The reference modulus of bituminous mixes, used for road design in France, is determined by the complex modulus measurement, carried out in the frequency domain at 15°C, 10 Hz or by the secant modulus in direct tension at 15°C, 0.02 s. However, there is strong interest to access this modulus using the indirect tensile (IT) test carried out in the time domain. Actually, this test appears to be easier to implement due to the sample manufacturing and numerous laboratories are already equipped. The present study proposes to determine the experimental condition to be applied to the IT test in order to get the reference modulus. A theoretical approach, based on the Maxwell and Huet viscoelastic linear models, is used to find the testing conditions at which the IT test has to be performed to give as precisely as possible the reference modulus. This procedure is applied on a large database of materials for which the viscoelastic spectrum has been provided. The influence of the loading waveform, which cannot be systematically controlled, is studied. For a force increasing linearly with time, the temperature at which the IT test has to be performed is close to 10°C, whatever the bituminous mix type. Moreover, the error that would have been committed by applying the equivalent temperature Teq = 10°C during the indirect tensile test is evaluated. This error depends on the loading waveform but always remains, theoretically, under 1000 MPa. This study has been carried out in the framework of collaboration between LCPC and USIRF (Union des Syndicats de l’Industrie Routière française).

For mechanistic based pavement design, it is necessary to know the stiffness of materials used for road construction. In the European standard EN 12697-26, several experimental tests are proposed to measure the modulus of bituminous mixtures. However, bituminous materials exhibit strong viscoelastic behavior. Hence, the stiffness of this specific material depends not only on the sample geometry but also on the loading law (strain or stress curve versus time). As a consequence, comparison between these different tests, performed in time or frequency domain, is not straight forward. The present paper focuses on measurement of secant modulus using the Indirect tensile (IT) test and comparison with complex modulus. In the IT test, the loading law is not systematically controlled. So, it is important to investigate the influence of the loading waveform on the test result. For this study, the case of a High Modulus Bituminous Mixture for base course (French EME) has been considered. Its viscoelastic behavior has been firstly determined using complex modulus measurements and using Prony series model. An intermediate step, based on an original master curve construction method is used. Doing so, the viscoelastic model is validated on the time domain by simulating direct tensile tests. The case of IT test is then considered. The Prony series model is conformed against IT test results for which the force waveform is known. For this last test, the dependance of the Poisson ratio with temperature and loading time is highlighted. Assuming a purely elastic behavior in isotropic compression, a formula, able to derive the viscoelastic Poisson ratio from the complex modulus is presented. Finally, a theoretical parametric study considering the IT loading waveform is undertaken. It appears that the correction factor given in the standard EN 12697-26 cannot be applied at all temperatures. Actually, this correction factor should be material dependent. This study has been carried out in the framework of collaboration between LCPC and USIRF (Union des Syndicats de l’Industrie Routière française).

A feasibility study has been carried out in order to estimate in laboratory the fumes emission potential during hot mix asphalt production in relation to manufacturing parameters. Temperature, binder content as well as some proceeding kinetics parameters have been studied through an experiment plan. The laboratory mixer was equipped with a stack for gas emission sampling in normalized conditions. A polycyclic aromatic hydrocarbons (PAH) sampling system has been designed and a continuous total organic compounds (TOC) analyzer has been used for hot mix asphalt fumes. Classical laboratory characterizations have been performed in order to estimate material performances. Among the possible ones, complex modulus of recovered binder or mix seems the most relevant as its result reflects mechanical properties as well as binder oxidation and ageing. The study performed allows to link potential fume emissions to mixing parameters for hot mix asphalts, in relation with mix mechanical properties.

Considering that most of time, cracks in bituminous mixes occur inside the binder/mastic film, it can be assumed that fatigue properties of the bitumen itself could give accurate information on the fatigue behaviour of mixes. It has been already demonstrated that a strong link exists between bitumen and mixes fatigue properties. From these previous studies, it is clear that when the same composition is used, mixes can be ranked as the corresponding binders. Whereas numerous studies have been conducted on shear fatigue properties using Dynamic shear Rheometer (DSR), only a few data obtained with other modes of loading are available. Knowing that mixes are tested in mode I crack opening, it seems interesting to test binders under the same fracture mode. For these reasons, the present study focuses on the design of a tension-compression fatigue test as well as the interpretation of the first results taking into account the rheological phenomena occurring during the test. The paper describes the design and the preparation of the "diabolo" shaped used samples. Fatigue test results from three binders are given and compared to the corresponding mixes results. Then, a particular attention is drawn on the phase angle versus stiffness (black diagram) during the fatigue experiments.

Fatigue tests of bituminous materials performed on trapezoidal specimens induce a loss of stiffness, combined with a significant extension of the specimens at the beginning of the test when applying high strain levels. This paper presents the development of a viscoelastic constitutive law with unilateral damage for asphalt materials, based on the observation of tensile failure tests on asphalt binder lenses between two metal spheres. The unilateral damage is defined as the opening of the micro-cracks in the bitumen during extension and their closing during contraction. When put into a semi-analytical structural model of the trapezoidal specimen, this constitutive law leads to results close to those highlighted at the beginning of fatigue experiments for various experimental conditions (temperature, load frequency and strain amplitude level). According to the used modelling, the damage evolution law describes the fast initial homogeneous creation of the micro-cracks and then their slow development. The modelling is also able to explain a given percentage of the loss of stiffness. These results show mainly that the bituminous material extension is due to the dissymetric behaviour induced by the unilateral damage coupled with viscoelasticity. Therefore the asphalt mix extension is a way to quantify the level of the material damage.

ABSTRACT Fatigue life of bituminous mixes is analyzed through two-points bending fatigue tests, the experimental protocol having been modified such as to manage rest periods. Fatigue sequences and rest sequences are analyzed and the parameters enabling to quantify the variation of stiffness with time (during rest) or loading (during fatigue) are defined and modeled. It is shown that the re-damaging rate is a good indicator of non-recoverable damage. Varying the experimental parameters that drive damage evolution, the respective weights of fatigue and healing are addressed.

A nonlocal damage model is proposed to predict the behavior of pavement fatigue cracking. This constitutive relation has been implemented in a finite-element code, along with a self-adaptive jump-in-cycle procedure for high cycle fatigue computations. Strain localization analysis shows that during uniaxial fatigue tests, bifurcation due to strain softening occurs much later than in monotonic tests. The incorporation of an internal length into the constitutive model is advocated since the model should encompass loading histories with very different amplitudes of cycles, in which localization may still occur. The influence of the internal length on the fatigue life of bending beams is also investigated. Calibration of the damage model is performed after thermal effects have been evaluated and accounted for in a simplified way, uncoupled to damage. Parameter identification is performed in bending and uniaxial tests. The resulting calibrated constitutive relation is found to yield a good description of several different uniaxial tests.

This paper presents an interlaboratory test campaign organized by the RILEM 182-PEB Technical Committee. In the campaign, 11 different test methods, comprising uniaxial tension/compression, 2-, 3- and 4-point bending and indirecttension tests, were utilized in order to investigate fatigue characteristics of a dense graded asphalt concrete mixture. The testing conditions specified were sinusoidal excitation at 10Hz and 10°C using controlled strain and stress modes. In total, more than 150 fatigue tests were carried out during the investigation. The fatigue test results were analyzed using both classical as well as continuum damage mechanics approaches. The fatigue test results obtained using the classical fatigue approach are considerably influenced by test type and mode of loading (controlled stress or strain) used. Consequently, this approach has limited use in realistic fatigue characterization of bituminous materials and pavement structures. In contrast to the classical approach, models founded on continuum damage theory may serve to isolate intrinsic fatigue characteristics from the influence of so-called biased effects, which are largely caused by the accelerated laboratory testing. The continuum damage models investigated may constitute steps, towards a rational mechanistic fatigue characterization model, which are important for effective future pavement design.

Introducing rest periods during fatigue tests can significantly improve the remaining lifetime of bituminous mixes. An experimental protocol has been designed such as to study the range and kinetics of loss of stiffness and recovering of properties during rest periods. The range of the maximum recovery depends on the number of cycles previously applied. Recovery is not permanent and the loss of stiffness speeds up under repeated fatigue sequences. The speed of loss of stiffness. seems to be a damage indicator. It evolves differently according to the fact that the specimen has (or not) previously been subjected to more aggressive solicitations. This points out the fact that the mix "keeps in memory" previous loadings.