ARAMID FIBER BASED LINEAR REINFORCING ELEMENTS

Although a majority of projects discussed in this chapter involve the use of carbon fiber reinforced tendons and/or cables, considerable work has also been conducted with aramid fiber reinforced tendons, bars and cables. Aramid fiber reinforced composites are increasingly popular in Japan because of their high toughness/tenacity, high elongation at break (significantly exceeding that of carbon fiber based elements) and nonmagnetic/nonconductive characteristics. Three main classes of aramid fiber based reinforcing elements are currently available:

• Arapree, which is developed and marketed by Kajima Corp. and Nippon Aramid Co.
• FiBRA, which is developed and marketed by Shinko Wire Co., Ltd.
• Technora, which is developed and marketed by Sumitomo Construction Co., Ltd. and Teijin, Ltd.

The first was not included in the draft document on "Design and Construction Guidelines for Prestressed Concrete Highway Bridges Using FRP Tendons" issued by PWRI in March, 1994. In this section, brief descriptions of each of these types and applications using these elements are discussed. Additionally, descriptions of the use of these elements can be found in the Akashi Kaikyo Bridge, the Kamiooka Station Parking Structure, the Rainbow Bridge, the Sone Viaduct and the Sumitomo Bridges site reports in Appendix B of this report.

Arapree Elements

Arapree, or Aramid Prestressing Elements, are fabricated from high modulus Twaron fibers (a product of Aramide Maatschappij vof, a joint venture of Akzo and N.V. Noordelikje Ontwikkelingsmaatschappij), which are a form of aramid developed through the combination of p-phenylenediamine and terephthaloydichloride (Fig. 3.42).

Fig. 3.42. Chemical structure of aramid.

The prestressing elements, which are available in the form of rectangular bars, or circular rods (Table 3.9), were initially developed through a collaboration between HBG-Hollandsche Beton Groep and Akzo Fibers. The elements are formed by passing non-twisted Twaron fibers through eyelets and combs. This enables the simultaneous impregnation of the bundles and the structure such that 35-45% by volume consists of the almost 100% paracrystalline, highly oriented, unidirectional rigid chain type molecule based fibers, and 65-55% consists of a chemically resistant epoxy resin. The surface of the element has a knobbed topography, which facilitates good mechanical interlock for stress transfer. Except for the two smallest sizes, all tendons are formed in multiples of 100,000 individual Twaron high modulus filaments. Overall properties for individual circular tendons are given in Table 3.10.

These elements have a creep coefficient of 0.2 after 10⁶ hours and exhibit a 15% drop in prestress level due to relaxation after 10⁶ hours in air, as well as a 20% drop in level after exposure to alkaline solution. Service temperature above 100°C is not recommended due to the low melting point of Twaron fibers. At 60% of ultimate strength (short term) the elements have a predicted life of 10⁵ hours (11.41 years). Twaron manufacturers recommend that the elements be stressed to a level no greater than 50% of their ultimate short-term strength levels.

Wedge type anchorages are employed for short-term use (Fig. 3.43). For long-term use, anchorages in which steel tubes are filled with cement mortar are used (Fig. 3.44).

Fig. 3.43. Short-term tendon anchorage assembly.

Fig. 3.44. Anchorages for long-term use.

A listing of sample application projects is given in Table 3.11. Except for the last case, Arapree was used in conjunction with another reinforcing element.

FiBRA Elements

FiBRA (meaning Fiber in Portuguese) elements are available using glass, carbon or aramid fibers in a braided form, although in most cases aramid fibers are used as the reinforcement. Aramid-based FiBRA is available in two varieties based on the type of resin systems used - flexible/elastomeric and rigid. In both cases, ultimate elongation is restricted by the strain capacity of the fiber, which gives a composite elongation level of 2%. The tensile modulus of FiBRA elements is 68.6 kN/mm² and the linear coefficient of thermal expansion is -5.2 x 10^-6/°C. Specifications of standard FiBRA elements are given in Table 3.12.

These elements have a relaxation ratio of 25% and retain 95% of their tensile strength at 220°C. At 280°C, despite the braid, strength retention is only 55%.

In general, two types of anchorages exist for FiBRA elements, the wedge type and the resin-cone type. The first type is more popular and consists of a three- or four-section inner conical wedge that is forced into a sleeve with the tension rod in the center. Figure 3.45 gives schematics of some of these anchorages depicting their use for multi-rod configurations.

Fig. 3.45. Schematics of wedge type anchorages.

In the bonded resin type anchorage, the tension rod is placed at the center of a conical hollow in a stainless-steel sleeve and resin is poured around it (Fig. 3.46).

Fig. 3.46. Bonded anchorage.

Limited use is made of non-metallic sleeves and anchor wedges. A listing of sample application projects is given in Table 3.13.

Technora Elements

Technora elements are made from PPODTA (poly-para-phenylene-3,4-oxidiphenylene terephthalamide) and vinylester resin. The PPODTA fiber is a type of polyaramid with slightly lower density and higher performance than conventional polyaramid fibers. They are available in the form of round rods, deformed rods, twisted strands (Fig. 3.47), and occasionally as flat bars. The fibers are produced by Teijin, Ltd. and the Technora elements are developed by Teijin, Ltd. and Sumitomo Construction Co., Ltd. Generic materials characteristics are given in Table 3.14.

Fig. 3.47. Types of Technora elements.

Specifications for the most commonly used varieties of the round rod and spiral wound (deformed) bar/rod elements are given in Table 3.15.

Technora rods are often used in multiples, with groups of rods loosely held together but anchored within the same anchor sleeve. Characteristics of these groups are given in Table 3.16.

Anchoring of Technora elements is enabled through the use of either wedge type (Fig. 3.48) or bonding type (Fig. 3.49) anchorages. Although non-metallic/composite anchorages are still not widely used there is considerable interest in this development, with composite bonding type anchorages being used on some demonstration sites. A schematic cut-away of a bonding type anchorage is shown in Fig. 3.50.

(a) single wedge type anchorage

(b) multi-wedge type anchorage
Fig. 3.48. Wedge type anchorages.

(a) metallic bonding anchorage (19 - 6? rods)

(b) non-metallic/composite bonding anchorage (7 - 6? rods)
Fig. 3.49. Bonding type anchorages.

Fig. 3.50. Schematic cut-away of a bonding type anchorage.

A listing of sample projects is given in Table 3.17.

Examples of Applications

As described earlier in this section, aramid fiber based linear reinforcing elements have been used in a large variety of applications in structures ranging from bridge decks and girders to precast irrigation channel sections. Descriptions of their use in the Kamiooka Station Parking structure entrance slab, the Rainbow Bridge, the Sone Viaduct, and the Sumitomo Bridges at the Oyama Works are in the site reports (Appendix B). Two additional examples are discussed below.

Use in a Prestressed Concrete Berth

Due to their non-corrosive nature, aramid fiber reinforced Arapree and Technora rods were used to reinforce the prestressed girders making up the concrete berth at the Ehime Works of the Sumitomo Chemical Corp. Girder geometry and placement of the prestressing tendons are shown in Fig. 3.52. The berth consists of 17 hollow girders, each of which is 8.72 m in length. Ten used Technora elements and 7 used Arapree bars (Table 3.18). The girders were designed as simply supported beams with a TL-20 live load rating.

Fig. 3.51. Overall view of the prestressed concrete berth.

Fig. 3.52. Details of girder geometry.

Use in Irrigation Channels

Irrigation channels consisting of precast concrete sections are commonly used in Japan. The channels are often built on soft ground, which is likely to settle unevenly and cause cracks, especially at joints. Sections are precast using aramid tendons, both as reinforcement and for connecting adjacent sections, avoiding the need to undertake costly ground improvements or support the channels using piles.

The use of these aramid based elements enables the sections to undergo some relative movement at joints without significant distress to the concrete. (Aramid tendons have about 25% of the stiffness of steel cables.) In the Nakagoh drainage works, precast culverts in the form of hollow boxes 25 m long were used. Both steel cables (32 mm diameter at corners) and groups of four 6 mm diameter Technora rods were used, as shown in Fig. 3.53. The Technora rods were laid continuously with prestress levels of 80%, 70% and 60% of ultimate strength, at jacking, transfer and under design load, respectively. L shaped precast segments (Fig. 3.54) were used in conjunction with a cast-in-place concrete bed to form an irrigation channel in the central Saga district on soft ground. The use of aramid Technora tendons on a 30 m test section was undertaken to demonstrate their use in connecting the precast L wall sections.

Fig. 3.53. Schematic of the culvert at the Nakagoh Drainage Works.

(a) cross-section

(b) view of the L sections
Fig. 3.54. Use of Technora for connection of precast linings.

The Technora elements were used in groups of three 6 mm diameter rods, two per L section, with stainless steel anchorages. The cable assemblies were designed to withstand differential forces due to the movement of the adjacent sections. The Kubota trunk line project used U-shaped precast units of 1.5 m in height by 1.8 m in width by 2.0 m in length. Units connected through the use of 6 mm diameter Technora rods. A schematic of the section is shown in Fig. 3.55a, and an individual unit is shown being placed in Fig. 3.55b.

(a) schematic of precast units

(b) placement of a precast unit
Fig. 3.55. Precast units on the Kubota trunk line for irrigation.