B. Rahimzadeh

VISCOELASTIC LINEARITY LIMITS FOR BITUMINOUS MATERIALS

As bituminous materials are viscoelastic in nature, their performance must be characterised using test methods and analytical techniques that account for time (or rate) of loading and temperature. In addition, it is usually advisable to confine the characterisation of a bitumen to its linear viscoelastic response (small strains) to simplify the mathematical modelling of the material, as non-linear response, particularly for viscoelastic materials, is extremely difficult to characterise in the laboratory and model in practical engineering problems. This paper describes an investigation of the linearity limits of a range of unmodified and modified bituminous binders and mixtures using a dynamic shear rheometer and a purpose-built dynamic, direct tension-compression, servo-hydraulic testing apparatus. The results show that there are strain dependent linearity criteria for both binders and asphalt mixtures at high stiffness values (low temperatures for binders and low to intermediate temperatures for mixtures) as well as a high temperature strain dependent linearity criterion for elastomeric modified binders. The linearity strain criterion for the mixtures was found to be in the order of 100 microstrain with the criterion for the binders being at least 100 times greater at just over 10,000 microstrain and the polymer network strain criterion at 1,000,000 microstrain.RésuméÉtant donné que les matériaux bitumineux sont naturellement viscoélastiques, la caractérisation de leur tenue en service doit faire appel à des méthodes expérimentales et des techniques analytiques qui tiennent compte du temps (ou de la vitesse) de chargement et de la température. En outre, il est généralement recommandé de se baser sur la réponse viscoélastique linéaire (petites déformations) dans la caractérisation du bitume. Ceci permet de simplifier la modélisation mathématique du comportement matériel, vu que la réponse non linéaire des matériauxviscoélastiques est difficile à caractériser expérimentalement et à modéliser dans le cas des problèmes pratiques de l’ingénieur. Cet article décrit une étude des limites de linéarité concernant une gamme de liants et mélanges bitumineux non modifiés et modifiés, à l’aide d’un rhéomètre de cisaillement dynamique et d’une machine à commande hydraulique, conçue en vue d’effectuer des essais dynamiques de tension-compression directe. Les résultats montrent que des critères de linéarité dépendant de la déformation existent aussi bien pour les liants que pour les mélanges bitumineux, pour des valeurs élevées de la rigidité, à de basses températures pour les liants et à des températures basses ou intermédiaires pour les mélanges. Dans le cas des liants élastomères modifiés, un critère de linéarité dépendant de la déformation à haute température est également mis en évidence. Le critère de linéarité dépendant de la déformation pour les mélanges s’est avéré de l’ordre de 100 micro-déformation. Le critère correspondant aux liants étant au moins 100 fois supérieur et dépasse les 10000 micro-déformation et celui du réseau de polymère se situe à 1000000 micro-déformation.

Linear Viscoelastic Performance of Asphaltic Materials

ABSTRACT This paper describes the linearity limits of various unmodified and modified bituminous binders as well as asphalt mixtures incorporating these binders. The linearity limits of the binders and mixtures have been determined using a dynamic shear rheometer (DSR) and a purpose-built dynamic, uniaxial tension-compression, servo-hydraulic testing apparatus respectively. Both continuously graded as well as gap-graded asphalt mixtures have been tested together with a fines/filler mortar (mastic). The results show that there are strain dependent linearity criteria for both binders and asphalt mixtures at high binder stiffness values as well as a high temperature strain dependent linearity criterion for elastomeric modified binders at temperatures greater than 60°C. The linearity strain criterion for the asphalt mixtures was found to be in the order of 100 microstrain with the criterion for the binders being at least 100 times greater at just over 10,000 microstrain (1% strain) and the elastomeric polymer network strain criterion at 1,000,000 microstrain (100% strain). In addition, the conventional, process modified and plastomeric polymer modified binders displayed a stress dependent linearity criterion of approximately 5kPa at low binder stiffness values.

EMPIRICAL AND FUNDAMENTAL RHEOLOGICAL PROPERTIES OF POLYMER MODIFIED BITUMENS

Although penetration, softening point and high temperature viscosity are empirically or semi-empirically based tests, specifically designed to determine the consistency of conventional, penetration grade bitumens, they are still being used in practice to quantify the effects of polymer modification. However, depending on the degree of polymer modification, the modification process can significantly alter the rheological properties of the binder, which are not necessarily characterised by conventional binder properties. This has led to an increased interest in the use of fundamental rheological parameters to specify polymer modified bitumens (PMBs). This paper provides a critical evaluation of the effect of polymer modification on the rheological properties of elastomeric and plastomeric PMBs as measured by conventional (empirical and semi-empirical) methods as well as fundamental rheological methods using a Dynamic Shear Rheometer (DSR). The results indicate that the penetration and high temperature viscosity tests can only indicate the effect of polymer modification as a filler-type mechanism. Softening point tests provide some indication of the presence of a dominant polymer network, but only dynamic mechanical analysis using a DSR and fundamental rheological parameters, such as complex modulus and phase angle, can completely quantify the effects of polymer modification and the unique rheological characteristics associated with different polymer types. For the covering abstract see ITRD E117423.