Development of requirement values for DSR tests after RTFOT aging
Dipl.-Ing. Hülya Sacihan
Dipl.-Ing. Hülya Sacihan
Civil Engineer Asphalt Road Construction NRW / International Projects / Bitumen Laboratory

Background

Short-lived asphalt road pavements and greater damage in recent years due to the varying aging behaviour of bitumen of the same grade pose ever increasing challenges for asphalt road construction. That is why bitumen, as a visco-elastic component in asphalt, is regarded as the weaker link. In particular, the susceptibility of bitumen to oxygen and UV-aging is not insignificant. Both oxidative aging and structural aging change the bitumen so disparately that the desired properties are no longer retained after a short period of time. The requirements for bitumen tests according to TL Bitumen-StB, which have been empirically determined in Germany, are mainly based on the delivery condition and only partially beyond the short-term aged condition. It is known among experts that the conventional tests are not suitable for determining the above mentioned changes in properties.

Findings:

In order to make a substantial contribution to further development in this area, a system was developed as part of the project on RTFOT aging (Rolling Thin Film Oven Test) of bitumen based on road construction bitumen 70/100, which takes performance tests with the dynamic shear rheometer (DSR) into account. According to bitumen standards, there are no specification values in Germany for these tests. For this reason, based on empirical values from TPA GmbH from 2016 to 2018, requirement values for tests with the dynamic shear rheometer were to be developed.

The results of this research project indicate that a holistic view of bitumen samples from different delivery points is no longer expedient when generating requirement values for the DSR, because the bitumen production methods now vary strongly due to the rationalisation of the refineries and the associated, different bitumen production methods. The examination of test results differentiated according to delivery points, however, has proven to be useful. It becomes clear that there are measured value ranges with clearly recognisable upper and lower limits for the respective delivery points, into which the DSR results can be classified. The stiffness as well as the elastic and viscous components of a bitumen can be determined based on the spans of the complex shear modules and phase angles.

This results in a separate range for short-term aged bitumen samples at a test tem-perature of 30 °C (Fig. 1). At delivery point A, particularly high ranges are achieved, almost all bitumen samples can be included here. There seems to be no constant production in delivery point A. The producing process of delivery point D appears to be as constant as possible; delivery point E is even more constant, but since the number of samples here is very small, this is initially ignored.

Figure 1: Minimum and maximum values for the complex shear modulus at 30 °C of RTFOT-aged bitumen 

Figure 2 shows the short-term aged bitumen samples from the same delivery points at a test temperature of 60 °C. Here it was found that the ranges changed and become smaller. It is conspicuous, that samples from delivery point B have higher complex shear modulus, which is a sign for a higher resistance to permanent deformation, especially at higher temperatures. Just like  the 30 °C tests, the range of delivery point D is small. Despite the large range differences at 30 °C and 60 °C, the mean values of all delivery points are in similar ranges; especially at 60 °C, the differences for the complex shear modulus and the phase angle do not differ significantly.

Figure 2: Minimum and maximum values for the complex shear modulus at 60 °C of RTFOT-aged bitumen

Although the project goal of generating requirement values for the DSR after RTFOT-aging could not be fully achieved, it is still possible to derive maximum and minimum values for the complex shear modulus and the phase angle by taking a differentiated view of the delivery points. Thus, a pattern can be derived from the empirical values from the years 2016 – 2018, with which it can easily be checked whether new deliveries are within these ranges. Outliers can be recognized immediately and the consequences can be drawn immediately after the DSR test.

Looking Ahead

In future projects, it must be validated whether the same procedures can also be used for other types of bitumen. It would also be advisable to examine the aging condition using the bending beam rheometer. These findings would considerably simplify the selection of bitumen with regard to the state characteristics of grain breakouts and top-down cracking for projects of different weights.

Figure 3: DSR samples for the determination of the complex shear modulus and the phase angle (source: TPA)
Figure 4: RTFOT oven (source: Eurobitumen.eu) 

Organisation unit:

TPA Germany / Switzerland / Netherlands / International

Duration of the project:

2018