Fourth International Conference on FRP Composites in Civil Engineering (CICE2008) 22-24July 2008, Zurich, Switzerland

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1 INTRODUCTION

In the last several decades, it has been widely recognized that bonding CFRP strip on concrete structures is one of the effective ways for strengthening or rehabilitation because of its high modulus and high strength compared to concrete (Chajes et al 1995; Finch et al. 1994). For re-habilitation of steel structures, several research works have indicated significant improvements of the fatigue life of the steel plates rehabilitated by bonded CFRP strips (e.g., Okura et al. 2000; Okura et al. 2001). In addition, because CFRP strips are easy to handle and apply, a total cost for rehabilitation work is expected to be decreased compared with that for other methods, such as bolting plates.

However, there are only few cases in which the rehabilitation method by bonding CFRP strips has been applied to existing steel structures due to relatively small number of research works and insufficient understanding on the strength of adhesives. In addition, the fact that Young’s modulus of CFRP strips is close to or less than that for steel seems to affect the appli-cation of this strengthening method.

To overcome the problem in Young’s modulus, pre-tensioning CFRP strips and releasing pre-tension after bonding is one possible solution, because resultant compression stress in a steel plate is able to compensate the tension stress induced by live loads. By considering the relation-ship between stress amplitude (Δσ) and fatigue life (N) for steel, C=N･Δσ3 (C=constant for a certain detail), reduction of stress amplitude due to live loads would result in significant im-provement in the fatigue life.

In this study, a potable pre-tensioning device for CFRP strips is developed and the reduction of bond strength due to pre-tensioning are experimentally investigated. In Section 2, details of pre-tensioning device are illustrated including mechanism of introducing the pre-tension in CFRP strips. Then, in Section 3, tensile tests were conducted on specimens fabricated by bond-

Effect of pre-tensioning on peeling failure of bonded CFRP strips

K. Nozaka & K. Ueda Ritsumeikan University, Shiga, Japan

ABSTRACT: In the last several decades, it has been widely recognized that bonding Carbon Fi-ber Reinforce Polymer (CFRP) strips on structures is one of the effective ways for strengthening or rehabilitation. Bonding CFRP strips has been mainly applied to concrete structures because of its high modulus and high strength compared to concrete. However, it has been hardly ap-plied to steel structures due to fewer studies on the strength of adhesives in the application for steel bridges. In addition, the Young’s modulus of CFRP strips is close to or less than that for steel. In order to overcome the disadvantage in Young’s modulus, pre-tensioning CFRP strips and releasing pre-tension after bonding is one possible solution, because resultant compression stress in the steel plate compensate the tension stress induced by live loads after strengthening. In this study, a potable pre-tensioning device of CFRP strips was developed and the reduction of bond strength due to pre-tensioning was experimentally investigated. The results indicated that the decrease in the peeling stress due to pre-tensioning was approximately 46% of that for spe-cimens without pre-tension.

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ing CFRP strips with and without pre-tension to compare the peeling stress. Lastly, conclusions are listed in Section 4.

2 PRE-TENSIONING DEVICE

Fig. 1 shows a sketch of the pre-tensioning device, and Fig. 2 shows an enlarged view of a part of the device (indicated by a red oval in Fig. 1). The pre-tensioning device is consisted of a box tube, two threaded rods, four angle members (two type ① and two type ②), two reaction plates, and two tensile blocks.

Procedures for introducing pre-tension into CFRP strips are as follows. First, two type ①angle members were bonded on edges of a CFRP strip (600 mm long, 50 mm width and 2 mm thick). Thickness of the adhesive layer was maintained by sprinkling several fishing lines (di-ameter = 0.5 mm) over the adhesive layer. Two type ② angle members were also bonded on edges of a CFRP strips.

After adhesive was hardened, these CFRP strips with angle members were placed on upper and lower sides on the box tube as shown in the side view in Fig. 1. Because there are holes on the box tubes, the angle members were able to be placed inside of the box tube. The angle members (type ① and ②) on CFRP strips have holes on their flanges which were not boned to the CFRP strips; and these angle members were connected to the tensile blocks placed be-tween the angle members by the threaded rods as shown in Fig.2. Ten, the threaded rods were connected to the box tube using reaction plates and nuts placed on both sides of the box tube as shown in Fig. 2.

By tightening the nuts at both ends (blue arrow in Fig. 2), two tensile blocks were moved to-ward the opposite direction each other (red arrow in Fig. 2). Then, the tensile strain was intro-duced into the CFRP strips.

Using this device, two CFRP strips were pre-tensioned at the same time; however, only one of them was bonded on steel plates as described in the next section. Using this device, pre-tension can be easily introduced into the CFRP strips without any special mechanical device.

washer

adhesive

angle member①

CFRP

tensile block

angle member②

CFRP

box tubenutreaction plate

threaded rod

nut

Figure 1. Sketch of pre-tensioning device.

Figure 2. Magnified view of pre-tensioning device.

600106

100103

106

690

nut

side view

washer

CFRP

threaded rod

tensile block

box tube

plane view

CFRP angle member②CL

7550

reaction plate

45

abgle member②

abgle member①

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3 EXPERIMENTAL TESTS ON PEELING FAILURE OF CFRP PLATES BONDED ON STEEL PLATES

In this section, tensile tests on the steel plates with bonded CFRP strips were discussed to verify the difference in the peeling failure due to compressive stress introduced in the steel plate.

3.1 Material Properties Materials properties for this research are summarized in Table 1, where σy=yield strength, σu=tensile strength and E=Young’s modulus. These values were measured by tensile tests con-ducted by authors.

Table 1. Material properties.

Material σy

(N/mm2)

σu

(N/mm2)

E

(N/mm2)

CFRP ML520 - 2,140 290,000 Adhesive DP460 29 35 2,250

Steel SM570 495 644 206,000

3.2 Fabrication of Specimens The fabrication procedures of specimens were as follows.

In order to introduce compression stress in a steel plate (50 mm width, 19 mm thick and 1150 mm long), two sets of pre-tensioning device were fabricated and CFRP strips were pre-tensioned. The tensile stain introduced into the CFRP strips were approximately 637 (or 425) microstrain which was monitored by the strain gauge during pre-tensioning procedures. The strain value of 637 (425) microstrain was determined so that compression stress of 30 (20) N/mm2, which was corresponding to 146 (97) microstrain, was able to be introduced into steel plates. After pre-tensioning CFRP strips, strain data were recorded for 30 minutes without any disturbance in order to check the relaxation of pre-stress in the CFRP strips.

Pre-tensioned CFRP strips were bonded on two sides of the steel plates as shown in Fig. 3. The target thickness of adhesive layer was 0.5 mm and fishing lines were also used at this time. After adhesive was hardened, nuts at ends of the devices were un-tightened, and the devices were removed while strain data in CFRP strips and the steel plate were recorded to check the in-troduction of compression stress in the steel plate. At this time, two CFRP strips with angle members were left on the steel plate. Finally, the CFRP strips were cut into 300 mm in length as shown in Fig. 4.

In Fig. 3 and 4, location of strain gauges (①～⑦) was also indicated; gauge No.①～⑥ were bonded on the CFRP strips and gauge No.⑦ were bonded on the side surface of the steel plate. Gauges on CFRP strips were located on opposite surfaces of that on which the steel plate was bonded. A picture during fabrication of a specimen is shown in Fig. 5.

①　　　②③④⑤

⑥

steel plate

adhesive strain gage

⑦

Figure 3. Fabrication of specimens.

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3.3 Specimen Matrix and Experimental Procedures A total of 12 specimens was fabricated and tested in tension. Six of those were fabricated with pre-tensioned CFRP strips, while other six were fabricated without pre-tension. For those with pre-tensioned CFRP strips, three specimens were fabricated in order to introduce 30 N/mm2 of compression stress in steel plate; and the other three were fabricated in order to introduce 20 N/mm2 compression stress. The specimen matrix is summarized in Table 2.

Specimens were tested in tension by gripping both ends of the specimens at the speed of 1.0 mm per minute with a universal testing machine. Each test was continued until peeling failures of CFRP strips occurred, and strain and applied load were measured at every 0.1 seconds.

3.4 Results and Discussion Fig. 6 and 7 shows stress-strain curves for specimens P0-3 and P30-3. The vertical axis is the nominal stress (N/mm2), the load divided by the cross-sectional area of the steel plate, and the horizontal axis is strain (×10-6) of the CFRP strips. Numbers in circle correspond to the gauge number in Figs. 3 and 4.

Table 2. Specimen matrix.

Specimen Strain for Pre-tension

(x10-6)

Target Compression stress in Steel Plates

(N/mm2) n

P20-n 637 20 1～3

P30-n 425 30 1～3

P0-n 0 (no pre-tension) 0 1～6

20

③

CFRP⑥

①

80

②

20

4251150300

150

steeladhesive

20

④

10

⑤

425

Figure 4. Typical specimen with gauges.

Figure 5. Fabrication of a specimen with proposed pre-stressing device.

⑦

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As can be seen in the figures, strains increased linearly with different slope for different loca-tion. The gauges near the edge of the CFRP strips (e.g. ⑤) have a steeper slope due to shear lag. As nominal stress increased, gauge ⑤ shows non-linear behavior as can be seen in Fig.6; this behavior can be explained by the yielding of a part of the adhesive layer (Nozaka et al. 2005).

For all specimens, strains in CFRP suddenly decreased at certain nominal stress as can be seen in figures; this indicates that the peeling failure of the CFRP strips. The nominal stress at which peeling failure occurred is called “peeling stress (σp)” in this paper.

The peeling stresses for all specimens are summarized in Table 3. For specimens with pre-tensioned CFRP strips (P20-n), the peeling stresses were reasonably constant, resulting in the average of 223 N/mm2. However, for specimens with pre-tensioned CFRP strips (P30-n) and specimens without pre-tension (P0-n), there are large scatter in the peeling stresses. Based on observation during fabrication and testing procedure, this was mainly due to the unexpected er-ror in fabrication, such as uneven wetting on steel surfaces by the adhesive or excessive adhe-sive around steel plates. For specimens with pre-tensioned CFRP strips (P30-n), the average

Figure 7. Stress-strain curves for CFRP strips (P30-3).

Figure 6. Stress-strain curves for CFRP strips (P0-3).

0

100

200

300

400

500

600

0 500 1000 1500 2000 2500 3000

stre

ss σ

(N/m

m2 )

s train ε (x10-6)

①

②

③

④

⑤

⑥

0

100

200

300

400

500

600

0 500 1000 1500 2000 2500 3000

stre

ss σ

(N/m

m2 )

stra in ε (x10-6)

①

②

③

④

⑤

⑥

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peeling stress for three specimens is 231 N/mm2, which is close to that for P20-n specimens. The average peeling stress for six specimens without pretension is 427 N/mm2.

By comparing the average peeling stresses, the peeling stress was decreased by approximate-ly 46% due to pre-tension in CFRP strips. Due to the scatter in the results for specimens with pre-tensioned CFRP strips (P30-n), there was no significant difference between reductions in the peeling stress for P20-n specimens and P30-n specimens.

4 CONCLUSIONS

In this study, a potable pre-tensioning device for CFRP strips was developed and the reduction of bond strength due to pre-tensioning was experimentally investigated. Major findings in this study are as follows,

1) The device to introduce tensile stress into CFRP strips was developed. 2) It was confirmed that the compression stress was able to be introduced in a steel plate. 3) Due to pre-tension in CFRP strips, the peeling stress was reduced by approximately

46%. 4) A scatter in the experimental results was observed, and the peeling stress was not signif-

icantly affected by the difference in the amount of pre-tension based on this study.

Further experimental studies are required to confirm the reduction of the peeling stress due to pre-tension considering a scatter in the experimental results.

5 REFERENCES

Chajes, M. J., Januszka, T. F., Mertz, D. R., Thomson, T. A., Jr. and Finch, W. W., Jr. 1995. Shear Strengthening of Reinforced Concrete Beams Using Externally Applied Composite Fabrics. Ameri-can Concrete Institute Structural Journal, 92(3), 295-303.

Finch, W. W., Jr., Chajes, M. J., Mertz, D. R., Kaliakin, V. N. and Faqiri, A. 1994. Bridge Rehabilitation using Composite Materials. New Materials and Methods of Repair, Proceedings of the Materials Engineering Conference-ASCE. 804, 1140-1147.

Nozaka, K., Shield, C. K. and Hajjar, J. F. 2005. Effective Bond Length of Carbon Fiber Reinforced Po-lymer Strips Bonded to Fatigued Steel Bridge I-Girders. Journal of Bridge Engineering-ASCE，10( 2), 195～205.

Okura, I., Fukui, T., Nakamura, K., Matsugami, T. and Iwai, Y. 2000. Application of CFRP Strips to Re-pair of Fatigue Cracks in Steel Plates. Journal of Constructional Steel, 8, 689-696.

Okura, I., Fukui, T., Nakamura, K., and Matsugami, T. 2001. Decrease in Stress in Steel Plates by Carbon Fiber Sheets and Debonding Shearing Stress. Journal of Structural Mechanics and Earth Quake En-gineering, 689, I-57, 239-249.

Table 3. Summary of peeling stress.

P20 P30 P0

n 1 2 3 1 2 3 1 2 3 4 5 6 σp

(N/mm2) 220 234 244 357 150 186 516 540 520 267 325 392

Average 233 231 427