|Site:||Sone Viaduct - External Cable Anchor Block|
Japan Highway Public Corporation
Hyogo Prefecture, Japan
|Date Visited: ||October 1996||Host: ||Shin-Ichiro Kumagai, Sumitomo Construction Co., Ltd.||Summary: ||Post-tensioning of anchor blocks to existing girders using Technora multi-strand tendons for the rehabilitation of existing concrete bridge girders with external tendons.|
The strengthening of bridges using external cables requires that additional brackets be attached to girders in order to anchor the external cables. Conventionally, this is done through friction splicing using short steel bars to anchor the additional brackets to the girders. However, this often results in loss of prestressing force in the short post-tensioned steel tendons due to anchor set and relaxation, leading to sliding of the brackets. Furthermore, the method is difficult to use in tight, confined spaces, such as when retrofits have to be conducted on bridges having a number of adjacent T-girders.
Aramid tendons were used to attach the anchor blocks taking advantage of the lower modulus (1/4th - 1/5th that of steel) of the aramid fiber-reinforced polymer (AFRP) tendons and thereby resolving the problems associated with tension loss and displacement. A schematic of the bridge cross-section and girder dimensions is given in Figure B.56.
Fig. B.56. Bridge girder details.
Instead of the conventional method of post-tensioning each bracket separately, a new method of simultaneous tensioning through all main girders, using temporary reaction devices between girders, was used. Aramid tendons were pre-tensioned together perpendicular to the girders after which the anchorage blocks were cast. Once cured the tendons were cut between girders and the external steel cable was attached and jacked. The overall procedure is shown in Figures B.57-B.64.
Fig. B.57. Unrolling AFRP tendons from a drum.
Fig. B.58. Preparing the anchorage (bond-type).
Fig. B.59. Injection of mortar into the tendon mounted anchorages.
Fig. B.60. Drilling of holes into girders for the introduction of aramid tendons.
Fig. B.61. Placement of temporary reaction devices between girders.
Fig. B.62. Jacking of tendons.
Fig. B.63. Cutting of tendons between girders after cure of concrete in anchor blocks.
Fig. B.64. Installation and jacking of external steel cables.
A total of 56 anchor blocks, each with 6 AFRP tendons of 9 strands of 7.4 mm diameter spirally wound rods, were used on the 28 main girders (2 spans of 7 girders each). Close-ups of the anchorage blocks are shown in Figures B.65 and B.66 and a set of 9 strands of AFRP in the anchorage is shown in Figure B.67. Each 7.4 mm diameter tendon has a nominal fiber volume fraction of 65% with a unit weight of 62 g/m and a design breaking strength of 7.630 kgf. The overall length of rod used in this project until its completion in May 1995 was 3.02 km.
Fig. B.65. View of anchorage block showing position of 6 AFRP tendons.
Fig. B.66. End view of anchor blocks with external cables and anchorages.
Fig. B.67. Technora 9-bar tendon in a stainless steel bonded anchorage.