The Swiss climate is characterised by a broad spectrum of temperatures, making high demands on the right choice of bituminous binders for road construction. They have to fully comply with current and future demands on road pavements in winter as well as in summer. Global warming must not hide the sporadic occurrence of strong winters with extreme low temperatures like in 2008/2009, which lead to embrittled pavements and thermally induced cracks.
The conventional assessment of the low temperature properties of bitumen and bituminous binders is often insufficient and repeatedly results in costly misjudgements and damaged roads. Because the standardised test method “Fraass breaking point” shows a low reproducibility, the search for an alternative test method was initiated. Some years ago, the determination of the flexural creep stiffness with the bending beam rheometer (BBR) was proposed. As this was a creep test, the expectations have only partly been satisfied. At low temperatures mainly crack induced failures were observed, which are better simulated by a fracture test. Possible alternative test methods are:
a) Determination of the fracture toughness by a static fracture toughness test
b) Determination of the DSR-breaking point in a fatigue test with the dynamic shear rheometer (DSR).
Originally, the static fracture toughness test (SFTT) was adopted from the polymer science and was improved and adapted lately to bituminous binders by a task group of the standardisation committee CEN TC 336 WG. Meanwhile the test method is ready to be evaluated in a European round robin test, before it will become a European standard. In order not to be entirely left out of the European standardisation process, it is very important for Switzerland to use this unique opportunity to participate in the imminent European round robin test.
The second test method is based on a binder fatigue test on a DSR at decreasing temperatures until fracture occurs. This temperature at fracture is defined as DSR-breaking point. In principle, the goal of this project consists in refining and validating the test method to obtain reliable and repeatable values. Only afterwards, the significance of the results can be verified.
In the last meeting of CEN TC 336 WG in February it was decided to give SFTT a high priority. Therefore, it is planned to carry out a round robin test already in the second half of 2009, to collect the mandatory precision data.
The main purpose of the research project is the replacement of the current test method, which has been proved to be insufficient for the assessment of the low temperature behaviour of bitumen and polymer modified binders. The new test method should be performance related und give more accurate and reliable results. The intended participation in the imminent European round robin test gives the opportunity to verify the results. Hence, the goal is a new test method for bituminous binders (PmB, non-modified and wax-modified bitumen):
- With an extended application field
- Giving proper and fundamental results with mechanical and physical characteristics based on a fracture test
- And particularly, give improved precision data.
Within this research project, the following work is planned:
- Commissioning and validation of the testing device adapted for static Fracture Toughness Test
- Participation in the European collaborative study
- Simple characterisation of the binder as delivered by penetration, ring and ball
- Study of 9 binders as supplied by SFTT and "DSR Breaking Point" (fatigue) including comparison with Fraass breaking point
- Investigation of 9 binders after aging with RTFOT and PAV by SFTT and "DSR Breaking Point" (fatigue) including comparison with Fraass breaking point.
- Evaluation of the results with consideration of the report to European collaborative study
- Be the research report
This research work focused on studying and further developing two new methods for the characterisation of the low temperature properties of bitumen and bituminous binders. The first test method, a three-point bending test on a notched specimen pursuant to Technical Specification CEN/TS 15963, provided consistent results after improvements had been made to the production of the test specimens. In addition to investigating the broadest possible range of types of bitumen, it was also checked whether ageing behaviour could be identified. The laboratory-induced ageing using RTFOT and PAV resulted in an increase in the fracture temperature of all the binders, which can be explained by an embrittlement of the binder. Furthermore, a significant difference between polymermodified and unmodified binders of similar viscosity was detected, which is not always the case with the bending beam rheometer. A collaborative study that was participated in as part of this project showed that reproducibility was still inadequate, and had approximately the same magnitude as with the Fraass breaking point test. This was mainly due to the fact that uniform test instruments were not used, as such instruments are not yet commercially available. The tests were performed in air or in different coolants depending on the respective test instrument. As a result of this, it can be concluded that the use of a uniform test instrument is essential in order to improve the reproducibility between laboratories.
The cyclic shear cooling (CSC) failure test has been redeveloped at Empa and is performed with a commercially available shear rheometer (DSR). The repeatability is very good in most cases, but as yet there is no reproducibility. The CSC fracture points of the polymer-modified binders in original state were significantly lower than those of the unmodified bitumen and thus clearly demonstrate the influence of polymer modification on the binder. After ageing (RTFOT/ PAV), however, these differences disappeared to some extent. In particular, the ageing behaviour of unmodified bitumen, which had lower CSC fracture points after PAV ageing, cannot be explained. The results are highly dependent on the test parameters selected, such as frequency, stretch amplitude and starting temperature. However, it was not possible to measure all the binder types using the same parameters. Further improvements and clarifications are therefore necessary for the standardisation of the