Determination of the failure limit in a repeated-load fatigue test in the laboratory has relied entirely on the arbitrary selection of a fixed criterion. The constant strain and constant stress modes of fatigue loading have been described by a consistent definition of failure in flexural fatigue testing because of the distinctly different application of energy during the loading history. The most widely accepted definition is a decrease in initial stiffness by 50 percent. Procedures examining energy input and dissipated energy have required different schemes for each mode in an attempt to describe similar states of damage in the mixture. A proposed method is presented for examining dissipated energy to select a consistent level of material behavior that is indicative of the damage accumulation in the mixture. This procedure shows the similarity between the constant stress and constant strain modes of testing and is shown to provide the potential for unifying the now phenomenological description of fatigue with a more rational energy-based description.
The existence of a fatigue endurance limit has been postulated for a considerable time. With the increasing emphasis on extended-life hot-mix asphalt pavement, or perpetual pavement, verification of the existence of this endurance limit, a strain below which none or very little fatigue damage develops, has become a substantial consideration in the design of these new multilayered full-depth pavements. Fatigue data are presented that were collected on a surface mix and a binder mixture tested for an extended period from 5 million to 48 million load repetitions at strain levels down to 70 microstrain. The fatigue results are analyzed in the traditional manner and using the dissipated energy ratio. This analysis shows that there is a difference in the data at normal strain levels recommended for fatigue testing and at the low strain levels. This difference cannot substantiate an endurance limit using traditional analysis procedures, but the dissipated energy approach clearly shows a distinct change in material behavior at low flexural strain levels, which supports the fact that at low strain levels the damage accumulated from each load cycle is disproportionately less than what is predicted from extrapolations of fatigue testing at normal strain levels. This reduced damage may be attributed to the healing process. The conclusion of this study is that laboratory testing can verify the existence of a fatigue endurance limit in the range of 90 to 70 microstrain below which the fatigue life of the mixture is significantly extended relative to normal design considerations.
An asphalt concrete damage-energy fatigue approach based on the concept of change in dissipated energy is presented in this paper. The damage-energy based fatigue approach is simple and based on a sound theoretical background. The central concept of the energy approach is the energy fatigue curve, which is based on two key elements, namely the plateau value (PV) and the number of load cycles to true failure (N tf ). The plateau value represents the constant value of the percentage of dissipated energy that produces damage to the material under cyclic loading. Failure is defined as the number of load cycles at which this percentage of dissipated energy begins to increase rapidly, indicating instability. Flexural fatigue testing was used to test hundreds of asphalt concrete beams, mainly under controlledstrain testing conditions. It was found that PV is highly dependent on the initial loading conditions, stress, strain, and dissipated energy. As a result, it can be used conveniently in pavement design. The number of load cycles to 50% reduction in initial stiffness was found to be highly correlated with the new failure point (N tf ). Using the dissipated energy concepts in fatigue analysis makes it possible to account for damage accumulation in a straightforward manner.Key words: fatigue of asphalt concrete, dissipated energy, damage, energy ratio. Résumé :Cet article présente un rapprochement de détermination de la fatigue par dommage-énergie du béton bitumineux basée sur le changement dans le concept de l'énergie dissipée. Ce rapprochement de détermination de la fatigue basée sur le dommage-énergie est simple et basée sur une théorie saine. Le concept principal de l'approche énergé-tique est la courbe fatigue énergie. Cette courbe fatigue énergie est fondée sur deux éléments clés : la valeur plateau (PV) et le nombre de cycles de chargement jusqu'à la défaillance réelle (N tf ). La valeur plateau représente la valeur constante du pourcentage d'énergie dissipée qui produit des dommages au matériel sous chargement cyclique. La dé-faillance est définie par le nombre de cycles de chargement auquel ce pourcentage d'énergie dissipée commence à s'accroître rapidement, indiquant ainsi une instabilité. Des essais de fatigue en flexion ont été utilisés pour mettre à l'épreuve des centaines de poutres en béton bitumineux, principalement sous des conditions contrôlées de contrainte. Il a été découvert que la PV dépend grandement des conditions initiales de charge, de la contrainte et de déformation et de l'énergie dissipée. Cela permet de l'utiliser dans la conception des chaussées. Le nombre de cycles de charge pour une réduction de rigidité initiale de 50 % est fortement corrélé au nouveau point de défaillance (N tf ). En utilisant les concepts d'énergie dissipée dans l'analyse de la fatigue, il est possible de tenir compte directement de l'accumulation des dommages.Mots clés : fatigue du béton bitumineux, énergie dissipée, dommage, rapport d'énergie.[Traduit par la Rédaction] Ghuzlan and Carpenter 901
Polyethylene (PE) is the most common type of plastic. In daily life, plastic bags, plastic bottles, and many other PE products are seen everywhere. Significant amount of plastics are not disposed properly and therefore present as waste material in the environment. Using polyethylene as an additive to asphalt binders may be considered a good way to utilize this material. However, modified asphalt binder properties should be investigated. Rheological properties at higher temperatures of asphalt binders modified with PE are investigated in this study. PE was added to asphalt binder at different percentages by volume of asphalt binder. These percentages were: 3, 4, 5, 6, and 7%. The rheological properties included: the rotational viscosity (RV), asphalt binders complex shear modulus (G*), and the phase angle (δ). It was found that the increase of PE to asphalt binder (PE/A) ratio increased the complex shear modulus (G*) and the rotational viscosity (RV) of asphalt binders. Furthermore, the rutting parameter (G*/sin δ) was improved. However, the PE/A ratio have no significant effect on the phase angle.
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