CHIMNEYS AND SMOKESTACKS

The use of carbon fiber for the seismic retrofit of smokestacks was pioneered by Mitsubishi Kasei and the Obayashi Corp. in 1987. The use of wet winding drew considerable attention to the potential use of composites for the retrofit of civil infrastructure. This application entailed the application of carbon fiber tape/prepreg to the structure in the longitudinal direction, followed by the automated wet-winding of carbon tow in the hoop direction. Once the procedure was completed, the composite was allowed to cure and was then coated with a fire-resistant coating. The steps in the application process are shown in Figure 4.45. Details of the machine and procedure are shown in Figure 4.46. A list of applications using this process is given in Table 4.10. In many cases, wet-winding and the use of the automated winder, which required setting up the ring and creels, was not considered efficient. Lately, the method has been almost completely replaced by the use of carbon fiber sheet material (FORCA tow sheet from Tonen, and Replark from Mitsubishi Kasei), which is applied to the concrete substrate using a wet-layup process similar to that used in the retrofit of columns and decks. Figure 4.47 shows the application of Replark on a rectangular chimney. Figure 4.48 shows the application of tow sheet onto the Wakayama Refinery chimney.

Fig. 4.45. General execution procedure.

Fig. 4.46. Details of the winding machine and procedure.

(a)

(b
Fig. 4.47. Application of Replark onto a rectangular chimney.

Fig. 4.48. Overall view of retrofit of the chimney at Tonen's Wakayama Refinery.

Completed in April 1991, the Wakayama project used two plies of tow sheet. One was applied in the longitudinal direction and the second in the circumferential, or hoop, direction (Fig. 4.49). The retrofit was necessitated by the exfoliation and severe cracking of the concrete as well as corrosion of the reinforcing steel. Prior to the use of composites, the stack had been reinforced by steel bands installed in the longitudinal and lateral directions to close cracks of 0.3 mm to 1 cm width and to prevent the large-scale break away of concrete. The steel bands, however, were not able to prevent further degradation of concrete and corrosion of steel, resulting in a severely weakened structure. The stack had a total height of 45.5 m with outside diameters of 3.63 m and 2.12 m at the bottom and top respectively. The dimensions are given in Figure 4.50 and general material characteristics are given in Table 4.11.

Fig. 4.49. Detail showing placement of the tow sheet in the circumferential direction.

After the removal of the steel bands and filling of cracks and crevices, two layers of tow sheet were applied to the concrete surface. The design was based on the assumption of a 20% loss in steel reinforcement cross-section. Comparisons of the original strength (as designed), strength before composite retrofit (assuming 20% loss of steel section), and strength after composite strengthening are given in Table 4.12.

Fig. 4.50. Cross-sectional details.

The strengthening from the use of tow sheet can be seen by comparing the three cases and in comparison with levels in Table 4.11. The stress levels in the tow sheet are extremely low. The retrofit construction was undertaken by the Taisei Construction Co. and the Hanshin Construction Co. Table 4.13 lists examples of chimney applications using tow sheet.

Between 1987 and 1994, a total of 28 sites were reported by Mitsubishi Kasei where retrofit had been conducted using either the tow winding or Replark application processes. The retrofit of chimneys and smokestacks accounted for about 2% of the use of Tonen's tow sheet in 1996. In the period 1987 to 1995, 18.3% of Mitsubishi's volume in retrofit was in this area. Besides the use of carbon fiber forms, aramid fiber tapes have also been used for the retrofit of chimneys. The FITS system consisting of FiBRA elements also has been used successfully, albeit in smaller quantities. This system shows flexibility through the use of aramid tapes of different widths, as shown in Table 4.14.