- International Symposium on NDT Contribution to the Infrastructure Safety Systems, 1999 NOV 22-26 Torres, published by UFSM, Santa Maria, RS, Brazil

TABLE OF CONTENTS

Abstract Objective The Dynamic Rheometer Experimental setup and main features of the measurements Calculation of Global Material parameters Summary and conclusions References

Abstract

When a rod is performing torsional vibrations at one of its resonance frequencies, any interaction with a viscoelastic medium will change both its resonance frequency and its damping characteristics. By measuring this change, one can obtain the viscoelastic material properties of the medium. The new dynamic rheometer [1] developed at the Institute of Mechanics, ETH Zurich, is based on this principle and possesses features, which makes it particularly versatile for various applications [2]. Since the working frequencies are in the kHz range, the device is very robust in environments involving spurious low frequencies. Even occasional impact disturbances do not affect the readings after a few measuring cycles of the order of milliseconds. Hence the device is also adapted for in situ measurements by placing the rheometer on the surface of the viscoelastic medium. A series of experiments were initially performed in the controlled laboratory environment and subsequently extended to field measurements. In the present case an asphalt plug joint of a Swiss highway is being monitored since August 1998. The ongoing experiments indicate that the viscoelastic properties of the subject plug joint have not altered significantly up to the time this paper is written (one year). As a result we can conclude the material has not changed or aged significantly.

Keywords: Viscoelasticity, Bitumen, Torsional Dynamic Resonance Rheometer

Objective

This paper is part of an on going research of ETH and EMPA, the main motivation of which is to characterize bituminous binders using the dynamic rheometer in situ. Furthermore, evaluating in situ the status of such binder materials as a function of temperature and climate will eventually lead to improvements in the quality, safety and durability of bituminous binders.

The Dynamic Rheometer

The dynamic rheometer (material: 18-8 CrNi steel) consists of an external tube rigidly joined to an internal cylindrical rod (Fig. 2). The tube is free of loading along its lateral surface and attached at one end to a thick plate PL of large diameter. Since the torsional rigidity of the plate is much larger than the tube, it acts as a decoupling mass enforcing a node of the torsional vibration mode in its immediate vicinity. Thus the end F of the tube beyond the decoupling mass can be fixed without any radiation energy losses at the attachment region. The other end E of the tube where it joins the internal rod is solidly closed. With an electromagnetic transducer, TR, fixed at the free end G of the internal rod, the system is forced to perform high frequency vibrations of very low amplitude at one of its torsional eigenmodes. The frequency (5.3 Hz in the present case) is stabilized within 0.005 Hz with the help of a phase-locked loop fixing the phase between the applied torque and the measured angle of rotation. Further details including those of the electronic control circuitry can be found in [1]. The resonance frequency corresponds to a phase angle of 90°. The damping is obtained from the frequency difference df_a at two values 90°±a of the phase angle in the vicinity of resonance. When the free and closed end of the external tube is placed on the surface of a viscoelastic medium, only a boundary layer of the medium in the immediate vicinity of the contact surface participates in the motion, provided that the frequency is sufficiently high. Nonetheless, the measured resonance frequency and the damping will change in comparison to similar measurements taken in air.

Experimental setup and main features of the measurements

Fig 1: Rheometer on a bridge plug joint

Fig 2: Schematic view of the rheometer

Fig 3: Resonance frequency and temperature in 1999

Fig 4: Superposition of the frequency values as a function of temperature for the same month (August) after one year

The rheometer is currently installed on a highway plug joint near Basel, Switzerland (Fig. 1). The plug joint material is polymer-modified bitumen (PmB) B40/50 with about double the amount of polymer as used for pavement binders. Furthermore the plug joints are composed of a gap-graded aggregate skeleton with a 16mm maximum aggregate size. The following are the main features of the ongoing experiment:
• Before placing the rheometer on the bitumen, the values of resonance frequency f_A (phase angle 90°) and frequency differences df_aA for phase angles 90±a (in the present case a=15.4°) are measured.
• A support/protection shell was designed for the rheometer to allow movement in the vertical and stability in the lateral directions (Fig. 2). Light and temperature sensors are located on the surface of the bitumen. Air can circulate freely around the material so that the sensors register the actual environmental conditions.
• Subsequently, the values of resonance frequency f_B and frequency differences df_aB are measured on the bitumen continuously. In the present case six measurements every one half-hour.
• Material inhomogeneities do not affect the results as long as the rheometer is not in immediate contact with large aggregates.
• Prior laboratory experiments showed that once the rheometer is placed on the surface of the bitumen and is free to move vertically, it begins to sink slowly in the viscoelastic bitumen due to its weight. Subsequently the vertical downward displacement reaches an asymptotic value of about 4-mm. The rheometer can continue to move up or down depending on the temperature.
• Readings from the rheometer up to August 1999, indicate fluctuations due to temperature only. Figure 3 shows the fluctuations of resonance frequency (phase angle of 90°) and df_aB at 90±15.4° in August 1999 with respect to temperature. As expected, the resonance frequency increases with decreasing temperature (about 80 Hz for 25°C) and can be interpreted as an increase in the rigidity of bitumen. On the other hand, dfa, which characterizes damping, is fairly constant over this range of temperature.
• Figure 4 shows resonance frequency in August 1998 and August 1999. Obviously resonance frequency has not changed significantly at similar temperatures after one year.
• From these readings we also calculated global viscoelastic parameters of the bituminous material as reported in the next section.

Calculation of Global Material parameters

With the help of an adequate theoretical model of the vibrating system {tube+rod+bitumen}, one can calculate from the measured frequencies for given phase angles the real (G') and imaginary part (G'') of the complex shear modulus G* of bitumen. The corresponding results will be reported elsewhere. For the present paper, in a first step, the influence of the bitumen has been modeled by a torsional spring with spring constant c and a torsional dashpot with damping constantl, neglecting the mass of the bitumen layer participating in the vibration. The rod and the tube of the rheometer have been modeled as linear elastic structures vibrating in the torsional mode of the whole system and transferring force and angular displacement to the neighboring structural part. Some values of c and l calculated from measured resonance frequencies f_B and frequency differences df_aB are given for a = 15.4° in Table 1. In this table c and l are dimensionless. The dimensional reference stiffness has been chosen as the torsional stiffness of the inner rod of the rheometer for a length of 1.0 m. The numerical value of this reference stiffness is 17.8 Nm/unit angle. Thus, the dimensional values for the spring constant follow from the numbers of Table 1 by multiplying with this reference value. For the dashpot constant the chosen reference value follows by dividing the reference stiffness with the resonance frequency f_A in air (5.315 Hz at 24°C). The damping of the bituminous material is characterized by the ratio l/c also shown in Table 1. For the temperature range 12 to 43.5°C this ratio varies between 0.45 (lower temperature) to 0.55.

The next step in this on going research will be to model the bitumen as a viscoelastic half space and to determine the real and imaginary parts of the complex modulus from k andl.

Temperature [°C]	Measured fB [Hz]	Meas. dfaB [Hz]	Calculated c [-]	Calculated l [-]	Damping ratio l/c
12	5 442.8	17.91	6.64	2.99	0.45
17	5 418.3	16.86	5.15	2.38	0.46
22	5 400.4	16.72	4.30	2.13	0.50
27	5 381.3	16.72	3.42	1.92	0.56
31	5 365.3	14.98	2.79	1.59	0.57
36	5 352.2	13.20	2.43	1.32	0.54
40	5 342.7	12.14	2.20	1.17	0.53
43.5	5 336.3	12.10	2.08	1.15	0.55
Table 1:

Summary and Conclusions

On line measurements of resonance frequency and frequency differences for given phase angles between the applied torque and the displacement are obtained with the help of a newly designed dynamic rheometer placed on an asphalt plug joint of a Swiss highway. From the measured values, the spring constant and the damping of the bituminous material were determined. Aging of the material was characterized using these properties of the medium.
In particular the following can be concluded:
• The dynamic rheometer can be used for continuous in situ characterization of bituminous binders non-destructively and in a cost-effective manner.
• High frequencies allows for measurement under traffic conditions without interference of traffic induced frequencies.
• The ongoing experiments conducted since August 1998 indicate that the viscoelastic properties of the subject plug joint have not altered significantly within one year of climatic exposure and traffic. As a result we can conclude that the material has not changed or aged significantly through climatic or temperature changes.

References

1. Goodbread, J., Sayir, M., Häusler, K., Dual, J., Method and Device for Measuring the Characteristics of an Oscillating System; US Patent No. 5,837,885, (1998), European Patent No. 0749570 (1998).
2. Sayir, M.B., Hochuli, A., Partl, M.N., Measuring the Complex Viscosity of Bitumen in the kHz Range with a New Resonance Rheometer, Workshop Briefing, Euro-Bitume Workshop 99 on Performance Related Properties for Bituminous Binders, 3-6 May Luxembourg, Paper No. 106.

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