In the course of preliminary investigations for a planned repair of the Altstadtring Tunnel, the material properties of the built-in prestressing steel should be determined.
For the prestressing steel test, the accompanying engineering office had openings made on the ceiling of the Altstadtring tunnel using high pressure water jets. The structural concrete, the cladding tube and the cement glue inside the cladding tube were removed.
For the further use of the tunnel, these opening points had to be closed again properly and the prestressing steel had to be protected from further corrosion.
The installed tendon is a prestressing steel of the type St145 / 165, Sigma oval, which is prone to stress cracking. When subsequently grouting tendons with prestressing steel at risk of stress cracking using cement suspension, further cracks can occur in the prestressing steel. The tendons should therefore be grouted with a special resin suitable for this purpose, which permanently prevents both moisture and oxygen from reaching the prestressing steel.
To create a compressible space, cladding tube half-shells were attached to the existing, separated cladding tube ends and injection openings were made in the cladding tube half-shells. After completion and sealing of the half-shells, the sections to be pressed were checked for leaks and at the same time the volume to be pressed was determined. To create a pressure cushion, all openings were reprofiled with shotcrete. After the sprayed concrete had hardened, it was grouted using the vacuum process.
The work had to be carried out in several steps in night work in order to minimize the closures and obstruction for inner-city traffic.