EXTERNAL STRENGTHENING OF BRIDGE GIRDERS AND DECK SOFFITS

The use of carbon fiber sheet material for the strengthening/repair of bridge girders and deck soffits has been widely practiced in Japan. The first use of this material was by the Shimizu Corp. and Tonen Corp. in 1984. In general, the application of composites in this area in Japan is driven by two criteria:

  1. Strengthening to enable higher wheel loads without punching shear
  2. Enhancement of the nominally provided longitudinal deck reinforcement

In a majority of cases, carbon fiber sheet materials have been used to either prevent further cracking (Fig. 4.22), to increase load carrying capacity (Figs. 4.23 and 4.24 and at the Hiyoshigura Viaduct, as described in the site report, Appendix B), or to strengthen overhangs to enable the addition of sound walls or barriers (Fig. 4.25).


Fig. 4.22. Application of longitudinally aligned tow sheet to repair a cracked deck soffit at the Koyahara Viaduct in Gunma Prefecture.


Fig. 4.23. Yumura Bridge in Hyogo Prefecture was strengthened to increase live load capacity by the application of one layer of Replark sheet each in the longitudinal and transverse directions.


Fig. 4.24. Close-up of resin overcoat ("topcoat") application for sheets in the transverse direction.


Fig. 4.25. Application of FORCA FTS-CF-300 tow sheet to the overhang of Hata Bridge in Fukuoka Prefecture to strengthen the cantilevered portion for load from additional sound walls.

Two methods are used for the application of the sheet material. In the first, the material is laid up in continuous fashion with overlaps between adjacent sheets, and with no gaps or areas where concrete is left uncovered. This presents more-or-less uniform coverage as shown in Figure 4.26.


Fig. 4.26. Application of Replark for the strengthening of a section of the road deck on the Hanshin Expressway (fabric was first laid continuously in the longitudinal direction, after which it was placed continuously in the transverse direction).

The second method has been introduced by the Sho-Bond Corp. and is known as the Sho-Bond method. In this method, layers are bonded to the concrete as in Figure 4.27, forming a lattice pattern. Future crack propagation, if any, can be observed, and water that permeates into the concrete slab can escape. Allowing the water to escape is important. If it were retained at the concrete-composite interface, it could cause degradation and peel. In the Sho-Bond method, the two components of the resin system are colored differently. When the dark pink resin is completely mixed with the hardener, the color changes to a pale yellow. This serves as an easy indicator of proper mixing in the field.


Fig. 4.27. Schematic of the lattice pattern arising from the Sho-Bond method of application.

Regardless of the method used, the original concrete surface has to be cleaned, chipped to remove loose and/or degraded material, filled with a mortar or paste to rebuild an even surface, and then covered with a primer before any adhesion/layup of the fibers can be conducted. In a number of cases, mortar or epoxy putty is applied to the primed surface to even out irregularities. In almost all cases, the composite is allowed to cure without the application of vacuum (for compaction) under ambient conditions. Although the Sho-Bond method was developed for use with the Torayca cloth system, the company also does field placement using the Tonen Corp.'s tow sheet.

An example of this use (without the lattice pattern) is in the retrofit of a section of the Hinobashi Bridge on National Road 20 conducted in May 1993. Cracks had developed in a section between supporting steel girders in a direction perpendicular to the girder. A single layer of FORCA FTS-C1-20 carbon fiber tow sheet was applied with fibers along the deck's longitudinal axis. The tow sheet was applied in 4 areas (Fig. 4.28) with overlaps of 10 cm in each area between adjacent layers of fabric. Load tests were conducted before and after the retrofit and deformation of the reinforcing steel bars was measured. Stress levels in the steel were estimated to have been reduced by 33% through the application of the tow sheet. Traffic flow was not interrupted by the retrofit.


Fig. 4.28. Schematic of geometry of retrofit on the Hinobashi Bridge.

The application of tow sheet to bridge decks/girders amounted to about 15% of Tonen's sales of sheet-type material in 1996. Similarly it amounted to an overall average of 10.1% of the projects conducted by Mitsubishi using Replark sheets from 1987 to 1995. A significant amount (>8,000 m2) of Toray's Torayca cloth was used in 1996. Most of these applications were for crack control, prevention of concrete exfoliation, upgrading of deck capacity for new sound barriers/walls on cantilevers, or for increase of live load levels to 25 T.


Published: November 1998; WTEC Hyper-Librarian