Static and fatigue behaviour of the bond interface between concrete and externally bonded CFRP in single shear

Engineering Structures 97 (2015) 54–67

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Static and fatigue behaviour of the bond interface between concrete and externally bonded CFRP in single shear Raid A. Daud ⇑, Lee S. Cunningham, Yong C. Wang School of Mechanical, Aerospace and Civil Engineering, The University of Manchester, UK

a r t i c l e

i n f o

Article history: Received 25 November 2014 Revised 2 March 2015 Accepted 30 March 2015

Keywords: CFRP Interface Pull-out tests Debonding strain Fatigue

a b s t r a c t This paper reports the results of an experimental investigation into the static and fatigue behaviour of the interfacial bond between carbon fibre reinforced polymer (CFRP) composite and the concrete substrate. Twenty-four single-shear pull-out tests with two different types of CFRP and different composite plate thicknesses have been carried out. Four additional specimens were tested using different concrete compressive strengths for the sake of comparison. Modes of failure, load–slip relationships, strain profiles of CFRP, interfacial shear stress distributions and interfacial bond stress–slip for monotonic, fatigue and post-fatigue loading have been obtained. The experimental results were used to examine the CFRP stiffness–fatigue life relationship, and CFRP stiffness level–debonding strain relationship. The results indicate that when considering post-fatigue loading regimes, the strain required to cause debonding of the CFRP and the ultimate load capacity of the strengthening system is reduced by the previous cyclic loading. Also, both the ultimate bond strength reduction and fracture energy reduction due to cyclic loading are dependent on the stiffness of the CFRP plate in the bond system for the range of the concrete strengths tested. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction At a practical level fatigue is a phenomenon which causes weakening of a material due to repeatedly applied loads. More fundamentally, fatigue may be considered as the propagation of damage. In the case of concrete externally strengthened with polymer composites, this is usually associated with degradation in cohesive behaviour of the adhesive and interfacial behaviour of the adhesive–concrete interface. The main risk associated with fatigue is that repetitive loading may lead to catastrophic failure at a much smaller load than that adopted for static design. In order to prolong service life of existing structures and to accommodate potential increases in cyclic loads various strengthening methods are adopted in practice. In particular, there has been a growing interest over the past three decades in strengthening concrete structures with externally bonded fibre reinforced polymer (FRP). Bond behaviour is considered the most critical issue in strengthening because the bond at the interface between concrete and FRP is relatively weak compared with the constituent materials in the strengthening system as a whole. An extensive amount of studies have been performed to experimentally investigate the behaviour and failure modes of the bond ⇑ Corresponding author. E-mail address: [email protected] (R.A. Daud). http://dx.doi.org/10.1016/j.engstruct.2015.03.068 0141-0296/Ó 2015 Elsevier Ltd. All rights reserved.

at the FRP composite–concrete interface under shear, in particular, bond behaviour and load transfer between FRP composite plate and concrete under monotonic loading [1–11]. This is understandable as the stress transfer mechanism is mainly reliant on the bond quality. In the above mentioned tests, the dominant failure mode observed was debonding within a few millimetres of the concrete layer beneath the adhesive, the two other failure modes were rupture of FRP and debonding at the adhesive–concrete interface. A significant amount of analytical research has also been undertaken to develop a bond slip model that describes the interfacial bond behaviour up to debonding [2,12–15,6,16,17]. However, as in the experimental investigations, these analytical studies have all focused on the monotonic load condition. Few of the previous research studies have investigated the bond behaviour under cyclic loading until debonding failure occurs. Amongst these studies, Bizindavyi et al. [18] investigated experimentally the influence of bond length, bond width, and cyclic bond stress level on bond characteristics between fibre reinforced polymer laminates and concrete under cyclic loading. Direct shear tests were performed by applying a force at the free end of the FRP and the concrete substrate was fixed in the direction of the force to prevent movement. They then proposed a stress level–fatigue life relationship. Yun et al. [19] observed bond behaviour by comparing five different bonding systems and they came to the conclusion that the fatigue endurance of a hybrid-bonded FRP (HB-FRP)

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R.A. Daud et al. / Engineering Structures 97 (2015) 54–67 Table 1 Experimental programme. Test type Loading rate (mm/s)

a

Static (monotonic) 0.0005

Fatigue (70–15%) Pult.a 1 Hz

Fatigue (80–15%) Pult. 1 Hz

(Post-fatigue test) [(70–15%) Pult.a + monotonic] 1 Hz + 0.0005

CFRP type

CFRP thickness (mm)

Concrete compressive strength (MPa)

Specimen ID

Specimen ID

Specimen ID

Specimen ID

T700

1 0.3 0.2 0.3

52.8 52.8 52.8 22.6

M2 M4 M5 M7

F2 F4 F5 F7

F9 F11 F12 F14

P-F2 P-F4 P-F5 P-F7

M46J

1 0.4 0.15

52.8 52.8 52.8

M1 M3 M6

F1 F3 F6

F8 F10 F13

P-F1 P-F3 P-F6

Pult.: ultimate load capacity in monotonic loading.

(b) Side view

(a) Front view

Bonded length = 300 mm 50 mm

70 mm

70 mm

70 mm

CFRP plate length = 500 mm

(c) Strain gauges detail Fig. 1. Test arrangement.

system with tight mechanical fastening gave the highest fatigue endurance among all of the tested systems. Nigro et al. [20] conducted an experimental study to investigate the effect of three different cyclic load paths in addition to a monotonic load path, on prismatic concrete specimens reinforced with carbon fibre reinforcing polymer (CFRP) sheets and plates. It was found that the design equations provided by the 3 standard design codes (ACI 2008; CNR 2004; fib 2001) gave good estimates of the effective bond lengths for sheets but more were conservative for plates.

Table 2 Concrete mix proportions (for 1 m3 concrete). Target 28 day compressive strength (MPa)

Compressive strength at time of test (MPa)

Cement (kg)

Water (kg or l)

Fine aggregate (kg)

Coarse aggregate (kg)

35 24

52.8 22.6

450 389

250 230

580 651

1078 1110

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Carloni et al. [21] performed seven direct shear tests of FRP composite–concrete interface under fatigue and monotonic quasi-static loading regimes. They observed that the monotonic quasistatic tests showed a different debonding mechanism from the post-fatigue test which was associated with a smaller stress transfer zone. The cyclic approach was also adopted in the development of the analytical cyclic hysteresis model of the bond stress–slip relationship, Ko and Sato [22]. To date, few studies have investigated the fatigue behaviour of adhesive joints. Among the relevant studies, Ferreira et al. [23] performed an experimental investigation for evaluating the effect of layer orientation, lap joint length and water immersion on adhesive lap joints produced from bi-directional woven E-glass fibres and polypropylene. The adhesive used was Bostik 7452-Super Glue 4, Rubber & Plastics Grade ethyl cyanoacrylate type, while the primer was Bostik 7480-Super Glue 4 based on n-heptane. The tests results suggest that the fatigue behaviour is not significantly influenced by the adhesive thickness. The fatigue strength was also shown to improve when using stiffer laminate adherent. Azari [24] also studied the fatigue performance of adhesive joints. The main parameters examined in this fatigue experiment were surface treatment, surface roughness and adhesive layer thickness. The results of this study indicate that surface treatment can change the failure mode. However, the surface roughness had no effect on the fatigue life threshold. Also it was found that, a thinner adhesive layer would lead to a shorter fatigue life. Furthermore the adhesive thickness had more pronounced influence on the crack growth rate than the fatigue life threshold. It is clear from the aforementioned discussions that although some work has been undertaken to investigate cyclic behaviour under shear, the fatigue behaviour of a FRP strengthened concrete structure is complex and affected by many parameters. The aim of this study is to obtain basic properties of one of the important structural components: the interfacial bond under shear. More research is required to better understand the debonding mechanism and to develop detailed design approaches for strengthened structural members subjected to cyclic loads. Specifically, none of the aforementioned research has considered the effect of CFRP type or CFRP thickness on fatigue and post-fatigue behaviour. In addition, there is still no accurate method to estimate bond failure due to fatigue. For example, one of the most widely used design guides for FRP strengthening of concrete, ACI 440.2R-02 [25], takes account of fatigue on the FRP composite behaviour by imposing stress limits of 0.2, 0.3, and 0.55 times the ultimate strength for Glass, Aramid, and Carbon FRP, respectively. It also suggests an FRP strain limit to prevent debonding. These crude limitations do not address degradation of the bond which may be induced by cyclic loading. The Italian design code, CNR-DT202 [26] recognise that ‘‘no accurate and reliable model (such as the S–N curve) is currently available for the evaluation of the fatigue resistance of the adhesive joint’’. The Japanese Society of Civil Engineers’ design guide, JSCE [27] proposes a constant reduction factor on the interfacial fracture energy to mitigate debonding fatigue failure. However, it states an accurate method has yet to be established. Guidance for addressing fatigue debonding failure of externally bonded FRP composites applied to the tension side of concrete has been evaluated by Harries and Aidoo [28]. However, this

Table 3 Physical properties of the bonding adhesive (Weber Building Solution, UK [31]). Colour Density Application viscosity Shear strengtha a

BS EN 1504-4:2004 [32].

White, transparent 1.3 kg/l 650 mPa s P12 N/mm2

evaluation is based on data obtained from a small number of bridge girders retrofitted with CFRP. A further limitation of the previous study is that, the common failure mode in flexural FRPstrengthened RC members is intermediate crack debonding (IC debonding) which in turn gives different failure mechanisms from that observed in a pull-out test. As is well known, the IC debonding mechanism has a highly brittle descending branch of the bond–slip response as described by Lu et al. [29], which causes a high level of dispersion in predicting strain at debonding. Yao et al. [5] proved a single shear test to be more reliable and robust in determining the debonding strain for strengthened RC members that fail by loss of composite action. This paper presents the results of physical testing of FRP composite to concrete bond under monotonic and fatigue loading, in single shear. The tests were performed to investigate the influence of both the stress range and CFRP stiffness on fatigue life of the bond. 2. Experimental programme Twenty-four single shear pull-out tests were conducted by changing the following three experimental variables: (1) two types of CFRP composite (T700, M46J); (2) CFRP plate thickness which ranged between 1 mm and 0.15 mm; and (3) loading hysteresis (static (monotonic), fatigue, and fatigue following static). The experimental programme consisted of four groups: the first group tested six specimens subjected to monotonic loading with a loading rate of 0.0005 mm/s; the second and third groups tested six specimens each subjected to fatigue loading at a frequency of 1 Hz with loading ranges of (70–15%) and (80–15%) of the ultimate load obtained from monotonic loading respectively; the fourth group tested six specimens subjected to fatigue loading with a loading range of 70–15% at a frequency of 1 Hz until a slip of 0.4 mm was reached followed by monotonic loading at rate of 0.0005 mm/s until failure. Also, four additional specimens were tested using different concrete compressive strengths for the sake of comparison. Table 1 summarises the experimental programme. 2.1. Details of test specimens Fig. 1 shows the single shear test arrangement. In all tests, the CFRP plate was 500 mm in length and 50 mm in width. The bonded length was 300 mm. The plain concrete substrate had dimensions of 150  200  500 mm. A notch of 40 mm was introduced at the interface between the CFRP plate and the concrete by leaving an un-bonded area of the CFRP plate near to the top edge of the concrete substrate (Fig. 1(a)) to facilitate crack growth and to avoid undesirable concrete failure. Two types of carbon fibre reinforced polymer were used (T700 and M46J). The CFRP plates were provided by Reverie Ltd. [30]. For the CFRP T700 composite plate, the mean modulus of elasticity was 127.2 GPa with a standard deviation of 11.7 GPa and the mean strength was 2160.4 MPa with a standard deviation of 166 MPa. The CFRP M46J composite plate had a mean Young’s modulus of 229.6 GPa with a standard deviation of 31.6 GPa, and a mean strength in the longitudinal direction of 1639.2 MPa with a standard deviation of 108.3 MPa. The ultimate strains for the different thicknesses of CFRP plates were obtained through uniaxial tensile tests and are given in Table 4. For the twenty-four concrete substrates, the average measured cube compressive strength and the mean tensile strength from the standard spilt tensile test on the day of testing were 52.8 N/ mm2 and 4.5 N/mm2 with a standard deviation of 2.4 N/mm2 and 0.43 N/mm2, respectively. For the four additional lower strength concrete substrates, the average measured cube compressive strength was 22.6 N/mm2 with a standard deviation of 0.55 N/

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Upper crosshead Testing specimen

Lower crosshead

Load cell

Hydraulic testing machine Fig. 2. Test setup.

(a)

(c)

(b)

Fig. 3. Failure modes: (a) Bond failure in the interfaces between concrete and adhesive layer, (b) CFRP composite plate rupture, and (c) concrete shearing beneath adhesive layer.

mm2 and the mean tensile strength from the standard spilt tensile test was 3.02 N/mm2 with a standard deviation of 0.37 N/mm2. Table 2 presents the mix proportions to produce 1 m3 of the concrete, the target 28 day cube strengths and the measured cube strength on the actual day of testing which varied from 28 to 90 days.

2.2. Surface preparation and bonding process Surface preparation has an effect on bond behaviour. A mechanically sound and clean surface is required prior to adhering the CFRP composite plate in order to achieve full bond with the plain concrete substrate. To achieve this, the concrete surface was first ground with a surface grinder and then cleaned to remove

dust and loose particles by vacuum cleaner. During preparation of the specimens, the adhesive layer was placed to both the concrete and CFRP surfaces with uniform thickness of 1–1.5 mm. This adhesive layer was made of approximately 2/3 Epoxy resin and 1/3 Epoxy hardener which was provided by Weber Building Solution, UK [31]. The physical properties of the bonding adhesive are listed in Table 3. The CFRP plate was then applied to the glued concrete surface and kept in place with weights to ensure there were no air voids and to ensure uniform coverage.

2.3. Test set-up Fig. 2 shows the test set-up. All specimens were tested by applying a tensile force to the loaded end of the CFRP composite

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Table 4 Monotonic and post-fatigue test failure loads and modes.

a

ID

Test type

CFRP elastic modulus (GPa)

CFRP thickness (mm)

CFRP stiffness (kN/mm)

CFRP ultimate strain (microstrain)

Test failure mode

Number of cycles, N

N/Nf

Debonding strain (microstrain)

Ultimate load (kN)

M1 M2 M3 M4 M5 M6 M7a P-F1 P-F2 P-F3 P-F4 P-F5 P-F6 P-F7a

Monotonic Monotonic Monotonic Monotonic Monotonic Monotonic Monotonic Post-fatigue Post-fatigue Post-fatigue Post-fatigue Post-fatigue Post-fatigue Post-fatigue

203.59 114.90 264.86 128.46 138.35 220.60 128.46 203.59 114.90 264.86 128.46 138.35 220.60 128.46

1 1 0.4 0.3 0.2 0.15 0.3 1 1 0.4 0.3 0.2 0.15 0.3

203.59 114.90 105.94 38.53 27.67 33 38.53 203.59 114.93 105.94 38.53 27.67 33 38.53

7810 20,130 6414 18,168 17,660 7980 18,168 7810 20,130 6414 18,168 17,660 7980 18,168

C-S-I C-S-I C-S C-S C-S C-S&P-R C-S C-S-I C-S C-S C-S C-S C-S C-S

– – – – – – – 1485 3750 4150 5600 9900 7800 3900

– – – – – – – 0.6 0.344 0.275 0.268 0.319 0.398 0.253

3926.47 4154.21 5395.91 8064.63 9213.24 7988.69 6489.64 2662 3224.82 4134.91 6729 8361 7303 5795.32

35.32 25.52 22.01 14.33 12 13.14 11.71 25.62 20.16 16.41 11.73 10.44 10.92 9.97

Concrete compressive strength of the concrete substrate was (22.6) MPa.

plate. The concrete substrate faces were restricted in the same direction of loading to prevent them from moving so that a direct shear force was applied at the CFRP-to-concrete interface. The specimen was inserted into a conventional steel loading rig for single-shear pull-out test. This rig consisted of three plates (bottom plate, top plate, and spacer plate). The bottom plate was securely mounted to the bottom crosshead of a 100 kN capacity hydraulic testing machine with 12 mm thick steel plates and four 12.5 mm diameter threaded steel at the corners of the bottom and top steel plates. As can be seen from Fig. 1, an additional plate, the spacer plate, was used to separate the concrete substrate from the rig top plate, so as to allow shearing fracture (i.e. when shear stress in the concrete exceeds its shear strength) through the concrete. The loaded end of the CFRP composite plate was clamped in a grip at the top of the crosshead of the hydraulic testing machine. 2.4. Instrumentation and testing procedure External instrumentations were placed on all test specimens. For the monotonic tests, five strain gauges were mounted along the bonded length of the CFRP plate Fig. 1(c) to measure the strain; and linear potentiometers (pots) were used to measure slip. For the post-fatigue tests, the same instrumentation was used during the monotonic loading phase. In addition, linear variable differential transducers (LVDTs) were used to measure slip in the fatigue loading phase. Only LVDTs were used in the fatigue tests. The strain gauges were installed in the fibre direction. The data was recorded every 5 s. by the data acquisition system. The fatigue loading was applied under load control at a frequency of 1 Hz. The cyclic reading data was saved at every 50 cycles.

40

M1

M2

M3

M4

M5

Load (KN)

20

10

0

0.5

1

Slip (mm) Fig. 4. Monotonic load–slip curves.

1.5

3.1. Failure modes Fig. 3 shows the three failure modes observed during the monotonic and post-fatigue tests, (a) bond failure in the interface between the concrete and the adhesive layer where there was little concrete attached to the FRP strip after failure (denoted as C-S-I), (b) CFRP composite plate rupture (denoted as P-R) and (c) concrete shearing beneath the adhesive layer in which a thin layer of concrete was attached to the FRP strip after separation (denoted as C-S). All the specimens that failed by CFRP rupture had comparatively thin CFRP plates. This failure mode started with crack initiation between the CFRP and concrete surface followed by crack propagation in the CFRP until rupture. The third failure mode was more dominant than the other two failure modes. This type of failure commenced with visible cracks in the concrete at the loaded end of the concrete substrate, and then the crack propagated towards the far end of the CFRP composite plate. Three specimens experienced adhesive failure (first failure mode). This may have been the result of inadequate surface preparation at the beginning of the test series, a problem duly rectified in the subsequent samples. The fatigue tests always experienced concrete shearing beneath the adhesive layer; this is due to the relatively high resistance of the CFRP plate to fatigue failure, ACI 440.2R-02 [25]. Table 4 lists the failure modes for all the tested specimens. 3.2. Load–slip behaviour The behaviour of bond between the CFRP plate and the concrete substrate can be described by the load–slip relationship. The following describes the experimental load–slip relationship for each of the loading regimes adopted.

M6

30

0

3. Discussion of experimental results

2

3.2.1. Monotonic tests Fig. 4 presents the recorded load-relative slip relationships for the monotonic single shear pull out tests. The relative slip refers to the relative movement of point (A) on the concrete substrate and (B) on the CFRP plate as shown in Fig. 1(a). It can be noted that all the specimens have similar load–slip curves: with an initial linear relationship, followed by a nonlinear portion before maintaining the maximum load with increasing slip. This load–slip relationship is typical of monotonic tests. Furthermore, specimens with lower CFRP stiffness also have lower ultimate load, but higher slip after reaching the maximum strength. This is a result of the shorter active bond zone required to transfer tension from the

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12 [1]

[2800]

[5600]

[11200]

[8400]

[14000]

[16000]

[19600] [20839]

10

Load (KN)

8

6

4

2

0

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Slip (mm)

(a) F4 12

[1]

[1600]

[3200] [4800]

[6400] [8000] [9600][11200][12600]

10

Load (KN)

8

6

4

2

0

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Slip (mm)

(b) F11 Fig. 5. Fatigue load–slip responses (the number above each curve indicates cycle number).

Visible crack

Fig. 6. Crack propagation.

CFRP plate to the concrete surface at lower CFRP plate stiffness based on the analytical model of Chen and Teng [12]. A shorter active bond zone also leads to a longer slip process, hence the increased slip after reaching the maximum strength. Higher ductility is obviously desirable from a design point of view. 3.2.2. Fatigue tests Fig. 5(a) and (b) presents the experimental load–slip relationships for two specimens with the same CFRP plate thickness of 0.3 mm, but different applied fatigue load ranges of

0.7Pult.–0.15Pult. (Specimen F4) and 0.8Pult.–0.15Pult. (Specimen F11), respectively, at 1 Hz frequency, where Pult. is the ultimate load from the corresponding monotonic test. In general, the ultimate slip of the specimen during the fatigue test was lower than from the monotonic test. As the number of cycles increased, the separation between the CFRP plate and the concrete substrate became visible to the eye along the side edges of the CFRP composite plates (Fig. 6). For high stiffness CFRP, failure occurred at or just after crack initiation, while for low stiffness CFRP failure occurred after significant crack propagation. The reloading path did not coincide with the unloading path due to fracture energy release during the load cycle. The amount of energy released during one cycle did not change significantly, indicating steady fracture energy release rate. It is also noticeable that cyclic loading caused minor, but a steady reduction in the bond secant stiffness (i.e. the upper load versus slip in a specific cycle). As a result, cumulative steady fracture energy release led to reductions in both the ultimate load and the debonding strain of the specimen. The reduction in the secant bond stiffness was more modest in specimens with higher CFRP plate thicknesses (see Fig. 7). The results in Table 5 show that using CFRP plate with lower stiffness gave a higher fatigue life under the same load amplitude range. Furthermore, the load amplitude range had a significant effect on the fatigue life. Fig. 8 plots the CFRP plate stiffness versus

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R.A. Daud et al. / Engineering Structures 97 (2015) 54–67 [1]

[1000]

[3000]

[5000]

[7000]

[9000]

[11000]

[13000]

[15050]

(a) F3 [1]

[500]

[1000]

[1500] [2000] [2500]

[3000]

[3500] [4000] [4500] [5100]

(b) F10 Fig. 7. Fatigue load–slip responses (the number above each curve indicates cyclic number).

fatigue life Nf relationship for both loading ranges (0.7Pult.–0.15Pult. and 0.8Pult.–0.15Pult.). 3.2.3. Post-fatigue tests Fig. 9 shows the load–slip relationships from the post-fatigue tests (see Table 1 for details of the loading sequence). Comparing the results in Fig. 6 for the monotonic tests, it can be seen that after cyclic loading, the ultimate load capacity was reduced for all six specimens. The ultimate load capacity reduction ranged from 27.5% for the 1 mm M46J CFRP plate to 13.3% for the 0.2 mm T700 CFRP plate. This reflects the steady fracture energy release under cyclic loading prior to monotonic loading to failure. These results are contradictory to the conclusions of Carloni et al. [20], who reported that the ultimate load of their composite system did not change much after applying cyclic loading between 15% and 80% until a threshold global slip equal to 0.4 mm was reached. This is due to the fact they tested in the post-fatigue situation only a very limited number of specimens (precisely one specimen) for a single thickness of 0.167 mm. The effect of this small thickness on the ultimate bond strength reduction and the fracture energy degradation is insignificant. As an indication of the level of system utilisation, a comparison of the number of cycles (N) required to achieve 0.4 mm slip in the post-fatigue (P-series) tests against the number of cycles at

failure (Nf) in the corresponding fatigue (F-series) tests is given in Table 4. Across the number of specimens, the N/Nf ratio ranged from a minimum of 0.25 to a maximum 0.6.

Table 5 Fatigue test results.

a

ID

Fatigue test

CFRP thickness (mm)

CFRP elastic modulus (GPa)

CFRP stiffness (kN/mm)

Failure mode

Fatigue life (Nf)

F1 F2 F3 F4 F5 F6 F7a F8 F9 F10 F11 F12 F13 F14a

(0.7–0.15)Pult. (0.7–0.15)Pult. (0.7–0.15)Pult. (0.7–0.15)Pult. (0.7–0.15)Pult. (0.7–0.15)Pult. (0.7–0.15)Pult. (0.8–0.15)Pult. (0.8–0.15)Pult. (0.8–0.15)Pult. (0.8–0.15)Pult. (0.8–0.15)Pult. (0.8–0.15)Pult. (0.8–0.15)Pult.

1 1 0.4 0.3 0.2 0.15 0.3 1 1 0.4 0.3 0.2 0.15 0.3

203.59 114.90 264.86 128.46 138.35 220.60 128.46 203.59 114.90 264.86 128.46 138.35 220.60 128.46

203.59 114.90 105.94 38.53 27.67 33 38.53 203.59 114.90 105.94 38.53 27.67 33 38.53

C-S C-S C-S C-S C-S C-S C-S C-S C-S C-S C-S C-S C-S C-S

2475 10,900 15,050 20,839 31,000 19,550 15,400 450 2900 5100 12,600 19,200 9050 9600

Concrete compressive strength of the concrete substrate was (22.6) MPa.

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3.3. Tensile strain profiles 3.3.1. Monotonic tests Fig. 10 presents the recorded strain profile curves along the CFRP plate for the six monotonic test specimens at 5 different load levels. The profiles are nonlinear at low load levels, but attained a

linear shape for a period at higher load levels. Debonding (when the strain reached the maximum) marked the start of the linear strain shape. Fig. 10 shows that the CFRP composite stiffness had a pronounced effect on the debonding strain. The debonding strain ranged from 3926.47 micro-strain for the 1 mm M46J CFRP plate (M1) to 9213.24 micro-strain for 0.2 mm T700 CFRP plate (M5).

250

Load range: 0.7Pult.-0.15Pult.

Load range: 0.8Pult.-0.15Pult.

150 100 50

0

5000

10000

15000

20000

25000

30000

35000

fague life, Nf Fig. 8. CFRP stiffness–fatigue life relationship.

pre-fague

21

post-fague

18

Load (KN)

0

15 12 9 6 3 0 0

0.2

0.4

0.6

0.8

1

1.2

Slip (mm)

P-F1

P-F2 pre-fague

15

post-fague

12

Load (KN)

N.Ef. (KN/mm)

200

9 6 3 0

0

0.5

1

1.5

Slip (mm)

P-F3

P-F4

P-F5

P-F6 Fig. 9. Post-fatigue load–slip responses.

2

2.5

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The M46J specimen with a CFRP plate thickness of 0.15 mm shows nonlinear behaviour through all loading levels because the failure mode of this specimen was rupture in the CFRP plate at the beginning of the debonding process. 3.3.2. Post-fatigue tests Fig. 11 presents the strain profiles for the post-fatigue tests. Pre-failure cyclic loading had significant effects on the strain distributions in the CFRP composite plate during the monotonic loading phase. The reduction in the maximum strain, compared to the results in Fig. 10 for monotonic loading, was caused by fracture energy release. Application of a pre-failure cyclic load caused reduction in the debonding strain ranging from 35% for the 1 mm M46J CFRP plate (M1) to 5.6% for the 0.2 mm T700 CFRP plate (M5). Moreover, the strain profiles indicate a steady level of debonding strain in the initial 50 mm of the bonded length in the earlier stages of loading. This is due to crack initiation induced by the previous cyclic loading. 3.4. Interfacial shear stress distributions Based on the strain distribution along the bonded length of the CFRP composite plate, recorded from the tested specimens in the

monotonic as well as the post-fatigue tests, the mean experimental shear stress between the two strain gauges mounted on the centre line of the CFRP plate were calculated using the following relationship:

s¼

Ef t f  ði  i1 Þ Dx

ð1Þ

where Dx is the distance between strain gauges (i) and (i  1); i and i1 are the strains in the CFRP plate at strain gauges (i) and (i  1), and Ef and t f are the Young’s modulus and thickness of the CFRP plate respectively. It is clear from Eq. (1) that the shear stress mainly depends on the CFRP plate stiffness. However, both the adhesive and concrete stiffness also implicitly affect the estimate of shear stress given by Eq. (1). 3.4.1. Monotonic test Fig. 12 shows the evolution of the mean interfacial shear stress of specimen M4, at four different locations [(0–50) mm, (50– 120) mm, (120–190) mm, (190–260) mm, see Fig. 1(c)] the relative load level (P/Pult.) is increased. Herein, P denotes the applied load level and Pult. is the ultimate load. In the first region [(0–50) mm] of the bonded CFRP plate, a gradual increase of the shear stress was observed until reaching a value of 8.2 MPa which represents the bond strength. As the relative load level was increased further,

M1

M2

M3

M4

M5 Fig. 10. Strain distributions along CFRP plate in monotonic tests.

M6

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the shear stress decreased abruptly and eventually reached zero. Simultaneously, increases in the mean shear stress in the adjacent region start to occur. This explains the process of debonding during different stages of loading. This phenomenon was observed progressively from one region to another until total failure of the bond interface occurred.

3.4.2. Post-fatigue tests Fig. 13 shows the corresponding mean shear stress distribution relative load level relationships at different regions along the bonded CFRP plate for all the six post-fatigue tests. Due to debonding from the concrete substrate resulting from the previous cyclic loading, the shear stress for the first region [(0–50)] for most of the

P-F1

P-F2

P-F3

P-F4

P-F5

P-F6

Fig. 11. Strain distributions along CFRP plate in post-fatigue tests.

Fig. 12. Shear stress as a function of relative load level (M4).

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P-F1

P-F2

P-F3

P-F4

P-F5

P-F6

Fig. 13. Shear stress as a function of relative load level for the post-fatigue tests.

Fig. 14. Interfacial bond stress–slip curves for single shear pull-out test in monotonic test.

test specimens (P-F2, P-F4, P-F5) are equal to zero for the whole static loading stage. Furthermore, for the 0.15 mm M46J test specimen, the last region [(190–260) mm] of the specimen has zero mean shear stress due to the fracture failure mode of this specimen. Fig. 13 indicated that the peak mean shear stress varied from

one specimen to another. When the CFRP stiffness is 27.67 kN/mm (specimen P-F5), the mean shear stress is 6.3 MPa and there are minor peak shear stress variations found between the four different regions. When the CFRP stiffness is increased to 203.59 kN/ mm (specimen P-F1), the peak shear stress decreases to

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2.75 MPa. The peak shear stress progressively moved from one region to the adjacent region as the relative load level increases until complete debonding of the interface, similar to the observation from the monotonic tests. 3.5. Interfacial bond stress–slip model Based on Eq. (1), the interfacial bond stress between the first two consecutive strain gauges in the bonded region was computed. In this section, firstly the static results of interfacial bond stress–

slip curves are compared with the model developed by Chen and Teng [12]. This model was developed based on a fracture energy concept with rational simplification. Moreover, it is suitable for predicting bond strength for single-shear or double-shear pullout tests for two failure modes; either shearing of the concrete directly beneath the bond or debonding at adhesive–concrete interface. The experimental results of the tested specimens under monotonic loading show that the interfacial bond stress–slip relationship does not vary for the six specimens that have approximately the same tensile concrete strength (4.5 MPa). The

10

10 Chen and Teng model [12]

P-F2

P-F4

Chen and Teng model [12]

P-F5

P-F3

P-F6

8

Shear Stress (MPa)

8

Shear Stress (MPa)

P-F1

6

4

6

4

2

2

0

0 0

0.1

0.2

Slip (mm)

0.3

0.4

0

0.1

0.2

0.3

Slip (mm)

Fig. 15. Interfacial bond stress–slip curves for single shear pull-out test in post-fatigue test.

Fig. 16. Interfacial bond stress–slip curves for single shear pull-out test (a) fc = 22.6 MPa, (b) fc = 52.8 MPa.

Fig. 17. (a) Interfacial bond stress reduction CFRP stiffness relationship. (b) Fracture energy reduction CFRP stiffness relationship.

0.4

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bond–slip curves are very close to the analytical bond–slip model proposed by Chen and Teng [12] in terms of the maximum value of shear stress and the slip at debonding, which underlines the capability of their model. However, the analytical model of Chen and Teng [12] shows a softer response during the descending branch (i.e. between the maximum shear stress and debonding, see Fig. 14). Secondly, for the post-fatigue results, the interfacial bond stress–slip curves are compared with static interfacial bond stress–slip (i.e. M1 was chosen as representative of the static specimens). Fig. 15 presents the estimated interfacial bond stress–slip relationships based on the measured tensile strain profile along the bonded CFRP plate. It can be observed that both the ultimate bond strength and fracture energy for two different types of CFRP (T700 & M46J) are reduced with increase in CFRP plate thickness. However, these six specimens were subjected to cyclic loading until reaching 0.4 mm slip. This experimental observation for the bond slip curves had the same response as reported by other researchers. For example, Camanho et al. [33] reported a decrease in both stiffness as well as interfacial bond strength of the bonding system with an increasing number of cycles. Travesa [34] reported that the fracture energy release rate increased as fatigue loading amplitude ratio increased. Fig. 16 shows an example of comparison between the static test experimental bond–slip curve with the analytical model of Chen and Teng [12] as well as the bond–slip curve after applied cyclic loading. The concrete compressive strength was 22.6 MPa and CFRP stiffness was 38.53 kN/mm. The fracture energy can be estimated by measuring the area under the bond–slip curve. Based on this calculation method, the ultimate bond strength reduction and fracture energy degradation ratios are 0.48 (from a value of 5.2 to 2.7) and 0.28 (from a value of 0.71 to 0.51), respectively. These reduction values are approximately equal to the reductions when concrete compressive strength is equal to 52.8 MPa and the CFRP stiffness is 38.53 kN/mm, being 0.42 (from 8.3 to 4.8) and 0.21 (from 0.9 to 0.71). Therefore, it can be said both the ultimate bond strength reduction and fracture energy reduction due to cyclic loading is dependent on the stiffness of the CFRP plate in the bond system, not the concrete strength, for the range of the concrete strength tested. The relationship between the CFRP plate stiffness with the ultimate bond strength reduction and the fracture energy degradation are shown in Fig. 17(a) and (b), respectively. Both the ultimate bond strength reduction and the fracture energy degradation are increased with increase the CFRP plate stiffness.

4. Conclusions In the present paper, single shear pull-out tests were undertaken to investigate the behaviour of the bond interface between concrete and externally bonded CFRP under different loading hysteresis (static (monotonic), fatigue, and fatigue followed by static (monotonic)). The following conclusions may be drawn:  Three different failures (CFRP rupture, concrete shearing, and concrete–adhesive interface failure) were observed in both monotonic and post-fatigue tests while the fatigue tests exhibited one predominant type of failure, that of concrete shearing.  The analytical model of Chen and Teng [12] gives an accurate prediction of the static test bond load slip relationships.  The load amplitude ranges have been shown to have a significant effect on fatigue life of the bonding system for the same CFRP stiffness. The reduction in the secant bond stiffness in fatigue tests resulting from steady fracture energy release with repeated fatigue cycles, leads to a decrease in the ultimate load capacity as well as in debonding strain of the single shear

pull-out specimens. These reductions in both the ultimate load capacity and debonding strain of the bonding system due to fatigue have to be considered in practice.  The cyclic load prior to the static load caused reductions in debonding strain ranging from 35% for the 1 mm M46J CFRP plate (M1) to 5.6% for the 0.2 mm T700 CFRP plate (M5). At the same time, the reduction in ultimate load carrying capacity ranged from 27.5% for the1 mm M46J CFRP plate to 13.3% for the 0.2 mm T700 CFRP plate.  The concrete compressive strength had little effect on the ultimate bond strength and fracture energy degradation induced during fatigue loading. These reductions are predominantly dependent on the stiffness of the CFRP plates. Further tests are required in order to consider the effect of the reduction in the ultimate load capacity and debonding strain for fatigue and post-fatigue loading regimes in practice. In particular, testing on full scale structural elements is needed in order to understand the extent of size effects and associated in situ stress states on the behaviour observed herein. Additionally, in practice, effective bond length is a key parameter for design effectiveness and further testing is needed to investigate this property under cyclic loading.

Acknowledgments Our thanks to ‘‘The Higher Committee for Education Development in Iraq’’ for funding this research. Sincere thanks are expressed to School of Mechanical, Aerospace and Civil Engineering of the University of Manchester for providing the experimental facilities and technical support to undertake this research.

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