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Dynapac’s Ronald Utterodt explains how hot-on-hot asphalt laying is a construction method with many advantages.

“Over the entire lifecycle of a road, the operator saves one relaying every 25 years”
-Ronald Utterodt

The Compactasphalt hot-on-hot asphalt laying method offers big advantages to roadbuilders and their clients. Roads surfaced using this method last longer and are cheaper to maintain. The method was patented in 1993 by Professor Elk Richter at Erfurt, Germany and is now becoming recognised as standard practice. The main difference with conventional asphalt laying methods is that the binder and wearing course are applied to the loadbearing layer, one after the other in a single pass. Regular asphalt paving involves two separate passes, with the wearing course being applied after the binder has cooled.

With conventional paving methods, the binder layer cools before the wearing course is applied. Especially in bad weather, where the asphalt temperature falls rapidly, this can cause problems and lead to uneven compaction. Also water, snow or dirt can come between the layers and negatively influence the end result. According to the official ZTV Asphalt regulations in Germany, these problems can be eliminated by using the new method. Not only does it cut construction time by half, helping to reduce motorway tailbacks, it also brings tangible technical benefits. It makes use of the heat in the thick binder layer that has just been laid to gain time for the compaction of the two upper layers.

Good compaction takes time

Good compaction of the asphalt is crucial in road building, but the rollers take time to do their job. The cooling of the asphalt during compaction reduces the degree of compaction and with it the durability of the road surface.

By laying the binder and the wearing course at the same time, a total of twelve centimetres of asphalt is applied. Because the applied asphalt is thicker, the useful compaction time is extended by a factor of at least seven.

When the two layers are applied 'hot-on-hot', it is possible to reduce the thickness of the uppermost layer to two centimetres, as opposed to four centimetres with the conventional method. Since the wearing course mix is particularly costly, this means substantial savings for the contractor and ultimately for the taxpayer. Compared with the conventional method, Compactasphalt also gives better bonding between the individual layers, helping to prevent crack formation in the road surface.

On average, the load-bearing layer of a road needs to be replaced after 40 to 50 years, the binder layer after 15 to 25 years and the wearing course after 10 to 15 years. So the wearing course needs replacing at least once during the normal service life of a binder layer.

"This isn't the case with 'hot-on-hot' asphalt laying," says Ronald Utterodt, Application Manager for Dynapac Competence Center for Paving Applications in Wardenburg, Germany. "With this method we can extend the service life of a wearing course to match that of the binder layer. Over the entire lifecycle of a road, the operator saves one re-laying every 25 years. Road owners should be interested in this information."

Successful long-term trial

Evaluation of a long-term trial on a stretch of road in Alabama, USA, provides firm evidence of the method's benefits. The trial stretch was driven over continuously by 60-tonne trucks, simulating an 18-year service life in just two years. The trial stretch showed not the slightest sign of fatigue in the form of tracks or cracks.

Environmental benefits

The 'hot-on-hot' laying method also provides a number of positive effects on the environment. The opportunity to reduce the mix temperatures will reduce pollutant emissions. Also, by reducing the mixed good temperature at 10 °C, approx.0,23l heating oil per one ton can be saved. No bitumen emulsion is needed (which is sprayed on the binder layer in case of the conventional laying method). An additional reduction of the asphalt binder course from 10 cm to 8 cm (highway projects) is possible.

Ronald Utterodt is a recognised authority in the field of road construction with more than 20 years of experience. He is an expert in asphalt paving and has been instrumental in the successful development of Dynapac Compactasphalt method, which is becoming a standard for sustainable hot-on-hot asphalt paving.