FATIGUE AND HEALING CHARACTERISTICS OF BITUMENS STUDIED USING DYNAMIC SHEAR RHEOMETER

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1 48 6th RILEM Symposium PTEBM'3, Zurich, 23 FATIGUE AND HEALING CHARACTERISTICS OF BITUMENS STUDIED USING DYNAMIC SHEAR RHEOMETER Xiaohu Lu, Hilde Soenen and Per Redelius Nynas Bitumen, SE Nynashamn, Sweden Abstract: This paper characterizes the fatigue and healing behavior of bitumen using a dynamic shear rheometer (DSR). Different types of bitumen, before and after aging, were investigated in strainand stress-controlled modes. Procedures of storage recuperation and intermittent loading were used to determine the effect of healing on bitumen fatigue. The results indicated that, at sufficiently high stiffness, bitumen exhibited fatigue, observed by a progressive decrease in complex modulus (G*) with load applications. The fatigue life was dependent on load amplitude, and in a log-log plot, straight lines were obtained. After oxidative aging, the slope of fatigue line may increase. Regarding the effect of rest periods (healing) on fatigue, it was found that bitumens differed widely both in strain-controlled and stress-controlled tests. In the strain-controlled fatigue and storage recuperation tests, recovery of the stiffness was not reflected in an extension of the fatigue life. In the stress-controlled tests using intermittent loading, the minimum ratio of rest time versus loading time for a significant benefit to fatigue life was found to vary from half to 1 between the bitumens studied. 1. Introduction During service, cracks of asphalt pavement may be attributed to fatigue from repeated traffic loads. The fatigue cracking is determined by various factors, including material properties. Examples of the binder properties influencing mix fatigue and field performance are stiffness, viscoelasticity, and temperature susceptibility. For predominantly strain-controlled fatigue (typical for thin pavement layers), loss modulus (or G*sin ) may be a criterion. Relationship of this parameter to mix fatigue has been shown [1, 2]. However, in other investigations, this fatigue parameter was shown insufficient to predict mix fatigue [3-5], and field performance [6-7]. The controversy may be due to variations in test methods and their relevance. Laboratory fatigue tests are always run at certain controlled conditions, while the fatigue performance in the field is complicated by varying effects of traffic loads, rest periods, and environmental variables (e.g. temperature and aging). Besides investigations on the fatigue-related binder parameters, fatigue tests on binders alone have also been reported [8-11]. Using a dynamic shear rheometer (DSR), complex modulus as function of time can be determined under continuous oscillatory loading. The modulus may decrease after certain loading cycles due to fatigue damage of the binder. Limited tests also showed benefit of rest periods to fatigue life. The objectives of this paper are to further characterize the fatigue behavior of bitumens and to determine the effect of rest periods (healing) on binder fatigue using a

2 Performance Testing and Evaluation of Bituminous Materials 49 DSR. Fatigue tests are performed under strain- and stress-controlled modes, and procedures of storage recuperation and intermittent loading were used in healing tests. 2. Experimental Three bitumens were selected in this study, and typical parameters are given in Table 1. Aging was performed using RTFOT (163 C, 75 min) followed by PAV (1 C, 2 h). Table 1 Typical parameters of bitumens Bitumen code Penetration at 25 C, dmm Bit 1 86 Bit 2 86 Bit 3 87 Softening point, (R&B), C Kin. viscosity at 135 C, mm 2 /s Fatigue tests with three repetitions were carried out with a DSR (Rheologica) at three strain levels between.4 and.16, 25 Hz frequency, and at the temperatures corresponding to 3 MPa G*. The fatigue life was defined as the time at which G* decreases to 5% of its initial value. The initial G* was measured in a pre-oscillation at 14 kpa stress. To investigate effect of rest period (healing) on bitumen fatigue, procedures of storage recuperation and intermittent loading were used. The storage recuperation was conducted after strain-controlled fatigue tests, and the intermittent loading was based on stress-controlled mode. Testing conditions are detailed together with presentation of results in Section Results and Analysis 3.1 Rheological characterization Temperature-frequency sweeps were performed at temperatures from to 4 C, and at frequencies between.1 and 25 Hz. The results are presented as the Black diagrams in Figure 1. As indicated, Bit 3 is less structured (higher phase angle at given stiffness level, e.g. 1 MPa) than the other two bitumens. The dispersion in diagrams at high temperatures (phase angle between 5 and 9 ) for Bit 1 and Bit 2 suggests that the time-temperature superimposition principle (TTSP) may not be valid. Additionally, in the stress sweeps at stiffness levels of 1 kpa and 1 MPa, and at 1.59 Hz, Bit 2 exhibited a wider linear region than Bit 1 and Bit 3 (the limiting strain for linearity is defined as the strain where the complex modulus decreases by 3%).

3 41 6th RILEM Symposium PTEBM'3, Zurich, 23 Complex modulus, Pa 1.E+9 1.E+8 1.E+7 1.E+6 1.E+5 1.E+4 1.E+3 1.E+2 Bit 1 Bit 2 Bit 3 1.E+1 1.E Phase angle, deg Figure 1. Black diagrams of bitumens 3.2 Fatigue tests The strain-controlled fatigue tests were performed at 25 Hz and at the temperatures corresponding to 3 MPa complex modulus (Cf. Figure 3). Examples of complex modulus and phase angle changing with loading time in the tests are shown in Figure 2, and fatigue lines of the binders are plotted in Figure E E+7 Complex modulus, Pa 2.5E+7 2.E+7 1.5E+7 1.E+7.12 strain.8 strain.6 strain Phase angle, deg strain.8 strain.6 strain 5.E+6.E+ 2.E+4 4.E+4 6.E+4 8.E+4 Time, s 2.E+ 2.E+4 4.E+4 6.E+4 8.E+4 Time, s Figure 2. Examples of strain-controlled fatigue tests for bitumen Bit 1 (unaged) As indicated, for the unaged bitumens, the fatigue lines seem to be parallel. After the RTFOT- PAV, bitumen Bit 2 shifts to a higher number of cycles to failure, while for bitumen Bit 1, the fatigue line becomes steeper. These differences might be attributed to the compositional changes of the aged binders, but might also be due to changes in testing temperatures (increase by 7 C after RTFOT-PAV). At the testing temperatures, the aged bitumens display a phase angle of 5-1 lower than original ones in the pre-oscillation tests. The fatigue life at.8 strain is found to correlate with bitumen initial phase angle, as illustrated in Figure 4.

4 Performance Testing and Evaluation of Bituminous Materials 411 Number of cycles to failure 1.E+8 1.E+7 1.E+6 1.E+5 Bit 1 (unaged) Bit 1 (RTFOT+PAV) Bit 2 (unaged) Bit 2 (RTFOT+PAV) Bit 3 (unaged) 1.E Strain Figure 3. Summary of fatigue tests 1.E+8 Number of cycles to failure 1.E+7 1.E+6 Bit 1 (RTFOT+PAV) Bit 2 (RTFOT+PAV) Bit 2 (unaged) Bit 1 (unaged) Bit 3 (unaged) 1.E Phase angle, deg Figure 4. Fatigue life at.8 strain as function of initial phase angle 3.3 Effect of rest periods on bitumen fatigue Effect of rest periods on bitumen fatigue was studied firstly using a procedure of storage recuperation, i.e. Fatigue I Rest I Fatigue II Rest II Fatigue III. In the storage recuperation, all the fatigue tests were performed at.16 strain, 25 Hz and 15 C, and the rest periods were 4 h for Rest I and 17 h for Rest II) at the same temperature. A typical example of the tests is shown in Figure 5. To quantitatively compare the binders, fatigue life is defined as the time at which G* decreases to 5% of the starting value of G* in Fatigue I. As illustrated in Figure 6, the binders tested differ in their fatigue life, stiffness recovery, as well as re-fatigue life. The dramatically reduced fatigue life in the re-fatigue tests implies that the stiffness recovery does not relate to recovery of the originally intrinsic strength or structure of the bitumen. The quick drop in G* suggests the presence of micro-cracks (micro-damage) in the specimen. Unlike complex modulus, phase angle changes slightly (a few degrees) in the fatigue tests, and this parameter recovers almost completely after rest periods. The same procedure was also tried for the bitumens

5 412 6th RILEM Symposium PTEBM'3, Zurich, 23 after RTFOT-PAV aging. Unfortunately, relevant results have not been obtained due to high stiffness of the samples, which lead to the maximum level of the DSR instrument. 2.5E+7 6 Complex modulus, Pa 2.E+7 1.5E+7 1.E+7 fatigue I fatigue II after fatigue I and 4 h rest fatigue III after fatigue II and 17 h rest Phase angle, deg fatigue I fatigue II after fatigue I and 4 h rest fatigue III after fatigue II and 17 h rest 5.E+6.E+ 5.E+2 1.E+3 1.5E+3 2.E+3 Time, s 4.E+ 5.E+2 1.E+3 1.5E+3 2.E+3 Time, s Figure 5. Effect of rest periods on fatigue tests (Bit 3, unaged) 25 2 In Fatigue I In Fatigue II (after Fatigue I and 4h rest) In Fatigue III (after Fatigue II and 17h rest) 5 4 Fatigue I Fatigue II (after Fatigue I and 4h rest) Fatigue III (after Fatigue II and 17h rest) Starting G*, MPa 15 1 Fatigue life, sec Bit 1 Bit 2 Bit 3 Bitumens Bit 1 Bit 2 Bit 3 Bitumens Figure 6. Effect of rest periods on bitumen fatigue (average of two tests) Effect of rest periods on bitumen fatigue was also studied using a method of intermittent loading. The intermittent loading with this type of DSR was found only suitable for the stress-controlled mode, but not for the strain-controlled. When the strain-controlled mode is used, in every loading period, the rheometer needs to adjust the stress for achieving a set strain, and then can maintain this stress. This means at beginning of the loading period, the actual strain can vary from the set value. A series of fatigue tests with continuous and intermittent oscillation loading have been conducted on the unaged samples at the following conditions: 15 C, 25 Hz, and constant stress corresponding to initial strain level of.1 (Bit 1 at 21 kpa, Bit 2 at 184 kpa, and Bit 3 at 237 kpa). In the tests, different combinations of loading and rest duration were used, such as 2s loading/2s rest and 2s loading /1s rest. Typical diagrams are shown in Figure 7. The fatigue life is defined as the oscillation loading time when the specimen is broken (upper and lower plates are no longer connected). The results of the tests are presented graphically in Figure 8.

6 Performance Testing and Evaluation of Bituminous Materials 413 3,E+7 2,E+7 no rest 2s loading/2s rest 2s loading/1s rest G*, Pa 1,E+7,E+,E+ 5,E+3 1,E+4 1,5E+4 Real oscillation loading time, s Figure 7. Effect of rest periods on fatigue tests of Bit 1 (unaged) at 21 kpa stress, 25Hz, and 15 C As can be seen, at a given duration (2s) of loading, the effect of rest period on fatigue life is largely dependent on the bitumen. For Bit 1, the rest periods have a significant effect at the test with 2s loading/2s rest. For Bit 2, benefit of rest periods is even more evident, and this has already been shown at the test with 2s loading/1s rest. In other tests with 2s loading/1s rest, and 2s loading/2s rest, failure is not reached for this bitumen (Bit 2) within the DSR maximum measurement point. On the other hand, for Bit 3, no significant healing effect is observed if the rest period is less than 1 times the loading time. It was also observed that, during the fatigue and healing tests, especially at low strain levels and long rest period, steric hardening may occur (Cf. Figure 7). The steric hardening (increase in G*) is believed to be dependent on bitumen composition, and its effect on fatigue and healing needs further investigation. 2 2 Loading time to failure, s Loading time to failure, s No rest 2s/2s 2s/1s No rest 2s/1s 2s/2s 2s/1s Oscillation loading time/rest time (s/s) Oscillation loading time/rest time (s/s) Bit 1 Bit 2

7 414 6th RILEM Symposium PTEBM'3, Zurich, Loading time to failure, s Loading time to failure, s No rest 5s/25s 1s/5s 2s/1s Oscillation loading time/rest time (s/s) No rest 2s/2s 2s/1s 2s/2s 2s/4s Oscillation loading time/rest time (s/s) Bit 3 Bit 3 Figure 8. Effect of rest periods on bitumen fatigue tests and comparison of the binders 4. Conclusions The dynamic shear rheometer is applicable for characterizing the fatigue and healing properties of bituminous binders. Continuous oscillation loading may be performed at constant strain or constant stress. Bitumen fatigue occurs only at sufficiently high stiffness (complex modulus). The fatigue life is dependent on load amplitude, and in a log-log plot, straight lines are obtained. For different bitumens, the fatigue lines appear to be parallel, and after aging, may shift or become more strain dependent. In the strain-controlled fatigue and storage recuperation tests, recovery of complex modulus is not necessarily related to extension of fatigue life. In the stress-controlled tests with intermittent loading, the duration of rest periods for a significant effect is largely dependent on the bitumen, and the minimum ratio of rest time versus loading time for obvious benefit is observed to vary from half to 1 between the bitumens studied. In the fatigue/healing tests, steric hardening may occur simultaneously, and its effect on fatigue life has not yet been studied. 5. References 1. SHRP-A-398, Stage 1: Validation of the relationship between asphalt properties and asphalt-aggregate mix performance, Oliver, J. W. H., The development of Australian specification for polymer modified binders, Eurobitume Workshop 99, Luxembourg, Vanelstraete, A., Francken, L. and Reynaert, R., Influence of binder properties on the performance of asphalt mixes, Eurasphalt & Eurobitume Congress, Strassbourg, France, Claxton, M., Lesage, J. and Planque, L., When can bitumen rheological properties be used successfully to predict asphalt mix performance, Eurasphalt & Eurobitume Congress, Strassbourg, France, Chappat, M., Elastomeric bitumens: verification of Superpave criteria in situ behaviour, Eurobitume Workshop 99, Luxembourg, Deacon, J. A., Harvey, J. T., Tayebali, A. and Monismith, C. L., Influence of binder loss modulus on the fatigue performance of asphalt concrete pavements, Journal of the Association of Asphalt Paving Technologists, 66 (1997)

8 Performance Testing and Evaluation of Bituminous Materials Reese, R. E., Development of a physical property specification for asphalt-rubber binder, Journal of the Association of Asphalt Paving Technologists, 63 (1994) Bahia, H. U., Zhai, H., Bonnetti, K. and Kose, S., Non-linear viscoelastic and fatigue properties of asphalt binders, Journal of the Association of Asphalt Paving Technologists, 68 (1999) Phillips, M. C., Multi-step models for fatigue and healing, and binder properties involved in healing, Eurobitume Workshop 99, Luxembourg, Soenen, H and Eckmann, B., Fatigue testing of bituminous binders with a dynamic shear rheometer, Eurasphalt & Eurobitume Conference, September 2, Barcelona. 11. D. A. Anderson, Y. M. Le Hir, M. O. Marasteanu, J.-P. Planche and D. Martin, Evaluation of fatigue criteria for asphalt binders, Paper for TRB 21.