Proceedings of the International Symposium on Bond Behaviour of FRP in Structures (BBFS 2005) Chen and Teng (eds)

© 2005 International Institute for FRP in Construction

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EXPERIMENTAL RESEARCH ON BOND BEHAVIOR OF CFRP TO STEEL

Yang Yong-xin, Yue Qing-rui, and Peng Fu-ming Central Research Institute of Building and Construction, MCC Group,China

ABSTRACT There are several advantages of bonding carbon fibre reinforced polymer (CFRP) laminates to the deteriorated steel members over the traditional retrofit methods. The bond behaviors of CFRP to steel affect the load transfer and the effectiveness of strengthening. Some adhesives suitable for strengthening steel structures with CFRP were chosen out of many adhesives in this paper. Appropriate specimens were employed to test the bond strength and bond durability between CFRP and steel. Test results show that the three adhesives chosen have excellent properties and are suitable for steel structures strengthened with CFRP. KEYWORDS adhesive; bond strength; bond durability; seawater; hot/wet cycle INTRODUCTION Lots of steel structures have been constructed in the civil engineering structures. Because of corrosion or fatigue caused degradations, many steel structures used in practice should be strengthened or repaired. A traditional strengthening method is to connect steel plate or profiled to the damaged position by welding or bolting. The practical experience had indicated that this traditional strengthening method is effective but has some shortages, while the strengthening method by bonding FRP would avoid these problems. There is massive research and widespread applications at home and abroad on FRP strengthening concrete structures, but those on FRP strengthening steel structures are less. Three FRP materials CFRP, GFRP and AFRP are commonly used in strengthening concrete structures. Because of the high strength and rigidity of steel, CFRP may be more effective in strengthening steel structures compared with the others due to its higher strength and rigidity. For utilizing the capacity of CFRP adequately, a good bond performance both on the strength and durability is therefore important for CFRP bonded steel structures. Although the bond behavior of CFRP to steel will affect the load transfer and the effectiveness of strengthening significantly, there is lack of research on this field in China mainland. In this paper, experimental studies are first carried out to choose adhesives suitable for steel structures strengthened with CFRP. Appropriate specimens are next used to investigate the bond strength and durability between CFRP and steel. EXPERIMENTS ON SELECTING BONDING MATERIALS There are many resins for strengthening concrete structures with FRP at present in the world, and the key mechanical properties of adhesives for strengthening structures are the strength, elastic modulus, elongation percentage. For the significant differences existing between steel and concrete in many aspects such as mechanical property, physical property and surface state, some special requirements for the adhesives should be highlighted in strength, elastic modulus and elongation and the indices to reflect the bond behavior between CFRP and steel such as bond strength, deformation capacity and bond durability should also be proposed, which is very important for strengthening steel structures with CFRP. The bond capability of adhesives and steel are the resultant of the attraction between the adhesive molecules and steel molecules including the physical, chemical and mechanical effects between adhesives and steels. The specimens for the properties of adhesives in bonding CFRP to steel are fabricated according to the Chinese national standards GB/T 6329-1996 and GB7124-86. To minimize the eccentricity in the steel-steel single-lap specimens induced by the clamps at ends, circular holes were drilled at both ends of each specimen and the loads were applied by pins through holes at both ends (Figure 1). An extensometer with a gauge length of 50mm was used to measure the axial extension of adhesives on the lapped length in testing. The loading rate is 5mm/min. The photos of the experiments are shown in Figure 1.

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In this series of experiments, the properties of seven kinds of adhesives (numbered A to G) were tested. The bond stress and ultimate stress of each adhesive are shown in Figure 2.

As Figure 2 shows, the bond capabilities of adhesive A, B and C in the steel-steel joints are better than that of the other adhesives. The bond strength between adhesive A and C with the steel is higher than the bond stress of themselves. The failure modes of those specimens are found to be the cohesive failure in adhesive. The shearing stress-axial strain curves of the three adhesives are shown in Figure 3. Figure 3 shows that the shear strength and shear deformation capability of specimens with adhesive A is similar with those with adhesive B, while the specimens with Adhesive C have better shear capacity. According to this part of experiments, Adhesives A, B and C have suitable properties as the bonding material to reinforce the steel structure. TEST FOR BOND STRENGTH OF STEEL-CFRP JOINTS As there is no standard available in China for determining the bond strength of CFRP-steel joints, the double overlap specimen (Figure 4) in this paper is chosen to illustrate the bond strength and deformation ability of the CFRP-steel joints. As shown in Figure 4, the specimen consists of two pieces of stainless steel plates connected with CFRP on both sides. The bond lengths of both CFRP at the two steel plates were different to ensure that failure always occurs at the side with a shorter bond length. The larger one is 50mm while the shorter one is 25mm which is the actual working bond length. The gap between two steel plates is 2mm.

Fig.1 Bond Test of Adhesive and Steel

(a) Tensile Test (b) Shear Test

Specimen

Specimen

极限纵向应变

20.0

2.5

拉伸剪切强度(MPa)

0.0

G

纵向应变( )1.51.00.5

剪切强度

2.010 3

C

E

F

D

A

B

0.0 12.08.04.0 16.0

3.0

24.0

Fig. 2 Shear Strength and axial Srtain

Axial Strain (× 10－3) □Shear Strength ■ Ultimate axial Strain

Shear Strength (MPa)

纵向应变( )2.4

粘结剂A

剪应力

(

0

3.0

0

9.0

6.0

12.0

1.20.80.4 2.01.610 3

24.0

)

MPa 15.0

18.0

21.0

3.22.8

粘结剂B

粘结剂C

Fig. 3 Shear Stress vs Axial Strain

Axial Strain (× 10－3)

Adhesive B

Adhesive C

Adhesive A

Shea

r stre

ss (M

Pa)

Fig4 Specimen with double overlap in tensile shear

Steel plate

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The test results of double overlap CFRP-steel joints are shown in Table 1.

Table 1 The test results of the bond strength of CFRP-steel joints Bond strength(MPa)

Adhesive FRP Average value

Coefficient of variation

Failure description

B CFRP sheet I 10.93 0.171 Adhesive-steel interfacial debonding (Figure 5a),

survival adhesives existing on the steel surface.

A CFRP plate II 15.31 0.051 Delamination of CFRP plate (Figure 5b), good bond

performance at the steel-adhesive interface.

C CFRP plate III 9.31 0.091 Adhesive-CFRP plate interfacial debonding (Figure

5c), surface of adhesives is smooth. From Table 1 and Figure 5, it can be seen that the coefficients of variation are larger for the cases of failure occurring in the interfaces, including both the adhesive-steel interface and the adhesive-CFRP interface. This is because that the initial defects, such as inadequate adhesives and existence of air voids, are more likely to occur at the interfaces. As a result, for qualified CFRP plates, the specimens failed by the delamination of CFRP plate have more stable bond strengths than those failed by interfacial failures.

Fig.5 Photos of failure modes TEST ON DURABILITY OF STEEL-CFRP For a composite structure composed of steel structures, bonded CFRP and adhesive, the adhesive layer and the interfaces between adhesive and adherend may be the most weakly part which controls the strength of the whole structure. The durability of this complex system is not only subjected to the durability properties of the steel structures, adhesive and CFRP, but is more closely related to the exterior environment which may corrode the interfaces seriously. It is therefore important to test the durability of the interfaces in CFRP bonded steel structures to study the long time capacities of this type of composite structures. A single-lap specimen, shown in Figure 5, is adopted for the durability test. As summarized in Table 2, the Adhesives B, C and A were also chosen to bond CFRP sheet I, CFRP plate II and CFRP plate III to steel plates, respectively. Two environments were employed in this series of tests: 1) man-made seawater and 2) hot/wet cycle. The man-made seawater was made according to GB/T 15748-1995 and hot/wet cycle environment was simulated according to GB2574-89.

Table 2 Specimens for durability test

FRP Adhesive Test environment Quantity

CFRP sheet I Adhesive B Man-made seawater 35

CFRP plate III Adhesive C 35 CFRP plate II Adhesive A Hot/wet cycle 35

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After cured at the room environment for 7 days, two series of specimens were tested in the man-made seawater and hot/wet cycle environments respectively. After the lasting times of 500h, 1000h, 1500h, 2000h, 2500h and 3000h, five specimens of each series were tested their bond strengths respectively. CFRP sheet-steel joints in seawater The curve of the shear strength versus the lasting time in man-made seawater of CFRP sheet-steel joints were shown in Figure 6. It can be seen that the shear stresses of the joints decrease significantly due to the lasting time in the man-made seawater at first 1000h, while after 1000h, the shear stresses decrease slowly with the increase of the lasting time. The water molecules can penetrate easily to the adhesive and the interfaces after the specimens were put into the man-made seawater. The penetration though the interfaces is even more easily compared to the adhesive layer. Since the adsorption ability of the metal surface to the polar hydro-molecule is stronger than that of the adhesive surface to the polar hydro-molecule, the adhesion force between the adhesive and metal surface is weakened. Therefore, the bond strength decreases and the failure occurred in the interface of the CFRP sheet and metal. At the same time, the penetration of water to the adhesive destroys the cohesive force (hydrogen bond or other valence bond) of the adhesive molecules, which may make the polymer inflating. After the water penetrates from interface edge to the interior to a certain degree, a new balance system is then formed. When the external condition is invariable, the movement of the water molecules will reduce obviously or basically stop. Since the destruction to the interface of the CFRP and the metal was not active any more, the decrease rate of the bond strength slows down. A bilinear model is proposed to predict the shear stress of the CFRP sheet-steel joint lasting time in this man-made seawater.

⎩⎨⎧

>+−=≤+−=

)1000(728.110003.0)1000(235.15004.0

2

1

ttSttS

(1)

where 1S 、 2S is the shear strength; t is the lasting time.

CFRP plate-steel joints under hot/wet cycle environment Curves of shear strength versus lasting time of single-lap CFRP plate-steel joints in hot/wet cycle environment are shown in Figure 7 while the failure modes are shown in Figure 8. Adhesive A and C were used in these specimens. It can be seen from Figure 7 and 8 that the shear strengths of both series of specimens with Adhesive A and Adhesive C increase after being aged at the hot/wet cycle environment. And the improvement of the shear strength is more notable for the specimens with Adheisve C than those with Adhesive A. This is because that the failure modes for all specimens with Adhesive C are the interfacial failure at CFRP plate-adhesive interface while for those with Adhesive A the delamination of CFRP plate is the key failure mode. The effects of the hot/wet cycle environment on the overlapped CFRP plate-steel joints exist in the following three aspects: Firstly, temperature increment is helpful for the further diffusing, filtering and entwisting of the adhesive molecules, and will cause the chemical reaction of adhesive more completely, which enhances the degree of resin cure and crosslinking and improves the cohesive strength and the bond strength between

0

4

8

12

16

0 500 1000 1500 2000 2500 3000老化时间 /h

拉剪强度

/MPa

Fig.6 Shear strength vs lasting time

Lasting time/h

Shea

r 0

4

8

12

16

0 500 1000 1500 2000 2500 3000老化时间 /h

拉剪强度

/MPa

粘结剂A

粘结剂C

Lasting time/h

Fig.7 Shear strength vs lasting time

Adhesive A Adhesive C

Shea

r Stre

ngth

(MPa

)

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adhesive and steel. Secondly, molecules of water filter into the bond interface along the hydrophilic steel surface and replace the physical adsorption of steel surface to adhesive molecules, which results in the decline of the bond strength. Finally, for the obvious differences of the thermal expansion coefficients between steel, adhesives and CFRP, the cyclic thermal stress will be produced on the CFRP plate-steel interface under the hot/wet cycle, which is adverse to the bond strength between CFRP and steel. The change of bond strength was caused by above three factors. Among them, the fist factor is helpful to bond strength while the other two factors have negative effects. If the effect of fist factor is greater than the other two, the bond strength will increase, otherwise it will decrease. As Adhesive A and Adhesive C perform very well in bonding CFRP to steel, the effect of the fist factor was dominant for the joints, consequently, the bond strength after hot/wet cycle increased. CONCLUSIONS According to experimental studies in this paper, the following conclusions can be drawn: Firstly, after comparing several kinds of adhesives, the Adhesive A, B and C show better performances in bonding CFRP to steel. Secondly, the chosen adhesives for CFRP-steel in this study have excellent durability, and there is no failure occurred in interface of CFRP and steel for all specimens. Finally, an elementary aging model of the bond behavior of CFRP-steel in man-made seawater environment has been proposed. ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the National High Technology Research and Development Program of China (863 Program) and the Special Foundation for Institute of Science and Technology of China in 2003. . REFERENCES Peng Fu-ming, Yue qing-rui etc.( 2003). “CFRP for Strengthening and Repairing of Damaged Steel Structures”, Industrial Construction, , 33(9): 7-10. Yue Qing-rui etc. (2004). “Experimental Research on the Effective Bond length of CFRP Sheets Repairing Steel Structure”. Industrial Construction., Vol.34(z1), 321-324. Chinese National Test Standard. GB/T 15748-1995. Chinese National Test Standard. GB 2574-89.

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