Horst Falkner

STRENGTHENING BRIDGES WITH UNBONDED PRESTRESSING

This paper outlines the advantages and some of the practical considerations of unbonded prestressing, which has become a major trend in strengthening new and existing bridges. Its advantages include: (1) considerable simplification of the layout of tendons; (2) substantial loss reduction of prestressing forces due to friction; (3) easy control and correction of prestressing forces; (4) easy addition and replacing of tendons; (5) easy application of anti-corrosion systems; and (6) reduction of crack widths, especially in segmental bridges. The usual procedure to strengthen a bridge with additional prestressing tendons is to: (1) install additional anchorages, deviation supports, and tendon bundles; and (2) prestress the tendons to raise the prestressing forces. Some examples are given of practical considerations arising from unbonded prestressing. The cross girders may have insufficient load-carrying capacity, or may be closed. For T-team bridges, the prestressing anchorages can be taken up through the anchorage block. Linear prestressing is most often applied along a bridges central axis. Curved bridges require a polygonal arrangement for the tendons.

Bond Strength and Damage of Long Rebar Anchored in HPC under Static and Fatigue Loading

This paper presents an experimental study on the anchorage behaviour of long high-strength steel rebars embedded in high-performance concrete (HPC) under both static loading and fatigue loading. The HPC was designed as C60 with its cube compressive strength larger than 60 MPa, and the high-strength steel bar was adopted as HRB500 with its characteristic yield strength equals 500 MPa. Under 3×106 fatigue loading cycles and then followed by a monotonous static loading, the strain and the stress state of the reinforcement bar, and the bond stress between the concrete and the 700 mm-long bar were investigated. Based on the test results and the ANSYS finite element analysis, the bond behaviour between HPC and long high-strength steel bars is discussed.

Steel fibre concrete for underwater slabs at Potsdamer Platz : Structures in Berlin

Potsdamer Platz in Berlin is the site of a 70 000 m 2 development that is creating a new city center, with a myriad of offices, housing, shops, entertainment and recreational facilities. As most of the buildings reach far below the groundwater level, the construction of deep building pits is necessary. Due to underground conditions and environmental protection requirements, sealing injection layers can not be used for the deeper parts of these buildings pits. The remaining solution is to construct an underwater concrete slab, back anchored by tension piles. These building pits are irregularly shaped, with extremely high water pressure on the concrete slab. In order to increase overall safety, steel fibre concrete has been used to achieve a robust and ductile construction.