J. Murali Krishnan

Mechanics of air voids reduction of asphalt concrete using mixture theory

Abstract Asphalt concrete used in flexible highway pavements has 5–8% air voids immediately after laying of the roadway. Constitutive laws for asphalt concrete developed till now have modelled the mix as a linear elastic or viscoelastic material and have not taken into account the effect of voids concentration on the mechanical behaviour of the material. In the present study the theory of mixtures is used to model asphalt concrete. Asphalt concrete is considered to be a mixture of aggregate matrix, asphalt and air in a purely mechanical system in which the thermal effects and chemical reactions are ignored. Constitutive relation for each component of the mixture is assumed to be dependent only on the kinematical quantities associated with each component. The resulting hyperbolic conservation equations are solved by an upwind finite volume scheme coupled with an operator splitting technique for a quasi-static type of loading. The numerical scheme is used to simulate the variation of air voids content across the thickness of a typical road pavement.

Rheology of Complex Fluids

Background.- Non-Newtonian Fluids: An Introduction.- Fundamentals of Rheology.- Mechanics of Liquid Mixtures.- Rheology.- Oscillatory Shear Rheology for Probing Nonlinear Viscoelasticity of Complex Fluids: Large Amplitude Oscillatory Shear.- PIV Techniques in Experimental Measurement of Two Phase (Gas-Liquid) Systems.- An Introduction to Hydrodynamic Stability.- Applications.- Statics and Dynamics of Dilute Polymer Solutions.- Polymer Rheology.- Active Matter.- Mathematical Modelling of Granular Materials.

Influence of the Type of Binder and Crumb Rubber on the Creep and Recovery of Crumb Rubber Modified Bitumen

This investigation reports the influence of the type of binder and crumb rubber gradation and dosage rate on the creep and recovery properties of crumb rubber modified bitumen (CRMB). Two types of binders, air blown and blended, were used with two gradations of crumb rubber, fine and coarse, at three dosage rates, 8, 10, and 12%. The morphology of these 12 binders were characterized by using environmental scanning electron microscopy (ESEM). Furthermore, energy dispersive X-ray spectroscopy was carried out on the samples to identify the chemical composition of the network structure as seen from ESEM. All the samples were subjected to creep and recovery tests by using a dynamic shear rheometer at five different temperatures of 46, 52, 58, 64, and 70°C. A generalized Voigt-Kelvin model was used to model the creep and recovery response of the material for all the temperatures tested. It was found that the nature of the binder plays a critical role on the development of morphology of CRMB. This morphology influences the creep and recovery characteristics of the final blend. The fibril diameter and the number of network junctions as determined through the image processing of ESEM images and the Voigt-Kelvin model parameters were found to be related.

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...

Air voids reduction phenomena of asphalt concrete – A continuum approach

Asphalt concrete used in flexible highway pavement construction has 5–8 percent air voids immediately after laying of the roadway. Constitutive laws for asphalt concrete developed till now have modelled the mix as a linear elastic or viscoelastic material and have not taken into account the effect of void concentration on the mechanical behaviour of the material. In this paper, the theory of linear elastic material with voids is used to model asphalt concrete under isothermal conditions. Two cases of void reduction behaviour are studied, one in which the void volume reduces asymptotically under a constant load and the other in which it reaches the refusal air void content. The model is used to predict the creep behaviour under constant compressive stress as well as to obtain the hysteretic stress-strain behaviour. Solutions for the case of uniaxial deformation are derived and the strains are simulated for a constant compressive stress. Use of the air voids reduction measure as a possible damage parameter is also examined.

Modelling constant displacement rate experiments of asphalt concrete using a thermodynamic framework

This study is concerned with the constitutive modeling of asphalt concrete mixtures. The response of the asphalt concrete pavement depends on its internal structure. The internal structure of the asphalt concrete mixture evolves during the loading process. Here, we develop a single constituent model for asphalt concrete mixture by associating different natural configurations (stress-free configurations) with distinct internal structures of the body. The evolution of the natural configurations is determined using a thermodynamic criterion, namely the maximization of the rate of dissipation. Making appropriate assumptions concerning the manner in which the body stores and dissipates energy, the constitutive relations for the stress is deduced. Constant displacement rate experiments are carried out at different confinement pressures on asphalt concrete specimens made of two different aggregates—granite and limestone. The efficacy of the model in predicting the mechanical response of asphalt concrete mixtures is shown by corroborating the model predictions with the experimental results.

Influence of confinement pressure and air voids on the repeated creep and recovery of asphalt concrete mixtures

A laboratory investigation was conducted to capture the influence of confinement pressure and specimen air voids on the creep and recovery response of asphalt concrete (AC) mixtures. AC specimens were fabricated at 2% and 7% air voids and tested at three temperatures (20, 40 and 55°C) and at unconfined and confined conditions (100 and 200 kPa). A total of 20,000 repetitions of a repeated trapezoidal loading and recovery cycle were applied. The resulting creep curves showed four distinct patterns of the three-stage creep curve depending on the loading condition and specimen density. To quantify the mechanical response during the secondary stage where the response was found to be linear, linear viscoelastic modelling was carried out. Using creep time, energy stored and energy dissipated, which were determined from model parameters; the influence of air voids and confinement pressure was quantified.

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.

Nonlinear viscoelastic response of asphalt binders in transient tests

This paper presents the results of a set of transient experiments conducted on asphalt binders that serve to highlight the nonlinearity of the viscoelastic response of asphalt binders. Two grades of binders were used in this study. A transient test that combines both creep and stress relaxation tests was conducted using a dynamic shear rheometer at several loading conditions and at different temperatures. The acquired experimental data was satisfactorily fit using a nonlinear six-parameter viscoelastic model.

Rheological characterisation of modified binders at mixing and compaction temperature

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.