Most important transit route in Austria compacted by HD rollers from HAMM

Austria‘s A2 motorway – also known as the „southern motorway“ – has to cope with an enormous traffic load of 145,000 vehicles daily. Trucks fill the right-hand lane like pearls on a string. Their Italian, Slovenian, Croatian, Czech and Russian number plates prove beyond any shadow of a doubt that this motorway is used as a transit route. It is therefore hardly surprising that the pavement of Austria‘s oldest motorway showed signs of damage due to this heavy load. The right-hand lane and the hard shoulder were consequently rehabilitated in summer 2006. The managers in charge of the project were not willing to accept any compromises when it came to compacting these lanes with their extremely high traffic loads: They preferred to rely on the technology offered by Hamm. No fewer than ten HAMM rollers of the series HD were in operation at the same time on various sections of the 10 km long motorway rehabilitation site.

The road rollers were kept busy, for four asphalt courses had to be compacted. Following removal of the old asphalt-topped concrete pavement, the tandem rollers first compacted a 0/16 levelling course to ensure reliable drainage of water seeping through from the superstructure. This was followed by two base courses: a 10 cm thick BT 32 course (bituminous base course with 32 mm top size of the aggregate) and another 10 cm thick BT 32 HS base course (fully stable bituminous base course). This was finally topped by a 3 cm thick surface course of SMA 11 chipped mastic asphalt.

More economical with oscillation

The decision in favour of HAMM rollers with oscillation was made during the planning phase, as maximum efficiency and compaction was needed. Construction engineer Michael Kraeftner, technical manager of the „ARGE A2 Pannenstreifensanierung 2006“ consortium formed by the companies Lang & Menhofer and Granit for this motorway project, is familiar with the advantages of oscillating rollers: „This technology gives us a lead both economically and in regards to quality. A few passes are enough to produce excellent compaction, the surfaces are extremely level and the pavement offers good skid resistance from the very beginning.“

When the work started, it was decided on the basis of tests that the asphalt courses would be compacted by two passes with vibration and three passes with oscillation. Each pass comprises a return journey. This rolling pattern rapidly yielded a high degree of compaction: After just five passes, all courses had been compacted to a level well over 100 %. Such results are typical for compaction with oscillation. Two or three passes with the roller are usually sufficient to achieve more than 95 % compaction. And the degree of compaction increases with each additional pass – without over-compacting or shattering the aggregate. The reason is revealed by comparing oscillation with vibration: In a vibratory roller, the drums are driven by an eccentric drive causing them to lift off the surface momentarily so that they compact the ground by a series of impacts in rapid succession. In an oscillating drum, on the other hand, masses rotating in the same direction build up a moment about the drum axis. In the course of one revolution, this moment changes its direction of action so that the drum performs an oscillating movement while at the same time remaining in constant contact with the ground and delivering shearing forces to the substrate. The great advantage of this technology is that, due to the constant ground contact, oscillating rollers compact the asphalt not only dynamically, but also constantly with 100 % of their static linear load.

Adjacent lane is not affected

There are also other quality criteria in favour of oscillating rollers. The remaining pavement should not be damaged by the compaction, for instance: Oscillation performs outstandingly here, as there is no shattering of the aggregate even when compacting colder areas, a useful effect when compacting the transition between hot, new asphalt and cold, existing asphalt. As a result, the A2 motorway was dynamically compacted with oscillation without simultaneously damaging the adjacent cold asphalt pavement. Oscillating rollers also display another advantage when paving surface courses, for pavements compacted with oscillation are very level. This levelness is combined with high initial skid resistance due to the grinding action of the drum on the mix. Studies have shown that final compaction with oscillation yields far better results than „static smoothing“ because the asphalt is compacted further with each pass of the oscillating roller.

More convenience, less wear

Working with oscillation also has another very pleasant secondary effect: it only produces about 15 % of the vibrations produced by vibratory drums. Even sensitive areas, such as on bridges, can consequently be compacted dynamically. This characteristic was also welcomed by the crews paving the A2 motorway, as the 10 km site included two bridges on which only static compaction had previously been possible until then. Thanks to dynamic oscillation, the paving crews were able to achieve the desired degree of compaction with considerably fewer passes.

Low-vibration compaction ultimately also pays off financially, as the roller components suffer considerably less stress and as such achieve a long service life.

HD series with perfect all-round view

The HD series of machines working on Austria‘s southern motorway were equipped as tandem or combination rollers with different operating weights. A whole series of characteristic advantages make these machines very popular with their operators: One is the HD rollers‘ panorama cab which offers a complete all-round view of the site. Even the drums and spray bars are always within the driver‘s field of vision, thus improving safety and ensuring good quality.

Reliability and good service

A tight schedule made it necessary to pave the asphalt without interruptions – another reason in favour of using HAMM rollers, for the name HAMM has been synonymous with reliability and durability for more than 125 years. Should a machine break down on site, the contractors Lang & Menhofer and Granit were able to rely on the straightforward support of a professional team from Wirtgen Austria. As engineer Michael Kraeftner succinctly said: „the service and support provided by Wirtgen Austria are excellent, as our colleagues in the workshop have confirmed.“

Oscillation and vibration: 1+1 = 3

Economical compaction means achieving the desired degree of compaction with fewer passes. In this way, the number of rollers and their operating time can be reduced. HAMM tandem rollers with two dynamic compaction systems on board are particularly effective. They have one oscillating and one vibratory drum and combine the advantages of both technologies . Identified by the letters „V“ and „O“ in their designation (e.g. HD O75V or DV 70 VO), these machines permit more flexible adjustment to the prevailing conditions, such as the weather or asphalt temperature, than with any other system.

Moreover, both compaction systems can be used simultaneously. Analysis of more than 200 trials with over 8000 measured values has shown that the combination of vertical forces applied by the vibratory drum and shearing forces from the oscillating drum produces a greater increase in compaction than achieved with two vibratory or two oscillating drums. This is why the formula for efficient asphalt compaction by HAMM is „1+1 = 3“.