Doubling the benefits

Laying asphalt is normally a time consuming task, particularly on a new road or full depth reconstruction where at least three layers have to be laid and carefully compacted separately. However, an innovative paver developed by Dynapac is allowing the binder and surface course to be laid in one process, offering significant time savings, performance and cost benefits.

The Dynapac Compact Asphalt Paver is currently being used to fast track surfacing on a motorway widening project in Germany, which will improve links between Hannover and Dortmund. Main contractor Kirchner is in the final stages of a two part project to widen a 6 km section of the A2 motorway, near Hamm, from two lanes to three, and is the penultimate project in the strategic upgrade between the two cities. The contractor started work on the west bound carriageway last year and is expected to finish the widening work on the east bound route by the end of 2006.

Approved Alternative

The project is not the first in Germany to benefit from this new paving technique - a number of road projects have already been successfully completed since Dynapac first launched it at Bauma in 2004. However, the project will be the last ‘trial’ before the ‘hot-on-hot’ surfacing technique is officially accepted by the German highway authority this autumn as an approved alternative to conventional surfacing.

The paver has two hoppers - the upper 25 tonne capacity one holds the surface wearing course, while the lower 45 tonne capacity hopper holds the binder course. The machine has a screed for each hopper, which enables it to lay the two layers of asphalt ‘hot-on-hot’ with the lower screed not only placing the binder course but also compacting it to 92% density.

According to Kirchner’s asphalt manager Karsten Lips, the surfacing phase of the work could have taken up to +50% longer if conventional techniques had been used. “Using the compact asphalt technique has enabled us to save both time and money on materials,” he said.

The route is used by up to 50000 vehicles a day - 25% of which are trucks - so Kirchner needed to construct a durable pavement that could cope with this heavy load. But it also needed to minimise the disruption to traffic, which made the dual asphalt technique ideal.

Cost Effective

In Germany, the standard road construction design involves overlaying an 80 mm binder course with a 40 mm surface course. But the use of the compact paver has enabled, Kirchner to reduce the thickness of the more expensive surface course to 20 mm and increase the binder course layer to 100 mm to maintain the pavement profile.

“The surface course is formed with a nominal 8 mm aggregate bound with a polymer modified bitumen binder and costs around € 55 tonne. But the binder course, which is formed from 22 mm aggregate and 70/200 penetration bitumen, costs about € 45 per tonne,” said Mr Lips.

“Despite the thinner surface course, the finished pavement is stronger and more durable than conventional surfacing. The thinner surface course is more stable in hot weather and is also more resistant to rutting.”

The combined binder and surface courses are being laid over a pre-applied base course, which have been constructed in a 220 mm layer over the new lane and between 50 and 60 mm elsewhere.

The Dynapac paver is being used en echelon - almost side-by-side - with another dual asphalt paver, built by Kirchner and based on a Vögele paver, to surface the full 16,5 m width of the road in one go. “Because the asphalt is being laid in an 120 mm thick layer, the joint between the paved widths could create a weak point but by laying the two widths ‘hot-by-warm’ results in an almost jointless finish,” said Dynapac German sales director Freidhard Johannes.

The paver’s ability to lay up to 1000 tonnes of asphalt an hour means that the logistics of keeping the machine supplied with sufficient material needs to be carefully planned. “The asphalt being used to surface this section of the A2 is being supplied by three separate asphalt plants to ensure the surfacing can be carried out as a continuous process,” said Mr Johannes.

Asphalt is fed into the paver by a mobile feeder, which can be moved to feed either the upper or lower hopper, and can take a 25 tonne load from a lorry in under 40 seconds. “The paver needs to be supplied with three trucks of binder to every truck of surface course,” said Mr Johannes. “The trucks are marked at the asphalt plant to avoid the two asphalts being mixed up and loading is carefully controlled by a manager, a role which is particularly important when the contractor is operating two dual asphalt pavers.”

Compaction Control

The binder course being used for the A2 project has been design to have an open texture to allow the paver’s screed to achieve the 92% compaction and to give a high level of interlock between the binder and surface course, without using a tack or bond coat. The progress of each paver is followed by a specially modified 4 tonne Dynapac CC142 roller, which has been widened from 1,3 to 1,6 m to apply a low linear load.

“In testing we found that the combined heat of both asphalt layers meant that the material was very sensitive, so we developed the static paver for use on the first two passes,” said Mr Johannes.

“The static paver is followed by around six passes of an 8 to 9 tonne vibratory roller, such as the CG233, and then a 12 tonne, 1,9 m wide CC522 roller completes another two to four passes to level out the pavement, and only uses vibration if necessary.”

Like all German resurfacing projects, the stone chippings are laid over the finished surface course and compacted to remove the bitumen from the upper surface of the asphalt to give maximum skid resistance.

Work on the A2 is on schedule to be completed by the end of 2006 and use of the compact asphalt paver means that the onset of colder weather will not hold up the project. “The heat from the two layers of asphalt reduces the rate of cooling, allowing the technique to be used in colder conditions than would normally be possible with conventional surfacing,” said Mr Johannes.

Dynapac’s compact asphalt paver already looks set for wider use in Germany and other machines are already in use Poland and Russia, while the first US trials have just been completed.

“Compact asphalt paving isn’t ideal for every resurfacing project,” said Mr Johannes. “The paver can be dismantled in two to three hours, allowing it to be used as a conventional machine so it need not stand idle between larger contracts.