Evaluation of fatigue properties of bituminous binders - Springer Link

Materials' and Structures / Matdriaux et Constructions, Vol. 37, June 2004, pp 356-359

Evaluation of fatigue properties of bituminous binders

J.-P. Planche l, D. A. Anderson 2, G. Gauthier ~, Y. M. Le Hir 1 and D. Martin 1 (1) TotalFinaElf, France (2) Penn State University, USA

ABSTRACT

RI~SUMI~

The original asphalt binder Superpave specification criterion for fatigue, G'sinS, has received considerable criticism as a specification requirement. A time sweep using the dynamic shear rheometer (DSR) has been proposed as an alternative test method for the Superpave specification. The "modulus versus number of cycles" relationships generated in the time sweep test have the appearance of typical fatigue curves. In this paper, the data was examined with respect to its validity as a measure of fatigue. Special attention was given to parameters that affect or could be confounded with "true" fatigue response: apparent fatigue can generate phenomena susceptible to interfere with the fatigue behavior: - Initial value of the complex modulus of the material. Behavior in repeated shear depends markedly on the initial stiffness of the binder. If the initial stiffness is low, damage can occur as a result of plastic flow at the outer edge of the asphalt binder. The influence of the rest periods at different time of the test. - The drop of the modulus at the beginning of the curves. - Steric hardening occurring with time, apparently even during loading, may have some effect on fatigue like when the time sweep is applied over an extended period of time. Binder type: this procedure showed significant differences in the behavior of various modified binders.

Le critbre original G'sin 6, sur la fatigue des liants dans les sp6cifications Superpave est aujourd'hui trbs fortement remis en question. Un essai rh6ologique de balayage dans le temps employant le rh6omOtre g~ cisaillement dynamique (DSR) a 6t6 propos6 comme m6thode d'essai alternative. L'Ovolution du module de rigiditO ainsi obtenue en fonction du nombre de cycles a l'apparence d'une courbe de fatigue classique. Dans cet article, les donn6es ont 6t6 6tudi6es au regard de leur validit6 pour mesurer la fatigue. Une attention particuliOre a ~t6 donnOe aux paramOtres qui affectent ou pourraient 6tre confondus avec une rOponse en fatigue vraie. Une fatigue apparente peut en effet g6nOrer des ph6nom~nes susceptibles d'interf6rer avec le comportement en fatigue: La valeur initiale du module complexe du mat6riau. Le comportement sous cisaillement rOp6t6 en d6pend beaucoup. Si cette rigidit6 est trop faible, un endommagement peut se produire par suite d'un 6coulement plastique du liant glla circonf6rence de l'6chantillon. L'influence de p6riodes de repos gl diffe'rents moments de l 'essai. La chute du module au dObut des" courbes. Le durcissement st6rique qui se produit en fonction du temps, m6me sous chargement, peut avoir des effets sur la fatigue lorsque le balayage est appliqu6 pendant des temps extrOmement longs. La nature du liant: cette proc6dure a montr~ des comportements significativement diff6rents pour des' bitumes modifies vari6s.

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INTRODUCTION

criterion for fatigue [1], G'sin6, has received considerable criticism as a specification requirement [2]. A time sweep using the dynamic shear (DSR) has been proposed as an

The original asphalt binder Superpave specification

Editorial Note Presented at the 6th International RILEM Symposium on Performance Testing and Evaluation of Bituminous Materials (PTEBM'03), held on 14th-16th April 2003, in Zurich, Switzerland, this'paper was' selected as an outstanding communication and peer-reviewed by the Scientific Committee of the Journal Materials and Structures.

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Materials' and Structures / Mat6riaux et Constructions, Vol. 37, June 2004 alternative test method for the Superpave specification [2]. The "modulus versus number of cycles" relationships generated in the time sweep test have the appearance of typical fatigue curves. Recent studies already mentioned that apparent fatigue could generate phenomena susceptible to interfere with the fatigue behavior [3, 4]. In this paper, special attention was given to parameters that affect or could be contbunded with "true" fatigue response, some already described in the literature, others not yet with respect to fatigue interference: Initial value of the complex modulus of the material [4], The influence of the rest periods, The drop of the modulus at the beginning of the curves, Steric hardening occurring with time, Binder type, including polymer modified binders.

carefully examined, in addition photographs were taken periodically during the testing to observe the failed surfaces. 3. R E S U L T S

AND DISCUSSION

3.1 General DSR time sweep The loss in apparent modulus of bitumen by repeated shear testing in the DSR is well known. The authors have conducted such testing on binders over a range of temperature and applied strain. Two different modes of failure have been observed - one in which internal micro damage appears to occur and another, which appears to be the result of testing artifacts. One purpose of this paper is to report on these observations and to identify the mechanism(s) responsible for the loss in modulus during repeated shearing in the DSR.

2. E X P E R I M E N T A L

2.1 Materials

3.2 Data analysis

The different binders used for the study are presented hereafter. All polymer-modified bitumens were prepared at a laboratory scale. Polymer dispersion and/or cross linking were carried out at high temperature (180~ under moderate shear for four hours. The Superpave grades of all the binders were also determined according to the procedure AASHTO MP1-98 "Standards specifications for performance graded asphalt binder" [5]. Unmodified bitumen, labeled B12, Superpave grade PG 82, penetration 15x0.1mm. Unmodified bitumen: labeled B13, Superpave grade PG 64, penetration 60x0.1 mm. Polymer modified binder: Styrelf| in-situ crosslinked, labeled M16; made of B13 with 4 % SB copolymer, grade PG 76; Physical blend, labeled M18, made of B13 with 6% EVA copolymer, grade PG 76. Special bitumen (standard NF EN 12597), labeled B16, grade PG82.

As described earlier [3], the authors defined the fatigue failure point, as the point (Nf) where the curve dissipated energy ratio DER vs. number of load repetition N, becomes nonlinear (i.e. DER/N > 1.05). This slope change occurs where all of the properties change very rapidly with additional load repetitions. Typical fatigue curves show three distinct phases on a variation of the stiffness modulus plotted versus the number of cycles: an initial decrease assumed to be self heating of the material under shear in the literature [4], then, a slight decrease corresponding to damage accumulation inside the sample, and finally a drop related to the sample failure.

3.3 Effect of various parameters 3.3.1 Initial value of the complex modulus of the material

Because the stiffness of the binder affects the type of failure, the tests were conducted at equi-stiffness temperatures (complex modulus equal to 45 and 5 MPa) obtained by using the modulus temperature dependency for a given binder. For an initial complex modulus at 45 MPa, Fig. 1 shows fatigue curves for each of the binders that were tested. In

2.2 Dynamic shear rheometer time sweep procedure

For the dynamic shear rheometer (DSR) testing, all specimens were prepared by using the standard test method for determining the rheological properties of bitumen, AASHTO Designation TP5-98. Adhesion between the plates and the binder was never suspect with respect to E the initiation of failure. All tests were performed on binders aged in the Rolling Thin Film Oven Test .g (RTFOT). .~ 2 The experiments were carried out using a controlled strain machine, fitted with 8-mm diameter parallel plates, 1 ~3SB crosshnked I 2 mm gap, running under a 10 Hz continuous sinusoidal * EVA modified I frequency, for a total testing time up to 16 hours z~Special binder (576,000 cycles). The tests were performed at different strain levels 1000 10000 100000 until the test specimens evidenced failure as defined Number of cycles at failure, N f later in this paper. For each test, the number of cycles at failure was determined and the failed surface was Fig. 1 - Fatigue curves for binders at 45 MPa.

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Planche, Anderson, Gauthier, Le Hir, Martin As a consequence of the above findings, the authors concluded that the DSR with its current limitations is promising for characterizing the fatigue behavior of bitumen but that additional study on the mechanism of instability flow and edge fracture is necessary. Other rheometer limitations have to be considered such as adequate torque for testing in the region where true fatigue (internal micro damage) dominates. For the materials tested, the results obtained where the initial stiffness of the binder was approximately 45 MPa, the polymer modification and more especially in-situ crosslinked binder did result in increased fatigue resistance.

all cases failure appeared to be the result of internal damage. No distortion was observed in the binder at the periphery of the plates. The polymer modified and the special bitumen displayed enhanced fatigue lives, the insitu crosslinked SB modification having with the most improved fatigue life. Another significant observation, however, was that failure occurred as the result of internal micro-damage and not from any apparent testing artifacts. At initial stiffness at 5 MPa, the sample was distorted at its circumference when the chamber was opened at the point where the modulus began to fall suddenly [3]. Photographs taken show the beginning of the drop of modulus coincided in all cases with the formation of an axisymmetric indentation in the meniscus located halfway between the two plates. With increased loading, the indentation then propagated inwards causing a significant reduction in the cross-section of the test specimen. The literature documents this phenomenon as edge fracture [6], generally associated with steady shear experiments but also observed in oscillatory shear [7]. The limit between "true" fatigue and instability flow was determined at different effecting stiffness at a single strain level. For each of the binders the pattern was the same contrary to reasonable assumption where fatigue life should increase with temperature, the fatigue life increased with temperature (in actuality with decreasing stiffness) and reached a maximum value after which it decreased (Fig. 2).

3.3.2 Influence of the rest periods at different time of the test. Healing capacity of bituminous binders is an important issue. It assesses the capacity of the binder to recover part of its rheological properties after having endured damage such as fatigue. In the present study, B12 was subjected to full-length fatigue tests including two rest periods of different durations (40min and 6h). These rest periods were placed before and after the material reached its failure point (Fig. 3). After a rest period the binder does recover a substantial part of the mechanical properties (stiffness) that were lost during the test period. This recovery seems to be more efficient when rest periods are longer, and especially when they are applied before the failure phase as defined earlier. When rest periods are located close or after the failure point, regardless of their duration, their healing effect does not seem to last more than a few thousands of cycles once the test is resumed.

3.3.3 Steric hardening Steric hardening can be defined as a progressive, reversible, isothermal increase in stiffness modulus due to molecular rearrangements. This phenomenon was observed and reported in asphalt binders. In opposition to physical hardening occurring at low temperature, steric hardening is present at intermediate or ambient temperature. In our study, this phenomenon was investigated to determine whether it has a significant influence on the fatigue behavior of asphalt binders. First, for each binder, the modulus was measured once per hour for the total. A significant hardening was observed over the duration of a regular fatigue test (16h). This hardening is also likely to occur during a fatigue test, artificially increasing the modulus of the binder. If so, in the case of a binder that would be particularly resistant to

Fig. 2 - Number of cycles at failure versus temperature, for the different binders.

The temperature at which the number of cycles at failure reaches a maximum is assumed to be the transition where internal micro damage and instability flow dominate fatigue. Based on the materials and equipment used in this study two mechanisms were observed to control the failure occurring in the dynamic shear rheometer when bitumens are subjected to repeated sheafing. True fatigue, as associated with internal micro damage, occurs at temperatures where the stiffness level is greater than 15 MPa. In the region of 5 MPa edge fracture or instability flow dominates. The transition between both types of behavior occurs over a range in stiffness rather than Fig. 3 - Effect of rest periods on fatigue recovery of bitumen binders. at a single temperature.

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Materials and Structures / Matdriaux et Constructions, Vol. 37, June 2004

Fig. 4 - Correction of a fatigue curve to account for steric hardening. fatigue, the modulus would not steadily decrease as usually observed in a fatigue curve, but rather be stable, or even increase while the material is subjected to fatigue. This very tendency was observed for some of the binders studied here. The modulus increase can be modeled mathematically as a logarithm of time. We can then obtain a rectified fatigue modulus curve that takes into account the steric hardening by subtracting its hardening effect from the original fatigue curve. In Fig. 4, the rectified curve shows a steady decrease in modulus, as a usual fatigue. From this data, it is not possible to establish with certainty that steric hardening and fatigue both occur with cumulative and opposite effects. As a matter of fact, it may be so that, when the applied strain is too high, the solicitation prevents the molecular rearrangements and avoids steric hardening. On the opposite, when the applied strain is low, steric hardening could very well play a significant role. Therefore, it is important to be aware that steric hardening does exist for bitumen binders and could influence their laboratory testing if resting times and thermal history of each specimen are not strictly controlled and reported.

- The influence of the rest periods at different time of the test, showing some "healing like" effect. - The drop of the modulus at the beginning of the curves, assumed to be self-heating of the material, as seen also in fatigue testing of mixes [8]. - Steric hardening occurring with time, even during loading may have some effect on fatigue like when the time sweep is applied over an extended period of time. This is believed to be binder composition dependent. Binder type: this procedure showed significant differences in the behavior of various binders, giving more credit to the crosslinked elastomer modified binder. This work unquestionably needs to be pursued towards mix validation, by testing hot mixtures employing similar binders and evaluating their fatigue behavior. One can expect to see such effects at different temperatures due to binder-filler and aggregate interactions.

ACKNOWLEDGEMENTS The authors are indebted to a number of people from both PennState and CReS (TotalFinaElf Research Center at Solaize, France) bitumen laboratories, including respectively D. Hunter, C. Lacour and L. Lapalu.

REFERENCES [1] Anderson, D.A. and Kennedy, T.W., 'Development of SHRP binder specification', Journal of AAPT 62 (1993). [2] Bahia, H., Zhai, H., Bonnetti, K. and Kose, S., 'Non-linear viscoelastic and fatigue properties of bitumen', Journal of AAPT 68 (1999). [3] Anderson, D.A., Le Hir, M.Y., Marasteanu, M., Planche, JP., Martin, D. and Gauthier, G., 'Evaluation of fatigue criteria for asphalt binder', Transportation Research Record, 1766 (2001) 48-56. [4] Soenen, H. and Eckmann, B., 'Fatigue testing of bituminous binders with a dynamic shear rheometer', Proceedings of the 2na Eurasphalt and Eurobitume Congress, Barcelona, 2000. [5] American Association of State Highway and Transportation Offices MP-1-98, 'Standard Specifications for Performance Asphalt Binder'. [6] Keentok, M., 'Edge fracture in rheometry', PhD thesis, Department of Mechanical and Mechatronic Engineering, University of Sydney (1997). [7] Keentok, M. and Xue, S., 'Edge fracture in cone-plate and parallel plate flows', Rheological Acta 38 (1999) 321-348. [8] Di Benedetto, H., Soltani, M.A. and Chaverot, P., 'Fatigue damage for bituminous mixtures: a pertinent approach', Journal of AAPT (1996).

4. CONCLUSIONS The time sweep using the dynamic shear rheometer (DSR) proposed as an alternative test method for the Superpave specification on the binder fatigue leads to "modulus versus number of cycles" relationships having the appearance of typical fatigue curves. In this paper, a number of parameters were found to possibly interfere with "true" fatigue response: The initial value of the complex modulus of the material, able to markedly change the binder behavior in repeated shear. A low initial stiffness can induce damage as a result of plastic flow at the outer edge of the asphalt binder.

Paper received. February 2, 2003,"Paper accepted." October 20, 2003

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