Ivan Isailović

Energy Dissipation in Asphalt Mixtures Observed in Different Cyclic Stress-Controlled Fatigue Tests

During cyclic fatigue testing of hot mix asphalt (HMA) under stress control, a continuous change of the strain amplitude is observed in each loading cycle either under pure tensile or under tensile-compressive stress conditions. Depending on the type of the applied load and on the specific viscoelastic behavior of the HMA this strain change can dramatically vary in association with a change in various mechanical properties. In order to study the variation of mechanical properties during cyclic fatigue tests under stress control an experimental program was performed using an approach based on dissipated energy. This study considers the following stress-controlled fatigue tests: indirect tensile test, uniaxial tension test and uniaxial tension-compression test. The hysteresis loops are drawn, and the dissipated energy is calculated. Based on the similar number of loading repetitions at failure, the tests are comparatively analyzed, and the changes in mechanical properties are identified. As a result, the tension-compression test shows low permanent deformation and high variation of dissipated energy which can be attributed to a distinct change in the material’s mechanical properties. On the other hand, the uniaxial tension test and indirect tensile test exhibit high accumulation of permanent deformation with very few changes in mechanical properties during cyclic excitation. Based on these results the uniaxial tension test and the indirect tensile test in stress control are not well-suited for fatigue analysis since failure most likely occurs by accumulation of permanent deformation.

Investigation on mixture recovery properties in fatigue tests

In this paper the recovery properties of hot mix asphalt (HMA) mixture in fatigue tests are investigated based on dissipated energy approach. In order to determine the influence of material ageing, bitumen content, polymer modification and compaction energy on material ability to recover its initial characteristics, different variations of an asphalt mixture for surface course AC 11 D S were prepared. Recovery of material mechanical properties is observed during a single rest period, introduced into tension–compression cyclic fatigue tests under stress control. The analysis of the experimental results reveals that there is a significant influence of the duration of rest period on recovery capability. A plateau phase is achieved when the length of the rest period does not have any impact on asphalt recovery properties. Asphalt ageing plays an important role in reducing the effects of material recovery. The degree of compaction used in this research seems to have no influence on material ability to restore its initial properties. Mixtures prepared with additionally 0.5% of bitumen by mass show the best recovery characteristics compared to other variations.

Fatigue investigation on asphalt mixture layers’ interface

In this paper, the interface shear fatigue performance is investigated based on a new fatigue test protocol, which includes both cyclic shear and normal static compressive load. Several asphalt structure types are tested at five temperatures (from −10°C to 50°C) and three normal stress levels (from 0 to 0.50 MPa). Based on fatigue tests at three different shear amplitude levels unique fatigue functions of the layers’ interface were identified. It was found that the interface shear fatigue performance is highly dependent on normal stress state and increases with increasing normal stress. This is a direct consequence of the enhanced friction, adhesion and interlock at the shear interface. Observing the fatigue lives at different test temperatures, the best resistance was observed at lowest temperature. From a practical viewpoint, it is highly recommended to perform the test without normal stress application, since this can better reflect the research and practice experience, while keeping the testing protocol to a reasonable level of complexity.

Assessment of Healing Properties of Asphalt Mixtures

Fatigue failure due to repeated traffic loading is a significant distress for asphalt pavements. Nevertheless, the damage associated to this phenomenon can be partially or entirely recovered during rest periods between loading repetitions, potentially resulting in a longer pavement service life. In this paper, the results from a comprehensive investigation on healing potential of asphalt materials are presented based on cyclic uniaxial tension compression fatigue tests under stress-control mode with and without rest periods. Then a new healing index parameter is proposed and evaluated using a testing procedure with different rest period durations. The analysis of the experimental results reveals that the newly developed healing index effectively asses the mixtures healing properties with an increasing trend for longer rest periods. A limit plateau value is then achieved when the duration of the rest period does not any longer affect the asphalt healing properties. The proposed healing index provides a simple and practical tool for optimal pre-selection of pavement materials, which can be used on a routine basis.

Sweep test protocol for fatigue evaluation of asphalt mixtures

This paper presents a laboratory test protocol for evaluating fatigue properties of asphalt mixtures through sweep tests at multiple amplitudes and frequencies. Conventional fatigue evaluation of asphalt mixtures typically relies on a limited number of single fatigue tests, and each test is performed at constant test temperature. Moreover, in order to reduce laboratory effort to a minimum, testing is quite often limited to a single test temperature, which means that the dependency of asphalt fatigue on temperature is ignored. Such test procedure is in best accordance with the European Standards. However, so-derived results may provoke misleading conclusions, as exemplarily shown in this study. In order to solve this shortcoming, a sweep test protocol for fatigue evaluation of asphalt mixtures at different temperatures is proposed. Frequency–amplitude sweeps are used for complex modulus evaluation and fatigue evaluation, but without significantly extending the laboratory effort compared to conventional fatigue testing. This sweep test protocol is applicable for fatigue evaluation of any asphalt mixture.

Permanent deformation of hot mix asphalt under compression and tension

In this paper, the existence of a correlation between the permanent deformations measured under different stress conditions is investigated. The results from cyclic penetration test, which is performed under compressive loading, are compared with the experimental values obtained with the indirect tensile fatigue test, which presents a dominant tensile stress field. Four different asphalt mixtures for surface and base layers were prepared and tested with both tests. The results indicate a significantly different material behavior between the two testing methods. In the case of the cyclic penetration test a first material densification is followed by deformation associated to shear effects with no change in material properties. On the other hand, during indirect tensile fatigue tests, a progressive accumulation of permanent deformation is linked to a continuous change in the mechanical properties of the material. It is observed that the development of permanent deformation under compression does not correlate with the deformation accumulation due to a tensile stress state. This suggests that the experimental results obtained with indirect tensile fatigue test cannot be used to predict the permanent deformation properties of asphalt mixture layers under the compressive state of stress induced by traffic load.