Paving at Top Speed
VÖGELE pavers rehabilitate race track in Brno, Czech Republic.
Speed is essential on every race track. And that was also true for the extensive rehabilitation work on the Automotodrom Brno circuit carried out by German contractor Kirchhoff, Leipzig. Bad weather prevented work being started on schedule, meaning that the 5.4km circuit had to be rehabilitated in a record-breaking six weeks. The contract included stabilization of the base course by cold in situ recycling. Then three VÖGELE pavers – a SUPER 1800-2, SUPER 1900 and SUPER 2100 – had to place binder and wearing courses in just a few days. It goes without saying that a perfect evenness was to be presented.
The Automotodrom Brno circuit had remained unchanged more or less since 1987, so that it now was well due for an extensive rehabilitation. Already challenging because of the specified evenness and perfect paving of the special mix, the task became a true race against time on account of the extreme deadline pressure. The first race was scheduled for mid-May 2008, leaving just six weeks to rehabilitate the 15m wide and 5.4km long track. By the time the pavers joined the “race”, only a few days were left to place the asphalt mix. For race tracks, the requirements on surface evenness are far stricter than for conventional roads, and this was also true in Brno: the maximum tolerance over 4 metres in the transverse direction was 5mm for the binder course and just 3mm for the wearing course.
VÖGELE pavers provide circuit with top-speed surface
The “racing team” of VÖGELE machines was made up of a SUPER 1800-2, a SUPER 1900 and a SUPER 2100 paver. Together, they completed two laps of the 5.4km circuit. The first lap lasted 7 days, in which binder course was placed 6cm thick and in an overall width of 15m. This was followed by four days for paving 4cm wearing course. Each of the three machines operated in pave widths between 5 and 5.2m.
High precision required in terms of evenness
In order to achieve the specified surface evenness, the paving job’s most important challenge, a high level of accuracy had to be attained already when placing the binder course: a tolerance of no more than 5mm was permitted over 4 metres in the transverse direction. The paving team of German contractor Kirchhoff Leipzig managed to fully meet the specified accuracy requirements using NIVELTRONIC®, the VÖGELE System for Automated Grade and Slope Control. A tensioned wire served as a reference in combination with a slope sensor. As the circuit was paved in three strips hot to hot, only the two outer machines could make use of the tensioned wires. The paver in the middle – the SUPER 1800-2 – worked behind them in a slightly offset position and used their freshly laid strips as references on the left and right. The elevation was picked up by sonic grade sensors.
Use of High Compaction Screeds
All three SUPER pavers were combined with High Compaction Screeds in TP2 version, equipped with tamper and two pressure bars. On the job site in Brno, the use of High Compaction Screeds was a benefit, indeed, given the cold and wet weather which quickly cooled the asphalt. Thanks to the high pre-compaction achieved by the pavers, it was no problem for the rollers to produce the final density after just a few passes and in the small time slot left for their job. For the contract in Brno, high compaction was used not only for the binder course but also for paving wearing course. In this case, however, the high-compaction systems operated at a very low pressure only. Use of VÖGELE high compaction also for the wearing course is possible as this technology allows to regulate the pressure for the pressure bars between 50 and 120 bars.
“Race experienced” lab professionals supported the paving team
Paving base course, binder course and wearing course on the Brno circuit was supported by well-known materials testing laboratories. The race track specialists from Hart Laboratory in Melsbach, Germany, also channelled their know-how into the project when it came to placing the two asphalt layers. In addition to the usual sampling of mixes, extensive quality checks were carried out in advance at the quarry and mixing plant, trial fields were paved and experienced engineers continuously monitored the quality of the mix and the evenness during the paving operations.
The composition of the mix was recommended by the building materials specialists: together with race track designers Tilke from Aachen, Germany, they developed a special formula for the mix. This special mix takes into account not only the general requirements for motor racing, but also the local climate, the specific needs of the Brno circuit and Czech standards. Details of the formula are a company secret, but it can be disclosed that the special mix is a combination of stone mastic asphalt (SMA) and asphaltic concrete with modified grain size fractions. This material is easy to place and fulfils the specific requirements for motor racing tracks, especially when it comes to grip. The SRV (Skid Resistance Value), which describes the skid and slip resistance of road surfaces, must be at least 70. For conventional roads, an SRV of 65 is specified.
WIRTGEN on the start line with almost 1,200 HP
Before the SUPER pavers could start paving the asphalt layers, WIRTGEN cold milling machines and a cold recycler had already done a good job stabilizing the existing road base. Of course, top speed was also called for in this respect. The milling machines removed 10cm of the old pavement. In a second pass the remaining road base asphalt was loosened down to 15cm for cold in situ recycling. A WR 4200 Cold Recycler blended emulsion, water and cement into the loosened asphalt. After just 15 days, some 80,000 square metres of stabilized road base could be handed over to the asphalt paving team.
VÖGELE High Compaction Technology
When it comes to compaction, VÖGELE set the standards with the unique High Compaction Technology. Inside the screed operate 3 compacting systems exerting infinitely adjustable pressure on the mix being paved. The systems of tamper and 1 or 2 pressure bars bring about the highest degree of density a road paver can achieve. The pressure bars are the core proper of VÖGELE High Compaction Screeds.
Pulsed Flow Hydraulics
VÖGELE High Compaction Technology is based on the principle of pulsed-flow hydraulics. A pulse generator generates high-frequency pressure pulses for the pressure bars. In contrast to the beating tamper, these remain in permanent contact with the mix. Due to the high pavement density achieved by the pressure bars, fewer passes are required for subsequent compaction by rolling.
In VÖGELE High Compaction Screeds, the pressure bars P1 and P2 are located in the rear area of the screed. In this location, the highest possible compactive effort is achieved, as the mix is prevented from yielding to the front. Nor can it yield to the sides where it is confined by the screed’s end plates.
Influence on the Package of Asphalt Layers
A high degree of pre-compaction is crucial when it comes to perfectly building up a pavement in layers and achieving a pavement profile true to line and level. When paving layers varying in thickness in the transverse direction, VÖGELE High Compaction Technology provides that, although applying an identical pressure across the pave width, the pressure bars are forced down to varying depths. This guarantees uniform compaction.
High Compaction Also for Wearing Course
Though conceived primarily for compacting base and binder layers, the pressure bars can also be used for paving wearing course in appropriate cases. This requires a low pressure level. VÖGELE High Compaction Technology fulfils this requirement as it allows infinite variation of the pressure and selecting very low settings.
High Compaction Switched Off
A practical feature of the VÖGELE High Compaction Screeds: the high compaction systems can be switched off at the push of a button.
The Masaryk Ring, as the race track is also known in Brno, can look back on a 70-year history. At the beginning, a natural circuit ran 29.1km from Brno to Prague. In 1987 the new circuit, used primarily for motorcycle racing, was opened to the west of the city of Brno. The circuit with sweeping fast bends comprising six left-handers and eight right-handers covers 5.403km.
The circuit has since become a legend. The reasons were explained by WTCC driver (World Touring Car Championship) Jörg Müller of BMW‘s Team Germany: „Brno is one of my favourite events throughout the year. What I like most is the circuit’s extremely good flow. There aren‘t really any slow sections at all. The circuit is perfectly embedded in the surrounding countryside, always going up and down. This is what I believe a challenging circuit should be like.” On the third weekend of June 2008, Müller took part in two races here in the WTCC World Touring Car Championship. And the circuit is otherwise well booked throughout the racing season with various events, including a Grand Prix weekend for the Motorcycle Racing World Championships.