“VÖGELE Spray Paver Technology Meters Bitumen Emulsion with Excellent Precision!”

Interview with Klaus Graf (Dipl.-Ing.), expert on noise-absorbing asphalt pavements.

The option of spreading bitumen emulsion and placing asphalt in a single pass makes the SUPER 1800‑2 with SprayJet Module the perfect choice for paving thin layers on tack coat or spray seal, hot on hot (DSH‑V). These overlays can reduce traffic noise, comparable to porous asphalt pavements. Klaus Graf has rendered outstanding services to the development of this and similar methods of asphalt paving. In the interview with RoadNews, the former head of the test centre at Dr Schellenberg Institute for Materials Testing in Leipheim, Germany, discusses the importance of VÖGELE Spray Paver Technology for placing thin, noise-absorbing overlays.

Personal Details // Klaus Graf

Klaus Graf (Dipl.-Ing.) has devoted decades to the intensive study of thin asphalt layers. Until he retired, Klaus Graf headed the test centre at Dr Schellenberg Institute for Materials Testing in Leipheim, Germany. In this position, he was involved in dozens of projects for paving thin overlays and acquired expert knowledge of spray pavers. He is a member of the Bavarian Chamber of Construction Engineering, as well as of several committees in the German Road and Traffic Research Society (FGSV), in which he continues to participate in the development of different information sheets.

RoadNews: Mr Graf, you have worked on noise-absorbing asphalt pavements for many years. Where do you see the importance of thin overlays in this context?

Klaus Graf: That really is an expansive topic, as thin layers have gained a great deal of ground in recent years. One reason for this development is that public budgets have become exceedingly tight, so that the funds available often only suffice for rehabilitation of the surface course. Another reason is that paving thin overlay opens up immense possibilities for meeting the growing demands for noise-abating roads. With the right pavement, noise levels can easily be reduced by 5 dB(A) or more nowadays. Another advantage when paving thin overlay, hot on hot, is that less high-grade aggregate is needed.

RoadNews: Does this only apply to thin overlay paved hot on hot?

Klaus Graf: No, that is just one of the methods that can be applied. Noise-abating asphalt pavements can also be built with SMA LA (low-noise stone mastic asphalt) which was first used on Bavarian motorways in 2005. Another noise-abating paving material designated LOA 5 D was first used in North Rhine-Westphalia in 2007. Thin overlay paved hot on hot (DSH‑V) and LOA D have a maximum grain size of 5mm, with 8mm in the case of SMA LA. The quantities required vary between 30 and 60kg/m² depending on the paving method applied.

RoadNews: When paving thin surface course, the type and handling of the bitumen emulsion is a matter of great importance. What is your experience in this respect?

Klaus Graf: One thing is clear: when paving thin overlay, the base must be sealed, regardless of whether emulsion of the type C60BP1‑S or C67BP5‑DSH‑V is used. The emulsion must be evenly applied at a constant rate over the entire area and the emulsion film must not be damaged before being overlaid with asphalt.

That is also why I am such a supporter and proponent of the new spray paving technology, for this technology allows the asphalt to be laid directly after spreading the emulsion. This eliminates the risk of the emulsion film getting caught up under the tyres of the vehicles moving on the job site, or the paver. Moreover, the amount of emulsion applied can be controlled exactly. That is also crucial, as the amount of emulsion required is very closely related to the quality of the underlying surface and to the quality of the asphalt. This explains, for example, why the amount of emulsion needed on a milled surface differs from that needed when overlaying a fresh or newly paved binder course. And this is precisely what VÖGELE Spray Paver Technology achieves outstandingly well. With this technology, rates of spread ranging from 200g to more than 1.6kg per square metre can be selected – and that is paramount in practice.

RoadNews: People involved in practical road construction work have frequently asked what happens to the water contained in the emulsion when asphalt is paved on it immediately after spraying. After all, that water has to go somewhere!

Klaus Graf: That is a very good question – with a wide variety of possible answers, for it depends very much on the type of bitumen emulsion used and on the type of asphalt to be laid. Type C60BP1‑S emulsion contains some 40% of water, for example, as compared to some 33% for type C67BP5‑DSH‑V emulsion. Since the emulsion is heated to between 70 and 80  °C, the water immediately begins to evaporate as soon as the emulsion is applied to the underlying surface. Since we normally use a semi-permeable asphalt when paving thin overlay, the moisture remaining after paving can escape through the porous layer. In this way, water is extracted from the emulsion, leaving only bitumen. This process by which just a firmly adhering film of bitumen remains when the emulsion comes into contact with the aggregate is known as “breaking” of the emulsion.

RoadNews: What about the so-called tack coat?

Klaus Graf: Tack coat is no longer used in this form in Germany. The German ZTV Asphalt‑StB 07 Guidelines (Supplementary technical contract conditions and guidelines for the construction of asphalt pavements) provide that apart from polymer-modified emulsion, type C40BF1‑S emulsion can also be used. C40BF1‑S emulsion contains less bitumen, but more water and also a fluxing agent to reduce the viscosity. In the past, this was assured by the tack coat. This emulsion is used in construction classes with low traffic loads, i.e. classes IV to VI. Since the load is lower in these classes, considerably less emulsion is required. Application of a thinner film allows the water to evaporate more rapidly compared to larger rates of spread. Basically, I recommend use of the C40 emulsion in warm summer months in particular, as the water in the emulsion can then evaporate more easily.

RoadNews: Does this mean that the emulsion can "break" during paving, as required by the ZTV Asphalt Guidelines, regardless of the type used?

Klaus Graf: Absolutely. Naturally it has to be treated correctly. Above all, it is essential that the machine technology permits precisely metered application of the correct rate of spread. If too much C40 emulsion is applied, for example, the water may not be able to evaporate quickly enough, with the result that the bitumen begins to foam and possibly push through the overlay to the surface. The pavement becomes greasy and loses its grip. This is undesirable, of course, and must be prevented, something that can be achieved with the right machine technology and the necessary know‑how. Then durable, thin, low-noise overlays can be paved economically.

RoadNews: What happens when dense, i.e. non-porous asphalts are used?

Klaus Graf: They can also be used, but in this case it is even more important to spray exactly the right amount of emulsion so that the water has enough time to evaporate. However, semi-permeable asphalts are almost always used when paving thin layers on spray seal, hot on hot, so the question as to what happens when using dense asphalt does not normally arise. If necessary in individual cases, the technical feasibility should be verified on a test area.

There is one very important tip that should be heeded by all involved in the practical side of road construction: the milled surface should always be thoroughly cleaned and the cleaning water also removed by suction. To maintain the quality of the new pavement, it is absolutely essential to remove dirt and water as completely as possible from the surface on which the overlay is to be paved. Geometrically demanding areas, such as spandrels at traffic islands or junctions and the areas around water inlets, should first be sprayed by hand. If this is done and the right machine technology is used, and if the appropriate type of emulsion is applied at the right rate of spread, then the conditions for durable, thin, low-noise overlays will be very good.

How the Water Escapes from the Bitumen Emulsion

[1] Prepared base in the form of a milled surface or freshly paved binder course.

[2] Application of bitumen emulsion by the SUPER 1800‑2 with SprayJet Module: the paver operates in spray mode and applies exactly the required amount of emulsion – preheated to a temperature of 70 to 80 °C.

[3] This immediately triggers a chemical reaction known as "breaking", in which water is extracted from the emulsion to leave a firmly adhering bitumen film.

The thin overlay of porous asphalt is paved immediately after spraying. The hot mix causes more water to evaporate.

[4] Any water still remaining in the emulsion can evaporate through the voids in the asphalt pavement.

C67BP5‑DSH‑V is a cationic polymer-modified bitumen emulsion with class 5 breaking effect, meaning that the emulsion breaks very rapidly when it comes into contact with the base. The breaking effect and high bitumen content make this emulsion ideal for paving thin overlay on spray seal, hot on hot (DSH‑V).