NEW APPROACH FOR THE FATIGUE CHARACTERISATION OF BITUMINOUS BINDERS

Transcription

1 424 6th RILEM Symposium PTEBM'3, Zurich, 23 NEW APPROACH FOR THE FATIGUE CHARACTERISATION OF BITUMINOUS BINDERS Sándor Tóth Technical and Information Services on National Roads, Hungary Róbert Perlaki Technical and Information Services on National Roads, Hungary Abstract From pavement s lifetime point of view one of the most relevant binder parameters is that parameter which can characterise the fatigue behaviour of the binder. This property can be measured at an intermediate temperature range, as the penetration according to conventional standards or as the G*sin parameter prescribed by a performance related specification (SHRP). In our study we looked for a correlation between the above mentioned parameters and the fatigue cycle-numbers determined from the classical fatigue curve (modulus vs. number of cycles), which are applied in the asphalt mix mechanics already for a long time. 1. Introduction Based upon our previous conventional and SHRP investigations we can state that the most critical feature of Hungarian bitumens is the fatigue behaviour. The low penetration values, as well as the high G*sin values (close to the SHRP specification limit) point to the need of further investigation of fatigue. Taking into consideration these facts the Wöhler's fatigue concept used in asphaltmechanics for decades seemed to be one of the most promising alternatives [1]. In the performance related (SHRP) binder specification, the value of G*sin measured in linear viscoelastic range is used for characterising the fatigue behaviour of a binder. In the literature, there are some opinions, that a possible method to come closer to the real fatigue conditions could be a non-linear characterisation [2]. One of these possible methods is investigation of modulus vs. time (cycle number) and determination the fatigue time or fatigue cycle number, at which the stiffness is reduced to the 5% of its initial value [3]. In this work a non-linear fatigue characterisation of different conventional and modified binders is presented. Fatigue characterisation includes modulus vs. time (cycle number) measurements in a dynamic shear rheometer at different oscillation stress levels. The aim of this work was to find a correlation between the SHRP G*sin parameter and the fatigue behaviour obtained from a modulus - cycle number dependence, as well as a new evaluation method of fatigue behaviour.

2 Performance Testing and Evaluation of Bituminous Materials Theory It is known that the stiffness modulus of a bitumen sample submitted to a continuous stress oscillation decreases with time (or number of cycles). A fatigue lifetime or a fatigue cycle number could be defined to a selected level of this material response. According to the international agreement (general practice) the cycle number pertained to the 5% of the initial stiffness modulus called fatigue cycle number, N. In accordance with Wöhler there is a relation between the cycle numbers (N) pertained to different stress levels (3): = A N B or (1.) log = B log N + log A (2.) where A and B are regression coefficients, which can be determined from oscillation measurements at different stress levels. 3. Experimental Eight binders were selected for the investigation. Sample identifications are shown in Table 1. The samples were tested in dynamic shear testing and their rheology was measured on the original binders. The reason of investigating the original samples was the fact that the instrument compliance with the available measuring geometry does not make it possible to apply a required stress level at the selected temperature. In addition, penetration measurements were made according to the conventional standard. Table 1. Sample identification Sample ID Description Origin B1 B-65 conventional binder MOL Rt. Zalaegerszeg B2 B-65 conventional binder MOL Rt. Százhalombatta B3 B-8 conventional binder MOL Rt. Zalaegerszeg B4 B-8 conventional binder MOL Rt. Százhalombatta P1 PmB-A 6/1S SBS mod. MOL Rt. P2 PmB-A 3/6S SBS mod. MOL Rt. P3 PmB-B 6/9 EVA mod. MOL Rt. P4 PmB-B 9/12 EVA mod. MOL Rt. The rheological measurements were made in a TA Instruments dynamic shear rheometer at 25 C and 1 rad/s, under controlled stress conditions. For measuring geometry 8mm parallel plate with 2mm gap was selected. The measurements of G*sin parameter were made following the procedure defined in SHRP binder characterisation method. Fatigue testing were made at four stress levels from 6 to 9kPa. 4. Results and Discussion G*sin and penetration values of the investigated bituminous binders were determined. Averages of the two parallel measurements are shown in Table 2.

3 426 6th RILEM Symposium PTEBM'3, Zurich, 23 Table 2. Properties of bitumen samples Sample ID Description G*sin (original binder) Penetration 25 C, kpa 25 C,,1mm B1 B-65 Zalaegerszeg B2 B-65 Százhalombatta 91 5 B3 B-8 Zalaegerszeg B4 B-8 Százhalombatta P1 PmB-A 6/1S (SBS) P2 PmB-A 3/6S (SBS) P3 PmB-B 6/9 (EVA) P4 PmB-B 9/12 (EVA) In addition, we have examined the fatigue behaviour of the selected binders according to the method mentioned above. Fatigue (modulus vs. time) curves for a conventional, a SBS and an EVA modified bitumen are shown in Figures1-3. From the modulus vs. time data the fatigue cycle number (N) could be simply determined by interpolation. Plotting the applied stress ( ) vs. fatigue cycle number (N) data in log-log diagram, the Wöhler equation can be presented. Resulting fatigue cycle numbers with the corresponding regression coefficients for all binders are shown in Table 3. In contrast to the expectation it can be seen from slope (B) of the Wöhler curves there is not any significant difference between the fatigue behaviour of conventional and modified binders. The stress dependence of the N values determined from the modulus vs. time curves for conventional and modified binders are shown in Figure 4. and 5. As a part of the investigation correlation between penetration as well as G*sin values and the slope (B) of the Wöhler curves was investigated. Results for convetional and modified binder are shown in Figures 6-7.

4 Performance Testing and Evaluation of Bituminous Materials E6 B1 fatigue curve at 25 C 1.25E6 B1 6 kpa B1 7 kpa B1 8 kpa B1 9 kpa 1.E6 G* (Pa) 7.5E5 5.E5 2.5E time (s) Figure1. Modulus vs. time for a conventional binder (B1) 8 4.E5 P1 (SBS) fatigue curve at 25 C 3.5E5 3.E5 P1 6 kpa P1 7 kpa P1 8 kpa P1 9 kpa G* (Pa) 2.5E5 2.E5 1.5E5 1.E time (s) 2 Figure 2. Modulus vs. time curves for an SBS modified binder (P1)

5 428 6th RILEM Symposium PTEBM'3, Zurich, 23 3.E5 P4 (EVA) fatigue curve at 25 C 2.5E5 2.E5 G* (Pa) 1.5E5 1.E5 5 P4 7 kpa P4 8 kpa P4 9 kpa time (s) 1 Figure 3. Modulus vs. time curves for an EVA modified binder (P4) Table 3. Fatigue cycle number of binders Applied stress (Pa) Regression coefficient Sample ID B1 B2 B3 B4 P1 P2 P3 P * * A B -,1196 -,2377 -,3747 -,1678 -,2241 -,28 -,1692 -,14 R 2,83,97,78,98,94,92,94,8 * The stiffness modulus does not decrease to the 5% of its initial value at this stress level within a reasonable measuring time

6 Performance Testing and Evaluation of Bituminous Materials Appleid stress (kpa) Fatigue cycle number, N Figure 4. Stress vs. cycle number (N) curves for conventional binders B1 B2 B3 B4 1 Appleid stress (kpa) Fatigue cycle number, N Figure 5. Stress vs. cycle number (N) curves for modified binders P1 P2 P3 P4

7 43 6th RILEM Symposium PTEBM'3, Zurich, Penetration at 25 C (,1mm) 2 1 -,4 -,35 -,3 -,25 -,2 -,15 -,1 -,5 Slope, B alap Figure 6. Penetration vs. slope (B) of the Wöhler curve for conventional binders (R 2 =,96) Penetration at 25 C (,1mm) 2 1 -,25 -,2 -,15 -,1 -,5 Slope, B mod Figure 7. Penetration vs. slope (B) of the Wöhler curve for modified binders (R 2 =,2273) From Figures 6-7. it was ascertained that there is no correlation between penetration values and the slope of the Wöhler curve. Also no correlation could be found for the G*sin values.

8 Performance Testing and Evaluation of Bituminous Materials Conclusions In our study we looked for a correlation between the penetration as well as the G*sin parameters and the fatigue cycle-numbers determined from the classical fatigue curve (modulus vs. time or cycle-number), which meant a non-linear fatigue characterisation of different conventional and modified binders in different stress levels. Based upon our results we can state, that - the good correlation coefficient of the Wöhler curves for all binders indicate that the method is applicable for binders (conventional and modified binders equally), - there is no correlation between the penetration as well as the G*sin values and the slope of the Wöhler curve neither for conventional nor modified binders, Based on good regression results of Wöhler curves we continue and expand our investigation to the aged (RTFOT and PAV) samples. 6. References 1. L. Francken, C. Clauwaert, Characterization and Structural Assessment of Bound Materials for Flexible Road Structures, 6 th International Conference on Structural Design of Asphalt Pavement, Ann Arbor, Michigan, USA, H. U. Bahia, H. Zhai, K. Bonetti, S. Kose, Non-Linear Viscoelastic and Fatigue Properties of Asphalt Binders, July 1998 (a paper submitted for presentation at the 1999 Annual Meeting of the Association of Asphalt Paving Technologist) 3. H. Soenen, B. Eckmann, Fatigue Testing of Bituminous Binders With a Dynamic Shear Rheometer, 2 nd Euroasphalt & Eurobitume Congress Barcelona, 2