The New Cross-Border Combined (road and rail) Bridge over the Danube River, commonly known as the Danube Bridge 2, can be considered a bridge of innovations. Several technical innovations in construction techniques improving long-term durability and performance of the bridge were incorporated. Innovations include vacuum grouting of tendons in precast segmental construction, precast segmental duct couplers protecting tendon enclosures, a new cable-stay saddle design removing saddle installation from critical path, and tight radius corrugated plastic duct at stressing blisters allowing an impenetrable envelope for the entire tendon length.

An initial post-tensioning challenge on site was getting the grouting technique approved. Lack of space within precast segments led to adoption of vacuum grouting without intermediate vents and re-grouting using thixotropic grout. Complete suitability testing and full-scale – 80m long – tests were performed successfully.

This led to an additional challenge of adapting the duct system for the segments to insure that duct joints are leak-tight (absolutely essential when working with vacuum grouting). Tendons 205m long with 40 segmental couplers each were grouted using this method – there were no voids in anchorage vents after 24 hours. Knowing that tendons are properly filled with grout gives the owners’ confidence that the post-tensioning system satisfies their long-term durability needs.

Maintaining tendon duct continuity for vacuum grouting was essential on this project. Precast segmental duct couplers are successfully used throughout the project
at all segment joints. Because of vacuum grouting there could be no crossovers or leakage through segmental couplers – each tendon enclosure had to remain integral – this innovation was an absolute success for the project.

By using the segmental coupler system providing a watertight barrier across segment joints, the owner has confidence that long-term durability of the post-tensioning system is maintained.

An adaptation of standard cable-stay saddles was necessary on the project due to scheduling constraints. A new “bundle” saddle was chosen which consisted of a bundle of bare strands grouted within the pylon and attached to cable-stays outside of the pylon. As the first ever installation of the “bundle” saddle, challenges had to be overcome, such as stressing 4m long “bundle” saddle tendons with up to 55 strands to 80% over the pylon. Procedures allowed re-stressing strands from both ends to install shims compensating for load loss due to wedge penetration. Vacuum grouting from the top of the pylon of “bundle” saddle tendons was achieved.

The advantage of “bundle” saddles was that all saddles on one pylon were installed before starting cable-stay installation – removing saddles from the critical path. Even though the cable-stays are not very long or very big, four cable-stays per level had to be installed concurrently on the same day which would not have been possible with other bundles.

Tendon anchorage blisters are used throughout the project to stress tendons in precast segments. For design and construction flexibility, tendons are bent to severe angles at anchorage blisters. With corrugated plastic duct required throughout for durability, a specially formulated, proprietary composite of high-performance materials was developed to create a “tight-radius” plastic duct for use at anchorage blisters. Tight-radius plastic duct can be bent to more severe angles than standard plastic duct while still achieving the wear resistance requirements of Annex 7 of fib Bulletin No. 7.