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.

Table 4.10
Examples of Seismic Retrofitting of Chimneys Using the Winding Process

Location

Date

Height of Chimney (m above ground lvl)

Outer Dia., d and Top Wall Thickness, t (mm)

Outer Dia., d and Base Wall Thickness, t (mm)

Retrofit Details

Area (m2)

Details of Retrofit

Height (m)

Length (m)

Tokyo

Dec.

1987

15

d = 930

t = 180

d = 1385

t = 205

9 - 13.5

4.5

14.8

•UD Tape 1-2 ply

*Strand: 5 mm pitch

Aichi

April 1988

33.4

d = 1780

t = 120

d = 3020

t = 230

16.6 - 31.6

15

111

•UD Tape 1-2 ply

*Strand: 5 mm pitch

Chiba

Dec.

1988

45

d = 1190

t = 140

d = 2394

t = 200

5 - 43.2

38.2

226

•UD Tape 2-3 ply

*Strand: 2.5 mm pitch

Chiba

March 1989

35

3040 x 3040

t = 150

3040 x 3040

t = 300

24 - 35

11

135

•NA

*UD Tape 1 ply

Osaka

Sept. 1989

37

d = 2597

t = 166

d = 3657

t = 280

8 - 35.7

27.7

270

•UD Tape 2-4 ply

*UD Tape 1 ply

Shizuoka

Nov. 1989

90

d = 2560

t = 160

d = 6164

t = 496

66.5 - 86.5

20

220

•UD Tape 4 ply

*Strand: 5 mm pitch

Mie

July 1990

55

d = 2160

t = 130

d = 3880

t = 320

23.5 - 53.5

30

252

•UD Tape 3 ply

*Strand: 2.5 mm pitch

Okinawa

March 1991

40

d = 2900

t = 150

d = 4180

t = 310

20 - 40

20

230

•UD Tape 1 ply

*UD Tape 1 ply

Shizuoka

April 1991

60

d = 2260

t = 130

d = 3940

t = 310

23 - 58

35

309

•UD Tape 1-3 ply

*Strand: 2.5 mm pitch

Fukuoka

July 1991

70

d = 4230

t = 150

d = 7380

t = 420

20 - 68

48

814

•NA

*UD Tape 1-2 ply

Kanagawa

Dec. 1991

50

d = 2090

t = 130

d = 3780

t = 290

32.5 - 47.5

15

114

•UD Tape 1-2 ply

*Strand: 3.5 mm pitch

Yamaguchi

Dec.

1991

50

d = 2320

t = 150

d = 4178

t = 355

27.5 - 48.5

21

190

•UD Tape 1 ply

*Strand: 5 mm pitch

•Vertical; *Horizontal Pitch


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.

Table 4.11
Characteristics of Materials Used

Material

Performance Metric

Specified Level of Strength (kg/cm2)

Short-Term Allowable Stress Level (kg/cm2)

Elastic Modulus(kg/cm2)

Concrete

Compression

150

100

2 x 105

Steel

Tension & Compression

2400 (yield)

2400

2.1 x 106

Carbon Fiber Tow Sheet (t = 0.2 mm)

Tension

14,000

9350

1.4 x 106

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.

Table 4.12
Comparison of Stress Levels
   

Cross-Section

(see Fig. 4.50)

  
 

C

D

F

G

I

I'

Height above ground (m):

35.3

30.5

20.5

15.5

5.5

0.5

Dimensions:

Outside radius (cm)

Inner radius (cm)

Wall Thickness (cm)

 

245.0

209.0

18.0

 

262.0

220.0

21.0

 

296.0

242.0

27.0

 

312.0

252.0

30.0

 

346.5

274.5

36.0

 

363.0

305.0

39.0

Load:

Bending moment (ton•m)

Axial Force (ton)

 

50.98

24.30

 

122.15

41.52

 

388.67

87.66

 

605.06

117.30

 

1256.00

191.16

 

1707.95

236.04

As-built design:

Steel cross-sectional area (cm2)

Stress in steel (kg/cm2)

Stress in concrete (kg/cm2)

 

42.46

819.73

17.27

 

64.35

1493.98

30.50

 

136.08

1973.87

47.17

 

273.67

1905.8

56.05

 

471.28

2226.77

72.69

 

657.81

2131.49

76.38

Levels after 20% reduction in cross-sectional area:

Steel cross-sectional area (cm2)

Stress in steel (kg/cm2)

Stress in concrete (kg/cm2)

 

 

33.97

1000.3

18.74

 

 

51.48

1826.63

33.25

 

 

108.86

2407.34

51.44

 

 

218.94

2311.8

61.04

 

 

377.02

2717.53

79.83

 

 

526.25

2623.25

84.71

Levels after retrofit with carbon fiber tow sheet:

Number of layers

Steel cross-sectional area (cm2)

Stress in steel (kg/cm2)

Stress in concrete (kg/cm2)

Stress in tow sheet (kg/cm2)

 

 

1

33.97

764.32

16.66

535.33

 

 

1

51.48

1477.80

30.04

1038.88

 

 

1

108.86

2116.38

48.13

1501.34

 

 

1

218.94

2157.7

58.83

1540.8

 

 

3

377.02

2439.04

74.80

1755.73

 

 

3

526.25

2427.84

80.86

1756.35

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.

Table 4.13
Example Chimney Sites Using Tow Sheet

Date

Structure

Reason for Retrofit

April 1991

Chimney of Tonen's Wakayama Refinery

Cracking and severe structural degradation

November1991

Chimney of M Paper Co.

Repair of the upper section with 3 layers of tow sheet

July 1992

Chimney of S Electric Co.

Secondary reinforcement

September1992

Chimney of D Pharmaceutical Co.

Severe exfoliation of walls

September1992

Chimney of T Glass Co.

Severe exfoliation and degradation of light weight concrete

March 1993

60 m High Chimney of the Waste Facility in Fukui Prefecture

Repair of cracks in the upper sections

September1993

Chimney of a Public Bathhouse

Exfoliation of concrete

January 1994

Chimney of K Drink Co.

Severe corrosion of steel reinforcement

March 1994

Chimney of K Drink Co.

Repair of cracks and exfoliated sections

March 1994

Chimney of T Chemical Co.

Repair of exfoliated sections

April 1994

Chimney of N Electric Co.

Repair of cracks and strengthening of sections with loss in steel cross-section

May 1994

Chimney of D Co., Osaka

Repair of cracks and strengthening of sections with loss in steel cross-section

July 1994

Chimney in Tonen's Wakayama Refinery

Repair of cracks and removal of corroded steel circumferential bands

April 1995

Chimney in Matsuzaka City

Repair of severely exfoliated sections

Table 4.14
Examples of the Use of the FITS System in Chimneys and Smokestacks

Location

Period

Sizes of Tape

 
  

Width (mm)

Length (m)

Yamaguchi

February 1993

75

200

800

2500

Chiba

October 1995

200

300

200

525


Published: November 1998; WTEC Hyper-Librarian