for rut resistance for pavements constructed with WMA mixtures for 16 different locations in India.

Linear Viscoelastic Limits for Determination of Dynamic Modulus of Bituminous Concrete Mixture in AMPT

The current practices of determining dynamic modulus such as the AASHTO TP: 7910 procedure assumes that if the measured strain values are between 50–150 microstrain, the material is being tested in the linear viscoelastic regime. It is necessary that for the linear viscoelastic conditions to hold the applied load and measured strain should satisfy linear scaling and superposition and the determination of such limits when the material is subjected to static load (seating load) and haversine compressive loading can be non-trivial. In this investigation, the bituminous concrete mixtures were subjected to stress sweep test for select temperatures. The collected data was analyzed and the linear viscoelastic limit was determined for each temperature and frequency by appealing to linear scaling and superposition. On comparison, the stress amplitude values calculated using the current AASHTO protocols were found to be greater than the linear stress amplitude values determined using the stress sweep test data. This emphasizes the importance of incorporation of linear limits for dynamic modulus computation.

Evolution of energy dissipation during four-point bending of bituminous mixtures

In this investigation, the fatigue life of bituminous mixtures is characterised using the four-point bending test. Two sets of bituminous concrete samples, one produced with unmodified binder and other with plastomer-modified binder, are subjected to repeated strain-controlled sinusoidal loading at 10 Hz frequency and 20°C. The test is conducted for three different strain amplitudes of 200, 400 and 600 microstrain and the corresponding stress and strain data are collected at 1/1000 second interval. In the first approach, these data are used to calculate total energy dissipation. The dissipation due to damage is determined by separating viscoelastic dissipation from the total dissipation. For this purpose, a linear viscoelastic model is used. In the second approach, the discontinuity in the evolution of phase lag determined from the stress–strain plot is taken as the onset of fatigue damage. The fatigue failure, as quantified by these two approaches, was found to be closer to the fatigue life computed using energy ratio method.