Nonlinearity of bituminous mixtures

This paper presents an experimental characterization of the strain dependency of the complex modulus of bituminous mixtures for strain amplitude levels lower than about \(110~\upmu\mbox{m}/\mbox{m}\). A series of strain amplitude sweep tests are performed at different temperatures (8, 10, 12 and 14°C) and frequencies (0.3, 1, 3 and 10 Hz), during which complex modulus is monitored. For each combination of temperature and frequency, four maximum strain amplitudes are targeted (50, 75, 100 and \(110~\upmu\mbox{m}/\mbox{m}\)). For each of them, two series of 50 loading cycles are applied, respectively at decreasing and increasing strain amplitudes. Before each decreasing strain sweep and after each increasing strain sweep, 5 cycles are performed at constant maximum targeted strain amplitude.Experimental results show that the behavior of the studied material is strain dependent. The norm of the complex modulus decreases and phase angle increases with strain amplitude. Results are presented in Black and Cole–Cole plots, where characteristic directions of nonlinearity can be identified. Both the effects of nonlinearity in terms of the complex modulus variation and of the direction of nonlinearity in Black space seem to validate the time–temperature superposition principle with the same shift factors as for linear viscoelasticity.The comparison between results obtained during increasing and decreasing strain sweeps suggests the existence of another phenomenon occurring during cyclic loading, which appears to systematically induce a decrease of the norm of the complex modulus and an increase of the phase angle, regardless of the type of the strain sweep (increasing or decreasing).     

Fatigue and healing properties of nano-reinforced bituminous binders

The research work focused on fatigue and healing properties of bituminous binders containing carbon nanotubes (CNTs) and nanoclays (NCs) as reinforcing additives. Investigations were carried out by means of a dynamic shear rheometer and by employing specifically devised testing protocols. Experimental results were analysed with the specific goal of highlighting the role played by additive type and base bitumen. Although fatigue response of base bitumens was always improved by nano-modification, effectiveness of nano-particles was found to be highly dependent on the physico-chemical properties of blend components, which strongly influence the morphological configuration assumed by additives within bituminous media. Results obtained in healing tests were processed in order to discern between self-healing of cracks induced by fatigue damage and other artefact phenomena which are related to viscoelastic changes occurring in the bulk of the material. Outcomes of fatigue and healing tests were found to be coherent with interaction mechanisms which take place at the nano-scale.     

Energy dissipation and the deformation resistance of bituminous mixtures

The use of energy dissipation has been extended for assessing the resistance to permanent deformation of bituminous mixtures. The method is found beneficial for accurately determining the end of the linear region from the curves of permanent strain versus number of loading cycles, where the strain rate is constant. An analysis procedure based upon a combined technique of strain rate and energy dissipation method is presented.

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Résumé La méthode de l'énergie de dissipation a été étendue pour mesurer la résistance à la déformation permanente des mélanges bitumineux. Cette méthode s'avère avantageuse pour définir précisément la position de la fin de la région linéaire des courbes de déformation relative au nombre des cycles de chargement. Une procédure analytique basée sur une technique combinant la vitesse de la déformation et la dissipation de l'énergie est présentée.

     

Rutting of bituminous mixtures: wheel tracking tests campaign analysis

Rutting is one of the main failure modes in pavement structures subjected to mechanical loading. Wheel Tracking Testers (WTT) devices used to evaluate the rutting performance of bituminous mixtures were investigated by Working Group 3 (WG3) “Mechanical Tests for Bituminous Materials”, of the RILEM ATB Technical Committee 206. Three different bituminous layer systems (A, B and C) currently used in road construction were compared to evaluate the abilities of “small” and “large” WTT devices to quantify road materials rutting performance. The tested specimens were cut from large slabs extracted from the onsite pavement structures. Seven laboratories located in seven different countries, were involved in the testing program. Results show that the mean rate of rutting for “small” WTT devices is faster than for “large” WTT devices. Systems A and C show equivalent performance and can be considered good in terms of the European Standard’s requirement. Results from system B indicate poor performance and it does not comply with the specifications of the standard. Nevertheless, a visual evaluation of the rutting showed a very good performance on the road for systems A and B and no apparent difference could be noted between these two systems.     

Quantification of different effects occurring during fatigue tests on bituminous mixtures

Quantification of different effects (nonlinearity, heating, thixotropy, and fatigue) occurring during fatigue tests on bituminous mixtures is presented in this paper. A focus is given on the nonlinearity phenomenon. Continuous fatigue tests and a test with specific protocol (called fatigue tests to estimate biasing effects) were performed in tension/compression mode on cylindrical samples of the same material. The analysis of results reveals that reversible effects (nonlinearity, heating, and thixotropy) are important (more than 90% decrease at 100,000 cycles for a strain amplitude of 100 μm/m at 10 Hz) and cannot be ignored when interpreting classical fatigue tests. The nonlinearity effects respect the time‐temperature superposition principle, and they are more pronounced at “high” temperature (at the same frequency). Direction of nonlinearity curve in the Cole‐Cole axes is shown to be independent of temperature and frequency for the considered range.     

Impact of bitumen and aggregate composition on stripping in bituminous mixtures

The impact of bitumen and aggregate composition on stripping was investigated using four bitumens and four aggregates. Moisture sensitivity was assessed based on retained resilient modulus and tensile strength ratio (MRR and TSR, respectively). The results indicate that mixtures from the bitumen with a high acid number exhibited high resilient modulus and tensile strength in the dry condition for all the aggregates. In wet condition, this conclusion did also hold except for one aggregate. Regarding penetration grade, mixtures made with lower penetration grade bitumen exhibited higher resilient modulus and tensile strength, in dry and wet conditions, than those of higher penetration grade. Bitumen characteristics like acid number, penetration grade and molecular size distribution did not influence moisture sensitivity. Mixtures with aggregates containing alkali metals (sodium and potassium) exhibited relatively high moisture sensitivity, regardless of the bitumen used. In contrast, indications of moisture sensitivity were not apparent in mixtures made with aggregates containing calcium, magnesium and iron. Data analysis revealed that variability in moisture sensitivity is attributed to aggregate rather than bitumen. No significant interaction effect between bitumen and aggregate was found on moisture sensitivity. The results indicated good correlation between MRR and TSR in ranking mixtures for stripping.     

Three‐dimensional analysis of fatigue tests on bituminous mixtures

A tension–compression test on cylindrical specimens was used to study the three‐dimensional behaviour of bituminous mixtures during fatigue tests. The tests were carried out at 10 °C, 10 Hz at constant strain amplitude mode. The axial strain, radial strain and axial stress were measured using a prototype apparatus developed at the University of Lyon/“Ecole Nationale des TPE” (ENTPE). In addition to axial stress and strain analysis, the measurements of the radial strain made it possible to obtain the complex Poisson ratio and the volumetric strains during the tests. The results showed good correlations between the volumetric strains and global damage. The effects of the change of temperature due to viscous dissipation on the volumetric strain and on the complex modulus were also analysed.     

Fatigue and healing properties of bituminous mastics reinforced with nano-sized additives

The research work described in the paper focused on fatigue and healing properties of bituminous mastics reinforced with nano-sized additives.Commercially available multiwall carbon nanotubes (CNTs) and montmorillonite nanoclay (NC) were combined with a single base bitumen and a standard mineral filler to produce bituminous mastics. These blends were prepared in the laboratory by making use of a technique consisting in simple shear mixing followed by sonication.Fatigue behaviour of mastics under repeated loading was investigated by means of time sweeps performed in the strain-controlled mode at various amplitudes. Healing potential was assessed by adopting a testing protocol specifically conceived to discriminate between recovery of damage induced by fatigue loading and other artefact phenomena which may affect material response. All rheological measurements were carried out with a dynamic shear rheometer in the parallel plates geometry.Outcomes of the experimental investigation were found to be highly dependent on the nature of additive type, as a result of the key role played by interaction mechanisms that nano-particles can establish within the bituminous mastic.     

Mechanical properties of cement-treated mixtures of milled bituminous concrete and crushed aggregates

Milled bituminous concrete and crushed limestone aggregates were combined in the laboratory in various proportions to simulate some of the combinations of materials that can be obtained when the method of mix-in-place recycling with cement is used. The influence of the milled bituminous concrete content of the various mixes is examined in terms of compressive strength (equivalent cubes, uniaxial prism strength), tensile strength (flexural and cylinder splitting strength) and modulus of elasticity (quasi-static, electrodynamic resonance). The effect of these mechanical properties on the traffic and temperature induced stresses as well as on the crack susceptibility of these materials is discussed. It is concluded that the incorporation of milled bituminous concrete in cement-stabilised granular materials imparts interesting properties in these materials, yet further research is needed, including fatigue tests as well as tensile strength and modulus of elasticity determinations at early ages (1–2 days).

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Résumé On a mélangé en laboratoire du béton bitumineux et des granulats calcaires concassés, dans des proportions variées, afin de simuler quelques unes des combinaisons de matériaux qu'on peut obeenir en utilisant la méthode de recyclage sur place par malaxage avec du ciment. On a examiné l'influence de la teneur en béton bitumineux broyé des différents mélanges vis-à-vis de la résistance à la compression (cubes équivalents, résistance uniaxiale du prisme), la résistance à la traction (résistance en flexion et résistance au fendage du cyclindre) et le module élasticité (quasi statique, résonance électro-dynamique). On discute l'effet de ces propriétés mécaniques sur les contraintes génŕées par le trafic et la température, et sur la fissurabilité de ces matériaux. On conclut que l'addition de béton bitumineux broyé à des matériaux granulaires stabilisés par le ciment communique des propriétés intéressantes à ces matériaux mais que, toutefois, une recherche ultérieure doit être conduite, comprenant des essais de fatigue et la détermination de la résistance à la traction et du module d'élasticité aux jeunes ages (1–2 jours).

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Editorial note Dr. Stelios Kolias is a RILEM Senior Member. He is the RILEM Delegate in Greece.     

Precision of method for determining resistance of bituminous mixtures to de-icing fluids

This paper presents the results of a Round-Robin test to estimate the precision of European method EN-12697-41 “Test methods for hot mix asphalt-Resistance to de-icing fluids”. The purpose of the project was to determine precision data according to ISO 5725, ASTM E691 and ASTM C802. The examined test method is intended for use in requirements specifications for airfield de-icing chemicals and/or as a tool for development of such products. Precision statistics, repeatability and reproducibility standard deviations, are based on observed values from six laboratories and six levels, each level comprising four samples. From a general statistical analysis, which was conducted in addition to precision determination, it could be concluded that the most damaging de-icing agents (treatments) were identified by all participating laboratories both in terms of absolute values and by ranks.     

Properties of bituminous mixtures at low temperatures and relations with binder characteristics

Within the framework of a partnership between the “Département Génie Civil et Batiment” of the ENTPE, Eurovia and Appia, a research work including a large experimental campaign on the thermomechanical behavior of bituminous materials at low temperatures is proposed. The aim is to establish the links between the characteristics of the binder and the properties of bituminous mixes at low temperatures. Four different bitumens have been used with one type of aggregate and grading. The low temperature behavior of binders was evaluated with three fundamental tests: the complex modulus determination, the Bending Beam Rheometer and the tensile strength at a constant strain rate and constant temperatures. The thermomechanical behavior of bituminous mixes has been studied by performing complex modulus tests, measurements of the coefficient of thermal dilatation and contraction, tensile tests at constant temperatures and strain rates, and Thermal Stress Restrained Specimen Tests. The results are analyzed considering a rational approach. Some pertinent links between binders and mixes properties are established. Characteristics which are pertinent and discriminating enough with regard to the thermal cracking of bituminous mixes at low temperatures are presented.

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Résumé Dans le cadre d'un partenariat entre le “Département Génie Civil et Batiment” de l'école Nationale des TPE, Eurovia (groupe VINCI) et Appia (groupe EIFFAGE), une vaste campagne de recherche sur le comportement thermo-mécanique des matériaux bitumineux a été menée. Le but de cette étude est d'établir les liens existants entre les caractéristiques du liant et les propriétés de l'enrobé à basses températures. Quatre bitumes différents et une seule formulation d'enrobé ont ici été étudiés. Le comportement à basse température des bitumes a été évalué avec trois tests fondamentaux: i) l'essai de module complexe, ii) l'essai de fluage au rhéomètre BBR, iii) l'essai de traction directe SHRP à vitesse de déformation constante et températures constantes. le comportement thermo-mécanique des enrobés bitumineux a été étudié en réalisant i) des essais de module complexe, ii) des mesures du coefficient de dilatation-contraction thermique, iii) des essais de traction à vitesses de déformation constantes, ainsi que iv) des essais de retrait thermique empêché (TSRST). A partir des résultats obtenus, des liens pertiments entre les propriétés des liants et des enrobés, et des caractéristiques suffisamment discriminantes au regard des propriétés à basse température des enrobés sont mis en évidence.

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Editorial Note Presented at the 6^th International RILEM Symposium on Performance Testing and Evaluation of Bituminous Materials (PTEBM'03), held on 14^th–16^th April 2003, in Zurich, Switzerland, this paper was selected as an outstanding communication and peer-reviewed by the Scientific Committee of the JournalMaterials and Structures. Mr. Hervé Di Benedetto participates in RILEM TCs 182-PEB ‘Performance testing and evaluation of bituminous materials’, ATB ‘Advanced testing and characterization of bituminous materials’ and CAP ‘Cracking in Asphalt Pavements’.     

Bulk and shear characterization of bituminous mixtures in the linear viscoelastic domain

Traffic loads and temperature variations produce three-dimensional stress–strain fields inside road pavements, and therefore the characterization of bituminous mixtures in different deformation modes is important for prediction of the performance of pavement structures. This paper presents a methodology for the bulk and shear characterization of bituminous mixtures in the linear viscoelastic domain, under the hypothesis of material isotropy, by means of uniaxial harmonic tests with the measurement of axial and transverse strains. The theoretical approach was based on the application of the elastic–viscoelastic correspondence principle and was validated by performing tension–compression tests at selected frequencies and temperatures on asphalt concrete specimens characterized by different volumetric properties. The results showed that since uniaxial tests induced both volume and shape variations, the simultaneous measurement of the complex bulk and shear moduli was possible. The validity of the time–temperature superposition principle was also verified for both deformation modes, allowing the construction of master curves for the bulk and shear moduli. The results also showed that the total dissipated energy could be decomposed into its volumetric and deviatoric fractions with excellent accuracy.     

Influence of polymer modification on low temperature behaviour of bituminous binders and mixtures

Effects of polymer modification on low-temperature properties of bituminous binders and mixtures were studied. Three bitumens were blended with 6% SBS, SEBS, EVA or EBA. Dense graded asphalt mixtures were prepared using a gyratory compactor. The low-temperature properties of the binders were characterised using dynamic shear rheometer and bending beam rheometer, and the low-temperature cracking of the mixtures evaluated by tensile stress restrained specimen test. The results indicated that low-temperature parameters were greatly dependent on the base bitumen, and in most cases, polymer modification did not show significant benefits as compared to the corresponding base bitumen. The mixture cracking temperature was found to correlate with the limiting temperatures (in bending beam rheometer) of the binders, weakly with Fraass breaking point, but not with parameters obtained using dynamic shear rheometer. Upon isothermal storage at low temperatures, the bitumens displayed physical hardening, and effect of polymer modification was small. However, physical hardening as measured by TSRST (tensile stress restrained specimen test) was not observed for the mixtures studied.     

Analytical approach for the mix design optimisation of bituminous mixtures with crumb rubber

The present paper provides a basis for defining a mix design method specifically tailored to rubberised asphalt that takes into account the behaviour of crumb rubber. An analytical approach to quantifying the recovered deformation of crumb rubber in the post-compaction phase has been developed in order to adjust the number of gyrations proposed by the Superpave method for compacting specimens of hot mix asphalt using a Superpave gyratory compactor. The maximum allowable amount of rubber has been calculated. Finally, a step-by-step protocol has been proposed in order to fabricate and compact crumb rubber modified mixtures with the gyratory compactor.     

Analysis of the influence of binder properties on the mechanical response of bituminous mixtures

Asphalt mixtures are composed by a mass of aggregates (more than 90% of their total weight), which are bonded by a bituminous binder. Despite the fact that the binder is not the main component of these materials (around 5% of their total weight), it exerts a high influence on their mechanical response. In this sense, the service life of asphalt pavements will directly depend on the type of binder used, and thus an adequate choice is crucial to construct more durable roads. Because of this fact, it is necessary to know the characteristics of the bitumen in order to reduce the impact of different distresses that appear on roads. For this purpose, this paper studies the influence of the binder properties in the appearance of the main distresses that affect asphalt pavements around the world (stripping, fatigue cracking and plastic deformations). Five bitumens with different properties have been analysed during this research using diverse binder (UCL, multiple stress creep and recovery test and dynamic shear rheometer time sweep) and mixture (water sensitivity, wheel tracking and UGR-FACT) tests. The results obtained show that the properties of the binder influence the long-term performance of bituminous mixtures. In this sense, it can be said that flexible binders which are able to recover plastic deformations could extend the service life of the pavements.     

Exploring the recovery of fatigue damage in bituminous mixtures: the role of rest periods

Recovery capability of bituminous materials plays a significant role in the development of new technologies for extending the service life of asphalt pavements. This capability originates from various phenomena such as thixotropy, cooling, relaxation of hardening, or healing. However, their real effect on mechanical response is not clear. This article aims to investigate how rest periods (RPs) available between traffic loads can contribute to the damage recovery of bituminous materials. For this purpose, different types and durations of RPs were applied during the laboratory evaluation of fatigue resistance of these materials using the University of Granada Fatigue Asphalt Cracking Test method. The results indicate that the addition of RPs to the loading regime could lead to an extension in the fatigue life of bituminous materials. Additionally, an increase in the RP duration showed a positive impact on the resistance of the materials against cyclic loading. Nonetheless, these benefits are not only related to the recovery of lost properties during RPs, but also a growth in the amount of plastic deformations as a result of the applying RPs could delay the appearance of damages (i.e. cracking). Consequently, the bituminous material can tolerate a higher number of load cycles during fatigue test.     

The application of the dissipated energy method for assessing the performance of polymer-modified bituminous mixtures

Dissipated energy analysis has been adopted to assess the performance of bituminous mixtures with regard to their resistance to permanent deformation. The ranking given by the dissipated energy approach is in good agreement with the one obtained from the wheeltracking test. The dissipated energy analysis also provides a solution for the question which is posed by the conventional strain rate method,i.e. the exact determination of the end of the linear region in the curve of permanent strain versus number of cycles.

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Résumé L'analyse de l'énergie de dissipation a été adoptée pour évaluer la performance, des mélanges bitumineux. Le niveau relatif donné par cette méthode de l'énergie de dissipation correspond avec celui obtenu par le test de ‘wheeltracking’. L'analyse de l'énergie fournit aussi une solution à la question posée par la méthode de déformation normalement utilisée, en particulier la détermination exacte de la fin de la partie linéaire de la courbe des déformations permanentes relative au nombre des cycles de chargement.