Economical, Eco-friendly, Noise-Reducing:
Paving Thin Overlay is a Convincing Option

Perfected spray paver design gains ground.

Rehabilitating road pavements by paving thin asphalt overlays “hot on hot” on spray seal (DSH-V) is becoming increasingly widespread. A powerful spray paver is needed for these jobs, such as the SUPER 1800-2 with SprayJet Module from VÖGELE. Dipl.-Ing. Volker Schäfer explains how the DSH-V method works and which points must be noted when using a spray paver.

Personal Details // Volker Schäfer

Volker Schäfer is a recognized expert in the field of asphalt road construction and a specialist on the topics of structural preservation and noise-reducing asphalts as well as asphalt pavements for very high loads. He works as a consultant on all matters related to asphalt and sett paving methods and their implementation in accordance with construction contracts. He has been a publicly appointed and sworn expert on asphalt paving in road construction since 2003. Since it was first founded in 2005, Volker Schäfer has been a lecturer for the further education programme entitled “Asphalt Technology for the Specialist Fields of Asphalt Preparation and Asphalt Paving as well as Practical Implementation in Accordance with Construction Contracts”. He is also very active in the “Asphalt Paving Methods” work group of the German Association for Road and Transportation Research (FGSV). In 1991, Volker Schäfer was one of the founding members of the technical committee dealing with “Maintenance Technology”, which he has headed for the past three years.

RoadNews: Mr Schäfer, how important is this method for “pavement preservation” today and how is this likely to develop in future?

Volker Schäfer: The method’s growth potential is a long way from being exhausted – quite the contrary, in fact. Its significance for additionally reducing noise in particular is not being given nearly enough emphasis. Another advantage of this method is that it is a technically mature means of controlling noise which has now also been firmly anchored in our technical guidelines. With the exception of paving porous asphalt (OPA), other noise abatement methods are still in their infancy: we do not know just how long the life span of the respective pavements actually is. In the case of DSH-V, however, we have been observing all of its properties over the last 20 years and we can, therefore, reliably plan the application of this type of surfacing. Since this is important to the client, the importance of this method will continue to increase.

RoadNews: Which points must be observed if a thin asphalt overlay is to be successfully paved “hot on hot” on spray seal? Which skills are needed and what must the machine be capable of?

Volker Schäfer: The team must have received special training to work with the spray paver. After all, they are not just paving asphalt, but are also applying a spray seal of bitumen emulsion at the same time. Some additional technical know-how is needed in order to handle the emulsion correctly. The logistics of a spray paving job are naturally also a little different, as the work proceeds at 3 to 5 times the pave speed of conventional asphalt paving. The spray paver’s operating concept should be easy to learn and easy to understand. Because everything happens fairly fast, the machine must also respond quickly. That is basically all that is needed.

RoadNews: What are the advantages of the DSH-V method?

Volker Schäfer: Due to the composition of the asphalt mix, the texture is finer and more open-graded than that of conventional asphalt pavements. This enhances the road’s non-skid and noise abatement properties. Although noise is not reduced to quite the same extent as with porous asphalt, the reduction is still significantly greater than with classical dense asphalt surfacings. Initially, the focus was on optimizing the durability of the road and improving its non-skid properties. Noise abatement was more or less a secondary aspect of the development, though it has now become one of its principle uses.

RoadNews: In your opinion, will the DSH-V method essentially be used in municipal areas or is it equally suitable for use on motorways and national roads?

Volker Schäfer: Noise is produced wherever there is a lot of traffic and/or vehicles travelling at high speeds. Not all job sites meet the basic prerequisites for paving porous asphalt as a noise reduction measure. But the DSH-V method can be used to durably reduce noise levels very quickly on existing roads without requiring major structural changes. That is the great advantage of this method. That is also why it can be used not only on inner-city roads with heavy traffic, but also in residential areas, on national roads and even on motorways. A little more care is needed when working on inner-city roads with their kerbstones, but that is no problem. And that is what makes this method so good! With a spray paver, you can work on an inner-city road one day and on a heavily frequented motorway the next, all with the same machine. The Southern Bavarian Motorway Authorities in Germany are already applying the DSH-V method instead of conventional asphalt paving, not only for pavement rehabilitation jobs but for reconstruction, too. And they are using it at all locations where local residents are clamouring for noise reduction measures, but where a porous asphalt surfacing is not yet called for.

RoadNews: What can you tell us about the C67BP5-DSH-V bitumen emulsion?

Volker Schäfer: The bitumen emulsion C67BP5-DSH-V contains 67% of bitumen and 33% of water. Its viscosity is, therefore, very good in comparison to more highly concentrated bitumen emulsions with a 70% bitumen content or more. The emulsion is much more fluid, easier to handle and to spray, but is not so thin and therefore more “stable” than a 60% bitumen emulsion. Even though we're only talking about a few percent here, this makes a world of difference. The bitumen emulsion contains polymer modified bitumen as binder to improve the elasticity and ensure a strong bond.

RoadNews: The bitumen emulsion has to “break” in order to produce the desired adhesion. What does that mean?

Volker Schäfer: To produce a bitumen emulsion, the bitumen is finely shredded and mixed with water. Seen under a microscope, bitumen emulsion is hence made up of small, medium-sized and larger globules of bitumen which are suspended in water, so to speak. The water contains an emulsifying agent which encloses the bitumen globules like a film so that they repell one another instead of bonding. In this state, the bitumen emulsion can be stored stably and can hence be transported to its place of use at room temperature. Normally, the breaking process only begins when the bitumen emulsion comes into contact with raw rock, as occurs, for instance, during surface treatment. The bitumen and the water separate again, the globules bond together and the adhesive effect is obtained. But breaking can also be triggered by incorrect handling.

RoadNews: Some people wonder what actually happens during spray paving. First the bitumen emulsion is sprayed on the surface, to be followed immediately by the hot asphalt. Where and when can water escape from the bitumen emulsion? Does it escape during this short time after spraying or only once the asphalt overlay has been paved and the water evaporates through the porous composition of the asphalt?

Volker Schäfer: The classical dogma is that bitumen emulsion must be sprayed, have broken and the water have evaporated before being overlaid with asphalt. But everything is very different with the DSH-V method. Initially, a large amount of bitumen emulsion is sprayed, about three times as much as is normally used to build a pavement in layers. This means that 2½ times the amount of water must evaporate. Breaking effectively does not take place between spraying the emulsion and paving the asphalt, the time frame is simply too short. Instead, the hot asphalt is laid immediately. In combination with the porous composition, the energy emitted by the hot asphalt causes most of the water to evaporate upwards immediately. A small amount of the moisture can also escape into the voids of the asphalt layers below.

RoadNews: Can you describe the special mix used for this method?

Volker Schäfer: The mix used in 90% of all cases is a DSH-V 5 asphalt because it can be used everywhere, from motorways to roads in residential areas. However, DSH-V 8 mix can also be used. DSH-V 5 is an asphalt with an aggregate grading curve somewhere between stone mastic asphalt (SMA) and asphaltic concrete. It has a higher voids content than conventional asphalt, since the sprayed bitumen emulsion seals the pavement from below. In other words, binder is artificially accumulated at the bottom during the paving process. The asphalt mix itself only contains the quantity of binder required to ensure that there is no need to spread chippings over the surface of the DSH-V overlay, as would be the case with any other roller-compacted asphalt. The non-skid property is obtained from the outset.

RoadNews: How important do you consider the use of a feeder to be in this process? What effect does it have on the process?

Volker Schäfer: Three factors in particular support the use of a feeder when paving thin asphalt overlay “hot on hot” on spray seal. Firstly, as already mentioned, the pave speed is very high when working with this method. At such speeds, the mobile feeder can greatly simplify the fairly complex site logistics. It is no longer possible for the feed lorry to bump the paver. This prevents any unevenness in the asphalt surfacing. Secondly, the mobile feeder guarantees an uninterrupted supply of mix to the paver. This vastly improves the quality of the pavement. And thirdly: if a mobile feeder is used, an extra emulsion tank can be installed on the VÖGELE spray paver, with the result that bitumen emulsion is replenished less often. This also speeds up the paving process.

RoadNews: Long job sites and the associated traffic tailbacks are a recurrent topic in Germany. With the DSH-V method, pavement rehabilitation projects can be undertaken quickly and in high quality, with shorter construction times. Could this become a decisive factor for customers?

Volker Schäfer: It could, but speed must not be obtained at the expense of quality. The important point is that the basic conditions for paving are met. And this also applies to such methods as DSH-V. The great virtue of DSH-V is that the finished overlay does not require a long cooling time and can hence be driven over shortly after paving. Planning must focus on quality as early as during the construction of the road, and not just when it needs rehabilitating. In my view, compact asphalt pavements with a 2cm surface course should have a service life of 20 years – and even more for the compact, thick asphalt binder courses with very low voids content. All we then have to do after 20 years is mill off the remaining thin surface course and overlay it with a thin asphalt surfacing paved “hot on hot” on a spray seal. In the end you have a low-maintenance road that lasts for at least 30 years.

What happens when bitumen emulsion breaks?

[ 1 ] Prepared base: milled surface or freshly laid binder course.

[ 2 ] Hot bitumen emulsion at a temperature between 70 and 80 °C is applied by the spray paver.

[ 3 ] Paving a thin asphalt overlay according to the DSH-V method. The bitumen emulsion “breaks” immediately as the hot asphalt causes the water to evaporate, leaving a firmly adhering film of bitumen.

[ 4 ] Any water still remaining in the emulsion evaporates through the “open pores” of the asphalt overlay.