Low Temps, Low Emissions for Tunnel Paving
BY Wirtgen Group
Strict conditions were applied for paving in the tunnel under Karlsruhe’s Kriegsstrasse road: to reduce emissions in the space, the “Kriegsstrasse Tunnel” consortium of Schleith and Züblin used machines with diesel particulate filters and reduced-temperature asphalt.
The process of completely reorganizing the center of Karlsruhe around the Kriegsstrasse has been going on since 2019. What used to be a wide road for through traffic is to become a generous space for pedestrians plus a landscaped tram track and an avenue of trees with cycle paths. At the same time, an underground train line is being built and through traffic is being diverted into a road tunnel approximately 1 mile (1.6 kilometers) long.
The client Karlsruher Schieneninfrastruktur-Gesellschaft mbH (KASIG) gave a high priority to protecting the paving team for the paving of the two tunnel bores, which were 17 feet 5 inches and 22 feet 4 inches (5.3 meters and 6.8 meters) wide, respectively. As a result, only rollers and pavers with diesel particulate filters were allowed to work in the tunnel.
The use of low-temperature asphalt was also specified. As the paving temperature is approximately 86 degrees F (30 degrees C) below that of hot-mix asphalt (HMA), it can significantly reduce emissions and high temperatures to which the paving team would be exposed in the confined space.
Experts furthermore assume that reduced-temperature asphalts have a particularly high resistance to deformation. This is of particular benefit on such a heavily-used route as the Karlsruhe road tunnel.
Short window of opportunity for paving and compaction
There were challenges involved in processing this mix. For the Karlsruhe project, special waxes ensured that it was possible to compact the mix, even at low temperatures. However, asphalt modified with waxes hardened the instant a critical temperature was undershot, so the mix had to be paved and compacted as quickly and smoothly as possible.
The contractor, Martin Paschmann Asphaltbau GmbH, used two Vögele pavers of the SUPER 1800-3i type and three Hamm rollers—a DV+ 90i VT-S-type Combination Roller, a DV+ 70i VV-S Tandem Roller and an HD 14i VO Compact Roller.
A high degree of pre-compaction and automatic functions save time
In addition to clean, accurate paving, pre-compaction was key. The AB 500 and AB 600 extending screeds were used in the Karlsruhe Road tunnel, both equipped with tamper and vibrators as compacting systems. This allowed the pavers to achieve a high degree of pre-compaction and thus reduced the number of roller passes required.
A challenge in final compaction
In addition to the challenges faced during paving, this project also demonstrated that “compared to paving hot, it is harder for the fleet of rollers to achieve final compaction because the window available for achieving final compaction is much smaller with reduced-temperature asphalts,” reported Sebastian Boldt, foreman at Schleith GmbH. The smaller window meant the roller needed to work particularly close to the paver and in short, regular runs.
“This allows the rollers to generate a great deal of compacting power in the short amount of time available. However, it is also possible to compact reduced-temperature asphalt homogeneously across the whole carriageway in dynamic form, i.e. using vibrators or oscillation,” said Dr. Axel Mühlhausen, application expert at Hamm. If the fleet of rollers is to consist of tandem rollers and combination rollers, thermal aprons are helpful to prevent the tires cooling down quickly.
The tunnel bores were paved with asphalt in two construction sections. “On the first really short section, we adapted the paving process and rolling patterns to suit the ease of compaction of the mix,” reported engineer Christian Riede. As senior construction manager at general contractor Schleith, he coordinated all the asphalt work.
“Although it was possible to process the reduced-temperature asphaltic concrete used for a relatively long time, it would then suddenly become hard,” Riede said. “This instant rise in stiffness makes compaction work more difficult and is a key difference from conventional hot asphalt. This once again proves that use of the correct time window is a key factor in the success of high-quality compaction.”
The client had specified medium-weight rollers for the first pass to compact the combined base and binder courses made of asphaltic concrete. A DV+ 90i VT-S-type combination roller from Hamm weighing approximately 9 tons was used. It first used its pneumatic tires to compact the mix, which is initially susceptible to pushing. This working motion of the tires first generated a dense structure. The drum then ensured the necessary evenness. If the roller then approaches the paver at an angle, this effectively avoids corrugation.
When compacting mixes susceptible to pushing in binder, base or surface courses, the pneumatic tires knead and work the mix without pushing it or tearing it up. To do this, the pneumatic-tired roller requires good traction. This is designed into Hamm rollers with the traction control function. The smart design of wheel suspension with level compensation in the DV+ furthermore ensures that weight is evenly distributed on the road base.
Final compaction was completed by a team involving an HD 14i VO tandem roller (4.5 ton) and a centre-pivot steered DV+ 70i-VV-S tandem roller. The reduced time window meant the compacting power needed to be applied quickly. When paving a free asphalt surface course, the edge areas in particular need to be compressed and compacted quickly—as soon as the asphalt has sufficient resistance to deformation. This was not necessary in the tunnel, as the course was being paved between slot gutters, which were already complete.
Once the last section of tunnel had been paved, the teams from Martin Paschmann Asphaltbau GmbH and Schleith GmbH took stock. “Paving reduced-temperature asphalt is still new to a lot of people, so there is a lack of experience-based values, especially with regard to material characteristics during compaction, which was why we continuously checked density using an isotope probe,” Riede said. “Both the measurements and the laboratory examinations carried out subsequently confirmed that we produced a high-quality, homogeneously compacted carriageway using Vögele pavers and Hamm rollers.”