Long-term bond performance of GFRP bars in concrete under temperature ranging from 20 °C to 80 °C

Construction and Building Materials 25 (2011) 486–493

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Long-term bond performance of GFRP bars in concrete under temperature ranging from 20 °C to 80 °C Radhouane Masmoudi a,*, Abdelmonem Masmoudi b, Mongi Ben Ouezdou b, Atef Daoud b a b

Department of Civil Engineering, Faculty of Engineering, Sherbrooke of University, QC, Canada J1K 2R1 Civil Engineering Laboratory, National Engineering School of Tunis, Tunisia

a r t i c l e

i n f o

Article history: Received 28 March 2009 Received in revised form 8 December 2009 Accepted 17 December 2009 Available online 9 February 2010 Keywords: Bond Concrete FRP bars Pull-out testing Bond–slip modelling Temperature effect

a b s t r a c t Eighty pull-out specimens were used to study the effect of temperature ranging from 20 °C to 80 °C in dry environment on bond properties between Glass Fiber Reinforced Polymer (GFRP) bars and concrete. The pullout-test specimens were subjected during 4 and 8 months to high temperatures up to 80 °C and then compared to untreated specimens (20 °C). Experimental results showed no significant reduction on bond strength for temperatures up to 60 °C. However, a maximum of 14% reduction of the bond strength was observed for 80 °C temperature after 8 months of thermal loading. For treated specimens, the coefficient b in the CMR model, which predicts the bond–stress–displacement behavior, seems to be dependant with the temperature. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Corrosion of steel in concrete has been identified as the prime factor of deterioration and structural deficiency. Various remedies, including replacing deteriorated concrete and using epoxy-coated or galvanized steel, have been proven to be costly and inadequate over the long run. Fiber-reinforced polymer (FRP) bars are a promising solution to this problem. Other attractive properties of FRP materials include light weight, corrosion resistance, and high strength. Glass FRP (GFRP) bars are gaining popularity as reinforcement for concrete bridge deck slabs and other concrete structures due to their low initial cost compared to carbon FRP bars [1–6]. However, the surface deformation and mechanical properties of FRP reinforced bars are different from those of conventional steel bars. Therefore, the design guidelines for steel reinforcing bars cannot be directly used for FRP reinforcing bars, Benmokrane et al. [7]. FRP materials are an isotropic and characterized by high tensile strength only in the direction of the reinforcing fiber. The transverse coefficients of thermal expansion (CTE) controlled by the resin is up to three to six times the CTE of the concrete [8]. This anisotropic behaviour should affect the shear strength action of the FRP bar, as well as, the bond performance of FRP bars when

* Corresponding author. Address: Faculté de génie Local C1-3002-4, Université de Sherbrooke, 2500 Blvd. Université Sherbrooke, QC, Canada J1K 2R1. Tel.: +1 819 821 8000x62767; fax: +1 819 821 7974. E-mail address: [email protected] (R. Masmoudi). 0950-0618/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2009.12.040

embedded in concrete and these effects need to be evaluated. High temperatures, such as those occurring in extremely hot climates, may decrease the mechanical and bond properties of FRP bars. Many research studies have been carried out to evaluate the effect of high temperature on bond strength of FRP bars, Katz et al. [9–11], Nanni et al [12]. However, very limited experimental data is available on the bond effects due to high temperatures, when applied for a relatively long period of time. An experimental investigation Galati et al. [13], was carried out on concrete specimens reinforced with an FRP bar and subjected to thermal cycles. The testing was completed using direct pull-out specimens. A 9.5 mm GFRP bar with different embedment lengths was placed inside a 152 mm-cube of concrete. The treated specimens were placed into an environmental chamber for 200 h at a temperature of 70 °C and at a humidity of 80%. The testing of the specimens was undertaken at room temperature. The influence of the thermal treatment is more evident with the small values of the concrete cover. Such behaviour was explained with the micro-cracking of the concrete due to the thermal stresses induced during the thermal treatment. In most of the specimens, the thermal treatment induced degradation in the bond performance of about 16%. A more pronounced effect was observed for the bond stress–slip curves in terms of slip values due primarily to the degradation of the resin (Galati et al. [13]). Another study of the effect of high temperature on the bond between GFRP reinforcing bars (rebars) and concrete was studied by Katz and Berman [11]. Four types of rods (12.7 mm diameter),

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were embedded vertically (embedment length 5d), in a normal concrete cylinder (150 mm diameter and 300 mm long). Rod CB has molded deformations on the surface, similar to ordinary deformed steel rebars. The polymer at the surface is a urethane modified vinylester and the polymer at the core of the rod was recycled polyethylene terephthalate. Rod CPH contains wraps of helical braid of fibers on the surface and the polymer was vinylester throughout the rod. Rod CPI contains wraps of a wide braid of fibers on the surface. The polymer was vinylester throughout the rod. Rod NG has tight wraps of a narrow braid of fibers which produced large deformations in the rod. The polymer was polyester. Pull-out tests were conducted at high temperature from 20 to 250 °C. A comparison between the behavior of the different rods at 80 °C, showed that the NG rod have an early reduction in the

Fig. 1. Specimens submitted to accelerated ageing.

Table 1 Experimental program. Bar diameter (db, mm) 8

16

2. Experimental study

Embedment length

Temperature (°C)

Samples number for each ageing case

5db

20 40 60 80

5

20 40 60 80

5

5db

Time exposure (months) 4 8

4 8

Nominal diameter (mm)

Glass

8

Glass

16

2.1. Test program The main objective of the test program is to evaluate, under temperature ranging from 20 °C to 80 °C (in dry environment) the performance of the bond strength of FRP bars embedded in normal concrete. Specimens were submitted to three temperatures of 40, 60 and 80 °C for 4 and 8 months in specially designed rooms, where the temperature is controlled, as shown in Fig. 1. A total of 80 specimens were tested. Table 1 presents the details of the experimental program.

2.2. GFRP bars

Table 2 Properties of the GFRP bars used in this study [14]. Type of bar

bond strength (about 43%) which reflects its low glass transition temperature (Tg) of the resin. For the CPH and CPI rods, the decrease of bond strength is relatively moderate, and they both have approximately the same reduction (20%). For the CB rod, the reduction is the smallest (3%), indicating that the bond relies mainly on the polymeric system. It is possible to conclude from the above that improving one or some of the followings can modify the bond at high temperature: (i) Use of a polymer with high Tg in order to increase the temperature at which the reduction in bond begins. (ii) Use of a polymer with a high extent of crosslinks to moderate the gradient of bond loss. (iii) Improvement of the inorganic system, which supports the bond at a high temperature where the polymer practically ceases to contribute to the bond. We noted that in the same study [11], and at 200 °C, the bond strength exhibited a severe reduction of 80–90% (for CB, CPH, CPI and NG rod). We conclude from these works, that reduction in the bond strength depends on the transverse coefficients of thermal expansion. A limited experimental data is available on the GFRP bond effects due temperature ranging from 20 °C to 80 °C, when applied for a relatively long period of time. The major focus of the present paper is to evaluate the long term effect of temperature ranging from 20 °C to 80 °C on bond properties of GFRP bars embedded in normal concrete. Results from a total of 80 specimens 8 mm and 16 mm diameters GFRP bars, after more than 5000 h (240 days) of exposure under high temperatures up to 80 °C are reported. The thermal effects on the average bond strength are compared to untreated specimens (20 °C). Based on the available experimental tests, the main parameters a of the Bertero–Popov– Elingehhausen (BPE) model and of the Cosenza, Manfredi Realfonzo (CMR) model (b, Sr) have been calibrated. The relationship between temperature and the parameters are established using the CMR and BPE models.

Tensile modulus of elasticity (GPa)

Ultimate tensile strength (MPa)

Coefficient of thermal expansion (mm/mm/°C)

Density

60 ± 1.9

738 ± 22

0.6  105 (axial) 2.2  105 (radial)

2.2

The Glass FRP bars ‘‘CombarÒ” used in pull-out specimens were manufactured by using fiber composites and were combined with synthetic resin to achieve improved properties, such as higher strength and elevated modulus of elasticity [14]. The tensile properties of the bars used in this investigation are presented in Table 2. These properties are based on the experimental tests conducted at the laboratories of Schock Bauteile GmbH, Munich Technical University, and Syracuse University [15]. Two nominal diameters were used in this study: 8 mm and 16 mm for the GFRP bars.

2.3. Concrete design Normal strength concrete was prepared in the laboratory. All the mixtures were prepared in a 204 liters mixer, using a Portland cement type CEM I 42.5, and aggregates with maximum size of 20 mm. Concrete mixture proportioning is presented in Table 3. Standard concrete cylinders 160  320 mm were cast and cured at room temperature (20 °C). The pull-out specimens and the standard concrete cylinders

Table 3 Concrete composition and characteristics. Water (kg/m3)

Cement I 42.5 (kg/m3)

Sand (kg/m3)

Aggregate 4/12 (kg/m3)

Aggregate 12/20 (kg/m3)

Compressive strength (MPa)

Slump (mm)

204

300

857

296

691

30 ± 3

90 ± 2

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R. Masmoudi et al. / Construction and Building Materials 25 (2011) 486–493 were cast in two layers and compacted using a vibrator. The compressive strength and slump were 30 ± 3 MPa and 90 ± 2 mm, respectively after 28 days of curing at 20 °C. The pull-out specimens were stored in dry environment under temperature ranging from 20 °C to 80 °C, until the date of testing.

2.4. Test procedure

Fig. 2. Temperature distribution versus time.

Pullout bond testing were performed on specimens which consist on a 500 mm long GFRP bar embedded vertically in 150  150  150 mm and 180  180  180 mm concrete cube, respectively for 8 and 16 mm bar diameters. This difference in concrete cube dimensions is intended to avoid the concrete splitting. The embedment length for all specimens is 5db, where db is the GFRP-bar diameter. The desired embedment length is obtained using PVC pipes which were placed around the bars and sealed with silicon to avoid the contact of the concrete in this area. All specimens were prepared following the specifications of ACI Guide Test Methods [16]. One additional specimen for GFRP rebars was instrumented with a thermocouple placed at the surface of the bar before casting the concrete, for temperature monitoring during the time that the specimens were subjected to different temperatures. After the thermal treatment (40, 60 and 80 °C) during 4 and 8 months, pull-out tests were performed. The pull-out specimen was installed on the machine testing immediately after removing it out from the environmental chamber. The pull-out test was performed about 3–5 min after the moment of removing it out from the environmental chamber. During the test, which lasts approximately 5 min, it took up to 10 min for the temperature at the FRP bar/Concrete Interface to decrease with a couple of degree Celsius as shown in Fig. 2. So, we can conclude that at the time of the pull-out test, the temperature at the FRP bar/Concrete Interface is close to the studied temperature levels (40, 60 and 80 °C). The pull-out tests were carried out using a calibrated LLoyed 50 KN testing machine with a displacement-rate control. The displacement-rate of loading was constant during the tests (1.2 mm/min). Four LVDTs, with accuracy equal to 0.001 mm,

Fig. 4. Load versus bar end slip behaviour.

Fig. 3. Setup of the pull-out test: (a) schematic and (b) photo.

Fig. 5. Failure mode of the rebar.

R. Masmoudi et al. / Construction and Building Materials 25 (2011) 486–493

Fm

489

were used for the GFRP bar to monitor the displacements. Three LVDTs were placed at 120° segment orientation at the loaded end, and one LVDT at the free end (Fig. 3).

sm ¼

2.5. Experimental results

where Fm, is the Peak recorded load (N) during the pull-out test, db is the nominal bar diameter (mm), and Leb is the embedment length of GFRP bar (mm). Table 4 presents the average bond strength values for each diameter and for each temperature after 4 and 8 months of ageing. Fig. 6 shows the maximal bond strength in dry environment after 120 and 240 days of ageing. The 16 mm-diameter bar developed lower bond strength than that of the 8 mm-diameter bar, (about 14 MPa for the 8 mm and 11 MPa for the 16 mm diameter).This diameter effect, is due to a difference in the contact surface (larger for 16 mm diameter) at the interface bar/concrete. The bond strength decreases when the diameter increases (diameter effect). This finding is in agreement with the results by Boyle and Karbhari [17], Nanni and Faza [18], Tighiouart et al. [19]. After 120 days of ageing of the GFRP bar in dry environment, and at temperature up to 60 °C, the average bond strengths do not show any significant reduction (1.81% and 3.36% respectively for the 8 mm and 16 mm). For the 80 °C temperature, the maximum reductions after 4 months of thermal loading were 9.39% and 13.71%, respectively for the 8 mm and 16 mm GFRP bars, compared to the reference results at 20 °C. After 240 days of ageing in dry environment at temperature up to 60 °C, the average bond strengths also did not show any significant reduction (1.96% and 3.54% respectively for the 8 mm and 16 mm). However, for the 80 °C temperature, the maximum reductions after 8 months of ageing in dry environment were 9.64% and 14.14%, respectively for the 8 mm and 16 mm, compared to the reference results at 20 °C. Fig. 7 presents the curve fittings of the thermal degradation of the bond strengths for the two GFRP bars tested in this study. As shown in Fig. 7a and b, it is concluded that the third degree polynomial equations sm = f(T) predict with good accuracy the thermal degradation of the bond strength from 20 to 80 °C. These equations are very useful to predict the bond strength for design purpose. In a similar study by Alvarez et al. [20], for the investigation of the thermal effect on bond properties with GFRP V-Rod bar, with a modulus of elasticity 44 GPa and a CTE of 3.4  105 mm/mm/°C, the average bond strength reduction is found to be up to 27% and 32% for the specimens which are subjected to 60 and 80 °C, respectively. This comparison shows that the thermal effect on bond strength of

2.5.1. Pullout load versus slip behavior The obtained experimental results are plotted in the form of load versus end slip curves. These curves contained mainly two phases as shown in Fig. 4. In the ascending phase, the load increases rapidly with small slip until it reaches the maximum load. In the descending phase, the load decreases gradually with significant slip increase. The maximum bond stress for GFRP bars 8 mm diameter was recorded at the free end at a slip of 0.55, 0.53, 0.49 and 0.43 mm respectively for 20, 40, 60 and 80 °C temperature. For 16 mm diameter, slips were 0.60, 0.58, 0.56 and 0.47 mm, respectively for 20, 40, 60 and 80 °C. It can be concluded that as the temperature increases, the slip corresponding to the maximum pullout load decreases. For all GFRP bars, the failure mode is shearing off the concrete corbels (Fig. 5). 2.5.2. Bond strength The maximal bond stress sm was calculated using the following equation:

Table 4 Specimens and summary of test results. Temperature (°C)

20 40 60 80 a

Average bonda (MPa) 4 months

8 months

8 mm (GFRP)

16 mm (GFRP)

8 mm (GFRP)

16 mm (GFRP)

14.37 ± 0.40 14.27 ± 1.04 14.11 ± 0.75 13.02 ± 0.22

11.01 ± 0.25 10.87 ± 0.36 10.64 ± 0.15 9.50 ± 0.27

14.32 ± 1.19 14.22 ± 1.99 14.04 ± 1.24 12.94 ± 1.49

11.03 ± 0.92 10.86 ± 0.21 10.64 ± 0.44 9.47 ± 0.93

Based on five identical tests.

Fig. 6. Loss in bond strength in dry environment after 120 and 240 days of ageing.

ð1Þ

pdb Leb

Fig. 7. Thermal degradation of the bond strength.

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GFRP Combar is less pronounced than that of GFRP V-Rod bar due to a lower CTE and higher elasticity modulus (60 GPa). The axial and radial GFRP coefficients of thermal expansion are respectively 0.6 and 2.2  105 mm/mm/°C. For comparison, the coefficient of thermal expansion of concrete is between 0.5 and 1.2  105 mm/mm/°C, which may explain why there is no significant thermal effect for the specimens subjected to temperatures from 20 to 60 °C.

3. Analytical models of the bond–slip behaviour In spite of a large number of formulations proposed in the past for steel reinforcements and FRP bars, even though many experimental programs have been conducted worldwide examining the bond characteristics of FRP bars, very little work has been published on analytical modelling. The available models for FRP reinforcement bond properties are reported hereafter. 3.1. Eligehausen, Popov and Bertero (BPE model) Fig. 7 shows a schematic of the BPE model, the ascending branch of the well-known bond–slip model proposed by Eligehausen et al. [21], given by:

s ¼ s1

Fig. 8. BPE model [17].

 a s s1

ð2Þ

where s1 is the maximum bond strength, (MPa), s and s1 is the slip and maximum slip at maximum bond strength, (mm).

Fig. 9. Local bond–slip relationships GFRP 8 mm.

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Therefore s1 = sm and s1 = sm. In Eq. (2), a is a curve-fitting parameter that must be not larger than 1 to be physically meaningful (a = 0.4 for steel bars). The value of parameter a, which the ascending branch depends on, is evaluated by equating the area, As1, underneath the ascending branch of the analytical bond–slip curve (see Fig. 8) given by Eq. (3), to the area, Ass, underneath the ascending branch of each actual curve:

Z s1 Z s1   a s s1  s1 As1 ¼ sðsÞ  ds ¼ s1  ds ¼ s1 1þa 0 0

ð3Þ

In Eq. (3), s1 and s1 represent the bond strength and the corresponding slip, respectively. Therefore, a can be expressed as a function of As1 given by:

a¼

sm  sm As 1

1

ð4Þ

Fig. 10. Local bond–slip relationships GFRP 16 mm.

Table 5 Mean values for each temperature and diameter of GFRP bars. GFRP 8 mm

4 months

8 months

CMR model

b Sr (mm)

BPE model

a

CMR model

b Sr (mm)

BPE model

a

GFRP 16 mm

20 °C

40 °C

60 °C

80 °C

20 °C

40 °C

60 °C

80 °C

0.458 0.134 0.087

0.463 0.149 0.088

0.476 0.145 0.09

0.496 0.137 0.095

0.416 0.155 0.085

0.425 0.172 0.089

0.456 0.149 0.087

0.512 0.105 0.084

0.458 0.148 0.093

0.463 0.146 0.089

0.477 0.147 0.092

0.498 0.136 0.094

0.417 0.166 0.088

0.425 0.161 0.090

0.458 0.147 0.086

0.515 0.112 0.089

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Diameter 8 mm :

3.2. CMR model Cosenza et al. [22], proposed a law to model the first branch of the s–s curve



s

sðsÞ ¼ sm 1  esr

b

ð5Þ

where sm is the maximum bond strength, (MPa), Sr and b are parameters based on curve-fitting of the experimental data. Parameters Sr and b were calibrated for each diameter of bar and temperature by the least-square method. The local bond–slip laws of the considered bars after thermal treatment have been determined via the BPE and the CMR models. The ascending branch is the most important branch because this branch gives the bond strength–slip of the bar below the ultimate load. A comparison of the ascending branch obtained from the analytical curves with the modified BPE and CMR models, and the experimental results submitted to different temperatures are presented in Figs. 9 and 10 respectively, for the diameter 8 and 16 mm after 8 months of ageing. Table 5 presents the mean values of a and b parameters for each temperature and each diameter of bar studied and calibrated to the experimental phase after 4 and 8 months of ageing in dry environment. The CMR model appears to be the most reliable for all specimens; the ascending branch of the bond–slip law is well interpreted by the CMR model valid for 0 6 s 6 sm. The average values obtained for the coefficient a, from the first branch of the BPE model for reference (20 °C) GFRP-Combars specimens is 0.089. It is noted that the average value found by Cosenza et al. [22] for sand-coated bars is 0.067. No significant effect was detected after 8 months of ageing on the calibrated coefficient a for specimens submitted to temperature ranging from 20 °C to 80 °C. For the CMR model and after 8 months of ageing, the coefficient b calibrated to the experimental data for specimens after conditioning depends on the temperature T. The coefficient b, from the first branch of the CMR model increases as temperature increases, as shown in Fig. 11. The third degree polynomial equations b = f(T) for each diameter predicts this dependence on temperature as presented by Eqs. (6) and (7). These equations are fit for temperature ranging from 20 °C to 80 °C with this particular kind of rebar and diameters used in this investigation

bðTÞ ¼ 0:0002T 3 þ 0:005T 2  0:0088T þ 0:462

ð6Þ

Diameter 16 mm : bðTÞ ¼ 0:0005T 3 þ 0:008T 2  0:0185T þ 0:426

ð7Þ

4. Conclusions The following conclusions are deduced from the experimental and analytical results: – For temperature up to 60 °C applied for periods of 4 and 8 months, the average bond strengths did not show any significant reduction. – For the 80 °C temperature, the maximum reductions after 8 months of ageing in dry environment were 10% and 14%, respectively for the 8 mm and 16 mm GFRP bars, compared to the reference results at 20 °C. – No significant damages were observed on the interface GFRP rebars/concrete after 240 days of thermal loading in dry environment. – No significant effects were detected on the coefficient a of the BPE modified model submitted to temperature ranging from 20 °C to 80 °C. – The bond strength decreases when the diameter increases (diameter effect). – The thermal effect on bond strength of GFRP Combar is less pronounced than that of GFRP V-Rod bar due to a lower CTE and higher elasticity modulus. – To predict the bond–stress slip behavior, the CMR model provides better accuracy with the experimental results, than the BPE model. – The coefficient b, from the first branch of the CMR model increases as temperature increases. This finding is fit for temperature ranging from 20 °C to 80 °C with this particular kind of rebar and diameters used in this investigation.

Acknowledgments The authors would like to thank the manufacturer of the GFRP CombarÒ (Schöck, Baden-Baden, Germany) for providing the GFRP bars. The opinion and analysis presented in this paper are those of the authors. References

Fig. 11. Temperature dependence of parameter beta after 8 months of ageing.

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