CARBON FIBER BASED LINEAR REINFORCING ELEMENTS

Due to their light weight (about one fifth that of steel), high tensile strength (higher than steel) and good overall environmental durability, carbon fiber based tendons and cables are increasingly being used for reinforcement of concrete structures in Japan. The reduction in weight facilitates better handling and easier field installation compared to steel. These elements also cause significantly less sag under their own weight, which increases load capacity while enabling the construction of longer bridge spans. There are three major product lines currently available in Japan:

• Leadline rods/tenders, manufactured and distributed by Mitsubishi Chemical Company
• Carbon Fiber Composite Cable (CFCC), manufactured and distributed by Tokyo Rope Manufacturing Co., Ltd.
• Nippon Steel Advanced Carbon Fiber Composite (NACC) strand cables, manufactured and distributed by the Suzuki Metal Industry Co., Ltd. and Nippon Steel Corp.

Leadline and CFCC are fairly well established and have a history of use going back to 1987-88. CFCC and NACC are multi-strand twisted cables. Leadline are individual rods.

Leadline Rods/Tendons

Leadline reinforcing elements are circular rods that are pultruded using unidirectional carbon fibers at 65% fiber volume fraction with an epoxy resin. The rods have a specific gravity of 1.6, a relaxation ratio of 2-4% at 20°C, and a coefficient of thermal expansion of 0.68 x 10^-6 /°C in the longitudinal direction. The rods have a tensile modulus of 147 GPa and a 1.5 to 1.7% elongation at break. Rods are available in a number of diameters with four major surface types as shown in Fig. 3.11

Fig. 3.11. Types of surface appearances on Leadline rods.

The indentations are provided to facilitate stress transfer and interlock between the rod and the surrounding concrete. Basic properties for the rods are given in Table 3.2. The values of tensile force represent guaranteed loads. Values of force for indented rods are guaranteed with a wedge type anchorage system.

Two main types of anchorages, the wedge type and the grout type, are used as shown in Fig. 3.12a and b respectively.

Fig. 3.12. Types of anchorages used with Leadline tendons/rods.

Capacity details for the two types of anchorages shown in Fig. 3.12 are given in Table 3.3

The wedge type anchorages are amenable for use in both pre-stressing and post-tensioning applications. The cement grout anchors are primarily used in post-tensioning applications. As described earlier, Leadline rods have been used in a number of applications ranging from long-span structures to ground anchors. These rods are easily coiled in lengths of 500 m using a ring of 2 m diameter (Fig. 3.13) for transportation and storage.

Fig. 3.13. Leadline rods ready for transportation.

Carbon Fiber Composite Cable

CFCC elements are made from PAN (polyacrylonitrile) based carbon fiber. The fiber is used in prepreg tow form that is stranded to make a primary strand. The surface of the strand is specially coated. Then multiple strands are used to form a twisted cable. Once the strands are twisted together, the entire assembly is heated to cause adhesion between the coatings and resin around individual strands. CFCC is available in the form of single strands and cables using 7, 19 and 37 strands. A flow chart for the process is shown in Fig. 3.14 and the form of the elements is shown in Fig. 3.15.

Fig. 3.14. Schematic of the process used to manufacture CFCC elements.

Fig. 3.15. Appearance of CFCC elements.

The elements are available in three types based on the use of different resin systems, each being applicable within a specific temperature range as shown in Table 3.4.

CFCC elements have a tensile strength of 1.8 GPa, a modulus of 137 GPa, 1.6% elongation at break, and a specific gravity of 1.5. The coefficient of stress relaxation is 0.65 at 20°C over a period of 10 hours and 2.5 at 20°C over a period of 50 hours. The elements exhibit creep at a level of 0.04% over 1000 hours at 180°C and have a coefficient of thermal expansion of 0.6 x 10^-6/°C in the longitudinal direction at temperatures between 20° and 180°C. Details related to single elements and stranded cables are given in Table 3.5.

End anchorages are available in 5 different types classifiable into four major groups:

"Resin Filling" Type. In this type the CFCC is bonded to the terminal using a special resin with the length being approximately 13.5 times the cable diameter. The tube can be either metallic or nonmetallic. A schematic of the sleeve and lock nut is shown in Fig. 3.16 and the metallic and non-metallic versions are shown in Fig. 3.17.

Fig. 3.16. Schematic of terminal end of the "Resin Filling System."

(a)
Metallic Terminal (M-S-R)

(b)
Metallic Terminal With Multi-Fixing (M-M-R)

(c)
Non-Metallic Terminal (N-M-S)Fig. 3.17. Ends of the "Resin Filling" type.

Die-Cast Wedge System. In this type, a molten alloy is molded onto the end of the CFCC. A steel pipe is then installed onto the die-cast end with the application of pressure such that the CFCC, die-cast end, pipe and wedge are integrated. The wedges are reusable and the alloy can be melted and reused. The terminal end is shown in Fig. 3.18 and a schematic of the system is shown in Fig. 3.19.

Fig. 3.18. End for the die-cast wedge system.

Fig. 3.19. Schematic of the die-cast wedge system (M-DS-C).

Multiple Wedge System with Die-Casting. In this type, multiple cables are simultaneously cast together. The end consists of an anchor head, lock nut and wedge (Fig. 3.20), and is similar to systems used with steel cables. Figure 3.21 shows the end.

Fig. 3.20. Schematic of the multiple wedge system with die-casting. (The schematic shows an arrangement for a six piece, 1 x 7, 12.5 mm diameter cable.)

Fig. 3.21. Multiple wedge system with die-casting (M-DM-C).

Multiple Resin Filling Wedge System. This type is a variation of the first system. Three cables are brought together to form a larger cable within a resin-filled thin-walled steel pipe, which is fixed in place with a wedge as in Fig. 3.22.

Fig. 3.22. Schematic of the multiple resin filling wedge system (shown with three ends of 1 x 7 cable, each of 12.5 mm diameter).

CFCC cables have been extensively used for pre- and post-tensioned concrete structures, external post-tensioning of wooden members and as earth anchors. The cables are available in maximum lengths of 600 m coiled onto drums 1 m in diameter (Fig. 3.23). The units are also available in the form of shear reinforcement and as continuous spiral hoop reinforcement for columns.

Fig. 3.23. Spool of CFCC.

NACC Strand

NACC strand is similar to CFCC and was developed by a consortium comprising the Kajima Corporation, Nippon Steel Corporation, Nippon Steel Chemical Co., Ltd., and Suzuki Metal Industry Co., Ltd. The strands are made from carbon fiber and are available in 7, 19 and 37 wire cables, using both PAN- and pitch-based carbon fibers. The pitch based fibers are used exclusively in the high modulus version. Figure 3.24 shows the three types of NACC strand, and typical properties are listed in Table 3.6.

Fig. 3.24. Types of NACC strand elements.

The material has a relaxation coefficient of 0.5 to 1.5% at 1,000 hours. Two types of anchorages are available for this system. Both use pressure grouting. The first anchor type is for single strands. The anchoring system is nonmetallic and is fabricated using both carbon and glass fibers, as shown in Fig. 3.25. The second system uses a metallic socket for multiple strands that are wedged tight with resin within an end-tapped section with a lock nut (Fig. 3.26).

(a)
schematic of the single strand anchorage

(b)
overall view of the single strand anchorage
Fig. 3.25. Non-metallic anchor for the NACC strand.

(a)
schematic of the multi-strand system anchor socket

(b)
overall view
Fig. 3.26. Anchor socket details for a multi-strand system.

Applications

This section provides an overview and examples of the application of carbon fiber based composite cable/tendon elements. A description of the use of Leadline cables in the Tsukuba Creation Center is given in the site report in Appendix B. Lists of applications using the Leadline and CFCC systems are given in Table 3.7 and 3.8, respectively. Total length in these tables refers to the overall length of individual cable elements.

Hexagonal Marine Structure (1987 - 1988)

As part of an investigation into floating platforms, the Ministry of Transportation commissioned the construction of two large hexagonal shaped floating concrete structures. The geometry and details are shown in Fig. 3.27. Leadline tendons of 8 mm diameter in sets of 8 were used for the tensioning of the diagonals. An overall view of the structure showing the tendons in the central hollow is seen in Fig. 3.28, and the structure is seen floating in Fig. 3.29. The first structure was constructed in Kanagawa Prefecture. The second was constructed Fukushima Prefecture a year later. In both case, tendons were initially tensioned to 80% of their ultimate strength, but under design loads the prestress was lowered to 60% of their ultimate strength.

Fig. 3.27. Geometry and detail of the hexagonal marine structure.

Fig. 3.28. Overall view of hexagonal structure showing tensioning diagonals.

Fig. 3.29. Completed structure floating on water.

Hisho Bridge - Tsukude Country Club (1993)

The 111 m long bridge with a clear span of 75 m and a width of 3.6 m (Fig. 3.30) was built in June 1993 by the Taisei Corp. It was the first example of a new combination technology using both inner and outer cables with the "free cantilever erection with cable (FCC)" method of construction (Fig. 3.31).

Fig. 3.30. Overall views of Hisho Bridge.

Fig. 3.31. Free cantilever construction procedure.

As shown in Fig. 3.32, the external cables were placed inside the hollow of the box girder, towards the top. The internal cables, each consisting of 7 strands of 12.5 mm diameter CFCC elements, were post-tensioned using a multi-type die-cast anchorage as shown in Fig. 3.33. In this system, a steel pipe was placed over the end of each strand and molten zinc alloy was injected into it to form a casing. Wedges were then applied to grip this assembly. Cast nylon buffer material was placed at the narrow end of the trumpet sheath to avoid fretting and fatigue damage. The total length of CFCC elements used in the internal cable system was 17,177 m. The external post-tensioning was done using the same grouping of CFCC elements, 7 strands of 12.5 mm diameter each, using a multi-fixing resin fill anchorage (Figs. 3.34 and 3.35). CFCC elements 2,033 m long were used in the external post-tensioning system. There was a consistent 5% difference in stressing levels as a percentage of ultimate strength between the internal and external cables. The levels at jacking were 65% and 60%, at a transfer of 60% and 55%, and under design load of 55% and 50% for the internal and external cables, respectively.

Fig. 3.32. Details of geometry of the Hisho Bridge.

Fig. 3.33. Cut-away of anchorage used for internal post-tensioned cables.

Fig. 3.34. Cut-away of anchorage used for external post-tensioned cables.

Fig. 3.35. View of positioning of the four external post-tensioning cable systems.

Shin-Nishimoniya Yacht Harbor Jetty (1995)

NACC cables were used in the repair of the concrete jetty at the Shin-Nishimoniya Yacht Harbor in Hyogo Prefecture. A single cable consisting of 7 strands of 12.5 mm diameter each was used to anchor the edge beams as shown in Fig. 3.36. Special composite anchor sleeves filled with resin were used in this application.

Fig. 3.36. Geometrical details of jetty and position of NACC cables.

Birdie Bridge (1990)

The Birdie Bridge at the Southern Yard Country Club in Ibaraki Prefecture is a stress ribbon bridge built by the Kajima Corp. in 1990 using a combination of CFCC, Arapree and Leadline elements. CFCC elements were used as reinforcement in the lower part of the deck slab, which also served as the formwork for the rest of the construction. In addition to the 5 mm diameter, 7 strand CFCC cables, short viynalon fibers were also used in the concrete. The formwork can be seen in Fig. 3.37. A view of the completed bridge is shown in Fig. 3.38; overall details are given in Fig. 3.39. Sixteen sets of Arapree bands, each consisting of eight individual elements, were laid up as pretensioning tendons on top of the assembled "formwork," as the main stressing elements for the bridge (Fig. 3.40). The CFCC and Arapree elements constituted total lengths of 2100 m and 7150 m, respectively. Leadline rods of 8 mm diameter in groups of eight were used as post-tensioning ground anchors. Figure 3.41 shows the placement of one of these ground anchors into the abutment. All composite reinforcement was stressed to levels of 80%, 70% and 60% of ultimate, at jacking, transfer and under design load, respectively.

Fig. 3.37. CFCC-reinforced permanent formwork.

Fig. 3.38. Overall view of the completed Birdie Bridge.

Fig. 3.39. Details and geometry of the stress ribbon bridge.

Fig. 3.40. Placement of Arapree tendons.

Fig. 3.41. Placement of Leadline ground anchors.