RELIABILITY OF RHEOMETRIC MEASUREMENTS IN BITUMENS BY MEANS OF DYNAMIC SHEAR RHEOMETERS Antonio MONTEPARA, University of Parma, Parma, Italy Felice GIULIANI, University of Parma, Parma, Italy 1. INTRODUCTION The rheological characterisation of bitumen subjected to alternating loading is generally performed through frequency sweep tests at constant temperature. The purpose of these tests is the evaluation of the complex modulus G*, the phase angle?, as well as a number of derived viscoelastic parameters [1]. Sophisticate scientific instruments are exploited to determine the above parameters through rigorous tests. Instruments and tests are designed so as to minimise the influence of the measurement system on the measured quantity. The authors present a study on the reproducibility of frequency sweep tests in natural bitumen and modified bitumen by means of Dynamic Shear Rheometer. Three identical testing instruments, located in three different Italian research centres, were used to test the same material by the same technician according to a rigorous testing procedure. 2. EXPERIMENTAL INVESTIGATION 2.1 Materials Two types of bitumen were studied (Table 1): natural bitumen with penetration class 80/100 drawn from caisson (termed NB); a bitumen modified with linear SBS (termed PMB). In the laboratory, the bitumen was stored in metallic airtight containers. The containers were kept away from heat sources in order to avoid thermal chocs. Particular attention was paid to avoid undesired deformations in the containers. The overall thermal variations were taken under control. The material was supplied in 1.5 Kg containers and then liquefied and poured into 5 g aluminium containers. In the case of the modified bitumen (PMB), particular attention was paid to the mixing phase so as the polymer resulted homogeneously distributed in the bitumen. Table 1 Bitumen NB PMB Penetration at 25 C [dmm] 83 60 Softening Point [ C] 43.7 83.5 Viscosity at 60 C [Pa s] 69.0 54.5 2.2 The Dynamic Shear Rheometer The model of dynamic shear rheometer used in the comparative investigation allows different measurement systems to accommodate in the plate-cone (CP), plate-plate (PP) and coaxial cylinder coupling of the machine. The latter is used to convert the couple applied to the electric engine shaft under shear loading to the specimen. An elettronical device drives the motion of the measurement system both under stress and rate control. Moreover, the device controls the setting of the gaps between plates containing bitumen by adjusting its mechanical (Poisson's forces) and thermal fluctuations.
The model of dynamic shear rheometer presents temperature conditioning system of the specimen that is controlled by two platinum thermoresistances via an electronical equipment. The device allows a setting of the specimen temperature within an accuracy of 0.1 C. The dangerous presence of thermal gradients was limited by positioning around the measurement system, having a ceramic support, an isolating cover that surrounds a chamber with inert (nitrogen) atmosphere. The transducer measurements of rheometer have to be adjusted to take into account of inertial and friction effects of the components which are in motion during the test. The corrections, which are quite complex, were undertaken by means of appropriate algorithms directly implemented in the software used to control the testing equipment. The corrections are based on input values of the friction of bearings and the inertia of rotating components. As the stiffness of the material increases, difficulties related to the machine compliance, which might be erroneously attributed to the material specimen itself, can be encountered. A specific contribution on the subject can be found in the work of Santagata [2], who schematically represented the rheometer as a system constituted by two elements: instrument and specimen. The overall stiffness of bitumen specimen measured during the test depends on the contribution of the measurement equipment and of that of the specimen itself. The dynamic shear rheometers employed in the present study were identical. Two of them were located in two different Italian academic research centres and one in a private testing laboratory. During the testing procedure, an identical measurement system was used in the apparatus by the technician, that is, two parallel plates of 8mm in diameter (PP8) with a 2mm gap. 2.3 Dynamic Mechanical Analisys The performance characterisation of the road bitumen requires in general isothermal curves (master curve) of a rheological parameter with respect to vibration frequency. Hence, the measurement of the parameter under consideration (usually the complex modulus G*) is performed for tests under constant temperature in the range of frequency allowed by the instrument used (frequency sweep). The tests were performed according to SHRP guidelines [1,3]. The stress level was limited to a maximum strain of 5% at the lower testing frequency so as to maintain the specimen in linear viscoelastic conditions. The range of frequencies adopted is between 0.1 and 10 Hz and the tests were performed at 10, 25 and 35 C. The results of dynamic tests under oscillating loading were used to plot master curves at the reference temperature of 25 C by exploiting Christensen's and Anderson's mathematical model [4], which defines the complex modulus G* with respect to the vibration frequency? by means of three rhelogical parameters: the vitreous modulus G g, the crossover frequency? c and the Rheological Index R. The experimental result considered for each configuration is the average of two measurements. 2.4 Reproducibility (inter-laboratory variations) According to the analysis of the results obtained in the three laboratories (, 2 and 3), the master curves of complex modulus and phase angle for the two materials are reported in figures 1 and 2. In Table 2, rheological parameters and shift factors are presented. The curves show a good correlation between the results of and 2 for both materials in terms of modulus and phase angle. A significant difference is shown by the comparison with the results of, although the trend of the curves is similar. In order to improve precision and repeatability of the rheological measurement, it is important to isolate error sources [5] which are possible even in the case of the present experimental investigation performed by the same technician and by using the same equipment.
Excluding errors in the manipulation of bitumen and in the correct preparation of specimens [3-6], it was observed after the investigation that the measurement system PP8 utilised in the three laboratories, despite its geometric similarity presented a different mass in comparison to that of the system PP8 adopted in the corresponding dynamic shear rheometer (Figure 3). Differences in the results of the three laboratories were also found by the comparison of the rheological parameters defined according to PSU Model. NB - Master Curve (G*-? ) NB - Master Curve (??-?? ) 1E+08 90 G* (Pa) 1E+07 1E+06 1E+05 1E+04 1E+03 Fig. 1 - Master Curve (G*,? ) and (?,? ) at 25 C for Natural Bitumen. 80 70 60 50 40 30 20 PMB - Master Curve (G*-? ) PMB - Master Curve (??-?? ) G* (Pa) 1E+08 1E+07 1E+06 1E+05 1E+04 1E+03 90 80 70 60 50 40 30 20 Fig. 2 - Master Curve (G*,? ) and (?,? ) at 25 C for Polymer Modified Bitumen. Table 2 BITUMEN TYPE NB PMB Complex Modulus Data log(gg) 8.6874 8.8178 9.8866 8.9660 9.8032 10.1576? c (rad/s) 711.8909 842.0905 98090.7496 486.3033 765.0189 12873.1898 Rheological Index 1.3513 1.5226 1.5524 1.6676 2.4462 2.3686 shift factor a(t=10 ) 1.5219 1.5926 1.2006 1.3724 1.3499 1.1832 shift factor a(t=35 ) -0.8165-0.7812-0.9929-0.9784-1.0696-1.0242 Phase Angle Data? c (rad/s) 1229.8590 1561.4570 3922.0204 633.6292 648.4763 3723.1244 Rheological Index 1.4254 1.4152 0.9791 1.7084 1.6555 1.7455 shift factor a(t=10 ) 1.8205 1.8734 0.8942 1.7398 1.6428 1.1673 shift factor a(t=35 ) -0.6354-0.5881-0.4234-0.6953-0.5012 0.2733
WEIGHT OF A SENSOR Side A PP 8mm Side A - : 22.29 gr PP 8mm Side A - : 22.27 gr Side B PP 8mm Side A - : 21.71 gr Fig. 3 - Geometry and weight of sensor. The final part of master curves which define the vitreous asynthot presents a small slope and, hence, it defines high values of the vitreous modulus G g. This is due to the high testing temperature and to the range of frequency adopted. Generally speaking, the rhelogical parameters obtained from the tests of the rheometer in and 2 are comparable. The anomalous results obtained from the rheometer tests in are confirmed from the distorted values of crossover frequency and from the scatter of the values of phase angle and shift factors. 3. CONCLUSIONS By using DSR for rheological measurement of bitumen, the reliability depends on different factors such as thermic history, manipulation of material and position of specimen in the measurement system. The control of the testing temperature, the efficiency of the instrument in its structural and logging components and the interpretation of data are essential aspects. An experimental investigation has been performed in order to evaluate the reproducibility of DSR measurement. Tests under oscillating loading were performed on two bitumens by means of three identical rotational rheometers adopted in three different research centres. The analysis of the results has shown that two rheometers give almost identical results, whereas the other gives significantly different results in terms of complex modulus and phase angle. In addition, the distributions of master curves with respect to loading frequency are quite scattered and irregular. Thus, the reproducibility of DSR measurement can be strongly related to the condition of the instrument and of the logging software. The study has highlighted the importance of the reliability of the equipment employed in order to perform reproducible tests. The results obtained suggest, once a common test procedure is established, to extend further the inter-laboratory testing experience. REFERENCES [1] Anderson D.A., et al., Binder Characterization and Evaluation, Volume 3, Physical Characterization, SHRP Report A-369, Strategic Highway Research Program, National Research Council, Washington DC, 1994.
[2] Santagata E., Misure reometriche su bitumi stradali - Indagine sperimentale sull uso di un reometro rotazionale, Convegno SIIV- I Materiali nella Sovrastruttura Stradale, Ancona, 1996. [3] ASTM 1995, Determining the Rheological Properties of Asphalt Binder for Specification Purposes Using a Dynamic Shear Rheometer, D-4 Proposal P246, Annual Book of ASTM Standards, Vol. 04.03, Philadelphia, PA, 1995. [4] Christensen D.W., Anderson D.A., Interpretation of Dynamic Mechanical Data for Paving Grade Asphalt Cements, Asphalt Paving Technology, Vol. 61, 1992. [5] Francken L., RILEM interlaboratory test on binder rheology, Proceedings of the Fifth International Rilem Symposium, Mechanical Tests for Bituminous Materials, MTBM Lyon, 1997. [6] Anderson D.A., Antle C., Marasteanu Y. & M., Knechtel K., Factors affecting the precision of the dynamic shear and bending beam rheometers, Proceedings of the Fifth International Rilem Symposium, Mechanical Tests for Bituminous Materials, MTBM Lyon, 1997.