“High Stability of the Binder Course!”

Interview with materials tester Professor Dr Kurt Schellenberg about InLine Pave®.

Personal Details // Kurt Schellenberg

Professor Dr Kurt Schellenberg is a renowned expert in the field of asphalt pavement construction. With his institute for materials testing, Dr. Schellenberg Rottweil GmbH, he has been involved in the development of new testing methods for decades. He received research awards and was entrusted with research projects by renowned companies and authorities, such as the German Road Research Institute (BASt). Prof Schellenberg has been dealing with “Two-layer paving of hot-rolled asphalts”, i.e. the “hot on hot” paving method for 14 years now, conducted a number of tests and developed new testing methods especially for this kind of paving.

Question: Professor Schellenberg, you and your materials testing institute observed the “hot on hot” paving process with VÖGELE InLine Pave® machinery from the very start. What are the results of your tests?

Prof Schellenberg: We verified that InLine Pave® and the method of paving “hot on hot” is capable of building pavements of high quality, with high process reliability. The machine technology allows to achieve a density of 98% and beyond, without rolling. This value proves the high stability of the binder course.

Question: But when, as is the case with InLine Pave® machinery, the paver for surface course passes over the freshly placed, hot binder course, is there no risk that its crawler tracks might leave imprints in the asphalt, as adverse “side effects”?

Prof Schellenberg: Imprints cannot generally be ruled out. However, it is a matter of fact that the load by the paver for surface course leaves just minimal imprints in the binder course. Given a correct composition of the mix – and I wish to emphasize that optimized mixes should be used – then the properties of the binder course, also under the crawler tracks, are identical with the ones under the rest of the pavement. This is what all our test protocols showed. There is no negative effect on the binder course, neither in terms of density nor in terms of voids content. And it is no problem to mix a binder course material so stiff that no imprints occur at all.

Question: And what about evenness?

Prof Schellenberg: In this context, too, our tests showed good results. On all projects, the currently valid tolerance requirement of ±4mm on a 4m section, in both the longitudinal and the transverse directions, was met.

Question: How good is interlocking of binder course and surface course?

Prof Schellenberg: Binder and surface courses are interlocked, this is what all our tests proved. In other words, the layers’ mixes are neither blended, nor can the layers become displaced. This is due to the monolithic interlocking of binder and surface courses. Monolithic interlocking means that an integrated whole is created that cannot be separated. The InLine Pave® method manages to create such an integrated whole as the paver for surface course follows right behind the paver for binder course. They do pave “hot on hot”.

Question: When applying the InLine Pave® method, surface course can be paved in reduced thickness whereas the thickness of the binder layer is increased. What advantages result from that?

Prof Schellenberg: This involves advantages primarily for the paving process proper and the quality of the asphalt pavement. When paving “hot on hot”, the binder layer prevents fast cooling of the surface course. The temperature inside the asphalt pavement is maintained for a longer period of time. As a result, subsequent compaction by rolling yields a higher degree of density along with a lower voids content. Thanks to this circumstance, high-quality paving is possible even in the cold season or in cold regions, as the time available for final compaction is prolonged considerably.

Question: How do you assess the service life, in the long term, of the layers paved “hot on hot”?

Prof Schellenberg: The crucial point for the lifespan of asphalt pavements – provided a road base of good load bearing capacity – is a binder course of sufficient thickness, perfectly compacted and highly stable. And this is the case with all the drill cores we have examined. At the same time, a surface course of minor thickness features a higher resistance to deformation than conventional 4cm surface course. And when the surfacing is worn, it can be milled off and a new one placed.

Question: What, in your opinion, should the design of a perfect road be like?

Prof Schellenberg: The perfect road is characterized by a road base of high stability, i.e. of high load bearing capacity. This not only refers to unbound bases, but also to asphalt bases and binder courses. Given this kind of base, the surface course can be regarded as a thin and water-tight coat, with surface properties optimized according to road users’ needs. Important for the surface course is a sufficient amount of binder. A high binder content increases a surface course’s impermeability so that water is prevented from penetrating into the asphalt pavement. Water is the asphalt’s natural enemy, so to speak. And, as mentioned before, the phenomenon of rutting is minimized due to the surfacing’s small thickness.

Question: Can such a perfect road pavement be built with InLine Pave®?

Prof Schellenberg: Yes, of course. Roads built with InLine Pave® do come very close to this ideal.