ACTIVITIES OF INTEREST

The aramid fiber-reinforced polymer (AFRP) rod reinforced demonstration bridges are two parallel bridge structures, each having a lane width of 4 m, and designed for 20-ton truck loads. The shorter span consists of 3 pretensioned box girders (each of 12.5 m length) whereas the longer span is made up of a single-cell post-tensioned box girder of 25 m length (Fig. B.68).

Fig. B.68. Plan and cross-section of the AFRP reinforced bridges.

Fig. B.69. View of the AFRP bridge.

Pretensioned Girders

The pretensioned girders were constructed using procedures similar to those used in conventional precast construction, with tensioning carried out in one operation through a movable tensioning plate. Each girder was reinforced in two layers with 12 aramid fiber reinforced tendons at the bottom and 4 similar tendons at the top that were pretensioned. Each of these tendons consisted of 3 strands of 6 mm diameter each. Eight millimeter aramid fiber reinforced bars were also employed for stirrups and reinforcement on the top (2 bars, one at each corner) and at the sides (3 bars per side). The initial stressing force for the tendons was 0.75 fpu assuming a 7% relaxation/prestress loss. Figure B.70 shows positioning of the prestressing tendons and Figure B.71 depicts the overall reinforcement arrangement (including stirrups). The overall fabrication diagram is shown in Figure B.72.

Fig. B.70. Prestressed configuration with tendons installed and pre-tensioned.

Fig. B.71. Overall reinforcement configuration.

Fig. B.72. Diagram of prestressing configuration.

Secondary reinforcement and stirrups were post-tensioned. The entire girders were reinforced with Technora tendons, with appropriate surface deformations. Total length of 6 mm diameter tendons was 2.82 km and that of the 8 mm diameter tendons was 0.6 km. The bridge was completed in July of 1990.

Post-Tensioned Girder

The single-cell cast-in-place box girder of 1.9 m depth is post-tensioned with both external and internal tendons. Internal post-tensioning is through ten 19-strand AFRP tendons of 6 mm diameter each with locations as shown in Figure B.73.

Fig. B.73. Location of internal tendons.

Tendons are anchored using a special bonding anchorage as shown in Figure B.74. The anchorage is designed for a tensile strength of 80 tons, has a cross-sectional area of 5.372 cm2, and is made of stainless steel. Anchorages are mortar filled. Six additional external tendons were also used, each consisting of seven 6 mm diameter AFRP rods in a plastic sleeve (Fig. B.74) and using a nonmetallic anchorage (Fig. B.75). The anchorages were made of short fiber reinforced composite with square threads machined into them. A close-up of a 12 strand anchorage is shown in Figures B.76 and B.77.

Fig. B.74. Sheathed external tendons.

Fig. B.75. Nonmetallic anchorage for post-tensioning.

Fig. B.76. Close-up of 12 strand nonmetallic anchorage (side-view).

Fig. B.77. Close-up of 12 strand nonmetallic anchorage (end-view).

Fig. B.78. View of external post-tensioned tendons at girder end.

Fig. B.79. View of external post-tensioned tendons at girder mid-span.

An automatic tensioning jack was used for post-tensioning, bearing in mind that aramid tendons show significant elongation (0.5 m in a 25 m length), necessitating frequent resetting of the tensioning jack. Views of the tendons at the ends and at midspan are shown in Figures B.78-B.79. The overall length of the deformed Technora rods in this girder was 5.9 km (4.8 km in the internal tendons) and 1.1 km in the external tendons. The AFRP bridge was completed in February 1991.

Tendon Characteristics

Technora rods are made by Teijin, Ltd. using aramid fiber and vinylester resin. The aramid fibers are made from PPODTA (co-poly-paraphenylen/3,4-oxidiphenylene telephthal amide). Basic properties of the fiber and resin are listed in Table B.7.

The rods are fabricated using the pultrusion process with 65% Vf and deformations where applicable are created using a secondary spiral winding process. The general appearance of the rods is shown in Figure B.80 and the geometry of deformations is depicted in Figure B.81.

Fig. B.80. AFRP rods.

Fig. B.81. Deformed bar geometry.

A comparison of rod characteristics is given in Table B.8.

ADDITIONAL POINTS OF INTEREST

• The development of the Technora system was initiated in 1984 by Teijin. In 1987 a joint research project was undertaken between Teijin, Ltd., Sumitomo Construction Co., Ltd. and the Institute of Industrial Science at the University of Tokyo (Kobayashi Laboratory). Materials characterization and structural testing for specifications were conducted in 1989 through a project funded by the Ministry of Construction.
• The use of AFRP rods rather than steel bars in concrete results in a change from tensile to compressive failure of concrete with an accompanying loss in toughness. In order to increase flexibility prior to failure and to limit failure to the tensile regime, limit values have to be established for the ratio of displacement at ultimate limit state for bending to the displacement when flexural cracking occurs. Factors of safety are also set at higher levels.
• Tests were conduced to assess degradation of AFRP bars due to their proximity to on-site welding; no detrimental effect was seen.
• Tests were conducted to assess accidental damage to the pretensioned rods by falling weights. It was ascertained that no detrimental effect was seen with loads up to 5 kgf weight dropped from a height of 25 m.
• The use of AFRP bars as stirrups results in a loss in strength at the corners where the bars are bent by 90ș. It was determined that the strength loss is about 60% in the corner region and special design guidelines for construction were prepared.
• Variation of tensile loads in tendons during prestressing was not greater than ±1.5%.