Effect of sulfates on bond behavior between carbon fiber reinforced polymer sheets and concrete

Materials and Design 43 (2013) 237–248

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Effect of sulfates on bond behavior between carbon fiber reinforced polymer sheets and concrete R. Al-Rousan ⇑, R. Haddad, K. Al-Sa’di Department of Civil Engineering, Jordan University of Science and Technology, Irbid, Jordan

a r t i c l e

i n f o

Article history: Received 19 May 2012 Accepted 9 July 2012 Available online 20 July 2012 Keywords: Repair Toughness Concrete Bond stress Slip Pull off

a b s t r a c t The bond–slip behavior between sulfate-damaged concrete and carbon fiber reinforced polymer-sheets was investigated using double shear pull-off concrete specimens. Concrete blocks (150  150  100 mm) were subjected to two different levels of sulfate cyclic treatment before and after bonded to carbon fiber reinforced polymer (CFRP) sheets. Other sets of concrete specimens bonded and unbounded to CFRP were maintained in lime water as controls. CFRP were attached to concrete at three different CFRP bonded widths (bf) of (50, 100 and 150 mm) and two CFRP bonded lengths (Lf) of (65 and 85 mm). Present and literature data were employed in the development of a statistical model to estimate the ultimate shear strength and slippage between CFRP sheets and sulfate-damaged concrete. The results showed reductions in the bond stress and corresponding slippage with further cyclic sulfate treatment, with specimens bonded to CFRP before showing better bond behaviors than those of specimens bonded to CFRP after sulfate treatment. The ultimate normal stress in CFRP sheets decreased with further sulfate treatment. The statistical model developed showed excellent fit of the data used and collected as indicated by the high R2 (coefficients of determination) values. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Recently, fiber reinforced polymers (FRPs) composite was considered as the best strengthening method of reinforced concrete (RC) structures [1–5]. In these strengthening methods, the bond behavior between the concrete and FRP composites is a key issue in design [6]. This behavior is the main factor controlling the debonding failure mode of RC structures strengthened with FRP composites. Therefore, it is necessary to understand the behavior of concrete to FRP composites interfaces in order to get economic and safe design of deficient RC structure strengthened with FRP composites [7]. In the last few years, some researchers have been investigated the behavior between concrete and FRP composites [6–14]. The main parameters of bond behavior such as the strain profile (distribution) along FRP bonded length, the effective bond strength, and the transfer mechanism of force and bond were studied using different setups [1]. The pull off test specimen in which a FRP composite bonded to concrete specimen subjected to pure axial tension force can be used to represent the actual debonding failure modes in terms of stress state at the concrete–FRP composite interface. The pull off test can be used to determine the local bond slip behavior as well as

⇑ Corresponding author. Tel.: +962 7 99887574; fax: +962 2 7201074. E-mail address: [email protected] (R. Al-Rousan). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.07.018

the ultimate load capacity of concrete–FRP interface [7–15]. The local bond slip behavior can be obtained using two well known methods: form the load displacement relationship [14] and from strain profile along FRP bonded length measured with strain gages distributed closely [11]. In the first method, the bond slip behavior is determined indirectly from the load displacement relationship with some differentiation. The second method can be consider as a simple method, but in reality cannot produce accurate local bond slip relationship. This is because the propagation of cracks caused a violent variation in FRP measured strains, the roughness of concrete area under the debonded FRP composite, and the heterogeneity of concrete. Therefore, the bond slip behavior varies significantly between the different pull off testes. Nowadays, deterioration of existing RC structures is a most important aspect for municipal agencies and governmental; the most common causes of which are freezing- and thawing action, corrosion of steel reinforcement, and especially sulfate attack. Sulfate attack can be considered as the major to the long term durability of concrete in foundations, surrounding retaining walls, and marine structures [16]. The main three different forms in of sulfate attack that caused deterioration of concrete can be classified as following: (a) eating away of the hydrated cement past in acidic sulfate environment; (b) shelling and scaling of the concrete surface in mixed sulfate environment; and (c) cracking and expansion of exposed aggregate due to the creation of ettringate in sodium sulfates environments [17,18].

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Nomenclature CFRP FRP RC LVDT UPV bf Lf fc0 ft

s s

smax c a1 a2 bw bL Lc

carbon fiber reinforced polymer fiber reinforced polymers reinforced concrete linear variable displacement transducer ultrasonic pulse velocity CFRP bonded width CFRP bonded length concrete compressive strength concrete tensile strength bond stress slippage the maximum bond stress parameter for the sulfate treatment level constant measured based on regression analysis for smax constant measured based on regression analysis slip so CFRP sheet to concrete width ratio factor CFRP sheet to concrete length ratio factor length of the concrete specimen

Sulfate attack for reinforced concrete elements results in cracking and disintegration of its concrete, generation of compressive and tensile stresses in concrete and steel, respectively, and loss of bond between reinforcing and concrete; leading to sever reduction in their load carrying capacity and loss of their durability as well. Traditionally, insulation of concrete, and use of rich concrete mixtures with sulfate resistant cements and pozzolanic additives was recommended to prevent or reduce the effect of sulfate attack on concrete [19,20]. Rehabilitation techniques involving the use of bonded FRP composites have been used to regain shear strength capacity and prevent failure of reinforced concrete flexural members [1–5]. The efficiency of this technique to regaining shear capacity would be maximized if the damage causative factor is eliminated. Therefore, in order to achieve durable repair of sulfate-damaged structures, sulfate intrusion into concrete must be stopped, otherwise the bond between the FRP repair layers and the original concrete may undergo significant degradation that leads to easy peeling of the FRP composite under relatively low external stresses. For this, there is a need to investigate the effect of different levels of sulfate attack severity on bond behavior between FRP composite and concrete. 2. Research significance The presence of sulfate in soil, sea water, groundwater, industrial effluent, and decaying organic matter cause a main menace to the concrete durability of surrounding RC structures. The main defects of sulfate attack are spalling and cracking, increase the permeability, and reduction in strength of concrete. Also, an effective strengthen of sulfate-damaged RC structures by sulfate attack using external FRP composite requires a strong bond between damaged concrete and FRP composites. Therefore, the bond strength and their reduction at various levels of sulfate attack is fundamental needed to evaluate for successful strengthen design. 3. Description of the test program 3.1. Concrete specimens details A total of 54 specimens (150  150  100 mm) and twelve standard cylinders (150  300 mm). The specimens were cast in a specially designed 20 mm thick wood mold of net internal dimensions

bc R2

c0

s0max s0o Control Stage I Stage II CL SL CS SS RS D RB

width of the concrete specimen coefficients of determination sulfate factor for concrete specimens treated with CFRP sheets maximum bond stress for concrete specimens treated with CFRP sheets maximum slippage for concrete specimens treated with CFRP sheets specimen immersion in lime water cyclic treatment in sulfate solution for 73 days cyclic treatment in sulfate solution for 123 days compressive load splitting load compressive strength splitting strength residual strength depth of peeled concrete residual bond

of (150  150  100 mm). A tilting drum mixer of 0.15 m3 capacities was used for mixing the concrete ingredients; the volume of each batch was enough to cast 24 specimens and eight cylinders.

3.2. Materials properties 3.2.1. Concrete specimens A single concrete mixture was designed using an effective w/c ratio of 0.65 using Portland cement Type I according to ACI-211 [21] mix design procedure to attain a slump of about   70 mm and a 28 days average concrete compressive strength fc0 of 43.6 MPa. The weight of cement, water, silica, coarse aggregate, and fine aggregate used in the concrete mixture was 411, 260, 227.4, 897, and 530.6 kg/m3.

3.2.2. Repair composites The Carbon fiber reinforced polymer sheet (CFRP) and Primer resin were used as external strengthening of concrete specimens. Primer resin is a two part solvent free, thixotropic epoxy based impregnating resin/adhesive. The quantity of resin coat for gluing the sheets to the concrete was 1.5 kg/m2. CFRP sheets had a 0.17 mm-thickness with unidirectional-continuous fiber in the form of tow sheet, manufactured in 300 mm wide-rolls that can be cut into appropriate lengths. The ultimate tensile strength, elastic modulus, and ultimate strain capacity were 3900 MPa, 230 GPa, and 0.015, respectively.

3.3. Experimental scheme and test variables Specimens were cured in water for 28 days before treated in 2.5% Na2SO4 and 2.5% Mg2SO4 solution using a special treatment chamber before bonded to CFRP sheets. Other set of specimens were cured in lime water for specific period before bonded to CFRP sheets and transferred to the sulfate solution chamber for treatment. A third set of specimens without and with bonded CFRP were kept in lime water, as controls. The compressive mechanical properties were evaluated for control specimens as well as for those subjected to different stages of sulfate treatment (stages I to II) using standard cylinders (150  300 mm). The type and number of specimens and their task designation is summarized in Table 1.

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R. Al-Rousan et al. / Materials and Design 43 (2013) 237–248 Table 1 Specimen type and task designation. Concrete specimen

Stage

Bonded to FRP before sulfate treatment

Control Stage I Stage II Control Stage I Stage II

Bonded to FRP after sulfate treatment

Control Stage I Stage II Control Stage I Stage II Control Stage I Stage II Control Stage I Stage II

Lf (mm)

bf (mm)

No. of specimens

85

50

33=9

85

100

33=9

85

50

33=9

85

100

33=9

85

150

33=9

65

100

33=9

Control: immersion in lime water. Stage I: Cyclic treatment in sulfate solution for 73 days. Stage II: Cyclic treatment in sulfate solution for 123 days. Lf: CFRP bonded length. bf: CFRP bonded width.

3.4. Sulfate treatment method After curing in lime water at room temperature, 18 CFRP bonded and 36 unbounded concrete specimens, designated for sulfate attack treatment, were subjected to cycles of immersion in (2.5% Na2SO4 and 2.5% MgSO4) solution followed by drying using a special conditioning unit, shown in Fig. 1. The unit consists of a treatment and a storage tanks that are connected by a two-way pumping system. The treatment tank is equipped with an electronic regulator that allows controlling the temperature, time of pumping forth and back, and the periods of immersion and drying. The temperature was set at 40 °C during immersion and drying cycles of 2 days each. Specimens were treated in the sulfate solution for about 123 days; the mechanical properties were evaluated for the conditioned pullout specimens at 73 days (Stage I) and 123 days (Stage II). 3.5. Bonding of CFRP composites Prior to application of the primer resin, weak concrete was removed and surface defects such as voids and blowholes were fully exposed. After that the bonded surface were perfectly cleaned from loose and friable materials by using brush and industrial vacuum cleaner. Then the surface was maintained and dried free of all contaminants such as coatings, grease, oil, and surface treatments.

Finally, the concrete bonded area is leveled using diamond grinding disk before bonding of CFRP composite. The fabric sheets were cut using a special scissor to the desired lengths and width to cover the required area at the surface of the specimens. A mixing spindle attached to a slow speed electric drill was used to prepare the resin, parts A and B was prepared by mixing together for at least three minutes, until the material becomes a uniform grey color and smooth in consistency with care to avoid aeration while mixing. Then for approximately one more minute, the whole mix was poured into a clean container and stir again at low speed to keep air entrapment at a minimum. The fabric sheets were placed onto the resin coating, which was already spread uniformly over the area, before carefully worked into the resin using a plastic roller that was moved in a parallel direction to the fiber until the resin was squeezed out between the fiber strands and scattered uniformly over the whole fabric surface, after which a resin layer was painted over the surface of the fabric sheets. Finally, specimens were left to cure the epoxy at laboratory temperature for 7 days. 3.6. Test procedure and instrumentation Fig. 2 shows the double shear pull-off testing setup used to determine bond stress versus slip behavior. A pulling off force (through a universal testing machine) was applied to a steel Ushaped arm connected to an aluminum cylinder that raps the CFRP middle part causing shear failure of the laminate, which is bonded to a concrete block fixed to the bottom platen of the machine using special fasteners. Two linear variable displacement transducer (LVDT) affixed to the right and left surfaces of the laminates were used to measure the slip. The load versus slip readings was acquired using a data acquisition system. 4. Discussion and test results 4.1. Evaluation of sulfate induced damage

Fig. 1. The conditioning unit used for cyclic treatment in the sulfate solution.

Compressive strength: In order to investigate the effect of sulfate treatment on fc0 and concrete tensile strength (ft) of the present concrete, the residuals for compressive and splitting strengths of concrete specimens were computed with respect to that of control specimens at different sulfate treatment ages of 73 and 123 days. At each age, core specimens were obtained from the concrete

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Fig. 2. Test setup and configuration of tested specimen.

4.2. Failure loads and modes The specimens did not display any signs of debonding during the first few seconds of the pull-off test procedure. With the increasing of applied load, small cracking and small popping sounds could be heard. Before a sudden shear failure of the tested specimen, cracking continued to occur in a sequence manner corresponding to an louder sound. This behavior is similar to the

30

123 days, 28.00%

25

UPV (DI), %

blocks (150  150  100 mm) and then tested under compression according to ASTM-C42 [22]. The average of three cores was considered as the test value. On the other hand, standard splitting test was carried on concrete cylinders (150  300 mm), which were prepared during casting, according to ASTM-C496-90 [22]. Results of Table 2 indicated that a decrease in the residual compressive strength was proportional to treatment period (stages I through II) similar to the results conducted by [5,16–18]. The residual strength for stages I and II averaged 94% and 89%, respectively, of that of control. Comparable with residuals trend for compressive strength, the corresponding residuals for splitting strength achieved 98% and 77%, respectively. Ultrasonic Pulse Velocity Measurements: the velocity of 54 kHz wave traveling across the length (longitudinally) and the width (transversely) of control and damage specimens was computed from direct measurements of lag time of the pulse wave transmitted and received by transducers of a pundit device according to ASTM test method C597 [22]. The ultrasonic velocity was computed in the lateral direction of the specimens at sixty square meshes covering the entire side area of the beams. The Ultrasonic Pulse Velocity (UPV) in the longitudinal direction of the damage specimens before FRP application was about 10.7% and 28% for stage I and II, respectively, as shown in Fig. 3.

20

Specimens treated in sulfate solution before bonded to FRP sheets

123 days, 17.50% 73 days, 10.70%

15 10

Specimens treated in sulfate solution after bonded to FRP sheets

73 days, 7.60%

5 0

0

20

40

60

80

100 120 140 160 180 200

Immersion time, days Fig. 3. UPV result for concrete prisms under sulfate cycle treatment.

Table 3 Mode of bond failure and depth of peeled concrete layer for specimens treated in sulfate solution before bonded to FRP sheets. bf (mm)

Lf (mm)

Treatment stage

Mode of failure

D (mm)

Control Stage I (73 days) Stage II (123 days) Control

Skin Skin Skin Skin

failure failure failure failure

2.37 1.96 1.45 2.82

50

85

100

85

Stage I (73 days) Stage II (123 days) Control

Skin failure Skin failure Skin failure

2.18 1.54 5.44

150

85

Stage I (73 days) Stage II (123 days) Control

Skin failure Skin failure Skin failure

2.76 2.49 2.1

100

65

Stage I (73 days) Stage II (123 days)

Skin failure Skin failure

1.6 1.26

D: depth of peeled concrete. Table 2 Compressive and splitting strength under sulfate treatment. Specimens

Control Stage I Stage II

Load (kN)

Strength (MPa)

RS (%)

CL

SL

CS

SS

CS

SS

188.3 177.0 168.0

89.1 87.0 68.3

43.6 41.0 38.9

2.8 2.8 2.2

100 94 89

100 98 77

CL: Compressive Load. SL: Splitting Load. CS: Compressive Strength. SS: Splitting Strength. RS: Residual Strength.

behavior conducted by Lopez [23]. The debonding failure mode was through shearing the concrete surface or peeling the concrete skin. Finally, the brittle failure and accompanied by large release of energy was final failure was particularly observed. Inspection of tested specimens reveals that the extension of cracks in the concrete was the top of the block to the edge of CFRP composite. Also, when the CFRP composite broke free, fragments of concrete surface were torn from the block.

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bf = 50 mm Lf = 85 mm

bf = 100 mm Lf = 85 mm

bf = 150 mm Lf = 85 mm

bf = 100 mm Lf = 65 mm

(a) Control Specimens

bf = 50 mm Lf = 85 mm

bf = 100 mm Lf = 85 mm

bf = 150 mm Lf = 85 mm

bf = 100 mm Lf = 65 mm

(b) treated in sulfate for 73 days before bonded to CFRP sheets

bf = 50 mm Lf = 85 mm

bf = 100 mm Lf = 85 mm

bf = 150 mm Lf = 85 mm

bf = 100 mm Lf = 65 mm

(c) treated in sulfate for 123 days before bonded to CFRP sheets

bf = 50 mm Lf = 85 mm

bf = 100 mm Lf = 85 mm

bf = 50 mm Lf = 85 mm

bf = 100 mm Lf = 85 mm

(d) treated in sulfate for 73 days after (e) treated in sulfate for 123 days after bonded to CFRP sheets bonded to CFRP sheets Fig. 4. Bond failure mode.

The Lf and bf and width had a direct relation with failure time, but the bf has excessive impact on failure time that Lf. The greatest bf of 150 mm provided more warning prior to final bond failure, while the shortest Lf of 65 mm provided a little warning before the final failure in the form of failure time and cracking sounds. After failure, the concrete bonded area was rough with several microcracks oriented to the direction of bond failure progression as well as the remainder of the concrete bonded region was free of the adhesive. The failure modes can be classified into two types: (a) concrete skin failure in which very small or thin layer of concrete surface peeled during the test and (b) shear failure within a cracked zone under the CFRP plate with thick layer of concrete surface peeled during the test as shown in Table 3. As seen from the Table 3, the depth of the peeled layer decreased with sulfate attack

progressed and increased as bf or Lf increased. Modes of failure for various pull-off specimens can be noticed through the photos of Fig. 4. 4.3. Bond–slip curves The bond stress–slip behavior is discussed in terms of CFRP bonded width and length in conjunction with sulfate treatment levels as shown in Fig. 5. The slope of linear part of the curves represents the stiffness of the bonded area which is almost the same for all curves regardless of the geometric parameters considered which represented the stage of prior to the beginning of debonding similar to the behavior conducted by [6–8,11]. After this stage, the FRP started to debond from the concrete surface and the slope of the curve started to change because of the nonlinearity of the

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5

Control Specimens bf = 50 mm, Lf = 85 mm

4 bf = 100 mm, Lf = 65 mm

bf = 100 mm, Lf = 85 mm

3

bf = 150 mm, Lf = 85 mm

2 1 0 0.00

0.05

0.10

0.15

0.20

Bond Stress (τ), MPa

Bond Stress (τ), MPa

5

Stage I Specimens

4 bf = 100 mm, Lf = 65 mm bf = 50 mm, Lf = 85 mm

3

bf = 100 mm, Lf = 85 mm

2

bf = 150 mm, Lf = 85 mm

1 0 0.00

0.25

0.05

0.10

Slip (s), mm 5

Bond Stress (τ), MPa

0.15

0.20

0.25

Slip (s), mm Stage II Specimens

4 3

bf = 100 mm, Lf = 65 mm bf = 50 mm, Lf = 85 mm bf = 100 mm, Lf = 85 mm bf = 150 mm, Lf = 85 mm

2 1 0 0.00

0.05

0.10

0.15

0.20

0.25

Slip (s), mm

(a) Specimens treated in sulfate solution before bonded to FRP sheets.

4

bf = 50 mm, Lf = 85 mm

3

bf = 100 mm, Lf = 85 mm

2 1 0 0.00

5

Control Specimens

0.05

0.10

0.15

0.20

Bond Stress (τ), MPa

Bond Stress (τ), MPa

5

Stage I Specimens

4

bf = 50 mm, Lf = 85 mm

3

bf = 100 mm, Lf = 85 mm

2 1 0 0.00

0.25

0.05

0.10

Slip (s), mm

0.20

0.25

Slip (s), mm 5

Bond Stress (τ), MPa

0.15

Stage II Specimens

4 3 bf = 50 mm, Lf = 85 mm

2

bf = 100 mm, Lf = 85 mm

1 0 0.00

0.05

0.10

0.15

0.20

0.25

Slip (s), mm

(b) Specimens treated in sulfate solution after bonded to FRP sheets. Fig. 5. Bond–slip curves.

interface contact area between the concrete and FRP composite. Inspection of Fig. 5 indicated that as bonded length increased the bond strength decreased while the corresponding slippage increased at varying treatment levels. The higher bond strength

values for the shorter Lf at 65 mm are attributed to that the shearing stress distribution between the CFRP sheet and concrete remain almost uniform across length, which usually occurs whenever Lf is approximately equal or slightly greater than the

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R. Al-Rousan et al. / Materials and Design 43 (2013) 237–248 Table 4 Tested result for tested specimens. Concrete specimens

CFRP application

Control

After sulfate treatment

Before sulfate treatment Stage I

After sulfate treatment

Before sulfate treatment Stage II

Lf (mm)

Pu (kN)

smax (MPa)

Slip (mm)

Loss in smax (%) (RB, %)

Loss in slip (%) (RB, %)

Toughness (MPa mm)

50

85

16.2

3.81

0.180

0 (100)

0 (100)

0.435

100 150 100 50

85 85 65 85

25.9 30.7 22.0 16.2

3.05 2.41 3.39 3.81

0.144 0.114 0.138 0.180

0 0 0 0

0 0 0 0

(100) (100) (100) (100)

0.278 0.174 0.316 0.435

100

85

25.9

3.05

0.144

0 (100)

0 (100)

0.278

50

85

13.0

3.05

0.140

20 (80)

22 (88)

0.271

100 150 100 50

85 85 65 85

20.7 24.6 17.6 15.3

2.44 1.93 2.71 3.60

0.112 0.089 0.095 0.161

20 (80) 20 (80) 20 (80) 6 (94)

22 22 31 11

(88) (88) (69) (89)

0.173 0.108 0.174 0.392

100

85

22.5

2.65

0.128

13 (87)

11 (89)

0.229

50

85

9.7

2.29

0.108

40 (60)

40 (60)

0.157

100 150 100 50

85 85 65 85

15.5 18.4 13.2 10.5

1.83 1.45 2.03 2.47

0.086 0.068 0.048 0.124

40 40 40 35

40 40 66 31

(60) (60) (34) (69)

0.100 0.063 0.066 0.207

100

85

16.9

1.98

0.099

35 (65)

31 (69)

0.132

bf (mm)

After sulfate treatment

Before sulfate treatment

(100) (100) (100) (100)

(60) (60) (60) (65)

RB: Residual Bond.

60

60

Specimens treated in sulfate solution before bonded to FRP sheets

40 30

50

39% 29%

39% 31%

33%

31%

23%

25% 19%

20

27% 20%

10 0

σtested/σCFRP, %

σtested/σCFRP, %

50

Specimens treated in sulfate solution after bonded to FRP sheets

49%

40

39% 33%

31%

30

27% 23% 20%

20 10

bf = 50 mm Lf = 85 mm

bf = 100 mm bf = 150 mm bf = 100 mm Lf = 85 mm Lf = 85 mm Lf = 65 mm

0

bf = 50 mm Lf = 85 mm

Stage Designation

bf = 100 mm Lf = 85 mm

Stage Designation

Fig. 6. Percent reduction in the ultimate normal strength.

bond width. Increasing the Lf to 85 mm resulted in none uniform stresses across length which caused bond failure at relatively low pull-off loads. 4.4. The characteristic of the bond–slip curves 4.4.1. Specimens treated in sulfate solution before bonded to FRP sheets Twenty-four concrete specimens were treated (damaged) in sulfate solution at different levels before being bonded to CFRP sheets at various geometrics. The characteristic of the bond stress versus slip curves for different pull-off specimens subjected to varying sulfate attack levels where obtained namely bond strength and slip at failure and are summarized in Table 4. Also, the percentage reduction in bond stress and corresponding slippage under sulfate treatment are summarized in Table 4. The results of Table 4 indicated that the detrimental effect of sulfate on concrete superficial stratum and hence bond with CFRP. Inspection of Table 4 reveals that the percentage reductions in bond strength were 20% and 40% after

stage I and stage II, respectively. The reduction in slip value, as expected, increased with sulfate treatment yet the percentage reduction depended upon the geometric configuration of bonded CFRP reaching its highest at 66%. Additionally, Table 4 indicated that the percentage reduction in bond stress and corresponding bond slip at failure were not affected by bf. The percentage reductions in bond strength were 20% and 40% and in corresponding slip 22% and 40% after treatment in sulfate solution for periods of 73, 123 days corresponding to stage I and stage II, respectively. The loss in bond strength was proportional to that in compressive and splitting strength indicated by results of Table 2. 4.4.2. Specimens treated in sulfate solution after bonded to FRP sheets Twelve concrete pull-off specimens, bonded to CFRP sheets of different geometrics, were treated for different levels of sulfate cyclic attack. The percent reduction in bond stress and corresponding slippage under sulfate treatment are computed based on the data of Table 4. The results indicate detrimental effect of sulfate on concrete superficial stratum and hence bond with CFRP. The

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4.50 3.50

τmax

3.00

γ 1β w β L f t

0.20

Control Specimens

s

2

2.50

α 2 = 0.035 γ 2 = 1.0

= α1 = 0.765

so, mm

τmax, MPa

0.25

Control Specimens

4.00

R = 0.942

2.00 1.00

α1 = 0.765 γ 1 = 1.0

0.50

Test Results

1.50

o = α 2 = 0.035 γ 2 β wβ L ft γ 2 = 1.0(Control )

0.15 0.10

R2 = 0.951

0.05 Test Results 0.00

0.00 0

1

2

3

4

5

0

6

1

2

4.50

0.25

Stage I Specimens

4.00

τmax

2.50

α1β w β L f t

= γ 1 = 0.81

so, mm

τmax, MPa

α 2 = 0.035 γ 2 = 0.78

0.20

3.50 3.00

2

R = 0.9335

2.00 1.00

α1 = 0.765 γ 1 = 0.81

0.50

Test Results

1.50

4

5

6

0.15

so

α 2 β wβ L ft

0.10

Stage I Specimens

= γ 2 = 0.78

R2 = 0.906

0.05 Test Results

0.00

0.00 0

1

2

3

4

5

6

0

1

2

α1βwβLft , MPa 4.50

3

4

5

6

α2βwβLft , MPa 0.25

Stage II Specimens

4.00

α 2 = 0.035 γ 2 = 0.62

0.20

3.50

Stage II Specimens

3.00 2.50

τmax

2.00

α1 β w β L f t

so, mm

τmax, MPa

3

γ2βwβLft , MPa

γ1βwβLft , MPa

= γ 1 = 0.60

1.00

α1 = 0.765 γ 1 = 0.60

0.50

Test Results

1.50

2

R = 0.926

0.15

so

0.10

α 2 β w β L ft

= γ 2 = 0.62

R2 = 0.978

0.05

Test Results

0.00

0.00 0

1

2

3

4

5

α1βwβLft , MPa

6

0

1

2

3

4

5

6

α2βwβLft , MPa

Fig. 7. Relationships between key bond–slip parameters and (a, c, bw, bL, ft).

percentage reductions in bond strength for bf of 50 and 100 mm were (6% and 13%) and 35% after stages I and II, respectively. The reduction in slip value, as expected, increased with sulfate treatment yet was depend upon the geometric configuration of bonded CFRP reaching as high as 11% and 13% stages I and II, respectively. In addition, Table 4 indicated that the percentage reduction in bond stress and corresponding bond slip at failure were not affected by CFRP bonded width (bf) except for the first stage of sulfate treatment (73 days) when the percentage reduction in bond stress was higher for a Lf of 100 mm than that at 50 mm. The percentage reductions in corresponding slip were 11% and 31% for stages I and II, respectively.

the CFRP sheets, the relative ultimate normal stress in the sheets (with respect to their ultimate normal stress capacity) were computed as shown in Fig. 6a. As can be seen, the relative stress levels were (49%, 39% and 31%) for control pull-off specimens having bf of (50, 100 and 150 mm), respectively. The corresponding values after sulfate treatment leading to stages I and II were (39%, 31% and 25%) and (29%, 23% and 19%), respectively. It is clear that, the rate of reduction in relative stress levels decreased with further sulfate treatment was not affected by bf. The normal stress for pull-off specimens having a bf of 100 mm and two different Lf (85 and 65 mm) were (39% and 33%), (31% and 27%) and (23% and 20%) pertaining to control, and sulfate treatment to stages I and II, respectively; indicating higher negative impact of sulfate treatment for specimens at higher bond strength.

4.5. Evaluation of normal stress in CFRP sheets 4.5.1. Specimens treated in sulfate solution before bonded to FRP sheets Fig. 6a shows that the ultimate normal stress in CFRP sheets decreased with further sulfate treatment for different pull-off specimens. In order to examine the effect of geometric dimensions of

4.5.2. Specimens treated in sulfate solution after bonded to FRP sheets To examine the effect of geometric dimensions of concrete pulloff specimens treated in sulfate solution after being bonded to CFRP sheets, the relative ultimate normal stress in the sheets (with respect to their ultimate normal stress capacity) were computed as

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R. Al-Rousan et al. / Materials and Design 43 (2013) 237–248 Table 5 Average ratio of predicted to test bond strength computed using various models and data from literature data. Existing data

Wu et al. [12] Takeo et al. [27] Tan [28] Zhao et al. [29] Ren [30] Ueda et al. [31] Present data

Proposed model

Lu et al. [7]

Neubauer and Rostasy [25]

Monti et al. [26]

Tested/ predicted

Coeff. of variation

Tested/ predicted

Coeff. of variation

Tested/ predicted

Coeff. of variation

Tested/ predicted

Coeff. of variation

4.29 2.23

0.41 0.844

6.98 3.5

0.413 0.893

13.14 5.55

0.477 0.893

12.27 5.68

0.445 0.893

1.82 2.33 1.66 2.67 1.00

0.168 0.239 0.221 0.856 0.048

2.91 3.78 2.68 4.12 1.58

0.178 0.25 0.229 0.91 0.45

4.56 5.46 3.76 6.48 2.28

0.183 0.25 0.23 0.977 0.05

4.52 4.98 3.78 6.47 2.09

0.177 0.250 0.232 0.977 0.029

shown in Fig. 6b. As can be seen, the relative stress levels were (49% and 39%) for control pull-off specimens having bf of (50 and 100 mm), respectively. The corresponding values of those treated in sulfate for stage I and stage II were (42% and 34%) and (32% and 25%), respectively. As can be noticed, the rate of reduction in relative stress levels decreased with further sulfate treatment. 4.6. Comparison between specimens treated in sulfate solution before and after bonded to CFRP sheets Results in Table 4 indicated that as CFRP width increased the bond stress and corresponding bond slip decreased for concrete specimens treated before and after bonded to CFRP sheets. The residual bond stress and corresponding bond slip are summarized in Table 4; the results indicated higher residual bond stress and bond slip for concrete specimens bonded to CFRP before exposure to sulfate attack. This may be referred to the protective impact of CFRP of concrete substrate during sulfate treatment. 4.7. Evaluation of toughness The characteristics for the load versus slip is summarized in Table 4. The characteristics of concern were the ultimate load capacity, ultimate bond stress, slippage (corresponding to ultimate load), and toughness. The slope of linear elastic portion of curve at bond stress–slip curve represents the stiffness of the concrete specimen, while the area underneath the bond stress–slip curves until ultimate load represents the energy ductility (toughness). Table 4 shows that the sulfate treatment had caused 38% and 64% reduction in the toughness for stage I and stage II specimen, respectively, exposed to sulfate solution before bonded to FRP sheets with respect to control ones, and that the specimens exposed to sulfate solution after bonded to FRP sheets helped enhancing the toughness significantly with an average of 24% and 10% for stage I and stage II specimen, respectively, with respect to the specimen exposed to sulfate solution before bonded to FRP sheets. Inspection of Table 4 indicated that the increase of Lf increased the toughness about 13% for control specimen. While, the increase of bf decreased the toughness about 37% and 60% for speciemns with bf of 100 and 150 mm, repsectivley, with respect to speciemen with bf of 50 mm. 5. Analytical modeling 5.1. Bond–slip models for CFRP sheets bonded to concrete Several empirical models were proposed to predict the behavior of the interfacial part of the concrete to CFRP composites in terms of local bond strength and corresponding slippage [7,11–14]. The main considering factors in theses empirical models war Lf, bf, and fc0 . However, the damaged factor especially due to sulfate attack was not considered in the existing proposed bond slip mod-

els concrete especially. Therefore, an empirical model is essential to develop in order to predict the maximum bond strength and the corresponding slippage values as function of key parameters and sulfate treatment reduction factor. These predictions can be used to estimate the shear strength and appropriate rehabilitation procedure of deteriorated RC structure by sulfate attack. The most impotent remarks of the bond slip curve were the portion of curve is completely linear elastic and an increasing trend behavior up to ultimate bond stress [7,11–14]. Based on these remarks, the bond slip model can formulate in similar manner to model proposed by Lu et al. [7]:

s ¼ smax

rffiffiffiffi s if s 6 so so

ð1Þ

where s and s are the bond stress and the corresponding slippage, respectively. The maximum bond stress smax and the corresponding slip so are proposed as following:

smax ¼ a1 c1 bL bw ft so ¼ a2 c2 bL bw ft

ð2Þ ð3Þ fc0

where ft was related to the according to ACI 318-08 [24], c is the parameter added to consider the sulfate treatment level, a1 and a2 are constants measured based on regression analysis for smax and the corresponding slip so, respectively. Finally, bw and bL are the CFRP sheet to concrete width ratio factor and concrete length ratio factor, respectively. The bL is proposed in the same manner as bw by Lu et al. [7] as following:

vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u u2:25  bbf c bw ¼ t b 1:25 þ bcf vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u u2:25  LLf c bL ¼ t L 1:25 þ Lcf

ð4Þ

ð5Þ

where Lf and Lc are the length of the bonded CFRP sheet and the length of the concrete specimen, respectively, whereas bf and bc are the width of the bonded CFRP sheet and the width of the concrete specimen, respectively. Fig. 7 shows the relationships between the key bond–slip parameters and c, a, bw, bL, ft for the control pull-off specimens and those subjected to sulfate treatment for stages I and II, respectively. The regression results show an excellent fit of the present proposed model of tested data in terms of multiple coefficients of determination (R2) for the bond strength and the corresponding slippage. The c factor was assumed equal to 1 for data obtained from measurement on control pull-off specimens; hence regression analysis of all the corresponding data combined yielded a1 and a2 of 0.765 and 0.035, respectively. Consequently, regression analysis of the data relevant to sulfate damaged pull-off specimens combined was carried out to obtain the c factors while keeping the factors a1 and a2 fixed.

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R. Al-Rousan et al. / Materials and Design 43 (2013) 237–248

9

9

8

8 7

2

R = 0.94

Predicted, kN

Predicted, kN

7

2

R = 0.93

6 5 4 3

Bond Force

2 1

Lu et al. [7]

0

6 5 4 3 2

Bond Force

1

Monti et al. [26]

0 0

1

2

3

4

5

6

7

8

9

0

1

2

3

Tested, kN 9

7

8

6 5 4 3

Bond Force

2 1

6 5

R2 = 0.96

4 3

Bond Force

2

Proposed Model

1

Neubauer and Rostasy [25]

0

0 0

1

2

3

4

5

6

7

8

9

0

1

2

3

Tested, kN

0.40

0.35

0.35

Predicted, mm

0.45

0.40 0.30 0.25 0.20 2

0.15

R = 0.9875

4

5

6

7

8

R2 = 0.991

0.30 0.25 0.20 0.15

0.10

Slippage

0.10

Slippage

0.05

Lu et al. [7]

0.05

Monti et al. [26]

0.00 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

0.00 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

Tested, mm

Tested, mm

0.45

0.45

0.40

0.40 0.35

2

Predicted, mm

R = 0.979

0.30 0.25 0.20 0.15

Slippage

0.10 0.05

9

Tested, kN

0.45

0.35

9

7

Predicted, kN

Predicted, kN

6

8

7

Predicted, mm

5

9

R2 = 0.95

8

Predicted, mm

4

Tested, kN

Neubauer and Rostasy [25]

0.00 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

0.30 0.25

2

R = 0.976

0.20 0.15 0.10

Slippage

0.05

Proposed Model

0.00 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45

Tested, mm

Tested, mm

Fig. 8. Tested bond strength and corresponding slippage versus predicted by existing bond slip models.

Installation Sequence: a and c factors for pull-off concrete specimens treated in sulfate after bonded to CFRP sheets are calculated based on the following equations:

c01 ¼ c02 ¼

s0max c smax 1 s0o so

 c2

ð6Þ ð7Þ

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R. Al-Rousan et al. / Materials and Design 43 (2013) 237–248 Table 6 Average ratio of predicted to test bond strength using the present proposed model and data from various literature data. Researcher

Predicted/test

Coefficient of variation

Correlation coefficient

Wu et al. [12] Takeo et al. [27] Tan [28] Zhao et al. [29] Ren [30] Ueda et al. [31]

2.72 1.56 1.27 1.53 1.11 1.68

0.384 0.785 0.140 0.785 0.187 0.827

0.564 0.160 0.786 0.865 0.950 0.800

where c0 and s0max and s0o are the sulfate factor, the maximum bond stress and the corresponding slippage for concrete specimens treated with CFRP sheets.

rior exposure condition, exterior exposure condition, and aggressive environment, respectively. Based on the experimental results in terms of c1, the stage I and stage II can be classified as aggressive environment and very aggressive environment conditions.

5.2. Predictability of various literature models versus present results 6. Conclusions The average value, coefficient of variation and correlation coefficient for bond strength and corresponding slip ratios predicted using various models were obtained statistically and were listed in Table 5. Inspection of Fig. 8 reveals that the trends of the test data are well described by present proposed bond–slip model. But the predict/test bond strength ratio is more than 2.00 for some of the existing bond slip models which is due to that the effect of Lf is not included. The model by Lu et al. [7] presented the best description of test bond strength values in terms of average as well as correlation coefficient, whereas, that by Neubauer and Rostasy [25] was the best in description of test slip values. 5.3. Predictability of present model versus the various literature results Results of pullout tests including geometric and strength parameters along with corresponding bond strength test data reported by various literatures were used to examine the predictability of the proposed model as compared to that of well-known models. The data provide by Takeo et al. [27], Tan [28], Zhao et al. [29], Ren [30], Ueda et al. [31], and Wu et al. [12] are used to check the predictability of the present proposed model. The ratio of predicted to test values indicated that the proposed model ability to predict depended upon the nature of the data used. The predictability of the model was poor when using the data by Wu et al. [12], poor-to-fair when using data by Zhao et al. [29] and Takeo et al. [27], and Ueda et al. [31], good when using data by Ren [30], and Tan [28]. Results of statistical analysis of Table 6 supported these conclusions, as predicted to test bond strength ratio when using the data by Ren [30] and Tan [28] were 1.11 and 1.27; close to 1, yet jumped to 2.72 using the data by Wu et al. [12]. To compare the predictability potential of the present proposed model to that of literature models, data presented in Table 6 were used and the average ratios of predicted to test bond strength are listed in Table 5 along with corresponding coefficient of variation. The results clearly indicate that the best prediction models were the proposed one followed in sequence by those of Ren [30] and Tan [28], whereas the worst was that by Wu et al. [12]. 5.4. Predictability of present model characteristic versus ACI 440-08 [30] For a misfortune, the manufactures do not consider the longterm exposure effect on the material properties such as the ultimate tensile. Therefore, the environmental conditions factor should be considered as initial properties because long-term exposure to various types of environments can diminish the fatigue endurance, creep-rupture, and tensile properties of FRP composites. As a result, the ACI 440-08 [32] proposed for CFRP composite an environmental reduction factor of 0.95, 0.85, and 0.85 for inte-

Based on the experimental results and analytical modeling, the major conclusions are: (1) The residuals for compressive strength, splitting strength, bond strength, normal stress in CFRP sheet, and toughness (energy ductility) decreased to 94%, 98%, 80%, 80%, and 62%, respectively, upon exposure to stage I as compared to 89%, 77%, 63%, 63%, and 36%, respectively, after stage II of sulfate treatment cyclic, respectively. (2) The protective impact of CFRP sheet in the case of bonding of CFRP to concrete prior to sulfate treatment imparted higher bond strength as compared to that when concrete was subjected to sulfate treatment prior to bonding to CFRP sheets. (3) The bond stress and corresponding slippage at failure were decreased significantly with the increase of CFRP bonded width, where as the bond stress and corresponding slippage were decreased and increased slightly, respectively, with the increasing of CFRP bonded length. (4) The proposed model that developed to predict maximum bond strength and corresponding slippage showed excellent fit with the used and collected results as indicated by a high R2 values in excess of 95% and 94%, respectively.

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