Internal Unbonded Longitudinal Post-Tensioning for a bridge

Context

Unbonded internal post-tensioning is used in bridge construction – both in box girders and in massive T girders. State of the art post-tensioning of box girders combine external transverse unbonded post-tensioning tendons inside the hollow box and bonded internal tendons in the deck and floor slabs. Unbonded internal post-tensioning can replace bonded internal post-tensioning both in box girders and massive T-beams. The advantages of the unbonded method are factory assembly, less weather dependency during installation, restressability, replaceability, better damage detection and thus better maintenance in general.

The bridge leading over the Kleine Laber consists of two separate double-webbed T-girders reaching a total length of 273m with eight 35m long internal spans and 31.5m long end spans. For the first time, internal unbonded tendons were used for post-tensioning a bridge that is erected in sections and incorporates overlapping tendons.

Solution

The structure has a maximum clearance of 12m. Pilaster strips that are located on both sides of the beams above the piers allow the statically even transfer of loads as well as accessibility to all anchorages. The unbonded internal SUSPA Type Tendons that were used had a tensioning force of Pm0,max = 2,430kN for 54 Ø 7mm, St1470/1670 post-tensioning strands and Pm0,max = 2,970kN for 66 Ø 7mm, St1470/1670 post-tensioning strands. Seven post-tensioning tendons and one empty duct were installed per superstructure in the standard range. If necessary, a tendon can be inserted into the empty duct for optional reinforcement at a later date. Due to the overlap connection above the piers, a minimum of 10 post-tensioning tendons are located there, in the area of the highest stress.

In September 2010, a replacement test was carried out using scientific monitoring. The aim of the test was to prove the installation quality of a post-tensioning tendon that had been inserted at a later time as well as the restoration of the appropriate corrosion protection on the project site. For this purpose, an empty duct running across two spans with a length of 84m and three tendon high points was fitted with an extra tendon as well as 23 temperature sensors before concreting the bridge deck. The replacement was carried out after the completion of the structure and post-tensioning of all tendons.

First, the extra tendon was detensioned and pulled from the duct while at the same time simultaneously inserting a cable. Afterwards, the corrosion protection material that had remained in the duct was removed using a bead chain and a brush chain, and the new tendon was inserted together with a preassembled stressing anchorage. This procedure was followed by the installation of the fixed anchorage and application of the tensioning load. Afterwards, the tendon was sealed, and the non grouted volume in the tendon was measured using the Volumess system in order to be able to compare it to the volume after grouting.

For grouting, the corrosion protection material was heated to more than 100°C and grouted starting from the stressing anchorage. Simultaneously, negative pressure was applied from the fixed anchorage. The temperature of the corrosion protection material inside the duct was measured by the sensors along the complete length, both during the operation and during the subsequent cool down. This way, progress could be followed along the length of the tendon. Afterwards, the completeness of the tendon grouting was demonstrated by comparing the volume of the injected material with the previously measured cavity volume and by endoscopic inspection at the tendon high points.

The replacement test with scientific monitoring at Laber Bridge has shown that a tendon can be replaced and corrosion protection can be re-established on site. The results obtained and the ensuing design principles will help to meet the ever increasing requirements of prestressed concrete bridge construction.