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Mechanical properties of FRP-strengthened concrete at elevated temperature
Construction and Building Materials 134 (2017) 424–432
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Review
Mechanical properties of FRP-strengthened concrete at elevated temperature Yue Li, Xiongfei Liu ⇑, Miaoke Wu The Key Laboratory of Urban Security and Disaster Engineering, MOE, Beijing Key Lab of Earthquake Engineering and Structural Retrofit, Beijing University of Technology, Beijing 100124, PR China
h i g h l i g h t s Ultimate tensile strength of FRP and MER decreased with temperature increasement. FRP-concrete bonding strength showed a decline trend as temperature increased. Fracture energy loss of FRP-strengthened members is more sensitive to temperature. Properties of FRP-strengthened concrete can be impaired due to high temperature.
a r t i c l e
i n f o
Article history: Received 6 April 2016 Received in revised form 2 December 2016 Accepted 26 December 2016
Keywords: Elevated temperature FRP Modified epoxy resin Interface bonding strength Fracture energy
a b s t r a c t This paper aims to illuminate the effects of elevated temperature on mechanical properties of fiber reinforced plastic (FRP) strengthened concrete glued by modified epoxy resin (MER) adhesive. Tensile strength, flexural strength and interface bonding properties of FRP-MER-concrete (C30 and C50) were measured after exposure to 80, 160 and 240 °C for 1.5 h and 3 h, respectively. Microstructures of the interface were analyzed by scanning electron microscope (SEM). The ultimate tensile strength and strain of MER-FRP, and bonding strength of FRP-concrete interface gradually decrease as exposure temperature and time increase. The ultimate capacity of FRP-strengthened concrete gradually decreases. The loss rates of ultimate capacity and fracture energy of high-strength specimens are greater than those of low-strength specimens. The fracture energy loss of FRP-strengthened concrete is more sensitive to temperature than to exposure time. The stress-strain relationship of FRP follows a bilinear behavior for both types of concrete. Micro-cracks in MER-concrete interface and concrete matrix degrade the mechanical properties of FRP-strengthened concrete at elevated temperature. Ó 2016 Elsevier Ltd. All rights reserved.
Contents 1. 2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Raw materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Modified epoxy resin adhesive (MER) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3. Fiber reinforced plastic (FRP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Experimental methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. High temperature test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. FRP-concrete interface bonding strength test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. FRP-strengthened concrete flexural strength test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. SEM imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Tensile strength test of MER and FRP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
⇑ Corresponding author. E-mail address: [email protected] (X. Liu). http://dx.doi.org/10.1016/j.conbuildmat.2016.12.148 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
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3.2. 3.3.
4.
Strength tests of FRP-concrete interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flexural strength test of FRP-strengthened concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. Failure modes of FRP-strengthened concrete specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction The behavior of external-bonded fiber-reinforced polymers (FRPs) for strengthening reinforced concrete (RC) structures has been intensively studied in the past two decades [1–4]. The effects of the service conditions (e.g. temperature, water, and humidity) on the structural performance represent an open topic that needs to be discussed in detail. Epoxy resin adhesive used to bond FRP and concrete is a thermosetting polymer, which is sensitive to temperature. The proper serving temperature of epoxy resin is related to its glass transition temperature (Tg). Most FRPs are susceptible to combustion of their polymer matrix, potentially resulting in increased flame spread, toxic smoke evolution, and the heating from thermal system. The Tg threshold, which depends on the specific polymer matrix composition and other factors, typically ranges from 65 °C to 82 °C [1,5]. At elevated temperatures (i.e. above Tg of polymer matrix/ adhesive), the external-bonded FRP materials are expected to show significant reduction in strength, stiffness, and bonding properties [5]. To date, information in this research area is extremely limited, and it should make a great effort to fill the gaps in knowledge. This paper aims to understand the performance of FRP-strengthened concrete at elevated temperature. Limited studies have been conducted on FRP-strengthened concrete structures at different heating temperatures. 1) The effects of elevated temperature on the bonding properties of FRP-concrete interface. Leone M [6] has investigated the variations in bonding strength in the interface between carbon fiber sheet, glass fiber sheet and concrete using double shear test after exposure to 20, 50, 65 and 80 °C. 65 °C was the transition temperature that adhesion shearing failure at the interface transforms into cohesion failure in the concrete matrix. Compared to the specimens tested at room temperature, the bonding strength of the interface between steel plates and concrete decreased by 0.9–31.8%, 48.6–55.3%, 70.9–72.2%, and 91.1–92.7% at 30, 60, 90 and 120 °C, respectively [7]. 2) The effects of elevated temperatures on the structural properties of FRP-strengthened concrete columns. 3) The protective effects of a thermal insulating layer on FRP-strengthened concrete structures at elevated temperature. Al-Salloum [8] performed an axial compression test on FRP-strengthened cylinders (dimensions: U100 200 mm) after exposure to 100 and 200 °C for 1, 2 and 3 h, respectively. The results have demonstrated that the reinforcement effectiveness of external-bonded FRP materials for strengthening concrete structures was extremely sensitive to temperature. The ultimate capacity of FRP-strengthened concrete members at temperature 2.5 times of Tg was 25% lower than that at room temperature. The ultimate axial compressive strength of GFRP-strengthened concrete cylinders decreased by 2%, 4%, 13% and 18% after exposure to 120, 130, 150 and 180 °C, respectively [9]. However, the ultimate compressive strength of GFRPstrengthened concrete cylinders with epoxy-based fireproofing coating decreased by about 3% and 10% at 150 and 185 °C, respectively. The failure modes of GFRP-strengthened concrete cylinders at different temperatures (i.e., fiber-dominated failure modes at lower temperatures and resin-dominated failure modes at higher temperatures) have been established. Chowdhury [10]
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has investigated the changes in structural properties of full-scale reinforced concrete cylinders (U400 3810 mm) with external fire insulation containing an FRP-reinforced layer in fire. Due to the external fire insulation, the internal reinforced bar and concrete of FRP-wrapped reinforced concrete structures remained at a lower temperature for up to 300 min. However, the temperature of the FRP-reinforced layer protected by 53 mm-thick fire insulation remained below its Tg for 34 min. To date, variations in performance of reinforcing materials and the mechanical properties of FRP-strengthened members at elevated temperatures have not been well understood, and the corresponding comprehensive studies have been limited. In this study, carbon FRP-strengthened concretes with MER adhesive were exposed to temperatures of 80, 160 and 240 °C for 1.5 and 3 h. This paper aims to comprehensively study the variations in adhesive, FRP and the mechanical properties of FRP-strengthened concrete structures at elevated temperatures. This work also includes SEM images of adhesive-concrete interface for analyzing the degradation of structural properties of FRP-strengthened concrete at elevated temperatures. 2. Experiments 2.1. Raw materials 2.1.1. Concrete The cement used in this study was Type I ordinary Portland cement. The concrete mixtures were prepared with locally available aggregates: siliceous river sand and crushed limestone gravel. The specific gravity of gravel was 2.7 g/cm3, the crush index of gravel was 4.5%, and its particle size distribution was continuous grading of 5–20 mm. The specific gravity of sand was 2.7 g/cm3, the fineness modulus was 2.56, and particle size distribution was from 0.16 to 5 mm [11]. The high-range water reducer JK-5 was naphthalene-based with water reduction rate of 23%. Two mixtures (C30 and C50) of concrete were prepared, the corresponding mix proportions and mechanical properties are shown in Table 1. 2.1.2. Modified epoxy resin adhesive (MER) The recipe of epoxy resin binder includes epoxy resin, diluents, curing agent, fillers and other additive (i.e., coupling agent and anti-foaming agent). The epoxy equivalent of epoxy resin E-51 and E-44 was 0.41–0.47 eq/100 g. The chemical structure of epoxy resin 501 (Fig. 1 [12]) was the reactive diluent n-butyl glycidyl ether. The flexible curing agents were polyether amine D-2000 and D-400. The flexibilizer QS-BE was obtained from Qishi Material SCI&TECH CO., LTD (Beijing). The coupling agent was c-glycidylpro pyltrimethoxysilane KH560 (A.R.) [13]. The chemical structure of KH560 is shown in Fig. 2 [13]. The anti-foaming agent of BYKA530 was a solution, which consists of anti-foaming polymer and polydimethylsiloxane. The average particle size of silica nanopowder as 20–30 nm and apparent density was 2.5 g/cm3. Flexible curing agent, flexibilizer, and nano-silica filler were added into epoxy resin to improve its strength and ductility, and FRPbonding adhesive was synthesized by mixing flexible curing agent, flexibilizer, nano-silica filler and epoxy resin.
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Table 1 Mix proportions of concrete and their mechanical properties. Mix
Cement kg/m3
Fine aggregate kg/m3
Coarse aggregate kg/m3
Water kg/m3
Water reducer kg/m3
28 d compressive strength/MPa
C30 C50
300 485
640 684
1290 1071
160 163
3.0 6.3
35.1 55.2
Fig. 1. Chemical structure of epoxy resin.
the impregnation of a fiber sheet with MER adhesive. After curing for 7 days under laboratory conditions, the FRP sheets were cut into strips (i.e. coupons) with required dimensions (Fig. 3). Thickness of FRP was 0.19 mm, its areal density was 300 g/m2, ultimate tensile strength was 4200 MPa, elastic modulus was 210 GPa, and the fracture strain was 0.02 [16].
Fig. 2. Chemical structure of KH560.
2.2. Experimental methods Base resin and thinner were mixed and stirred vigorously, followed by mixing with coupling agent and anti-foaming agent. Subsequently, the silica nano-powder was mixed into the epoxy mixture solution with stirrer. Epoxy curing agent was added into the mixture and stirred at 800 rpm for 2 min and 2000 rpm for 3 min to remove entrapped air. The well-mixed binder was cast into waxed molds and vibrated to remove the air bubbles. The tensile strength test of epoxy resin adhesive was performed in accordance with ASTM D638-08 [14]. The test results of MER and neat adhesive [15,16] are shown in Table 2. The tensile strength, tensile strain, elastic modulus and flexural strength of MER are 15.73, 15.38, 2.71 and 33.70% larger, respectively, than the neat adhesive. Also, the operation time of MER was improved [15,16]. The authors just simply claim that the MER is a high performance adhesive, which exhibits a higher strength and ductility than neat epoxy resin. 2.1.3. Fiber reinforced plastic (FRP) Flat coupon tensile test for FRP composites were performed in accordance with ASTM standard D3039M-08 [17] to determine tensile strength. Coupon preparation started with the preparation of an FRP sheet following the usual wet layup process involving
2.2.1. High temperature test The specimens were heated at 80, 160 and 240 °C for 1.5 h and 3 h, respectively. Control groups were stored at 19 °C. After
280
3 hours
240ć
240 200
3 hours
160ć
160 120
3 hours 80
80ć
40
Room temperature (19ć)
0
0
40
80
120
160
200
240
280
Fig. 4. Time-temperature curves. Table 2 Properties of adhesive. Adhesive type
Tensile strength/MPa
Tensile strain
Elastic modulus/MPa
Flexural strength/MPa
Operation time at 25 °C/min
MER Neat adhesive
48.03 41.50
0.030 0.026
2580 2512
81.56 61.00
30–35 27–33
Fig. 3. CFRP coupons, unit of mm (SG1, SG2 and SG3 are the strain gauges in the specimen).
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P
glue FRP
FRP-glue
Concrete
a)
b)
Fig. 5. a) Schematic diagram and b) real set-up of interface bonding strength test.
P
exposing at the target temperature for 1.5 h or 3 h, the specimens were stored at 19 °C for 1 day. Fig. 4 shows the time-temperature curves of specimens used in this study.
2.2.2. FRP-concrete interface bonding strength test Bonding strength of the interface of MER-glued FRP-concrete was performed in accordance with ASTM D4541-09 [18]. Sample preparation: A layer of MER was applied on a clean concrete surface. A piece of pre-cut carbon fiber sheet was laid on the air-dried MRE surface. Then another layer of MER was applied and rolled on a flat surface to enable MPC to penetrate into CFRP and to remove air bubbles. After curing at 19 °C for 7 days, the specimens were placed into a high temperature test chamber. The concrete surface was cut using a saw to create specimens of bonded surface with a dimension of 40 mm 40 mm and a thickness of 10-15 mm, and gaps with wideness of 2 mm. After 1 day of curing at 19 °C, the interface bonding strength of the specimens was measured using an interface bonding gauge (HC-6000C, Hichance, China). The set-up of the test is shown in Fig. 5.
2.3. FRP-strengthened concrete flexural strength test
force sensor displacement sensor load distributing girder
Glue
FRP
Strain gauges
SG1
SG2
a) Schematic diagrams of flexural strength test
The dimensions of the specimens for FRP-reinforced concrete flexural strength test were 100 100 400 mm. The FRP was longitudinally applied along one side of the concrete specimen, and a piece of U-shaped FRP (50 mm wide, with three sides of the specimen bonded to the FRP) was bonded to both ends of the FRP to form a yoke and prevent early delamination of the FRP [19,20]. Two parallel strain gauges (SG1, SG2) were attached longitudinally along the middle position on the surface of FRP to measure the stress of FRP. The length of the strain gauge was 60 mm. The deflection and load values of each specimen were measured by a build-in displacement and force sensor, respectively. All test data were collected by a computer data collection system. The schematic diagram and test devices are shown in Fig. 6.
2.4. SEM imaging The microstructures of fractured FRP-bonded concrete specimens were analyzed using SEM (FEI Quanta 200, Holland).
FRP
b) Test devices Fig. 6. Flexural strength test.
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3. Results and analysis 3.1. Tensile strength test of MER and FRP To minimize the errors in the test, average values are taken from five samples [21] in the tensile strength of MER and FRP. The degree of uncertainty of the specimens in each group is less than 5%. The stress-strain curves and failure modes of MER after high temperature exposure are shown in Fig. 7. The failure of MER in Fig. 7 is basically observed at the midpoint of the specimens. As the exposure temperature and time increase, the ultimate tensile strength and ultimate tensile strain of MER decrease, while the tensile elastic modulus of MER increases. When the temperature is above the glass transition temperature (Tg) of resin, the non-uniform expansion of different components in the epoxy occurs. Thus, tensile strength of MER decreases significantly [6,8] and the MER becomes more brittle. The epoxy turns darker with an increased temperature. The yellowing of resin epoxy is due to the break of chemical bonds during
thermal aging [22]. As the elevated temperature increases, the degree of aging and yellowing of epoxy increases. The stress-strain curves and failure modes of FRP after high temperature exposure are shown in Fig. 8. As shown in Fig. 8 and 1) As the exposure temperature and time increase, the gradual decreased tensile strength and fracture strain of the MER result in the corresponding reduction in the FRP. The tensile strength of fiber sheet generally does not change at temperature below 1000 °C [23]. Thus, the changes in mechanical properties of FRP composites reflect the changes in mechanical properties of MER after exposure to high temperature. 2) The FRP failure modes were basically identical and were observed in the center of the specimens. After exposure to high temperature, fractures, which form between the MER and fiber bundles, degrade the adhesion force between the two phases, leading to a non-uniform force on the FRP under tension. Thus, the ultimate tensile strength and fracture strain of the FRP decrease. Table 3 shows the loss of ultimate tensile strength and ultimate tensile (fracture) strain of MER and FRP observed from Figs. 7 and 8.
50
Stress/MPa
40
30
80ć/1.5h 80ć/3h 160ć/1.5h 160ć/3h 240ć/1.5h 240ć/3h Control/MER
20
10
0 0.00
0.01
0.02
0.03
Strain
a) Stress- strain curves of MER
b) Failure modes of MER
Fig. 7. Tensile strength test results of MER.
4500 4000
Stress(MPa)
3500 3000
80ć/1.5h 160ć/1.5h 240ć/1.5h 240ć/3h Control/FRP
80ć/3h 160ć/3h
2500 2000 1500 1000 500 0 0.000
0.005
0.010
0.015
0.020
0.025
Strain
a) Stress-strain curves of FRP Fig. 8. Tensile strength test results of FRP.
b) Failure modes of FRP
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Y. Li et al. / Construction and Building Materials 134 (2017) 424–432 Table 3 Ultimate tensile strength and strain of FRP and MER composites. Code
Ultimate strength (MPa)
Control/FRP(MER) 80 °C/1.5 h 80 °C/3.0 h 160 °C/1.5 h 160 °C/3.0 h 240 °C/1.5 h 240 °C/3.0 h
Ultimate strain
FRP
Loss (%)
MER
Loss (%)
FRP
Loss (%)
MER
Loss (%)
4154 3913 3861 3834 3671 3485 3352
– 5.80 7.05 7.70 11.63 16.11 19.31
48.03 40.23 38.64 38.50 37.13 36.31 34.76
– 16.24 19.55 19.84 22.69 24.40 27.63
0.020 0.019 0.018 0.017 0.016 0.014 0.013
– 5 10 15 20 30 35
0.030 0.021 0.019 0.018 0.016 0.014 0.013
– 30.00 36.67 40.00 46.67 53.33 56.67
Given the same time of exposure at high temperature (Table 3), the loss in ultimate tensile strength and ultimate tensile (fracture) strain of MER and FRP increases with an increased temperature. The ultimate tensile strength of MER decreases by 16.24–24.40% (for 1.5 h) and 19.55–27.63% (for 3 h), and the corresponding ultimate tensile strain decreases by 30.00–53.33% (for 1.5 h) and 36.67–56.67% (for 3 h), respectively. The ultimate tensile strength of FRP decreases by 5.80–16.11% (for 1.5 h) and 7.05–19.31% (for 3 h), and the corresponding fracture strain decreases by 5–23% (for 1.5 h) and 10–35% (for 3 h), respectively. When the temperature is above the Tg of MER, the mechanical properties of MER degrade. Meanwhile, due to the high temperature, delamination is observed in the areas between the adhesive and fiber bundles and in the fiber bundles matrix. These factors lead to a decreased ultimate tensile strength of FRP. 3.2. Strength tests of FRP-concrete interface Fig. 9 shows the bonding strength and failure modes of the interface between FRP and concrete of different strengths. Here,
Bonding strength (MPa)
4.5
C30/1.5h C50/1.5h
4.0
C30/3.0h C50/3.0h Control/C50
3.5 3.0 2.5 Control/C30 2.0 1.5 0
40
80
120
160
200
240
280
Temperature (ć䠅
a) Bond strength of FRP-concrete interface
Control/Rt represents a specimen at room temperature, and Control/C30 (C50) represents a specimen without FRP at room temperature. The degree of uncertainty of the specimens in each group is less than 5%. As shown in Fig. 9, the failure modes of all specimens are the concrete failure at different temperatures for different durations, which means FRP adhesive could guarantee sound binding between FRP and concrete at different elevated temperatures. Moreover, the bonding strength of the interface varies with the strength of the concrete, temperature and exposure time: 1) When the surface is not strengthened with FRP, the results of bonding strength specify the uniaxial tensile strengths of concrete. C50 concrete, which has a higher compressive strength, shows higher tensile strength than that of C30 concrete. The results are in good agreement with Raphael’s work [24]. 2) When FRP is bonded to the surface, the bonding strength is governed by both the tensile strength of concrete and the constraint of FRP to the concrete surface. The FRP bonded to the concrete surface is a strengthened layer with high stiffness. The propagation of cracks in concrete matrix caused by tensile strength is constrained by the confinement effect, which leads to a well distributed stress. During the tension process of concrete, the bonded FRP on the concrete surface is able to constrain the propagation of micro-cracks. This mechanism is similar to hoops in steel-reinforced concrete. 3) As the temperature or exposure time increases, the interface bonding strength between FRP and concrete shows a decreasing trend. The increased temperature causes the formation of micro-cracks in the concrete-MER interfaces and concrete, decreases the confinement effect, and further leads to the widening of cracks. As the temperature and time increase, the confinement effect decreases and further degrades the interface bond strength. 4) Meanwhile, free water in concrete gradually evaporates, leading to the formation of pores and cracks in the concrete matrix. At 300 °C, cement pastes shrink and aggregates expand, which also induce the micro cracking in the concrete. Meanwhile, the hydrates (e.g. the C-S-H gel, AFt and AFm phases) in hydrated cement paste dehydrate [25]. When bearing load is applied, stress is concentrated at the tips of cracks in the concrete, which leads to crack growth and failure of the concrete. Even when the confinement effect by FRP decreases, the bonding strength of the MER-concrete interface is still greater than the tensile strength of concrete [8], thus the failure is observed in a concrete. Also, the common epoxy adhesive melts at 200 °C for 3 h, and the mode of failure changes to bonding along with concrete failure [8]. This suggests that MER has excellent high temperature resistance. According to the experimental data, the formula of interfacial bonding strength of FRP-concrete for high temperature, durations and concrete strength was shown in Eq. (1)”
Z ¼ 4:03 0:77
b) Failure modes of interfaces of samples Fig. 9. Test results of interface bonding strength.
rffiffiffi T 0:07 t ln t; R2 ¼ 0:89 P
ð1Þ
where Z is the interface bonding strength of FRP-concrete (MPa). T is the exposure temperature (°C). t is the exposure time (h). P is the compressive strength (MPa) of concrete at room temperature.
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a) C30ˈControl /Rt
b) C30ˈ80䉝/1.5h
Fig. 10. SEM images of FRP-concrete interface.
The predicated results from Eq. (1) agreed well with the experimental data. The degradation degree of the interface bonding strength of FRP-concrete could be predicated. So the effective protective measure could be made to protect the concrete structure. The morphology of the adhesive-concrete interfaces at microscale is observed with SEM to analyze the mechanism of the reduction of interface bond strength. It illustrates that the interface between MER and concrete is dense at room temperature, but after high temperature exposure, micro-cracks form in the MERconcrete interface and concrete, which degrade the confinement effect. Due to the cracks propagation, the concrete strength decreases significantly and reduces the bonding strength of FRP-strengthened concrete interface. The failure is observed in the concrete (See Fig. 10). 3.3. Flexural strength test of FRP-strengthened concrete 3.3.1. Failure modes of FRP-strengthened concrete specimens Figs. 11 and 12 show the failure modes and the stress-strain curves of FRP-strengthened specimens in flexural test, respectively. Control/Rt represents the 1-lalyer-FRP bonded specimens FRP at room temperature, and Control/C30(C50) represents plain concrete specimens. The flexural strength (f) and fracture energy (W) of the R concrete prism is calculated by f = FL/bd2 and W = Fdx according to China national code-Design code on ordinary concrete (GB/T 50081-2002), respectively, where F is applied load (N), L is the support span (mm), b is the width of the prism (mm), d is the depth of R the prism (mm), and Fdx is the area under the load-displacement curves (Nm) in Fig. 13. To minimize the testing errors, average values are taken from three samples in the test. The degree of uncertainty of the specimens in each group is less than 5%. As shown in Fig. 11, the failure modes of the specimens are flexural or shearing failures [26] at different temperatures and for different durations. The trends in failure of the concrete specimens with different strengths are basically similar. The failure mode of FRP-strengthened concrete specimens changes from shearing failure to flexure failure once the temperature and time of exposure exceed the limit. For instance, the transition from shearing failure to flexure failure of C30 and C50 FRP-strengthened specimens is at 160 °C/3 h and 240 °C/1.5 h, respectively. The mechanical properties of FRP and the bonding strength of the FRP-concrete interface gradually decrease with increasing temperature and time. This results in a gradual loss in the utilization rate of FRP. When the specimens reach the critical load, micro-cracks form in the FRP and FRP-concrete interface. As the load increases, the micro-cracks
Fig. 11. Failure modes of strengthened concrete.
propagate toward the force-bearing region and widen until the specimen fails. As shown in Fig. 12 and 1) the trend in FRP stress-strain of the two types of concrete are similar, following a bilinear development behavior: both the slope of the curve and initial stiffness of each specimen at the first stage are basically identical; the second stage is plastic development with rapid development of tensile strain in the FRP. 2) As the exposure temperature and time increase, yield strength, ultimate strength and stiffness of the FRP-strengthened specimens significantly decrease. Due to the decrease in interface bonding strength and the degradation of FRP and MER, the mechanical properties of the FRP-strengthened specimens degrade. The load-displacement (mid-span) curves of FRP-strengthened concrete are shown in Fig. 13. ‘‘Displacement” represents the
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12
80ć/1.5h 160ć/1.5h 240ć/1.5h Control/Rt
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Strain
Strain
a) C30
b) C50
80ć/3h 160ć/3h 240ć/3h Control/C50
5000
6000
7000
Fig. 12. Stress-strain curves of FRP-strengthened concrete.
35
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80ć/1.5h 160ć/1.5h 240ć/1.5h Control/Rt
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80ć/3h 160ć/3h 240ć/3h Control/C30
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Load (kN)
40
25 20 15 10
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30 20 10
5 0 0.0
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2.0
2.5
0 0.0
0.5
Displacement (mm)
1.0
1.5
2.0
2.5
3.0
Displacement (mm)
a) C30
b) C50
Fig. 13. Load-displacement curves of FRP-strengthened concrete.
deflection in the mid-span of the specimens and ‘‘Load” represents the bearing load of the specimens. As shown in Fig. 13 and 1) the load-displacement curves of the two types of concrete develops linearly in the initial stage of loading, and as the load increases, the development of the curves becomes non-linear. 2) As the temperature and time increase, the initial elastic modulus and ultimate strength of specimens gradually decrease, which is similar to the stress-strain development of FRP. 3) For the plain concrete specimens, the loaddisplacement curve develops linearly from the initial load to the ultimate load. The specimens suddenly fail at the maximum loading. A typical brittle failure presents, and the mid-span ultimate deflection and ultimate load are obviously lower than those of the FRP-strengthened concrete. The ultimate load and fracture energy of the FRP-strengthened specimens are calculated on the load-displacement curves (Fig. 13), and are shown in Table 4. The correlations between the increasing rate of the strength of FRP-strengthened concrete and temperature (i.e. the ratio of the strength fu of FRP-strengthened concrete at a specific temperature to the strength f0 of unstrengthened concrete at room temperature) are shown in Fig. 14. Compared to the FRP-strengthened concrete specimens at 19 °C, the correlations between the fracture energy loss and exposure conditions of FRP-strengthened specimens are shown in Fig. 15. Chowdhury [10] claimed that concrete undergoes a slow increase in surface temperature at 400 °C for 300 min, with negligible variation in strength. The maximum temperature and time
Table 4 Results of FRP-strengthened specimens. Code
Control/C30(C50) Control/Rt 80 °C/1.5 h 80 °C/3.0 h 160 °C/1.5 h 160 °C/3.0 h 240 °C/1.5 h 240 °C/3.0 h
Ultimate load (kN)
Fracture energy (kNmm)
C30
C50
C30
C50
11.04 32.74 30.97 28.46 26.68 24.27 24.05 22.75
12.07 39.27 33.65 31.90 30.24 28.76 28.26 25.91
5.29 41.22 34.47 32.77 32.20 24.27 29.09 24.16
5.55 56.90 38.28 29.47 37.03 31.94 33.39 32.05
in the present work is 240 °C and 180 min, respectively, therefore the concrete compressive strength here is nearly constant (i.e. fu/f0 = 1). From Table 4 and Fig. 14 and 1) Compared to each of the plain concrete specimens (Control/C30 and Control/C50), the ultimate load of the FRP-strengthened specimen significantly increases. The increase rate of the strength of high-strength specimens is greater than that of low-strength specimens. However, the increase rate of ultimate strength gradually decreases with increasing temperature and time. 2) The effects of exposure temperature on the mechanical properties of FRP-strengthened specimens are greater than that of exposure time. The most distinct decrease in strength is observed in specimens, which are
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3.5
FRP-confined,C50,1.5h FRP-confined,C50,3h
3.0
4) After the high temperature exposure, micro-cracks form in the FRP-concrete interface, and therefore the structural properties of FRP-strengthened concrete are degraded.
2.5
fu /f0
Acknowledgements
FRP-confined,C30,1.5h FRP-confined,C30,3h
2.0 1.5 1.0
Unconfined 0.5 0.0
References
0
50
100
150
200
250
Temperatures/ć Fig. 14. Relationship between temperature.
60
C30 % Loss of fracture energy
The authors would like to acknowledge the financial support provided by Program for New Century Excellent Talents in University (NCET-12-0605), The Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions (CIT&TCD20150310), National Natural Science Foundation of China (51278014).
1.5h 3h
C50
50 40 30 20 10 0
80
160
240
80
160
240
Temperature(ć) Fig. 15. Loss of fracture energy and peak load enhancement ratio.
exposed to 240 °C (3 times of Tg) for 3 h. The strength decreases by 30.54% and 34.02% for C30 and C50 specimens, respectively. Table 4 and Fig. 15 indicate that the fracture energy and its loss rate of high-strength FRP-concrete are greater than those of lowstrength specimens. The fracture energy loss of FRP-strengthened concrete structures increases gradually with temperature but is not proportional to exposure time at elevated temperature, which suggests that the fracture energy loss of FRP-strengthened concrete structures is more sensitive to temperature. 4. Conclusions 1) As high temperature and exposure time increase, the ultimate tensile strength and ultimate tensile (fracture) strain of MER (FRP) decrease. 2) As temperature and exposure time increase, the FRPconcrete interface bonding strength shows a decreasing trend. The loss rate of the ultimate capacity of FRPstrengthened concrete increases gradually, and the loss rates of ultimate load and fracture energy of high-strength concrete structures are greater than those of low-strength concrete structures. The fracture energy loss of FRPstrengthened concrete structures is more sensitive to temperature than to exposure time. 3) FRP stress-strain in both two types of FRP-concrete with different strengths presents a bilinear behavior.
[1] C.E. Bakis, A. Ganjehlou, D.I. Kachlakev, Guide for the design and construction of externally bonded FRP systems for strengthening concrete structures Reported by, ACI Committee (2002) 440. [2] CSA S806-02, Design and construction of building components with fibrereinforced polymers, Canadian Standards Association, 2002. [3] ISIS Canada, Strengthening Reinforced Concrete Structures with Externallybonded Fibre Reinforced Polymers, ISIS, Canada, 2001. [4] Technical Report 55, Design Guidance for Strengthening Concrete Structures using Fibre Composite Materials, The Concrete Society, Blackwater, Camberley, Surrey, 2000. [5] S.K. Foster, L.A. Bisby, High temperature residual properties of externallybonded FRP systems, ACI Spec. Publ. 230 (2005) 1235–1252. [6] M. Leone, S. Matthys, M.A. Aiello, Effect of elevated service temperature on bond between FRP EBR systems and concrete, Compos. B Eng. 40 (1) (2009) 85–93. [7] A.J.B. Tadeu, F.J.F.F. Branco, Shear tests of steel plates epoxy bonded to concrete under temperature, J. Mater. Civ. Eng. 12 (1) (2000) 74–80. [8] Y.A. Al-Salloum, H.M. Elsanadedy, A.A. Abadel, Behavior of FRP-strengthened concrete after high temperature exposure, Constr. Build. Mater. 25 (2) (2011) 838–850. [9] D.B. Cleary, C.D. Cassino, R. Tortorice, et al., Effect of elevated temperatures on a fiber composite used to strengthen concrete columns, J. Reinf. Plast. Compos. 22 (10) (2003) 881–895. [10] E.U. Chowdhury, L.A. Bisby, M.F. Green, et al., Investigation of insulated FRPwrapped reinforced concrete columns in fire, Fire Saf. J. 42 (6) (2007) 452–460. [11] Y. Li, J. Li, Capillary tension theory for prediction of early autogenous shrinkage of self-consolidating concrete, Constr. Build. Mater. 53 (2014) 511–516. [12] X. Zhang, W. Xu, X. Xia, et al., Toughening of cycloaliphatic epoxy resin by nanosize silicon dioxide, Mater. Lett. 60 (28) (2006) 3319–3323. [13] Y. Li, X. Liu, J. Yuan, et al., Toughness improvement of epoxy resin mortar by incorporation of ground calcium carbonate, Constr. Build. Mater. 100 (2015) 122–128. [14] Standard A. D638-08. Standard Test Method for Tensile Properties of Plastics, 2008. [15] Y. Li, X. Liu, J. Li, Experimental study of retrofitted cracked concrete with FRP and nanomodified epoxy resin, J. Mater. Civ. Eng. (2016). 04016275. [16] Y. Li, X. Liu, J. Li, Bond properties of FRP-concrete interface with nano-modified epoxy resin under wet-dry cycles, KSCE J. Civil Eng. (2016) 1–7. [17] Standard A. D3039M-08. Standard test method for tensile properties of polymer matrix composite materials. Annual Book of ASTM Standards, 2008. [18] ASTM D4541-09, Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers, American Society for Testing and Materials, West Conshohocken (PA), 2009. [19] J. Dong, Q. Wang, Z. Guan, Structural behaviour of RC beams with external flexural and flexural–shear strengthening by FRP sheets, Compos. B Eng. 44 (1) (2013) 604–612. [20] A. El Menoufy, K. Soudki, Flexural behavior of corroded pretensioned girders repaired with CFRP sheets, PCI J. 59 (2) (2014) 129–143. [21] J.G. Dai, Y.L. Bai, J.G. Teng, Behavior and modeling of concrete confined with FRP composites of large deformability, J. Compos. Construct. 15 (6) (2011) 963–973. [22] D.C.R.S. De Oliveira, M.G. Rocha, A. Gatti, et al., Effect of different photoinitiators and reducing agents on cure efficiency and color stability of resin-based composites using different LED wavelengths, J. Dent. 43 (12) (2015) 1565–1572. [23] CE. Bakis, FRP Reinforcement: Materials and Manufacturing. Fiber-ReinforcedPlastic (FRP) Reinforcements for Concrete Structures: Properties and Applications, Elsevier Science Publishers, 1993. pp. 13–58. [24] J.M. Raphael, Tensile strength of concrete, ACI J. Proceed. 81 (2) (1984). [25] A.H. Buchanan, Structural Design for Fire Safety, Vol. 273, Wiley, New York, 2001. [26] L. Yan, Plain concrete cylinders and beams externally strengthened with natural flax fabric reinforced epoxy composites, Mater. Struct. (2015) 1–13.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Review
Mechanical properties of FRP-strengthened concrete at elevated temperature Yue Li, Xiongfei Liu ⇑, Miaoke Wu The Key Laboratory of Urban Security and Disaster Engineering, MOE, Beijing Key Lab of Earthquake Engineering and Structural Retrofit, Beijing University of Technology, Beijing 100124, PR China
h i g h l i g h t s Ultimate tensile strength of FRP and MER decreased with temperature increasement. FRP-concrete bonding strength showed a decline trend as temperature increased. Fracture energy loss of FRP-strengthened members is more sensitive to temperature. Properties of FRP-strengthened concrete can be impaired due to high temperature.
a r t i c l e
i n f o
Article history: Received 6 April 2016 Received in revised form 2 December 2016 Accepted 26 December 2016
Keywords: Elevated temperature FRP Modified epoxy resin Interface bonding strength Fracture energy
a b s t r a c t This paper aims to illuminate the effects of elevated temperature on mechanical properties of fiber reinforced plastic (FRP) strengthened concrete glued by modified epoxy resin (MER) adhesive. Tensile strength, flexural strength and interface bonding properties of FRP-MER-concrete (C30 and C50) were measured after exposure to 80, 160 and 240 °C for 1.5 h and 3 h, respectively. Microstructures of the interface were analyzed by scanning electron microscope (SEM). The ultimate tensile strength and strain of MER-FRP, and bonding strength of FRP-concrete interface gradually decrease as exposure temperature and time increase. The ultimate capacity of FRP-strengthened concrete gradually decreases. The loss rates of ultimate capacity and fracture energy of high-strength specimens are greater than those of low-strength specimens. The fracture energy loss of FRP-strengthened concrete is more sensitive to temperature than to exposure time. The stress-strain relationship of FRP follows a bilinear behavior for both types of concrete. Micro-cracks in MER-concrete interface and concrete matrix degrade the mechanical properties of FRP-strengthened concrete at elevated temperature. Ó 2016 Elsevier Ltd. All rights reserved.
Contents 1. 2.
3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Raw materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.1. Concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.2. Modified epoxy resin adhesive (MER) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1.3. Fiber reinforced plastic (FRP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Experimental methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. High temperature test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. FRP-concrete interface bonding strength test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. FRP-strengthened concrete flexural strength test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. SEM imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Tensile strength test of MER and FRP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
⇑ Corresponding author. E-mail address: [email protected] (X. Liu). http://dx.doi.org/10.1016/j.conbuildmat.2016.12.148 0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
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3.2. 3.3.
4.
Strength tests of FRP-concrete interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Flexural strength test of FRP-strengthened concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. Failure modes of FRP-strengthened concrete specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1. Introduction The behavior of external-bonded fiber-reinforced polymers (FRPs) for strengthening reinforced concrete (RC) structures has been intensively studied in the past two decades [1–4]. The effects of the service conditions (e.g. temperature, water, and humidity) on the structural performance represent an open topic that needs to be discussed in detail. Epoxy resin adhesive used to bond FRP and concrete is a thermosetting polymer, which is sensitive to temperature. The proper serving temperature of epoxy resin is related to its glass transition temperature (Tg). Most FRPs are susceptible to combustion of their polymer matrix, potentially resulting in increased flame spread, toxic smoke evolution, and the heating from thermal system. The Tg threshold, which depends on the specific polymer matrix composition and other factors, typically ranges from 65 °C to 82 °C [1,5]. At elevated temperatures (i.e. above Tg of polymer matrix/ adhesive), the external-bonded FRP materials are expected to show significant reduction in strength, stiffness, and bonding properties [5]. To date, information in this research area is extremely limited, and it should make a great effort to fill the gaps in knowledge. This paper aims to understand the performance of FRP-strengthened concrete at elevated temperature. Limited studies have been conducted on FRP-strengthened concrete structures at different heating temperatures. 1) The effects of elevated temperature on the bonding properties of FRP-concrete interface. Leone M [6] has investigated the variations in bonding strength in the interface between carbon fiber sheet, glass fiber sheet and concrete using double shear test after exposure to 20, 50, 65 and 80 °C. 65 °C was the transition temperature that adhesion shearing failure at the interface transforms into cohesion failure in the concrete matrix. Compared to the specimens tested at room temperature, the bonding strength of the interface between steel plates and concrete decreased by 0.9–31.8%, 48.6–55.3%, 70.9–72.2%, and 91.1–92.7% at 30, 60, 90 and 120 °C, respectively [7]. 2) The effects of elevated temperatures on the structural properties of FRP-strengthened concrete columns. 3) The protective effects of a thermal insulating layer on FRP-strengthened concrete structures at elevated temperature. Al-Salloum [8] performed an axial compression test on FRP-strengthened cylinders (dimensions: U100 200 mm) after exposure to 100 and 200 °C for 1, 2 and 3 h, respectively. The results have demonstrated that the reinforcement effectiveness of external-bonded FRP materials for strengthening concrete structures was extremely sensitive to temperature. The ultimate capacity of FRP-strengthened concrete members at temperature 2.5 times of Tg was 25% lower than that at room temperature. The ultimate axial compressive strength of GFRP-strengthened concrete cylinders decreased by 2%, 4%, 13% and 18% after exposure to 120, 130, 150 and 180 °C, respectively [9]. However, the ultimate compressive strength of GFRPstrengthened concrete cylinders with epoxy-based fireproofing coating decreased by about 3% and 10% at 150 and 185 °C, respectively. The failure modes of GFRP-strengthened concrete cylinders at different temperatures (i.e., fiber-dominated failure modes at lower temperatures and resin-dominated failure modes at higher temperatures) have been established. Chowdhury [10]
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has investigated the changes in structural properties of full-scale reinforced concrete cylinders (U400 3810 mm) with external fire insulation containing an FRP-reinforced layer in fire. Due to the external fire insulation, the internal reinforced bar and concrete of FRP-wrapped reinforced concrete structures remained at a lower temperature for up to 300 min. However, the temperature of the FRP-reinforced layer protected by 53 mm-thick fire insulation remained below its Tg for 34 min. To date, variations in performance of reinforcing materials and the mechanical properties of FRP-strengthened members at elevated temperatures have not been well understood, and the corresponding comprehensive studies have been limited. In this study, carbon FRP-strengthened concretes with MER adhesive were exposed to temperatures of 80, 160 and 240 °C for 1.5 and 3 h. This paper aims to comprehensively study the variations in adhesive, FRP and the mechanical properties of FRP-strengthened concrete structures at elevated temperatures. This work also includes SEM images of adhesive-concrete interface for analyzing the degradation of structural properties of FRP-strengthened concrete at elevated temperatures. 2. Experiments 2.1. Raw materials 2.1.1. Concrete The cement used in this study was Type I ordinary Portland cement. The concrete mixtures were prepared with locally available aggregates: siliceous river sand and crushed limestone gravel. The specific gravity of gravel was 2.7 g/cm3, the crush index of gravel was 4.5%, and its particle size distribution was continuous grading of 5–20 mm. The specific gravity of sand was 2.7 g/cm3, the fineness modulus was 2.56, and particle size distribution was from 0.16 to 5 mm [11]. The high-range water reducer JK-5 was naphthalene-based with water reduction rate of 23%. Two mixtures (C30 and C50) of concrete were prepared, the corresponding mix proportions and mechanical properties are shown in Table 1. 2.1.2. Modified epoxy resin adhesive (MER) The recipe of epoxy resin binder includes epoxy resin, diluents, curing agent, fillers and other additive (i.e., coupling agent and anti-foaming agent). The epoxy equivalent of epoxy resin E-51 and E-44 was 0.41–0.47 eq/100 g. The chemical structure of epoxy resin 501 (Fig. 1 [12]) was the reactive diluent n-butyl glycidyl ether. The flexible curing agents were polyether amine D-2000 and D-400. The flexibilizer QS-BE was obtained from Qishi Material SCI&TECH CO., LTD (Beijing). The coupling agent was c-glycidylpro pyltrimethoxysilane KH560 (A.R.) [13]. The chemical structure of KH560 is shown in Fig. 2 [13]. The anti-foaming agent of BYKA530 was a solution, which consists of anti-foaming polymer and polydimethylsiloxane. The average particle size of silica nanopowder as 20–30 nm and apparent density was 2.5 g/cm3. Flexible curing agent, flexibilizer, and nano-silica filler were added into epoxy resin to improve its strength and ductility, and FRPbonding adhesive was synthesized by mixing flexible curing agent, flexibilizer, nano-silica filler and epoxy resin.
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Table 1 Mix proportions of concrete and their mechanical properties. Mix
Cement kg/m3
Fine aggregate kg/m3
Coarse aggregate kg/m3
Water kg/m3
Water reducer kg/m3
28 d compressive strength/MPa
C30 C50
300 485
640 684
1290 1071
160 163
3.0 6.3
35.1 55.2
Fig. 1. Chemical structure of epoxy resin.
the impregnation of a fiber sheet with MER adhesive. After curing for 7 days under laboratory conditions, the FRP sheets were cut into strips (i.e. coupons) with required dimensions (Fig. 3). Thickness of FRP was 0.19 mm, its areal density was 300 g/m2, ultimate tensile strength was 4200 MPa, elastic modulus was 210 GPa, and the fracture strain was 0.02 [16].
Fig. 2. Chemical structure of KH560.
2.2. Experimental methods Base resin and thinner were mixed and stirred vigorously, followed by mixing with coupling agent and anti-foaming agent. Subsequently, the silica nano-powder was mixed into the epoxy mixture solution with stirrer. Epoxy curing agent was added into the mixture and stirred at 800 rpm for 2 min and 2000 rpm for 3 min to remove entrapped air. The well-mixed binder was cast into waxed molds and vibrated to remove the air bubbles. The tensile strength test of epoxy resin adhesive was performed in accordance with ASTM D638-08 [14]. The test results of MER and neat adhesive [15,16] are shown in Table 2. The tensile strength, tensile strain, elastic modulus and flexural strength of MER are 15.73, 15.38, 2.71 and 33.70% larger, respectively, than the neat adhesive. Also, the operation time of MER was improved [15,16]. The authors just simply claim that the MER is a high performance adhesive, which exhibits a higher strength and ductility than neat epoxy resin. 2.1.3. Fiber reinforced plastic (FRP) Flat coupon tensile test for FRP composites were performed in accordance with ASTM standard D3039M-08 [17] to determine tensile strength. Coupon preparation started with the preparation of an FRP sheet following the usual wet layup process involving
2.2.1. High temperature test The specimens were heated at 80, 160 and 240 °C for 1.5 h and 3 h, respectively. Control groups were stored at 19 °C. After
280
3 hours
240ć
240 200
3 hours
160ć
160 120
3 hours 80
80ć
40
Room temperature (19ć)
0
0
40
80
120
160
200
240
280
Fig. 4. Time-temperature curves. Table 2 Properties of adhesive. Adhesive type
Tensile strength/MPa
Tensile strain
Elastic modulus/MPa
Flexural strength/MPa
Operation time at 25 °C/min
MER Neat adhesive
48.03 41.50
0.030 0.026
2580 2512
81.56 61.00
30–35 27–33
Fig. 3. CFRP coupons, unit of mm (SG1, SG2 and SG3 are the strain gauges in the specimen).
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P
glue FRP
FRP-glue
Concrete
a)
b)
Fig. 5. a) Schematic diagram and b) real set-up of interface bonding strength test.
P
exposing at the target temperature for 1.5 h or 3 h, the specimens were stored at 19 °C for 1 day. Fig. 4 shows the time-temperature curves of specimens used in this study.
2.2.2. FRP-concrete interface bonding strength test Bonding strength of the interface of MER-glued FRP-concrete was performed in accordance with ASTM D4541-09 [18]. Sample preparation: A layer of MER was applied on a clean concrete surface. A piece of pre-cut carbon fiber sheet was laid on the air-dried MRE surface. Then another layer of MER was applied and rolled on a flat surface to enable MPC to penetrate into CFRP and to remove air bubbles. After curing at 19 °C for 7 days, the specimens were placed into a high temperature test chamber. The concrete surface was cut using a saw to create specimens of bonded surface with a dimension of 40 mm 40 mm and a thickness of 10-15 mm, and gaps with wideness of 2 mm. After 1 day of curing at 19 °C, the interface bonding strength of the specimens was measured using an interface bonding gauge (HC-6000C, Hichance, China). The set-up of the test is shown in Fig. 5.
2.3. FRP-strengthened concrete flexural strength test
force sensor displacement sensor load distributing girder
Glue
FRP
Strain gauges
SG1
SG2
a) Schematic diagrams of flexural strength test
The dimensions of the specimens for FRP-reinforced concrete flexural strength test were 100 100 400 mm. The FRP was longitudinally applied along one side of the concrete specimen, and a piece of U-shaped FRP (50 mm wide, with three sides of the specimen bonded to the FRP) was bonded to both ends of the FRP to form a yoke and prevent early delamination of the FRP [19,20]. Two parallel strain gauges (SG1, SG2) were attached longitudinally along the middle position on the surface of FRP to measure the stress of FRP. The length of the strain gauge was 60 mm. The deflection and load values of each specimen were measured by a build-in displacement and force sensor, respectively. All test data were collected by a computer data collection system. The schematic diagram and test devices are shown in Fig. 6.
2.4. SEM imaging The microstructures of fractured FRP-bonded concrete specimens were analyzed using SEM (FEI Quanta 200, Holland).
FRP
b) Test devices Fig. 6. Flexural strength test.
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3. Results and analysis 3.1. Tensile strength test of MER and FRP To minimize the errors in the test, average values are taken from five samples [21] in the tensile strength of MER and FRP. The degree of uncertainty of the specimens in each group is less than 5%. The stress-strain curves and failure modes of MER after high temperature exposure are shown in Fig. 7. The failure of MER in Fig. 7 is basically observed at the midpoint of the specimens. As the exposure temperature and time increase, the ultimate tensile strength and ultimate tensile strain of MER decrease, while the tensile elastic modulus of MER increases. When the temperature is above the glass transition temperature (Tg) of resin, the non-uniform expansion of different components in the epoxy occurs. Thus, tensile strength of MER decreases significantly [6,8] and the MER becomes more brittle. The epoxy turns darker with an increased temperature. The yellowing of resin epoxy is due to the break of chemical bonds during
thermal aging [22]. As the elevated temperature increases, the degree of aging and yellowing of epoxy increases. The stress-strain curves and failure modes of FRP after high temperature exposure are shown in Fig. 8. As shown in Fig. 8 and 1) As the exposure temperature and time increase, the gradual decreased tensile strength and fracture strain of the MER result in the corresponding reduction in the FRP. The tensile strength of fiber sheet generally does not change at temperature below 1000 °C [23]. Thus, the changes in mechanical properties of FRP composites reflect the changes in mechanical properties of MER after exposure to high temperature. 2) The FRP failure modes were basically identical and were observed in the center of the specimens. After exposure to high temperature, fractures, which form between the MER and fiber bundles, degrade the adhesion force between the two phases, leading to a non-uniform force on the FRP under tension. Thus, the ultimate tensile strength and fracture strain of the FRP decrease. Table 3 shows the loss of ultimate tensile strength and ultimate tensile (fracture) strain of MER and FRP observed from Figs. 7 and 8.
50
Stress/MPa
40
30
80ć/1.5h 80ć/3h 160ć/1.5h 160ć/3h 240ć/1.5h 240ć/3h Control/MER
20
10
0 0.00
0.01
0.02
0.03
Strain
a) Stress- strain curves of MER
b) Failure modes of MER
Fig. 7. Tensile strength test results of MER.
4500 4000
Stress(MPa)
3500 3000
80ć/1.5h 160ć/1.5h 240ć/1.5h 240ć/3h Control/FRP
80ć/3h 160ć/3h
2500 2000 1500 1000 500 0 0.000
0.005
0.010
0.015
0.020
0.025
Strain
a) Stress-strain curves of FRP Fig. 8. Tensile strength test results of FRP.
b) Failure modes of FRP
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Y. Li et al. / Construction and Building Materials 134 (2017) 424–432 Table 3 Ultimate tensile strength and strain of FRP and MER composites. Code
Ultimate strength (MPa)
Control/FRP(MER) 80 °C/1.5 h 80 °C/3.0 h 160 °C/1.5 h 160 °C/3.0 h 240 °C/1.5 h 240 °C/3.0 h
Ultimate strain
FRP
Loss (%)
MER
Loss (%)
FRP
Loss (%)
MER
Loss (%)
4154 3913 3861 3834 3671 3485 3352
– 5.80 7.05 7.70 11.63 16.11 19.31
48.03 40.23 38.64 38.50 37.13 36.31 34.76
– 16.24 19.55 19.84 22.69 24.40 27.63
0.020 0.019 0.018 0.017 0.016 0.014 0.013
– 5 10 15 20 30 35
0.030 0.021 0.019 0.018 0.016 0.014 0.013
– 30.00 36.67 40.00 46.67 53.33 56.67
Given the same time of exposure at high temperature (Table 3), the loss in ultimate tensile strength and ultimate tensile (fracture) strain of MER and FRP increases with an increased temperature. The ultimate tensile strength of MER decreases by 16.24–24.40% (for 1.5 h) and 19.55–27.63% (for 3 h), and the corresponding ultimate tensile strain decreases by 30.00–53.33% (for 1.5 h) and 36.67–56.67% (for 3 h), respectively. The ultimate tensile strength of FRP decreases by 5.80–16.11% (for 1.5 h) and 7.05–19.31% (for 3 h), and the corresponding fracture strain decreases by 5–23% (for 1.5 h) and 10–35% (for 3 h), respectively. When the temperature is above the Tg of MER, the mechanical properties of MER degrade. Meanwhile, due to the high temperature, delamination is observed in the areas between the adhesive and fiber bundles and in the fiber bundles matrix. These factors lead to a decreased ultimate tensile strength of FRP. 3.2. Strength tests of FRP-concrete interface Fig. 9 shows the bonding strength and failure modes of the interface between FRP and concrete of different strengths. Here,
Bonding strength (MPa)
4.5
C30/1.5h C50/1.5h
4.0
C30/3.0h C50/3.0h Control/C50
3.5 3.0 2.5 Control/C30 2.0 1.5 0
40
80
120
160
200
240
280
Temperature (ć䠅
a) Bond strength of FRP-concrete interface
Control/Rt represents a specimen at room temperature, and Control/C30 (C50) represents a specimen without FRP at room temperature. The degree of uncertainty of the specimens in each group is less than 5%. As shown in Fig. 9, the failure modes of all specimens are the concrete failure at different temperatures for different durations, which means FRP adhesive could guarantee sound binding between FRP and concrete at different elevated temperatures. Moreover, the bonding strength of the interface varies with the strength of the concrete, temperature and exposure time: 1) When the surface is not strengthened with FRP, the results of bonding strength specify the uniaxial tensile strengths of concrete. C50 concrete, which has a higher compressive strength, shows higher tensile strength than that of C30 concrete. The results are in good agreement with Raphael’s work [24]. 2) When FRP is bonded to the surface, the bonding strength is governed by both the tensile strength of concrete and the constraint of FRP to the concrete surface. The FRP bonded to the concrete surface is a strengthened layer with high stiffness. The propagation of cracks in concrete matrix caused by tensile strength is constrained by the confinement effect, which leads to a well distributed stress. During the tension process of concrete, the bonded FRP on the concrete surface is able to constrain the propagation of micro-cracks. This mechanism is similar to hoops in steel-reinforced concrete. 3) As the temperature or exposure time increases, the interface bonding strength between FRP and concrete shows a decreasing trend. The increased temperature causes the formation of micro-cracks in the concrete-MER interfaces and concrete, decreases the confinement effect, and further leads to the widening of cracks. As the temperature and time increase, the confinement effect decreases and further degrades the interface bond strength. 4) Meanwhile, free water in concrete gradually evaporates, leading to the formation of pores and cracks in the concrete matrix. At 300 °C, cement pastes shrink and aggregates expand, which also induce the micro cracking in the concrete. Meanwhile, the hydrates (e.g. the C-S-H gel, AFt and AFm phases) in hydrated cement paste dehydrate [25]. When bearing load is applied, stress is concentrated at the tips of cracks in the concrete, which leads to crack growth and failure of the concrete. Even when the confinement effect by FRP decreases, the bonding strength of the MER-concrete interface is still greater than the tensile strength of concrete [8], thus the failure is observed in a concrete. Also, the common epoxy adhesive melts at 200 °C for 3 h, and the mode of failure changes to bonding along with concrete failure [8]. This suggests that MER has excellent high temperature resistance. According to the experimental data, the formula of interfacial bonding strength of FRP-concrete for high temperature, durations and concrete strength was shown in Eq. (1)”
Z ¼ 4:03 0:77
b) Failure modes of interfaces of samples Fig. 9. Test results of interface bonding strength.
rffiffiffi T 0:07 t ln t; R2 ¼ 0:89 P
ð1Þ
where Z is the interface bonding strength of FRP-concrete (MPa). T is the exposure temperature (°C). t is the exposure time (h). P is the compressive strength (MPa) of concrete at room temperature.
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a) C30ˈControl /Rt
b) C30ˈ80䉝/1.5h
Fig. 10. SEM images of FRP-concrete interface.
The predicated results from Eq. (1) agreed well with the experimental data. The degradation degree of the interface bonding strength of FRP-concrete could be predicated. So the effective protective measure could be made to protect the concrete structure. The morphology of the adhesive-concrete interfaces at microscale is observed with SEM to analyze the mechanism of the reduction of interface bond strength. It illustrates that the interface between MER and concrete is dense at room temperature, but after high temperature exposure, micro-cracks form in the MERconcrete interface and concrete, which degrade the confinement effect. Due to the cracks propagation, the concrete strength decreases significantly and reduces the bonding strength of FRP-strengthened concrete interface. The failure is observed in the concrete (See Fig. 10). 3.3. Flexural strength test of FRP-strengthened concrete 3.3.1. Failure modes of FRP-strengthened concrete specimens Figs. 11 and 12 show the failure modes and the stress-strain curves of FRP-strengthened specimens in flexural test, respectively. Control/Rt represents the 1-lalyer-FRP bonded specimens FRP at room temperature, and Control/C30(C50) represents plain concrete specimens. The flexural strength (f) and fracture energy (W) of the R concrete prism is calculated by f = FL/bd2 and W = Fdx according to China national code-Design code on ordinary concrete (GB/T 50081-2002), respectively, where F is applied load (N), L is the support span (mm), b is the width of the prism (mm), d is the depth of R the prism (mm), and Fdx is the area under the load-displacement curves (Nm) in Fig. 13. To minimize the testing errors, average values are taken from three samples in the test. The degree of uncertainty of the specimens in each group is less than 5%. As shown in Fig. 11, the failure modes of the specimens are flexural or shearing failures [26] at different temperatures and for different durations. The trends in failure of the concrete specimens with different strengths are basically similar. The failure mode of FRP-strengthened concrete specimens changes from shearing failure to flexure failure once the temperature and time of exposure exceed the limit. For instance, the transition from shearing failure to flexure failure of C30 and C50 FRP-strengthened specimens is at 160 °C/3 h and 240 °C/1.5 h, respectively. The mechanical properties of FRP and the bonding strength of the FRP-concrete interface gradually decrease with increasing temperature and time. This results in a gradual loss in the utilization rate of FRP. When the specimens reach the critical load, micro-cracks form in the FRP and FRP-concrete interface. As the load increases, the micro-cracks
Fig. 11. Failure modes of strengthened concrete.
propagate toward the force-bearing region and widen until the specimen fails. As shown in Fig. 12 and 1) the trend in FRP stress-strain of the two types of concrete are similar, following a bilinear development behavior: both the slope of the curve and initial stiffness of each specimen at the first stage are basically identical; the second stage is plastic development with rapid development of tensile strain in the FRP. 2) As the exposure temperature and time increase, yield strength, ultimate strength and stiffness of the FRP-strengthened specimens significantly decrease. Due to the decrease in interface bonding strength and the degradation of FRP and MER, the mechanical properties of the FRP-strengthened specimens degrade. The load-displacement (mid-span) curves of FRP-strengthened concrete are shown in Fig. 13. ‘‘Displacement” represents the
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12
80ć/1.5h 160ć/1.5h 240ć/1.5h Control/Rt
8
10 8
f (MPa)
f (MPa)
10
12
80ć/3h 160ć/3h 240ć/3h Control/C30
6
6
4
4
2
2
0
0
1000
2000
3000
4000
0
5000
80ć/1.5h 160ć/1.5h 240ć/1.5h Control/Rt
0
1000
2000
3000
4000
Strain
Strain
a) C30
b) C50
80ć/3h 160ć/3h 240ć/3h Control/C50
5000
6000
7000
Fig. 12. Stress-strain curves of FRP-strengthened concrete.
35
Load (kN)
30
80ć/1.5h 160ć/1.5h 240ć/1.5h Control/Rt
50
80ć/3h 160ć/3h 240ć/3h Control/C30
40
Load (kN)
40
25 20 15 10
80ć/1.5h 160ć/1.5h 240ć/1.5h Control/Rt
80ć/3h 160ć/3h 240ć/3h Control/C50
30 20 10
5 0 0.0
0.5
1.0
1.5
2.0
2.5
0 0.0
0.5
Displacement (mm)
1.0
1.5
2.0
2.5
3.0
Displacement (mm)
a) C30
b) C50
Fig. 13. Load-displacement curves of FRP-strengthened concrete.
deflection in the mid-span of the specimens and ‘‘Load” represents the bearing load of the specimens. As shown in Fig. 13 and 1) the load-displacement curves of the two types of concrete develops linearly in the initial stage of loading, and as the load increases, the development of the curves becomes non-linear. 2) As the temperature and time increase, the initial elastic modulus and ultimate strength of specimens gradually decrease, which is similar to the stress-strain development of FRP. 3) For the plain concrete specimens, the loaddisplacement curve develops linearly from the initial load to the ultimate load. The specimens suddenly fail at the maximum loading. A typical brittle failure presents, and the mid-span ultimate deflection and ultimate load are obviously lower than those of the FRP-strengthened concrete. The ultimate load and fracture energy of the FRP-strengthened specimens are calculated on the load-displacement curves (Fig. 13), and are shown in Table 4. The correlations between the increasing rate of the strength of FRP-strengthened concrete and temperature (i.e. the ratio of the strength fu of FRP-strengthened concrete at a specific temperature to the strength f0 of unstrengthened concrete at room temperature) are shown in Fig. 14. Compared to the FRP-strengthened concrete specimens at 19 °C, the correlations between the fracture energy loss and exposure conditions of FRP-strengthened specimens are shown in Fig. 15. Chowdhury [10] claimed that concrete undergoes a slow increase in surface temperature at 400 °C for 300 min, with negligible variation in strength. The maximum temperature and time
Table 4 Results of FRP-strengthened specimens. Code
Control/C30(C50) Control/Rt 80 °C/1.5 h 80 °C/3.0 h 160 °C/1.5 h 160 °C/3.0 h 240 °C/1.5 h 240 °C/3.0 h
Ultimate load (kN)
Fracture energy (kNmm)
C30
C50
C30
C50
11.04 32.74 30.97 28.46 26.68 24.27 24.05 22.75
12.07 39.27 33.65 31.90 30.24 28.76 28.26 25.91
5.29 41.22 34.47 32.77 32.20 24.27 29.09 24.16
5.55 56.90 38.28 29.47 37.03 31.94 33.39 32.05
in the present work is 240 °C and 180 min, respectively, therefore the concrete compressive strength here is nearly constant (i.e. fu/f0 = 1). From Table 4 and Fig. 14 and 1) Compared to each of the plain concrete specimens (Control/C30 and Control/C50), the ultimate load of the FRP-strengthened specimen significantly increases. The increase rate of the strength of high-strength specimens is greater than that of low-strength specimens. However, the increase rate of ultimate strength gradually decreases with increasing temperature and time. 2) The effects of exposure temperature on the mechanical properties of FRP-strengthened specimens are greater than that of exposure time. The most distinct decrease in strength is observed in specimens, which are
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3.5
FRP-confined,C50,1.5h FRP-confined,C50,3h
3.0
4) After the high temperature exposure, micro-cracks form in the FRP-concrete interface, and therefore the structural properties of FRP-strengthened concrete are degraded.
2.5
fu /f0
Acknowledgements
FRP-confined,C30,1.5h FRP-confined,C30,3h
2.0 1.5 1.0
Unconfined 0.5 0.0
References
0
50
100
150
200
250
Temperatures/ć Fig. 14. Relationship between temperature.
60
C30 % Loss of fracture energy
The authors would like to acknowledge the financial support provided by Program for New Century Excellent Talents in University (NCET-12-0605), The Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions (CIT&TCD20150310), National Natural Science Foundation of China (51278014).
1.5h 3h
C50
50 40 30 20 10 0
80
160
240
80
160
240
Temperature(ć) Fig. 15. Loss of fracture energy and peak load enhancement ratio.
exposed to 240 °C (3 times of Tg) for 3 h. The strength decreases by 30.54% and 34.02% for C30 and C50 specimens, respectively. Table 4 and Fig. 15 indicate that the fracture energy and its loss rate of high-strength FRP-concrete are greater than those of lowstrength specimens. The fracture energy loss of FRP-strengthened concrete structures increases gradually with temperature but is not proportional to exposure time at elevated temperature, which suggests that the fracture energy loss of FRP-strengthened concrete structures is more sensitive to temperature. 4. Conclusions 1) As high temperature and exposure time increase, the ultimate tensile strength and ultimate tensile (fracture) strain of MER (FRP) decrease. 2) As temperature and exposure time increase, the FRPconcrete interface bonding strength shows a decreasing trend. The loss rate of the ultimate capacity of FRPstrengthened concrete increases gradually, and the loss rates of ultimate load and fracture energy of high-strength concrete structures are greater than those of low-strength concrete structures. The fracture energy loss of FRPstrengthened concrete structures is more sensitive to temperature than to exposure time. 3) FRP stress-strain in both two types of FRP-concrete with different strengths presents a bilinear behavior.
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