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Bond properties between carbon fibre-reinforced polymer plate and fire-damaged concrete
International Journal of Adhesion & Adhesives xxx (xxxx) xxx
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Bond properties between carbon fibre-reinforced polymer plate and fire-damaged concrete Chanachai Thongchom a, b, Akhrawat Lenwari a, *, Riyad S. Aboutaha c a
Composite Structures Research Unit, Department of Civil Engineering, Faculty of Engineering, Chulalongkorn University, Pathumwan, Bangkok, 10330, Thailand Department of Civil Engineering, Thammasat School of Engineering, Thammasat University, Pathumtani, 12120, Thailand c Department of Civil and Environmental Engineering, College of Engineering and Computer Science, Syracuse University, New York, 13244, USA b
A R T I C L E I N F O
A B S T R A C T
Keywords: Concrete Fire damage Strengthening Carbon fibre-reinforced polymer Bond Single-shear test
In this research, a single-shear test was conducted to investigate the bond properties between carbon fibrereinforced polymer (CFRP) plates and fire-damaged concrete prisms. The investigation focused on the effects of fire exposure condition and the presence of internal steel reinforcements on the effective bond length, failure mode, bond strength (maximum joint load) and interfacial fracture energy. After being air-cooled, the concrete prisms were adhesively bonded with CFRP strips of different lengths, and then quasi-static tested at the ambient condition. The pull-off test results showed that the tensile strength of concrete substrate decreased after being exposed to elevated temperatures. Such deterioration of concrete substrate caused an increase in the effective bond length. It also decreased the bond strength and interfacial fracture energy. The steel reinforcements in concrete structures are beneficial as they minimize the effect of fire exposure on bond strength and effective bond length. An application of the fracture mechanics-based model for strength prediction of bond between firedamaged concrete and the CFRP plate is proposed. A linear relationship between interfacial fracture energy and pull-off tensile strength of concrete could be assumed for the unreinforced concrete.
1. Introduction The use of externally bonded fibre-reinforced polymer (FRP) lami nates to strengthen deficient RC structures has been popular worldwide. The bond between FRP and existing concrete structures is critical for the effectiveness of the strengthening scheme, especially for flexural and shear strengthening. The soundness and tensile strength of the concrete substrate can limit the overall effectiveness of the externally bonded FRP system. In practice, the pull-off test is used for quality control to ensure the suitability and condition of the concrete surface after the surface preparation. Epoxy resins commonly used to bond the FRP and concrete are so strong that the debonding failures generally occur in the concrete, either well into the concrete or in the concrete adjacent to the adhesive/ concrete interface [1]. As concrete is the weakest link in the con crete/FRP system, all design recommendations on the debonding fail ures of FRP-strengthened concrete structures assume that the interfacial strength is governed by the concrete strength rather than the adhesive strength [2]. Currently, the strengthening design guidelines such as ACI 440.2R-17 [3] “Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures” recommends that
pull-off adhesion test be conducted to determine the tensile strength of concrete on surfaces where the FRP system will be installed. Tension adhesion strengths should exceed 200 psi (1.4 MPa) and should exhibit failure of the concrete substrate. However, the current design guidelines do not provide specific provisions for strengthening of fire-damaged RC members. Concrete is known as the material that exhibits a good behavior at the elevated temperatures due to its incombustible and low thermal diffusivity properties. However, the fire exposure is detrimental to concrete strength. While concrete is heated, an irreversible damage occurs as a result of chemo-physical transformations of aggregates, cement paste, and aggregate/paste interface [4]. Such damage includes micro cracking caused by thermal expansion mismatch between hard ened cement paste and aggregates, decomposition of aggregates of limestones and dolomites into lime (Cao) [4], and dissociation of cal cium hydroxide and destruction of calcium-silicate-hydrate gel in the cement paste [5]. While concrete is cooled down to the ambient con dition, further potential damage includes crack development, moisture absorption, and rehydration of the decomposed aggregates [6]. In reinforced concrete (RC) structures, the fire exposure also has a negative
* Corresponding author. E-mail address: [email protected] (A. Lenwari). https://doi.org/10.1016/j.ijadhadh.2019.102485 Available online 9 November 2019 0143-7496/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Chanachai Thongchom, International Journal of Adhesion & Adhesives, https://doi.org/10.1016/j.ijadhadh.2019.102485
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behavior between fire-damaged concrete and CFRP plate (CFRP-toconcrete bonded joints). Test variables are the elevated temperatures, amount of internal steel reinforcements, and CFRP plate length. The concrete prisms were exposed to fire (500 � C, 700 � C or 900 � C) for 3 h. After air-cooled, the concrete prisms were adhesively bonded with CFRP plate. The CFRP-to-concrete bonded joints were then statically tested at ambient condition. A single shear test was chosen. The studied bond properties include the effective bond length, failure mode, bond strength (maximum load of the bonded joint), and interfacial fracture energy. A fracture mechanics based-model was adopted for strength prediction of bond between fire-exposed concrete and CFRP plate. A correlation be tween interfacial fracture energy and pull-off tensile strength of concrete was studied. 2. Debonding analysis of FRP-to-concrete bonded joints Based on the linear elastic fracture mechanics, the bond strength (Pmax ) of FRP-to-concrete joints can be predicted by [14], pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pmax ¼ bf 2Ef tf Gf ðNÞ (1)
Fig. 1. Details of unreinforced and reinforced concrete prisms (dimensions in mm). (a) Plain concrete joint (J). (b) Reinforced concrete joint (RJ).
where bf and tf are width and thickness of the FRP plate (mm); Ef is the elastic modulus of FRP; Gf is the interfacial fracture energy (N/mm). In Eq. (1), the FRP plate should be longer than the effective bond length (Le ). Using Eq. (1), the interfacial fracture energy can be determined from the test data of specimens with sufficient bond length as follows [15], Gf ¼
P2max 2b2f Ef tf
(2)
Fib14 [16] adopted the results from Neubauer and Rost� asy (1997) [17]. The effective bond length in terms of the pull-off tensile strength of concrete substrate (ft) can be determined from, sffiffiffiffiffiffiffiffi ! Ef tf mm (3) Le;FIB ¼ 2ft
Fig. 2. Concrete prisms inside fire test furnace.
Eq. (3) was derived from the results of a double shear test on 51 CFRP-to-concrete bonded joints [17,18]. The equation provides a good prediction for joints with concrete having the compressive and tensile strengths of 42.5 and 3.5 MPa, respectively, and FRP having stiffness (Ef tf ) from 25 to 58 N/m [19].
effect on the bond between steel reinforcements and surrounding con crete. A significant reduction in bond strength can occur when tem perature at the steel reinforcement is higher than 500 � C [7–9]. Past experimental studies on the bond between FRP and firedamaged concrete showed that the elevated temperatures deteriorate the bond properties. Limpaninlachat (2012) [10] investigated the bond behavior between fire-damaged reinforced concrete and CFRP plates. Test variables included the CFRP plate length and thickness of concrete cover over steel reinforcements. The reinforced concrete was exposed to ASTM E119 standard fire [11] for 45 or 90 min. The modified beam test was carried out after the fire exposure. The test results indicated a reduction in bond strength and interfacial fracture energy after concrete was exposed to fire. The effect of concrete cover thickness on the post-fire bond strength was minimal. Haddad et al. (2013) [12] exper imentally investigated the bond-slip behavior between fire-damaged concrete and CFRP sheets. Test variables included the concrete strength and CFRP-to-concrete width ratio. Concrete prisms were exposed to 300, 400, 500, and 600 � C for 2 h. A double-shear test was conducted. The test results showed that the effect of fire deteriorated the bond strength and increased the slip at failure when the exposure tem perature was higher than 400 � C. Danie Roy et al. (2015) [13] investi gated the bond behavior between fire-damaged concrete and GFRP laminates. Test variables included the exposure temperature (200, 400, 600, and 800 � C for 3 h) and GFRP length (100, 150, and 200 mm). A single-shear test was conducted. The test results showed that the bond strength decreased significantly when concrete was exposed to tem peratures above 400 � C. Due to limited previous studies, this research investigates the bond
3. Experimental program on bonded joints 3.1. Test variables In the experimental program, a total of 30 concrete prisms (150 � 150 � 500 mm) were constructed to investigate the effects of steel reinforcements (J ¼ plain concrete and RJ ¼ reinforced concrete), fire exposure temperature (RT ¼ room temperature, 500, 700, and 900 � C) and bond length (Lb ¼ 100, 200, 300, and 400 mm) on the bond properties between the CFRP plate and concrete. Fig. 1 shows the details of the unreinforced and reinforced concrete prisms. Four 12-mm-diam eter deformed bars (DB12) were used as the longitudinal re inforcements. The transverse reinforcements (RB9) had a constant spacing of 75 mm. The concrete cover thickness was 25 mm. All concrete prisms were cast from the same ready-mixed concrete batch. After 24 h, the concrete prisms were demolded and cured with plastic sheeting at the ambient temperature for 28 days. 3.2. Material properties 3.2.1. Concrete The concrete consisted of Portland cement Type I, river sand (fine aggregate), and natural rock (coarse aggregate) which the maximum 2
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Fig. 3. Schematic fire exposure setup (Dimensions in mm). (a) Cross-sectional elevation view. (b) Plan view.
Fig. 4. Average temperatures inside fire test furnace for 500 � C, 700 � C, and 900 � C exposure conditions. 3
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Fig. 5. Pull-off testing on concrete prisms. (a) Installation of dolly. (b) Tension test apparatus.
Fig. 6. Procedure for installation of CFRP plate to concrete prism.
size was 19 mm. A mix proportion of the ready-mixed concrete consisted of cement (366 kg/m3), water (160 kg/m3), fine aggregate (750 kg/m3), and coarse aggregate (1150 kg/m3). The water to cement ratio was 0.44. The compressive strength of three standard concrete cylinders (∅150 � 300 mm) was 43 � 0.6 MPa (average � standard deviation) at 28 days. 3.2.2. Steel reinforcements A tension test on steel reinforcements was conducted in accordance with the standard test method ASTM A370 [20]. The measured yield, ultimate strengths, and elastic modulus of DB12 were 532, 640, and 200417 MPa, respectively. The measured yield, ultimate strengths, and elastic modulus of RB9 were 346, 550, and 194700 MPa, respectively. 3.2.3. CFRP plate and bonding agent A unidirectional CFRP plate, Sika Carbodur® S512, was used in the study. The plate had thickness and width of 1.2 and 50 mm, respectively. A tension test on CFRP plates was conducted in accordance with the standard test method ASTM D3039 [21]. The measured elastic modulus, tensile strength, and strain at break were 181 GPa, 3303 MPa, and 1.7%, respectively. The adhesive used for bonding the CFRP plates to concrete prisms was Sikadur®-30. According to the manufacturer, it is a thixotropic, structural two-component adhesive, based on a combination of epoxy resin and special filler, for bonding Sika Carbodur® plates to concrete, brickwork and timber (SIKA product data sheet [22]). This adhesive can be classified as a cold-cure adhesive, which is defined as one which is
Fig. 7. Concrete surface after mechanical grinding.
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Fig. 8. Schematic single-shear test setup for bonded joints.
Table 1 Summary of test results for plain concrete specimens. Plain concrete specimen
Fire exposure condition
Residual tensile strength from Pull-off test (MPa)
Pmax,test (kN)
Le,test (mm)
Gf,test (N/mm) Mean � SD
Gf,model (N/ mm)
Mode of failurec
J-RT-100 J-RT-200 J-RT-300 J-RT-400(A) J-RT-400(B) J-RT-400(C) J-500-100 J-500-200 J-500-300(A) J-500-300(B) J-500-400 J-700-100 J-700-200 J-700-300 J-700-400(A) J-700-400(B) J-900-100 J-900-200 J-900-300 J-900-400(A) J-900-400(B)
No exposure
N/Aa N/Aa 2.9 N/Aa 4.2 6.2 2.2 0.8 1.3 1.5 1.4 N/Aa N/Aa 0.5 0.3 0.4 Severely damaged
18.7 27.2 24.4 27.0 32.3 37.6 9.6 18.4 17.8 22.8 28.4 4.8 3.4 10.2 17.2 20.3
>100 120 145 150 150 175 >100 >200 >300 >300 320 >100 >200 >300 340 360
N/Ab 0.775 � 0.279 0.784 � 0.491 0.717 � 0.295 1.093 � 0.751 1.338 � 0.700 N/Ab N/Ab N/Ab N/Ab 0.677 � 0.409 N/Ab N/Ab N/Ab 0.317 � 0.058 0.223
N/Ab 0.680 0.548 0.671 0.964 1.305 N/Ab N/Ab N/Ab N/Ab 0.742 N/Ab N/Ab N/Ab 0.273 0.379
1 1 1 1 1 1 2 2 2 2 1 2 2 2 2 2
500 � C (3 h)
700 � C (3 h)
900 � C (3 h)
Remark. a No measurement. b bond length is lower than effective bond length. c Mode 1 ¼ debonding at concrete substrate; Mode 2 ¼ concrete prism failure.
capable of curing to the required strength between the temperatures of 10 � C and 30 � C [23]. Two components include the resin (component A: white color) and hardener (component B: black color). The mixing ratio of component A to B was 3:1 by volume. According to the manufacturer, the minimum compressive, shear, and tensile strengths of the hardened adhesive at 7 days (curing temperature of 35 � C) are 90, 18, and 29 MPa, respectively. The reported coefficient of thermal expansion and glass transition temperature were 2.5 � 10 5/ � C (temperature ranges from 20 � C to 40 � C) and 52 � C, respectively.
3.3. Fire exposure After 28 days, the concrete prisms were placed inside the fire test furnace, as shown in Fig. 2. Fig. 3 shows the schematic fire exposure setup of prisms inside the fire test furnace. The furnace had internal dimensions of 900 � 2500 � 1700 mm. The furnace was equipped with six LPG-fueled burners at two levels above the furnace floor. The average furnace temperature was obtained from six thermocouples installed alongside the burners. The top surface of each concrete prism was also 5
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Table 2 Summary of test results for reinforced concrete specimens. Reinforced concrete specimen
Fire exposure condition
Residual tensile strength from Pull-off test (MPa)
Pmax,test (kN)
Le,test (mm)
Gf,test (N/mm) Mean � SD
Gf,model (N/ mm)
Mode of failurec
RJ-RT-400(A) RJ-RT-400(B) RJ-500-400(A) RJ-500-400(B) RJ-700-300 RJ-700-400(A) RJ-700-400(B) RJ-900-400(A) RJ-900-400(B)
No exposure
3.7 N/Aa 0.5 1.4 N/Aa 1.0 0.7 Severely damaged
32.6 32.4 34.6 35.2 26.6 35.7 32.9
160 120 280 300 >300 310 340
1.060 � 0.544 0.928 � 0.361 1.089 � 0.732 0.963 � 0.593 N/Ab 1.389 � 0.307 1.376 � 0.090
0.981 0.968 1.101 1.144 N/Ab 1.174 0.997
1 1 1 1 3 3 3
500 � C (3 h) 700 � C (3 h) 900 � C (3 h)
Remark. a No measurement. b bond length is lower than effective bond length. c Mode 1 ¼ debonding at concrete substrate; Mode 3 ¼ debonding at concrete substrate and concrete cover separation.
directly exposed to fire. This simulated the condition when cracking of concrete due to the service load influences the heat propagation in fireexposed RC structures [24]. Fig. 4 shows the average temperatures inside fire test furnace for 500 � C, 700 � C, and 900 � C exposure conditions. The average furnace temperature was initially controlled to follow the ISO 834 standard fire curve [25]. Once the target temperature of 500, 700 or 900 � C was reached, the average temperature was maintained for a heating period of 3 h. After heating, the concrete prisms were left inside the furnace to allow natural air cooling to the ambient temperature.
probe, along with a bonded mass of concrete, by applying a direct ten sion force. 3.5. CFRP installation procedure Fig. 6 shows the procedure for installing the CFRP plate to the con crete prism. The concrete substrate was first prepared by manual grinding until the aggregates became clearly exposed. The grinding method removed loose or unsound materials and minimized the local ized out-of-plane variations. Then, the surface was cleaned by air vac uuming to remove any dust. All concrete surfaces were dry before the installation of FRP plates. Fig. 7 shows the concrete surface after the grinding. Finally, the CFRP plate was bonded to the concrete prism using an adhesive. The adhesive layer thickness was 2 mm. A 50-mm unbon ded length (without an adhesive) was provided near the loaded end of the CFRP plate to simulate the interfacial crack. Also, it could prevent the undesired wedge/concrete splitting failures [27]. After the instal lation, the bonded joints were left for one week to allow curing of the adhesive before a single-shear test.
3.4. Pull-off testing To understand the effect of fire on the residual tensile strength of concrete substrate before CFRP installation, the pull-off test was con ducted in accordance with the European Standard EN 1542 [26]. The testing was conducted on the concrete substrate on one side of the prism at mid-height. The CFRP plate was installed on a different side of the prism. Fig. 5 shows a circular steel probe that is bonded to the concrete surface and a specially designed portable apparatus used to pull off the
3.6. Static test setup and instrumentation Figs. 6(e) and 8 show the single-shear test setup for the bonded joints. Near the loaded end of the CFRP plate, strain gages were instrumented along the CFRP plate with a spacing of 20 mm. Close to the free end of the CFRP plate, the spacing between gages was double (40 mm). The steel grips were used to clamp the CFRP plate in order to apply the tension force from the testing machine. The specimen was restrained against the vertical movement by the steel support at the
Fig. 9. Visible damage of concrete prisms after exposure to fire.
Fig. 10. Failure of concrete substrate in pull-off testing. 6
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Fig. 11. Effect of fire exposure on pull-off strength of concrete.
Fig. 12. Determination of effective bond length from CFRP strain distributions [specimen J-RT-400(C)].
loaded end. The crosshead speed of the testing machine was 1 mm/min until failure. During the static test, the applied load and CFRP strain data were recorded with the data acquisition system.
4.1. Pull-off (tensile) strength of concrete substrate Fig. 9 shows the visible damage of concrete prism after exposure to fire. After exposure to 500 and 700 � C, the occurrence of thermal cracks was observed on the surface. The colors of concrete were whitish grey and pink for 500oC-exposed and 700oC-exposed concrete prisms, respectively. A severe spalling of concrete occurred in concrete prisms exposed to 900 � C. The aggregate color was changed to white. All plain concrete prisms failed inside the furnace and cannot be bonded with the CFRP plate. Fig. 10 shows the concrete substrate failure in the pull-off testing. It was observed that the thicknesses of pull-offed concrete layers were
4. Experimental results and discussion Tables 1 and 2 summarize the test results for plain concrete and reinforced concrete specimens, respectively. The test results include the residual tensile strength obtained from pull-off testing, bond strength or maximum load (Pmax,test), effective bond length (Le,test), interfacial fracture energy (Gf,test), and mode of failure. Effects of fire damage on the bond properties are discussed in this section.
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Fig. 13. Effect of exposure temperature on effective bond length.
Fig. 14. Prediction of effective bond length by pull-off strength of concrete.
similar regardless of the level of exposure temperature. Fig. 11 shows the effect of fire on the residual pull-off strength of concrete. The pull-off strength was not significantly affected by the presence of steel re inforcements. The pull-off strength of unexposed concrete prisms was 4.2 � 1.4 (average � standard deviation) MPa, which was about 10% of the compressive strength of concrete cylinders (43 MPa). After exposure to 500 � C and 700 � C for 3 h, the pull-off strength reduced to 1.3 � 0.5, and 0.6 � 0.3 MPa, respectively. No data could be obtained from all 900oC-exposed concrete prisms due to excessive spalling. Therefore, the pull-off strength of most fire-damaged concrete prisms were below the recommended value of 1.5 MPa (Fib14 [16]) or 1.4 MPa (ACI 440.2R [3]) for the application of externally bonded FRP reinforcement to RC structures.
4.2. Effective bond length The concept of effective bond length for the externally bonded FRP laminates is that no further load increases when the bond length is longer than the effective bond length [19]. According to previous research works [15,27,28], the effective bond length can be determined from the CFRP strain data. In this study, the effective bond length is defined as the distance from the loaded end position to the position where CFRP strain vanishes. Fig. 12 shows typical strain distributions along CFRP plate from station at the loaded end (x ¼ 0 m) to free end at various load levels (20%, 40%, 60%, 80%, and 100% of ultimate load) until failure. A high strain value was observed near the loaded end location. When load increased, the strain value at each location 8
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Fig. 17. Close-up view of aggregate and aggregate/paste bond failures in concrete substrate [specimen J-500-400].
unexposed, 500oC-exposed, and 700oC-exposed specimens were 161, 289, and 438 mm, respectively. The prediction tends to overestimate the effective bond length. The ratio between predicted and measured effective bond length of unexposed, 500oC-exposed, and 700oC-exposed concrete joints ranges from 0.92 to 1.34 (with an average value of 1.13), 0.90 to 1.03 (with an average value of 0.97), and 1.22 to 1.41 (with an average value of 1.30), respectively.
Fig. 15. –Three failure modes of CFRP-to-concrete bonded joints. (a) Plain concrete joint (J). (b) Reinforced concrete joint (RJ).
increased. At high load levels, the CFRP strain distributions became constant near the loaded end due to the occurrence of crack or debonding. Fig. 13 shows the effect of fire on the effective bond length. The effective bond length required by the CFRP plate increased when exposure temperature increased. The presence of steel reinforcements decreased the effective bond length. The average effective bond lengths of unexposed plain and reinforced concrete specimens were 148 and 140 mm, respectively. The average effective bond lengths of 500oCexposed plain and reinforced concrete were 320 and 280 mm, respec tively. The average effective bond lengths of 700oC-exposed plain and reinforced concrete were 350 and 325 mm, respectively. Fig. 14 shows comparison between the effective bond lengths pre dicted with Eq. (3) (using the pull-off strength of concrete) and the test data. The average values for predicted effective bond lengths of
Fig. 18. Effects of test variables on strength of CFRP-concrete bonded joints.
Fig. 16. Typical failures of bonded joints. 9
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in Fig. 16(a). The debonding started at the loaded end of the CFRP plate and then propagated towards the plate end. The presence of steel rein forcement has no effect on the failure mode. For 500 � C and 700 � C exposure conditions, Mode 2 governed the failure of plain concrete joints with short bond lengths, i.e., less than the effective bond length. The cracking occurred in the concrete prism at the termination of CFRP plate and then propagated towards the support, as shown in Fig. 16(b). As bond length increased beyond the effective bond length, the failure mode tended to be Mode 1. A layer of peeled-off concrete substrate was thicker than the case of unexposed specimens. Fig. 17 shows the close-up view of concrete substrate failure. A combi nation of aggregate and aggregate/paste bond failures was observed. For all 700oC-exposed unreinforced specimens, the failure mode was Mode 2. Possibly, the effective bond length, which increased due to fire damage, became close to the maximum 400-mm bond length used in the study. Mode 3 governed the failure of all 700oC-exposed steel-reinforced specimens. The debonding was observed as concrete cover separation along the steel reinforcements, as shown in Fig. 16(c). This failure could be attributed from the deterioration in bonding between steel and sur rounding concrete at steel temperature above 500 � C. 4.4. Bond strength Fig. 18 shows the effects of fire exposure and steel reinforcements on the strength of CFRP-to-concrete bonded joints. For unexposed speci mens, the bond strength increased as CFRP bond length increased. Beyond 200 mm, however, the strength did not significantly increase with the bond length. This implies that the effective bond length was about 200 mm for unexposed joints. The CFRP bond length of 400 mm was used to investigate the effect of steel reinforcement (plain and reinforced concrete specimens). For unexposed specimens, no signifi cant difference between the bond strength of plain concrete and RC joints was observed. Similarly, for 500oC-exposed specimens, the bond strength increased when CFRP bond length increased. The bond strengths of fire-exposed plain concrete specimens with CFRP plate length of 100, 200, 300, and 400 mm decreased from the unexposed specimens by 49%, 32%, 17%, and 12%, respectively. At 400-mm bond length, the presence of steel reinforcements enhanced the bond strength by 23% compared with the plain concrete specimens. For 700oC-exposed joints, the bond strengths of plain concrete specimens with CFRP plate length of 100, 200, 300, and 400 mm decreased from the unexposed specimens by 75%, 87%, 58%, and 42%, respectively. At 300-mm and 400-mm bond length, the presence of steel reinforcements enhanced the bond strength by 162% and 83%, respec tively, compared with the plain concrete specimens. As a summary, the bond strength between CFRP and unreinforced concrete specimen decreased as exposure temperature increased. In contrast, the effect of fire on the bond strength of reinforced concrete specimens was minimal. Therefore, the steel reinforcements in concrete structures could minimize the effect of fire exposure on bond strength between fire-damaged concrete and externally bonded FRP system. A possible reason was that the presence of steel reinforcements decreased the propagation of fire-induced cracks in concrete.
Fig. 19. Example bond stress-slip relationships of tested joints. (a) J-RT-400(C). (b) J-500-400. (c) J-700-400(B).
4.3. Failure mode
4.5. Interfacial fracture energy
Fig. 15 shows the failure modes of CFRP-to-concrete bonded joints. The observed debonding failure can be classified into three modes, namely (a) failure along concrete substrate (Mode 1), (b) failure in concrete prism (Mode 2), and (c) concrete cover separation (Mode 3). Tables 1 and 2 indicate the mode of failure of individual specimens for plain and reinforced concrete bonded joints, respectively. The failure mode for all joints with unexposed concrete was Mode 1. A thin layer of concrete substrate beneath bond area peeled off, as shown
The interfacial fracture energy (Gf ) is defined as the area under the bond-slip relationship. Using the CFRP strain data, the bond stress and slip relationship can be constructed as follows,
τi ;iþ1 ¼ Ef tf
10
ðεiþ1 ðxiþ1
εi Þ xi Þ
; i ¼ 1; :::; n
(4)
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Fig. 20. Interfacial fracture energies: bond strength model v.s. test data.
Fig. 21. Relationship between interfacial fracture energy and pull-off tensile strength of concrete. n � 1X si;iþ1 ¼ s lf þ ½ðεiþ1 þ εi Þ:ðxiþ1 2 i¼1
xi Þ�
Fig. 19 shows the bond stress-slip relationships of some tested joints of unexposed, 500oC-exposed, and 700oC-exposed conditions. When the bond length was higher than the effective bond length, the average interfacial fracture energy (Gf,test) was directly computed using the complete curves in the effective bond length zone. The Gf,test values of plain and reinforced concrete specimens are also shown in Tables 1 and 2, respectively. Fig. 20 shows comparison between the interfacial fracture energy values from the bond strength model (Gf,model from Eq. (2)) and test data. The mean and standard deviation of Gf,model to Gf,test ratios were 0.98 and 0.24, respectively. A comparison implies that the back calcu lation based on the fracture mechanics (Eq. (2)) provides a good agreement with the interfacial fracture energy derived from the test using the CFRP strain data.
(5)
where τi ;iþ1 is the average interfacial bond stress between station i and i þ 1 (from the loaded end); εi and εiþ1 are the measured axial strain on the FRP plate at section i and i þ 1, respectively; xi and xiþ1 are the distances along the FRP plate corresponding to strain εi and εiþ1 , respectively. Ef and tf are the elastic modulus and thickness of the FRP plate, respectively. si;iþ1 is the average local slip between FRP plates and concrete between station i and i þ 1 (from the loaded end); lf is total length of the FRP plate; sðlf Þ is the free-end slip of the FRP plate. The value of free-end slip was assumed to be zero in case of long bond lengths. 11
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The effect of fire deteriorated the interfacial fracture energy. The interfacial fracture energy of 500oC-exposed and 700oC-exposed con crete specimens decreased from unexposed specimens by 28% and 71%, respectively. The steel reinforcements restrained the reduction in interfacial fracture energy due to fire damage. Fig. 21 shows the relationship between the interfacial fracture en ergy and pull-off test data. For fire-exposed plain concrete specimens, the interfacial fracture energy decreased as pull-off strength of concrete decreased. A linear regression was performed to obtain the following equation for predicting the interfacial fracture energy using the pull-off test data, � � Gf ¼ 0:181ft þ 0:283 N mm (6)
Golden Jubilee Ph.D. Program (Grant no. PHD/0135/2556). Also, au thors would like to acknowledge Sika (Thailand), Co. Ltd. and Retrofit Structure Specialist, Co. Ltd. for supplying composite materials and assistance on the experimental work. References [1] Pellegrino C, Sena-Cruz J. Design procedures for the use of composites in strengthening of reinforced concrete Structures. RILEM State-of-art reports; 2016. [2] Teng JG, Chen JF, Smith ST, Lam L. FRP-strengthened RC structures. Chichester, West Sussex, UK: Wiley; 2001. [3] ACI 440.2R-17. Guide for the design and construction of externally bonded FRP systems for strengthening concrete structures. 2017. [4] Fib Bulletin No. 46. Fire design of concrete structures - structural behavior and assessment. State-of-art report; 2008. [5] Poon CS, Azhar S, Anson M, Wong YL. Strength and durability recovery of firedamaged concrete after post-fire-curing. Cement Concr Res 2001;31(9):1307–18. [6] Fib Bulletin No. 38. Fire design of concrete structures - materials, structures and modelling. State-of-art report; 2007. [7] Khalaf J, Huang Z, Fan M. Analysis of bond-slip between concrete and steel bar in fire. Comput Struct 2016;162:1–15. [8] Bratina S, Saje M, Planinc I. The effects of different strain contributions on the response of RC beams in fire. Eng Struct 2007;29(3):418–30. [9] Thongchom C, Lenwari A, Aboutaha RS. Effect of sustained service loading on postfire flexural response of reinforced concrete T-beams. ACI Struct J 2019;116(3): 243–54. [10] Limpanainlachat P. Bonding behavior between reinforced concrete after fire and carbon fiber reinforced polymer. MS Thesis. Chulalongkorn University; 2012. [11] ASTM E119-07. Standard test methods for fire tests of building construction and materials. 2007. [12] Haddad RH, Al-Rousan R, Almasry A. Bond-slip behavior between carbon fiber reinforced polymer sheets and heat-damaged concrete. Compos Part B 2013;45(1): 1049–60. [13] Danie Roy AB, Sharma UK, Bhargava P. Bond properties of GFRP laminate with heat-damaged concrete. J Compos Constr 2015;20(2):04015053. [14] T€ aljsten B. Defining anchor lengths of steel and CFRP plates bonded to concrete. Int J Adhesion Adhes 1997;17(4):319–27. [15] Diab HM, Farghal OA. Bond strength and effective bond length of FRP sheets/ plates bonded to concrete considering the type of adhesive layer. Compos B Eng 2014;58:618–24. [16] Fib Bulletin 14. Externally bonded FRP reinforcement for RC structures: technical report on the design and use of externally bonded fibre reinforced polymer reinforcement (FRP EBR) for reinforced concrete structures. 2001. [17] Neubauer U, Rostasy FS. Design aspects of concrete structures strengthened with externally bonded CFRP-plates. In: Forde MC, editor. Proceedings of the seventh international conference on structural faults and repairs. Edinburgh, UK: Engineering Technics Press; 1997. p. 109–18. [18] Holzenk€ ampfer O. Ingenieurmodelle des verbunds geklebter bewehrung für betonbauteile, Dissertation. TU Braunschweig; 1994. [19] Chen JF, Teng JG. Anchorage strength models for FRP and steel plates bonded to concrete. J Struct Eng 2001;127(7):784–91. [20] ASTM A370-19. Standard test methods and definitions for mechanical testing of steel products. 2019. [21] ASTM D3039/D3039M-17. Standard test method for tensile properties of polymer matrix composite materials. 2017. [22] SIKA product data sheet. Sikadur®-30. Thixotropic epoxy adhesive for bonding reinforcement. 2017. January 2017, Version 02.02. [23] May GC, Hutchinson AR. Adhesives in civil engineering. Cambridge University Press; 1992. [24] Ba G, Miao J, Zhang W, Liu C. Influence of cracking on heat propagation in reinforced concrete structures. J Struct Eng 2016;142(7):04016035. [25] ISO 834-1. Fire resistance tests-elements of building construction. Part 1: general requirement. Geneva, Switzerland: International Organization for Standardization; 1999. [26] CEN. Products and systems for the protection and repair of concrete structures – test methods – measurement of bond strength by pull–off. CSN EN 1542, Brussels (Belgium). European Committee for Standardization; 1999. [27] Gravina RJ, Aydin H, Visintin P. Extraction and analysis of bond-slip characteristics in deteriorated FRP-to-concrete joints using a mechanics-based approach. J Mater Civ Eng 2017;29(6):04017013. [28] Bizindavyi L, Neale K. Transfer lengths and bond strengths for composites bonded to concrete. J Compos Constr 1999;3(4):153–60.
when ft is the pull-off tensile strength of concrete (MPa). 5. Conclusions In this research, the effects of fire exposure (elevated temperature of 500 � C, 700 � C, or 900 � C for 3 h) on bond properties between the CFRP plate and concrete (with and without embedded steel reinforcements) were investigated. The linear elastic fracture mechanics-based model was adopted for strength prediction of the bonded joints. The main conclusions are as follows. o The bond properties between the CFRP plate and fire-exposed con crete deteriorated due to fire exposure. As exposure temperature increased, the bond strength decreased and the effective bond length required by the CFRP plate increased. o The pull-off testing showed that the elevated temperatures decreased the tensile strength of concrete substrate. After exposure to 500 � C and above, the tensile strength of concrete substrate decreased below the minimum value recommended for external FRP strengthening. o In reinforced concrete structures, the steel reinforcements are beneficial. They minimize the effect of fire exposure on the bond strength and effective bond length. o The debonding along concrete substrate (Mode 1) was a dominant failure mode when the CFRP plate was bonded to the unexposed concrete, This failure mode involved the peel of a thin layer of concrete substrate beneath bond area. o When the CFRP plate was bonded to the fire-exposed concrete, Mode 1 occurred if the CFRP plate was longer than the effective bond length. However, a thicker layer of peeled-off concrete substrate than unexposed specimens was observed. o The presence of steel reinforcements can lead to a concrete cover separation failure along steel reinforcements (Mode 3) when the effect of fire significantly deteriorates the bond between steel and surrounding concrete. o A fracture mechanics based-model can be used for strength predic tion of bond between fire-exposed concrete and the CFRP plate. A good correlation between interfacial fracture energy and pull-off tensile strength was obtained in case of unreinforced concrete. Acknowledgements This research was supported by the Thailand Research Fund (TRF) and Chulalongkorn University under the TRF Research Career Devel opment Grant (Grant no. RSA6080019). The first author would like to acknowledge the financial support from the TRF through the Royal
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Contents lists available at ScienceDirect
International Journal of Adhesion and Adhesives journal homepage: http://www.elsevier.com/locate/ijadhadh
Bond properties between carbon fibre-reinforced polymer plate and fire-damaged concrete Chanachai Thongchom a, b, Akhrawat Lenwari a, *, Riyad S. Aboutaha c a
Composite Structures Research Unit, Department of Civil Engineering, Faculty of Engineering, Chulalongkorn University, Pathumwan, Bangkok, 10330, Thailand Department of Civil Engineering, Thammasat School of Engineering, Thammasat University, Pathumtani, 12120, Thailand c Department of Civil and Environmental Engineering, College of Engineering and Computer Science, Syracuse University, New York, 13244, USA b
A R T I C L E I N F O
A B S T R A C T
Keywords: Concrete Fire damage Strengthening Carbon fibre-reinforced polymer Bond Single-shear test
In this research, a single-shear test was conducted to investigate the bond properties between carbon fibrereinforced polymer (CFRP) plates and fire-damaged concrete prisms. The investigation focused on the effects of fire exposure condition and the presence of internal steel reinforcements on the effective bond length, failure mode, bond strength (maximum joint load) and interfacial fracture energy. After being air-cooled, the concrete prisms were adhesively bonded with CFRP strips of different lengths, and then quasi-static tested at the ambient condition. The pull-off test results showed that the tensile strength of concrete substrate decreased after being exposed to elevated temperatures. Such deterioration of concrete substrate caused an increase in the effective bond length. It also decreased the bond strength and interfacial fracture energy. The steel reinforcements in concrete structures are beneficial as they minimize the effect of fire exposure on bond strength and effective bond length. An application of the fracture mechanics-based model for strength prediction of bond between firedamaged concrete and the CFRP plate is proposed. A linear relationship between interfacial fracture energy and pull-off tensile strength of concrete could be assumed for the unreinforced concrete.
1. Introduction The use of externally bonded fibre-reinforced polymer (FRP) lami nates to strengthen deficient RC structures has been popular worldwide. The bond between FRP and existing concrete structures is critical for the effectiveness of the strengthening scheme, especially for flexural and shear strengthening. The soundness and tensile strength of the concrete substrate can limit the overall effectiveness of the externally bonded FRP system. In practice, the pull-off test is used for quality control to ensure the suitability and condition of the concrete surface after the surface preparation. Epoxy resins commonly used to bond the FRP and concrete are so strong that the debonding failures generally occur in the concrete, either well into the concrete or in the concrete adjacent to the adhesive/ concrete interface [1]. As concrete is the weakest link in the con crete/FRP system, all design recommendations on the debonding fail ures of FRP-strengthened concrete structures assume that the interfacial strength is governed by the concrete strength rather than the adhesive strength [2]. Currently, the strengthening design guidelines such as ACI 440.2R-17 [3] “Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures” recommends that
pull-off adhesion test be conducted to determine the tensile strength of concrete on surfaces where the FRP system will be installed. Tension adhesion strengths should exceed 200 psi (1.4 MPa) and should exhibit failure of the concrete substrate. However, the current design guidelines do not provide specific provisions for strengthening of fire-damaged RC members. Concrete is known as the material that exhibits a good behavior at the elevated temperatures due to its incombustible and low thermal diffusivity properties. However, the fire exposure is detrimental to concrete strength. While concrete is heated, an irreversible damage occurs as a result of chemo-physical transformations of aggregates, cement paste, and aggregate/paste interface [4]. Such damage includes micro cracking caused by thermal expansion mismatch between hard ened cement paste and aggregates, decomposition of aggregates of limestones and dolomites into lime (Cao) [4], and dissociation of cal cium hydroxide and destruction of calcium-silicate-hydrate gel in the cement paste [5]. While concrete is cooled down to the ambient con dition, further potential damage includes crack development, moisture absorption, and rehydration of the decomposed aggregates [6]. In reinforced concrete (RC) structures, the fire exposure also has a negative
* Corresponding author. E-mail address: [email protected] (A. Lenwari). https://doi.org/10.1016/j.ijadhadh.2019.102485 Available online 9 November 2019 0143-7496/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Chanachai Thongchom, International Journal of Adhesion & Adhesives, https://doi.org/10.1016/j.ijadhadh.2019.102485
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behavior between fire-damaged concrete and CFRP plate (CFRP-toconcrete bonded joints). Test variables are the elevated temperatures, amount of internal steel reinforcements, and CFRP plate length. The concrete prisms were exposed to fire (500 � C, 700 � C or 900 � C) for 3 h. After air-cooled, the concrete prisms were adhesively bonded with CFRP plate. The CFRP-to-concrete bonded joints were then statically tested at ambient condition. A single shear test was chosen. The studied bond properties include the effective bond length, failure mode, bond strength (maximum load of the bonded joint), and interfacial fracture energy. A fracture mechanics based-model was adopted for strength prediction of bond between fire-exposed concrete and CFRP plate. A correlation be tween interfacial fracture energy and pull-off tensile strength of concrete was studied. 2. Debonding analysis of FRP-to-concrete bonded joints Based on the linear elastic fracture mechanics, the bond strength (Pmax ) of FRP-to-concrete joints can be predicted by [14], pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Pmax ¼ bf 2Ef tf Gf ðNÞ (1)
Fig. 1. Details of unreinforced and reinforced concrete prisms (dimensions in mm). (a) Plain concrete joint (J). (b) Reinforced concrete joint (RJ).
where bf and tf are width and thickness of the FRP plate (mm); Ef is the elastic modulus of FRP; Gf is the interfacial fracture energy (N/mm). In Eq. (1), the FRP plate should be longer than the effective bond length (Le ). Using Eq. (1), the interfacial fracture energy can be determined from the test data of specimens with sufficient bond length as follows [15], Gf ¼
P2max 2b2f Ef tf
(2)
Fib14 [16] adopted the results from Neubauer and Rost� asy (1997) [17]. The effective bond length in terms of the pull-off tensile strength of concrete substrate (ft) can be determined from, sffiffiffiffiffiffiffiffi ! Ef tf mm (3) Le;FIB ¼ 2ft
Fig. 2. Concrete prisms inside fire test furnace.
Eq. (3) was derived from the results of a double shear test on 51 CFRP-to-concrete bonded joints [17,18]. The equation provides a good prediction for joints with concrete having the compressive and tensile strengths of 42.5 and 3.5 MPa, respectively, and FRP having stiffness (Ef tf ) from 25 to 58 N/m [19].
effect on the bond between steel reinforcements and surrounding con crete. A significant reduction in bond strength can occur when tem perature at the steel reinforcement is higher than 500 � C [7–9]. Past experimental studies on the bond between FRP and firedamaged concrete showed that the elevated temperatures deteriorate the bond properties. Limpaninlachat (2012) [10] investigated the bond behavior between fire-damaged reinforced concrete and CFRP plates. Test variables included the CFRP plate length and thickness of concrete cover over steel reinforcements. The reinforced concrete was exposed to ASTM E119 standard fire [11] for 45 or 90 min. The modified beam test was carried out after the fire exposure. The test results indicated a reduction in bond strength and interfacial fracture energy after concrete was exposed to fire. The effect of concrete cover thickness on the post-fire bond strength was minimal. Haddad et al. (2013) [12] exper imentally investigated the bond-slip behavior between fire-damaged concrete and CFRP sheets. Test variables included the concrete strength and CFRP-to-concrete width ratio. Concrete prisms were exposed to 300, 400, 500, and 600 � C for 2 h. A double-shear test was conducted. The test results showed that the effect of fire deteriorated the bond strength and increased the slip at failure when the exposure tem perature was higher than 400 � C. Danie Roy et al. (2015) [13] investi gated the bond behavior between fire-damaged concrete and GFRP laminates. Test variables included the exposure temperature (200, 400, 600, and 800 � C for 3 h) and GFRP length (100, 150, and 200 mm). A single-shear test was conducted. The test results showed that the bond strength decreased significantly when concrete was exposed to tem peratures above 400 � C. Due to limited previous studies, this research investigates the bond
3. Experimental program on bonded joints 3.1. Test variables In the experimental program, a total of 30 concrete prisms (150 � 150 � 500 mm) were constructed to investigate the effects of steel reinforcements (J ¼ plain concrete and RJ ¼ reinforced concrete), fire exposure temperature (RT ¼ room temperature, 500, 700, and 900 � C) and bond length (Lb ¼ 100, 200, 300, and 400 mm) on the bond properties between the CFRP plate and concrete. Fig. 1 shows the details of the unreinforced and reinforced concrete prisms. Four 12-mm-diam eter deformed bars (DB12) were used as the longitudinal re inforcements. The transverse reinforcements (RB9) had a constant spacing of 75 mm. The concrete cover thickness was 25 mm. All concrete prisms were cast from the same ready-mixed concrete batch. After 24 h, the concrete prisms were demolded and cured with plastic sheeting at the ambient temperature for 28 days. 3.2. Material properties 3.2.1. Concrete The concrete consisted of Portland cement Type I, river sand (fine aggregate), and natural rock (coarse aggregate) which the maximum 2
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Fig. 3. Schematic fire exposure setup (Dimensions in mm). (a) Cross-sectional elevation view. (b) Plan view.
Fig. 4. Average temperatures inside fire test furnace for 500 � C, 700 � C, and 900 � C exposure conditions. 3
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Fig. 5. Pull-off testing on concrete prisms. (a) Installation of dolly. (b) Tension test apparatus.
Fig. 6. Procedure for installation of CFRP plate to concrete prism.
size was 19 mm. A mix proportion of the ready-mixed concrete consisted of cement (366 kg/m3), water (160 kg/m3), fine aggregate (750 kg/m3), and coarse aggregate (1150 kg/m3). The water to cement ratio was 0.44. The compressive strength of three standard concrete cylinders (∅150 � 300 mm) was 43 � 0.6 MPa (average � standard deviation) at 28 days. 3.2.2. Steel reinforcements A tension test on steel reinforcements was conducted in accordance with the standard test method ASTM A370 [20]. The measured yield, ultimate strengths, and elastic modulus of DB12 were 532, 640, and 200417 MPa, respectively. The measured yield, ultimate strengths, and elastic modulus of RB9 were 346, 550, and 194700 MPa, respectively. 3.2.3. CFRP plate and bonding agent A unidirectional CFRP plate, Sika Carbodur® S512, was used in the study. The plate had thickness and width of 1.2 and 50 mm, respectively. A tension test on CFRP plates was conducted in accordance with the standard test method ASTM D3039 [21]. The measured elastic modulus, tensile strength, and strain at break were 181 GPa, 3303 MPa, and 1.7%, respectively. The adhesive used for bonding the CFRP plates to concrete prisms was Sikadur®-30. According to the manufacturer, it is a thixotropic, structural two-component adhesive, based on a combination of epoxy resin and special filler, for bonding Sika Carbodur® plates to concrete, brickwork and timber (SIKA product data sheet [22]). This adhesive can be classified as a cold-cure adhesive, which is defined as one which is
Fig. 7. Concrete surface after mechanical grinding.
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Fig. 8. Schematic single-shear test setup for bonded joints.
Table 1 Summary of test results for plain concrete specimens. Plain concrete specimen
Fire exposure condition
Residual tensile strength from Pull-off test (MPa)
Pmax,test (kN)
Le,test (mm)
Gf,test (N/mm) Mean � SD
Gf,model (N/ mm)
Mode of failurec
J-RT-100 J-RT-200 J-RT-300 J-RT-400(A) J-RT-400(B) J-RT-400(C) J-500-100 J-500-200 J-500-300(A) J-500-300(B) J-500-400 J-700-100 J-700-200 J-700-300 J-700-400(A) J-700-400(B) J-900-100 J-900-200 J-900-300 J-900-400(A) J-900-400(B)
No exposure
N/Aa N/Aa 2.9 N/Aa 4.2 6.2 2.2 0.8 1.3 1.5 1.4 N/Aa N/Aa 0.5 0.3 0.4 Severely damaged
18.7 27.2 24.4 27.0 32.3 37.6 9.6 18.4 17.8 22.8 28.4 4.8 3.4 10.2 17.2 20.3
>100 120 145 150 150 175 >100 >200 >300 >300 320 >100 >200 >300 340 360
N/Ab 0.775 � 0.279 0.784 � 0.491 0.717 � 0.295 1.093 � 0.751 1.338 � 0.700 N/Ab N/Ab N/Ab N/Ab 0.677 � 0.409 N/Ab N/Ab N/Ab 0.317 � 0.058 0.223
N/Ab 0.680 0.548 0.671 0.964 1.305 N/Ab N/Ab N/Ab N/Ab 0.742 N/Ab N/Ab N/Ab 0.273 0.379
1 1 1 1 1 1 2 2 2 2 1 2 2 2 2 2
500 � C (3 h)
700 � C (3 h)
900 � C (3 h)
Remark. a No measurement. b bond length is lower than effective bond length. c Mode 1 ¼ debonding at concrete substrate; Mode 2 ¼ concrete prism failure.
capable of curing to the required strength between the temperatures of 10 � C and 30 � C [23]. Two components include the resin (component A: white color) and hardener (component B: black color). The mixing ratio of component A to B was 3:1 by volume. According to the manufacturer, the minimum compressive, shear, and tensile strengths of the hardened adhesive at 7 days (curing temperature of 35 � C) are 90, 18, and 29 MPa, respectively. The reported coefficient of thermal expansion and glass transition temperature were 2.5 � 10 5/ � C (temperature ranges from 20 � C to 40 � C) and 52 � C, respectively.
3.3. Fire exposure After 28 days, the concrete prisms were placed inside the fire test furnace, as shown in Fig. 2. Fig. 3 shows the schematic fire exposure setup of prisms inside the fire test furnace. The furnace had internal dimensions of 900 � 2500 � 1700 mm. The furnace was equipped with six LPG-fueled burners at two levels above the furnace floor. The average furnace temperature was obtained from six thermocouples installed alongside the burners. The top surface of each concrete prism was also 5
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Table 2 Summary of test results for reinforced concrete specimens. Reinforced concrete specimen
Fire exposure condition
Residual tensile strength from Pull-off test (MPa)
Pmax,test (kN)
Le,test (mm)
Gf,test (N/mm) Mean � SD
Gf,model (N/ mm)
Mode of failurec
RJ-RT-400(A) RJ-RT-400(B) RJ-500-400(A) RJ-500-400(B) RJ-700-300 RJ-700-400(A) RJ-700-400(B) RJ-900-400(A) RJ-900-400(B)
No exposure
3.7 N/Aa 0.5 1.4 N/Aa 1.0 0.7 Severely damaged
32.6 32.4 34.6 35.2 26.6 35.7 32.9
160 120 280 300 >300 310 340
1.060 � 0.544 0.928 � 0.361 1.089 � 0.732 0.963 � 0.593 N/Ab 1.389 � 0.307 1.376 � 0.090
0.981 0.968 1.101 1.144 N/Ab 1.174 0.997
1 1 1 1 3 3 3
500 � C (3 h) 700 � C (3 h) 900 � C (3 h)
Remark. a No measurement. b bond length is lower than effective bond length. c Mode 1 ¼ debonding at concrete substrate; Mode 3 ¼ debonding at concrete substrate and concrete cover separation.
directly exposed to fire. This simulated the condition when cracking of concrete due to the service load influences the heat propagation in fireexposed RC structures [24]. Fig. 4 shows the average temperatures inside fire test furnace for 500 � C, 700 � C, and 900 � C exposure conditions. The average furnace temperature was initially controlled to follow the ISO 834 standard fire curve [25]. Once the target temperature of 500, 700 or 900 � C was reached, the average temperature was maintained for a heating period of 3 h. After heating, the concrete prisms were left inside the furnace to allow natural air cooling to the ambient temperature.
probe, along with a bonded mass of concrete, by applying a direct ten sion force. 3.5. CFRP installation procedure Fig. 6 shows the procedure for installing the CFRP plate to the con crete prism. The concrete substrate was first prepared by manual grinding until the aggregates became clearly exposed. The grinding method removed loose or unsound materials and minimized the local ized out-of-plane variations. Then, the surface was cleaned by air vac uuming to remove any dust. All concrete surfaces were dry before the installation of FRP plates. Fig. 7 shows the concrete surface after the grinding. Finally, the CFRP plate was bonded to the concrete prism using an adhesive. The adhesive layer thickness was 2 mm. A 50-mm unbon ded length (without an adhesive) was provided near the loaded end of the CFRP plate to simulate the interfacial crack. Also, it could prevent the undesired wedge/concrete splitting failures [27]. After the instal lation, the bonded joints were left for one week to allow curing of the adhesive before a single-shear test.
3.4. Pull-off testing To understand the effect of fire on the residual tensile strength of concrete substrate before CFRP installation, the pull-off test was con ducted in accordance with the European Standard EN 1542 [26]. The testing was conducted on the concrete substrate on one side of the prism at mid-height. The CFRP plate was installed on a different side of the prism. Fig. 5 shows a circular steel probe that is bonded to the concrete surface and a specially designed portable apparatus used to pull off the
3.6. Static test setup and instrumentation Figs. 6(e) and 8 show the single-shear test setup for the bonded joints. Near the loaded end of the CFRP plate, strain gages were instrumented along the CFRP plate with a spacing of 20 mm. Close to the free end of the CFRP plate, the spacing between gages was double (40 mm). The steel grips were used to clamp the CFRP plate in order to apply the tension force from the testing machine. The specimen was restrained against the vertical movement by the steel support at the
Fig. 9. Visible damage of concrete prisms after exposure to fire.
Fig. 10. Failure of concrete substrate in pull-off testing. 6
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Fig. 11. Effect of fire exposure on pull-off strength of concrete.
Fig. 12. Determination of effective bond length from CFRP strain distributions [specimen J-RT-400(C)].
loaded end. The crosshead speed of the testing machine was 1 mm/min until failure. During the static test, the applied load and CFRP strain data were recorded with the data acquisition system.
4.1. Pull-off (tensile) strength of concrete substrate Fig. 9 shows the visible damage of concrete prism after exposure to fire. After exposure to 500 and 700 � C, the occurrence of thermal cracks was observed on the surface. The colors of concrete were whitish grey and pink for 500oC-exposed and 700oC-exposed concrete prisms, respectively. A severe spalling of concrete occurred in concrete prisms exposed to 900 � C. The aggregate color was changed to white. All plain concrete prisms failed inside the furnace and cannot be bonded with the CFRP plate. Fig. 10 shows the concrete substrate failure in the pull-off testing. It was observed that the thicknesses of pull-offed concrete layers were
4. Experimental results and discussion Tables 1 and 2 summarize the test results for plain concrete and reinforced concrete specimens, respectively. The test results include the residual tensile strength obtained from pull-off testing, bond strength or maximum load (Pmax,test), effective bond length (Le,test), interfacial fracture energy (Gf,test), and mode of failure. Effects of fire damage on the bond properties are discussed in this section.
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Fig. 13. Effect of exposure temperature on effective bond length.
Fig. 14. Prediction of effective bond length by pull-off strength of concrete.
similar regardless of the level of exposure temperature. Fig. 11 shows the effect of fire on the residual pull-off strength of concrete. The pull-off strength was not significantly affected by the presence of steel re inforcements. The pull-off strength of unexposed concrete prisms was 4.2 � 1.4 (average � standard deviation) MPa, which was about 10% of the compressive strength of concrete cylinders (43 MPa). After exposure to 500 � C and 700 � C for 3 h, the pull-off strength reduced to 1.3 � 0.5, and 0.6 � 0.3 MPa, respectively. No data could be obtained from all 900oC-exposed concrete prisms due to excessive spalling. Therefore, the pull-off strength of most fire-damaged concrete prisms were below the recommended value of 1.5 MPa (Fib14 [16]) or 1.4 MPa (ACI 440.2R [3]) for the application of externally bonded FRP reinforcement to RC structures.
4.2. Effective bond length The concept of effective bond length for the externally bonded FRP laminates is that no further load increases when the bond length is longer than the effective bond length [19]. According to previous research works [15,27,28], the effective bond length can be determined from the CFRP strain data. In this study, the effective bond length is defined as the distance from the loaded end position to the position where CFRP strain vanishes. Fig. 12 shows typical strain distributions along CFRP plate from station at the loaded end (x ¼ 0 m) to free end at various load levels (20%, 40%, 60%, 80%, and 100% of ultimate load) until failure. A high strain value was observed near the loaded end location. When load increased, the strain value at each location 8
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Fig. 17. Close-up view of aggregate and aggregate/paste bond failures in concrete substrate [specimen J-500-400].
unexposed, 500oC-exposed, and 700oC-exposed specimens were 161, 289, and 438 mm, respectively. The prediction tends to overestimate the effective bond length. The ratio between predicted and measured effective bond length of unexposed, 500oC-exposed, and 700oC-exposed concrete joints ranges from 0.92 to 1.34 (with an average value of 1.13), 0.90 to 1.03 (with an average value of 0.97), and 1.22 to 1.41 (with an average value of 1.30), respectively.
Fig. 15. –Three failure modes of CFRP-to-concrete bonded joints. (a) Plain concrete joint (J). (b) Reinforced concrete joint (RJ).
increased. At high load levels, the CFRP strain distributions became constant near the loaded end due to the occurrence of crack or debonding. Fig. 13 shows the effect of fire on the effective bond length. The effective bond length required by the CFRP plate increased when exposure temperature increased. The presence of steel reinforcements decreased the effective bond length. The average effective bond lengths of unexposed plain and reinforced concrete specimens were 148 and 140 mm, respectively. The average effective bond lengths of 500oCexposed plain and reinforced concrete were 320 and 280 mm, respec tively. The average effective bond lengths of 700oC-exposed plain and reinforced concrete were 350 and 325 mm, respectively. Fig. 14 shows comparison between the effective bond lengths pre dicted with Eq. (3) (using the pull-off strength of concrete) and the test data. The average values for predicted effective bond lengths of
Fig. 18. Effects of test variables on strength of CFRP-concrete bonded joints.
Fig. 16. Typical failures of bonded joints. 9
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in Fig. 16(a). The debonding started at the loaded end of the CFRP plate and then propagated towards the plate end. The presence of steel rein forcement has no effect on the failure mode. For 500 � C and 700 � C exposure conditions, Mode 2 governed the failure of plain concrete joints with short bond lengths, i.e., less than the effective bond length. The cracking occurred in the concrete prism at the termination of CFRP plate and then propagated towards the support, as shown in Fig. 16(b). As bond length increased beyond the effective bond length, the failure mode tended to be Mode 1. A layer of peeled-off concrete substrate was thicker than the case of unexposed specimens. Fig. 17 shows the close-up view of concrete substrate failure. A combi nation of aggregate and aggregate/paste bond failures was observed. For all 700oC-exposed unreinforced specimens, the failure mode was Mode 2. Possibly, the effective bond length, which increased due to fire damage, became close to the maximum 400-mm bond length used in the study. Mode 3 governed the failure of all 700oC-exposed steel-reinforced specimens. The debonding was observed as concrete cover separation along the steel reinforcements, as shown in Fig. 16(c). This failure could be attributed from the deterioration in bonding between steel and sur rounding concrete at steel temperature above 500 � C. 4.4. Bond strength Fig. 18 shows the effects of fire exposure and steel reinforcements on the strength of CFRP-to-concrete bonded joints. For unexposed speci mens, the bond strength increased as CFRP bond length increased. Beyond 200 mm, however, the strength did not significantly increase with the bond length. This implies that the effective bond length was about 200 mm for unexposed joints. The CFRP bond length of 400 mm was used to investigate the effect of steel reinforcement (plain and reinforced concrete specimens). For unexposed specimens, no signifi cant difference between the bond strength of plain concrete and RC joints was observed. Similarly, for 500oC-exposed specimens, the bond strength increased when CFRP bond length increased. The bond strengths of fire-exposed plain concrete specimens with CFRP plate length of 100, 200, 300, and 400 mm decreased from the unexposed specimens by 49%, 32%, 17%, and 12%, respectively. At 400-mm bond length, the presence of steel reinforcements enhanced the bond strength by 23% compared with the plain concrete specimens. For 700oC-exposed joints, the bond strengths of plain concrete specimens with CFRP plate length of 100, 200, 300, and 400 mm decreased from the unexposed specimens by 75%, 87%, 58%, and 42%, respectively. At 300-mm and 400-mm bond length, the presence of steel reinforcements enhanced the bond strength by 162% and 83%, respec tively, compared with the plain concrete specimens. As a summary, the bond strength between CFRP and unreinforced concrete specimen decreased as exposure temperature increased. In contrast, the effect of fire on the bond strength of reinforced concrete specimens was minimal. Therefore, the steel reinforcements in concrete structures could minimize the effect of fire exposure on bond strength between fire-damaged concrete and externally bonded FRP system. A possible reason was that the presence of steel reinforcements decreased the propagation of fire-induced cracks in concrete.
Fig. 19. Example bond stress-slip relationships of tested joints. (a) J-RT-400(C). (b) J-500-400. (c) J-700-400(B).
4.3. Failure mode
4.5. Interfacial fracture energy
Fig. 15 shows the failure modes of CFRP-to-concrete bonded joints. The observed debonding failure can be classified into three modes, namely (a) failure along concrete substrate (Mode 1), (b) failure in concrete prism (Mode 2), and (c) concrete cover separation (Mode 3). Tables 1 and 2 indicate the mode of failure of individual specimens for plain and reinforced concrete bonded joints, respectively. The failure mode for all joints with unexposed concrete was Mode 1. A thin layer of concrete substrate beneath bond area peeled off, as shown
The interfacial fracture energy (Gf ) is defined as the area under the bond-slip relationship. Using the CFRP strain data, the bond stress and slip relationship can be constructed as follows,
τi ;iþ1 ¼ Ef tf
10
ðεiþ1 ðxiþ1
εi Þ xi Þ
; i ¼ 1; :::; n
(4)
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Fig. 20. Interfacial fracture energies: bond strength model v.s. test data.
Fig. 21. Relationship between interfacial fracture energy and pull-off tensile strength of concrete. n � 1X si;iþ1 ¼ s lf þ ½ðεiþ1 þ εi Þ:ðxiþ1 2 i¼1
xi Þ�
Fig. 19 shows the bond stress-slip relationships of some tested joints of unexposed, 500oC-exposed, and 700oC-exposed conditions. When the bond length was higher than the effective bond length, the average interfacial fracture energy (Gf,test) was directly computed using the complete curves in the effective bond length zone. The Gf,test values of plain and reinforced concrete specimens are also shown in Tables 1 and 2, respectively. Fig. 20 shows comparison between the interfacial fracture energy values from the bond strength model (Gf,model from Eq. (2)) and test data. The mean and standard deviation of Gf,model to Gf,test ratios were 0.98 and 0.24, respectively. A comparison implies that the back calcu lation based on the fracture mechanics (Eq. (2)) provides a good agreement with the interfacial fracture energy derived from the test using the CFRP strain data.
(5)
where τi ;iþ1 is the average interfacial bond stress between station i and i þ 1 (from the loaded end); εi and εiþ1 are the measured axial strain on the FRP plate at section i and i þ 1, respectively; xi and xiþ1 are the distances along the FRP plate corresponding to strain εi and εiþ1 , respectively. Ef and tf are the elastic modulus and thickness of the FRP plate, respectively. si;iþ1 is the average local slip between FRP plates and concrete between station i and i þ 1 (from the loaded end); lf is total length of the FRP plate; sðlf Þ is the free-end slip of the FRP plate. The value of free-end slip was assumed to be zero in case of long bond lengths. 11
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The effect of fire deteriorated the interfacial fracture energy. The interfacial fracture energy of 500oC-exposed and 700oC-exposed con crete specimens decreased from unexposed specimens by 28% and 71%, respectively. The steel reinforcements restrained the reduction in interfacial fracture energy due to fire damage. Fig. 21 shows the relationship between the interfacial fracture en ergy and pull-off test data. For fire-exposed plain concrete specimens, the interfacial fracture energy decreased as pull-off strength of concrete decreased. A linear regression was performed to obtain the following equation for predicting the interfacial fracture energy using the pull-off test data, � � Gf ¼ 0:181ft þ 0:283 N mm (6)
Golden Jubilee Ph.D. Program (Grant no. PHD/0135/2556). Also, au thors would like to acknowledge Sika (Thailand), Co. Ltd. and Retrofit Structure Specialist, Co. Ltd. for supplying composite materials and assistance on the experimental work. References [1] Pellegrino C, Sena-Cruz J. Design procedures for the use of composites in strengthening of reinforced concrete Structures. RILEM State-of-art reports; 2016. [2] Teng JG, Chen JF, Smith ST, Lam L. FRP-strengthened RC structures. Chichester, West Sussex, UK: Wiley; 2001. [3] ACI 440.2R-17. Guide for the design and construction of externally bonded FRP systems for strengthening concrete structures. 2017. [4] Fib Bulletin No. 46. Fire design of concrete structures - structural behavior and assessment. State-of-art report; 2008. [5] Poon CS, Azhar S, Anson M, Wong YL. Strength and durability recovery of firedamaged concrete after post-fire-curing. Cement Concr Res 2001;31(9):1307–18. [6] Fib Bulletin No. 38. Fire design of concrete structures - materials, structures and modelling. State-of-art report; 2007. [7] Khalaf J, Huang Z, Fan M. Analysis of bond-slip between concrete and steel bar in fire. Comput Struct 2016;162:1–15. [8] Bratina S, Saje M, Planinc I. The effects of different strain contributions on the response of RC beams in fire. Eng Struct 2007;29(3):418–30. [9] Thongchom C, Lenwari A, Aboutaha RS. Effect of sustained service loading on postfire flexural response of reinforced concrete T-beams. ACI Struct J 2019;116(3): 243–54. [10] Limpanainlachat P. Bonding behavior between reinforced concrete after fire and carbon fiber reinforced polymer. MS Thesis. Chulalongkorn University; 2012. [11] ASTM E119-07. Standard test methods for fire tests of building construction and materials. 2007. [12] Haddad RH, Al-Rousan R, Almasry A. Bond-slip behavior between carbon fiber reinforced polymer sheets and heat-damaged concrete. Compos Part B 2013;45(1): 1049–60. [13] Danie Roy AB, Sharma UK, Bhargava P. Bond properties of GFRP laminate with heat-damaged concrete. J Compos Constr 2015;20(2):04015053. [14] T€ aljsten B. Defining anchor lengths of steel and CFRP plates bonded to concrete. Int J Adhesion Adhes 1997;17(4):319–27. [15] Diab HM, Farghal OA. Bond strength and effective bond length of FRP sheets/ plates bonded to concrete considering the type of adhesive layer. Compos B Eng 2014;58:618–24. [16] Fib Bulletin 14. Externally bonded FRP reinforcement for RC structures: technical report on the design and use of externally bonded fibre reinforced polymer reinforcement (FRP EBR) for reinforced concrete structures. 2001. [17] Neubauer U, Rostasy FS. Design aspects of concrete structures strengthened with externally bonded CFRP-plates. In: Forde MC, editor. Proceedings of the seventh international conference on structural faults and repairs. Edinburgh, UK: Engineering Technics Press; 1997. p. 109–18. [18] Holzenk€ ampfer O. Ingenieurmodelle des verbunds geklebter bewehrung für betonbauteile, Dissertation. TU Braunschweig; 1994. [19] Chen JF, Teng JG. Anchorage strength models for FRP and steel plates bonded to concrete. J Struct Eng 2001;127(7):784–91. [20] ASTM A370-19. Standard test methods and definitions for mechanical testing of steel products. 2019. [21] ASTM D3039/D3039M-17. Standard test method for tensile properties of polymer matrix composite materials. 2017. [22] SIKA product data sheet. Sikadur®-30. Thixotropic epoxy adhesive for bonding reinforcement. 2017. January 2017, Version 02.02. [23] May GC, Hutchinson AR. Adhesives in civil engineering. Cambridge University Press; 1992. [24] Ba G, Miao J, Zhang W, Liu C. Influence of cracking on heat propagation in reinforced concrete structures. J Struct Eng 2016;142(7):04016035. [25] ISO 834-1. Fire resistance tests-elements of building construction. Part 1: general requirement. Geneva, Switzerland: International Organization for Standardization; 1999. [26] CEN. Products and systems for the protection and repair of concrete structures – test methods – measurement of bond strength by pull–off. CSN EN 1542, Brussels (Belgium). European Committee for Standardization; 1999. [27] Gravina RJ, Aydin H, Visintin P. Extraction and analysis of bond-slip characteristics in deteriorated FRP-to-concrete joints using a mechanics-based approach. J Mater Civ Eng 2017;29(6):04017013. [28] Bizindavyi L, Neale K. Transfer lengths and bond strengths for composites bonded to concrete. J Compos Constr 1999;3(4):153–60.
when ft is the pull-off tensile strength of concrete (MPa). 5. Conclusions In this research, the effects of fire exposure (elevated temperature of 500 � C, 700 � C, or 900 � C for 3 h) on bond properties between the CFRP plate and concrete (with and without embedded steel reinforcements) were investigated. The linear elastic fracture mechanics-based model was adopted for strength prediction of the bonded joints. The main conclusions are as follows. o The bond properties between the CFRP plate and fire-exposed con crete deteriorated due to fire exposure. As exposure temperature increased, the bond strength decreased and the effective bond length required by the CFRP plate increased. o The pull-off testing showed that the elevated temperatures decreased the tensile strength of concrete substrate. After exposure to 500 � C and above, the tensile strength of concrete substrate decreased below the minimum value recommended for external FRP strengthening. o In reinforced concrete structures, the steel reinforcements are beneficial. They minimize the effect of fire exposure on the bond strength and effective bond length. o The debonding along concrete substrate (Mode 1) was a dominant failure mode when the CFRP plate was bonded to the unexposed concrete, This failure mode involved the peel of a thin layer of concrete substrate beneath bond area. o When the CFRP plate was bonded to the fire-exposed concrete, Mode 1 occurred if the CFRP plate was longer than the effective bond length. However, a thicker layer of peeled-off concrete substrate than unexposed specimens was observed. o The presence of steel reinforcements can lead to a concrete cover separation failure along steel reinforcements (Mode 3) when the effect of fire significantly deteriorates the bond between steel and surrounding concrete. o A fracture mechanics based-model can be used for strength predic tion of bond between fire-exposed concrete and the CFRP plate. A good correlation between interfacial fracture energy and pull-off tensile strength was obtained in case of unreinforced concrete. Acknowledgements This research was supported by the Thailand Research Fund (TRF) and Chulalongkorn University under the TRF Research Career Devel opment Grant (Grant no. RSA6080019). The first author would like to acknowledge the financial support from the TRF through the Royal
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