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Dynamic Behavior of PET FRP and Its Preliminary Application in Impact Strengthening of Concrete Columns

Dynamic Behavior of PET FRP and Its Preliminary Application in Impact Strengthening of Concrete... applied sciences Communication Dynamic Behavior of PET FRP and Its Preliminary Application in Impact Strengthening of Concrete Columns 1 1 2 3 1 , Yu-Lei Bai , Zhi-Wei Yan , Togay Ozbakkaloglu , Jian-Guo Dai , Jun-Feng Jia * and Jun-Bo Jia Key Laboratory of Urban Security and Disaster Engineering of Ministry of Education, Beijing University of Technology, Beijing 100124, China; baiyulei@bjut.edu.cn (Y.-L.B.); yanzhw@126.com (Z.-W.Y.); junbojia2001@yahoo.com (J.-B.J.) Ingram School of Engineering, Texas State University, San Marcos, TX 78667, USA; togay.oz@txstate.edu Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong 999077, China; cejgdai@polyu.edu.hk * Correspondence: jiajunfeng@bjut.edu.cn Received: 15 October 2019; Accepted: 15 November 2019; Published: 20 November 2019 Abstract: Polyethylene terephthalate (PET) fiber has attracted significant attention for reinforced concrete (RC) structure rehabilitation due to its large rupture strain (LRS; more than 7%) characteristic and recyclability from waste plastic bottles. This study presents a dynamic tensile test of PET fiber bundles performed using a drop-weight impact system. Results showed that the tensile strength and the elastic modulus of the PET fiber bundles increased, whereas the failure strain and the toughness decreased with the increasing strain rate from 1/600 to 160 s . In addition, the performance of concrete confined with the PET fiber-reinforced polymer (FRP) under impact loading was investigated based on a 75 mm-diameter split Hopkinson pressure bar (SHPB) device and a drop-weight apparatus. For the SHPB test, owing to the large rupture strain property of PET FRP, the PET FRP-confined concrete exhibited significantly better performance under impact loading compared to its counterpart confined with carbon FRPs (CFRPs). During the drop-weight test, the confinement of the PET FRP composites to the concrete columns as external jackets not only improved the peak impact force, but also prolonged the impact process. Keywords: composite materials; polymeric composites; PET FRP; dynamic behavior 1. Introduction Fiber-reinforced polymers (FRPs) have been widely used for seismic strengthening of existing reinforced concrete (RC) structures, owing to their advantages of high strength-to-weight ratios, corrosion resistance, and superior durability [1–14]. Considering the fact that the existing RC columns may be subjected to not only seismic loading but also dynamic loading such as vehicle/boat impact during their service life [15], the investigations of impact behaviors of RC structures strengthened with FRP composites are of vital importance for the application of FRPs in the impact-resistant design of RC structures. Although extensive studies of impact-resistant behaviors of conventional concrete and RC structures have been conducted [16–21], only a few studies of impact-resistant behaviors of FRP-confined concrete were conducted [22–27]. Xiao and Shen [23] conducted the drop-weight test to investigate the impact-resistant behaviors of the concrete-filled steel tube column confined with carbon FRPs (CFRPs) and found that the damage of the specimen could be reduced with the application of CFRPs as external jackets. Huang et al. [26] investigated the impact-resistant behaviors of the glass FRP (GFRP)–SR (spiral reinforcement)-confined Appl. Sci. 2019, 9, 4987; doi:10.3390/app9234987 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, x FOR PEER REVIEW 2 of 11 mm-diameter split Hopkinson pressure bar (SHPB) apparatus to investigate the dynamic mec Appl. hani Sci. cal 2019 behavio , 9, 4987rs of the concrete confined with aramid FRPs (AFRPs). The experimental result 2 of s 11 showed that the dynamic compressive strength, ultimate strain, and toughness of the concrete were remarkably increased with the application of the external AFRP jacket to confine the core concrete. concrete columns and found that an increase in the thickness of the GFRP tube led to an increase Considering the fact that the FRP mainly bears tensile force in the strengthening of the in the impact-resistant behaviors of the specimen. Yang et al. [27] used a 100 mm-diameter split structures and the fiber bundles are the main load-bearing element in FRP composites, the dynamic Hopkinson pressure bar (SHPB) apparatus to investigate the dynamic mechanical behaviors of the tensile mechanical properties of fiber bundles are necessary for studying the failure mechanism concrete confined with aramid FRPs (AFRPs). The experimental results showed that the dynamic between the FRP and the concrete under impact loading. To address this issue, some studies have compressive strength, ultimate strain, and toughness of the concrete were remarkably increased with been reported to date. Ou et al. [28] investigated the strain rate effect on the dynamic tensile the application of the external AFRP jacket to confine the core concrete. mechanical properties of the glass-fiber bundle specimen and found that the tensile strength Considering the fact that the FRP mainly bears tensile force in the strengthening of the structures −1 increased with the increasing strain rate from 1/600 to 160 s . Zhu et al. [29] investigated the effect of and the fiber bundles are the main load-bearing element in FRP composites, the dynamic tensile −1 the strain rate (20 to 100 s ) on the dynamic tensile mechanical properties of the Kevlar 49 fiber mechanical properties of fiber bundles are necessary for studying the failure mechanism between the bundle specimen. The experimental results showed that the tensile strength, the elastic modulus, FRP and the concrete under impact loading. To address this issue, some studies have been reported to and the toughness increased with the increasing strain rate. date. Ou et al. [28] investigated the strain rate e ect on the dynamic tensile mechanical properties of The past decade has witnessed the emergence of a new type of FRP composite, polyethylene the glass-fiber bundle specimen and found that the tensile strength increased with the increasing strain terephthalate (PET) [30–42]. Research has shown that its very large rupture strain (i.e., more than 1 1 rate from 1/600 to 160 s . Zhu et al. [29] investigated the e ect of the strain rate (20 to 100 s ) on 7%) may lead to enhanced structural ductility and impact energy dissipation in the impact-resistant the dynamic tensile mechanical properties of the Kevlar 49 fiber bundle specimen. The experimental retrofitting/strengthening of RC structures [43]. This paper briefly presents the authors’ recent results showed that the tensile strength, the elastic modulus, and the toughness increased with the studies on the dynamic tensile mechanical properties of PET fiber bundles and the dynamic increasing strain rate. compressive behavior of concrete confined by the PET FRP. The preliminary results illustrated the The past decade has witnessed the emergence of a new type of FRP composite, polyethylene great potential of the use of PET FRP in impact strengthening applications of RC structures at the terephthalate (PET) [30–42]. Research has shown that its very large rupture strain (i.e., more than material (i.e., PET fiber bundle and PET FRP-confined concrete) and component (i.e., PET 7%) may lead to enhanced structural ductility and impact energy dissipation in the impact-resistant FRP-confined concrete column) levels. It is expected that the results presented here will facilitate retrofitting/strengthening of RC structures [43]. This paper briefly presents the authors’ recent studies further investigation of the impact-resistant behavior of PET FRP-strengthened RC structures. on the dynamic tensile mechanical properties of PET fiber bundles and the dynamic compressive behavior of concrete confined by the PET FRP. The preliminary results illustrated the great potential 2. Materials and Methods of the use of PET FRP in impact strengthening applications of RC structures at the material (i.e., PET fiber bundle and PET FRP-confined concrete) and component (i.e., PET FRP-confined concrete 2.1. Dynamic Tensile Test of PET Fiber Bundles column) levels. It is expected that the results presented here will facilitate further investigation of the A typical specimen was fabricated with a gauge length of 25 mm (Figure 1) according to Ou et impact-resistant behavior of PET FRP-strengthened RC structures. al. [28]. A long fiber bundle with a 1800 mm length was extracted from the unidirectional PET fiber −3 fabric and weighed by an electronic balance to obtain a mass of 0.307 g. The linear density (1.7  10 2. Materials and Methods g/cm) could be calculated by dividing the mass by the length, and the area of the transverse section −3 2 2.1. Dynamic Tensile Test of PET Fiber Bundles of the bundle (1.23  10 cm ) could be obtained by dividing the linear density by the bulk density (1.38 g/cm ) that was provided by the manufacturer (Maeda Kosen Co., Sakai-shi, Japan). A fiber A typical specimen was fabricated with a gauge length of 25 mm (Figure 1) according to bundle with a length of 60 mm was cut from the extracted long fiber bundle. Two aluminum sheets Ou et al. [28]. A long fiber bundle with a 1800 mm length was extracted from the unidirectional with a length of 20 mm, a width of 15 mm, and a thickness of 0.2 mm were roughhewed by a PET fiber fabric and weighed by an electronic balance to obtain a mass of 0.307 g. The linear density serrated steel 3 plate and folded along the long side to catch the fiber bundle using the epoxy resin. (1.7  10 g/cm) could be calculated by dividing the mass by the length, and the area of the transverse After the resin was fully cured an 3 d the 2 redundant fiber bundle at both sides was cut off, a fiber section of the bundle (1.23  10 cm ) could be obtained by dividing the linear density by the bulk bundle specimen with 3 a gauge length of 25 mm was completed. density (1.38 g/cm ) that was provided by the manufacturer (Maeda Kosen Co., Sakai-shi, Japan). A Five PET fiber bundle specimens were tested for each displacement rate of 1, 2, 3, and 4 m/s fiber bundle with a length of 60 mm was cut from the extracted long fiber bundle. Two aluminum −1 with the respective strain rates of 40, 80, 120, and 160 s , which is the ratio of the displacement rate sheets with a length of 20 mm, a width of 15 mm, and a thickness of 0.2 mm were roughhewed by to the corresponding gauge length of the specimen, using an Instron drop-weight impact system a serrated steel plate and folded along the long side to catch the fiber bundle using the epoxy resin. (Figure 2). The impact height of the system ranged from 0.03 to 1.10 m, and the maximum mass of a After the resin was fully cured and the redundant fiber bundle at both sides was cut o , a fiber bundle drop hammer was 37.5 kg. The drop hammer dropped freely along the guide rail and impacted the specimen with a gauge length of 25 mm was completed. specimen after the release of the drop hammer. Figure 1. Schematic diagram of a fiber bundle specimen. Figure 1. Schematic diagram of a fiber bundle specimen. Five PET fiber bundle specimens were tested for each displacement rate of 1, 2, 3, and 4 m/s with the respective strain rates of 40, 80, 120, and 160 s , which is the ratio of the displacement rate to the corresponding gauge length of the specimen, using an Instron drop-weight impact system (Figure 2). Appl. Sci. 2019, 9, 4987 3 of 11 The impact height of the system ranged from 0.03 to 1.10 m, and the maximum mass of a drop hammer was 37.5 kg. The drop hammer dropped freely along the guide rail and impacted the specimen after the Appl. Sc release i. 2019of , 9, x FOR the dr PEER op hammer REVIEW . 3 of 11 Figure 2. Instron drop-weight impact system. Figure 2. Instron drop-weight impact system. For comparison, five PET fiber bundle specimens were also tested under quasi-static conditions at For comparison, five PET fiber bundle specimens were also tested under quasi-static conditions a crosshead speed of 2.5 mm/min (strain rate of 1/600 s ) using an MTS universal testing machine. The −1 at a crosshead speed of 2.5 mm/min (strain rate of 1/600 s ) using an MTS universal testing machine. loading capacity of the MTS system ranges from 0.1 to 30 kN with a data sampling rate of 1 kHz. The The loading capacity of the MTS system ranges from 0.1 to 30 kN with a data sampling rate of 1 kHz. tension of the PET fiber bundle specimen was measured with a force sensor, of which the maximum The tension of the PET fiber bundle specimen was measured with a force sensor, of which the range and the data acquisition rate are 1 kN and 20 Hz, respectively. The deformation of the PET fiber maximum range and the data acquisition rate are 1 kN and 20 Hz, respectively. The deformation of bundle specimen was considered as the displacement of the upper beam of the system, due to the fact the PET fiber bundle specimen was considered as the displacement of the upper beam of the system, that the sti ness of the specimen was far lower than that of the loading system [44]. due to the fact that the stiffness of the specimen was far lower than that of the loading system [44]. 2.2. Split Hopkinson Pressure Bar Test 2.2. Split Hopkinson Pressure Bar Test Concrete cylinders with a diameter of 150 mm and a height of 305 mm were cast with ordinary Concrete cylinders with a diameter of 150 mm and a height of 305 mm were cast with ordinary Portland cement, river sand, and crushed granitic rocks with a particle size ranging from 5 to 10 mm. Portland cement, river sand, and crushed granitic rocks with a particle size ranging from 5 to 10 mm. The average 28-day compressive strength of the concrete was 18.5 MPa. The average 28-day compressive strength of the concrete was 18.5 MPa. A total of 9 concrete columns with a 70 mm diameter and a 38 mm height were prepared. Among A total of 9 concrete columns with a 70 mm diameter and a 38 mm height were prepared. them, 3 concrete cylinders were wrapped with one layer of the PET FRP, of which the nominal thickness, Among them, 3 concrete cylinders were wrapped with one layer of the PET FRP, of which the the tensile strength, and the elastic modulus were 0.841 mm, 751 MPa, and 17.9 GPa, respectively, 3 nominal thickness, the tensile strength, and the elastic modulus were 0.841 mm, 751 MPa, and 17.9 concrete cylinders were wrapped with two layers of the PET FRP, and 3 specimens were wrapped with GPa, respectively, 3 concrete cylinders were wrapped with two layers of the PET FRP, and 3 one layer of the CFRP, of which the nominal thickness, the tensile strength, and the elastic modulus specimens were wrapped with one layer of the CFRP, of which the nominal thickness, the tensile were 0.165 mm, 4423 MPa, and 240 GPa, respectively, for the purpose of the comparison (Table 1), strength, and the elastic modulus were 0.165 mm, 4423 MPa, and 240 GPa, respectively, for the according to the manual wet layup way [30]. A continuous fiber sheet saturated with the epoxy resin, purpose of the comparison (Table 1), according to the manual wet layup way [30]. A continuous mixed at a ratio of the main resin component L-500AS to the hardener L-500BS of 2:1 (provided by fiber sheet saturated with the epoxy resin, mixed at a ratio of the main resin component L-500AS to SANYU Shanghai Company), was wrapped around the concrete cylinder with the fibers orientated the hardener L-500BS of 2:1 (provided by SANYU Shanghai Company), was wrapped around the in the hoop direction of the cylinder. To prevent the debonding failure, a fiber sheet with a length concrete cylinder with the fibers orientated in the hoop direction of the cylinder. To prevent the of 350 mm (i.e., one circumference + half circumference of the overlapping zone) was used for the debonding failure, a fiber sheet with a length of 350 mm (i.e., one circumference + half circumference fabrication of one-layer FRP-confined concrete specimens. To reduce the test scatter, three identical of the overlapping zone) was used for the fabrication of one-layer FRP-confined concrete specimens. specimens were tested at each impact velocity using a 75 mm-diameter SHPB device to investigate the To reduce the test scatter, three identical specimens were tested at each impact velocity using a 75 dynamic compressive behavior of the FRP-confined concrete (Figure 3). The apparatus comprised an mm-diameter SHPB device to investigate the dynamic compressive behavior of the FRP-confined energy source, a 600 mm-long projectile, a 5000 mm-long incident bar, a 3000 mm-long transmission concrete (Figure 3). The apparatus comprised an energy source, a 600 mm-long projectile, a 5000 bar, a digital oscilloscope, and a velocity testing system that was used to record the initial velocity of mm-long incident bar, a 3000 mm-long transmission bar, a digital oscilloscope, and a velocity the projectile. testing system that was used to record the initial velocity of the projectile. The specimen was sandwiched between the incident and transmission bars. The stress wave was reshaped with a rubber shaper (1 mm in thickness and 20 mm in diameter) installed on the impact end of the incident bar. The time histories of the strain ε(t), stress σ(t), and strain rate ε̇ (t) of the specimen can be obtained by Equations (1)–(3), according to the two-wave theory [27]: 2C  (t) [ (t) (t)]dt ,  (1) 0 i t s Appl. Sci. 2019, 9, 4987 4 of 11 Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 11 Table 1. Summary of specimens for the SHPB test. Nominal  (t) E Tensile  (t) , Unconfined (2) Volume Number Initial Number of Type of D  H Thickness Strength Concrete Type of FRP Fraction of Velocity Identical Test (mm) of FRP of FRP Strength of Fiber Layers (m/s) Specimen (mm) (MPa) (MPa) 2C Carbon FRP   (t) [ (t) (t)] , (3) i t 70  38 0.165 15% 4423 1 10.9 18.5 3 (CFRP) SHPB Polyethylene test 70  38 terephthalate 0.841 31% 751 1 11.4 18.5 3 where εi(t) and εt(t) are the strain of the incident and transmission bars, respectively; A is the (PET) FRP cross-sectional area of bars; As and Ls are the initial cross-sectional area and the height of the 70  38 PET FRP 0.841 31% 751 2 11.1 18.5 3 specimen, respectively. Figure 3. Split Hopkinson pressure bar (SHPB) apparatus. Figure 3. Split Hopkinson pressure bar (SHPB) apparatus. The specimen was sandwiched between the incident and transmission bars. The stress wave was Table 1. Summary of specimens for the SHPB test. reshaped with a rubber shaper (1 mm in thickness and 20 mm in diameter) installed on the impact end ofT the incident bar. The time histories of the strain "(t), stress (t), and strain rate " (t) of the specimen can be obtained by Equations (1)–(3), according to the two-wave theory [27]: 2C e Nominal Tensile Unconfined Number "(t) = [" (t) " (t)]dt, (1) Volume i Initial o D × H Type of thickness 0 strength Number concrete of fraction velocity f (mm) FRP of FRP of FRP of layers strength identical of fiber (m/s) (t) = E " (t), (2) t (mm) (MPa) (MPa) specimen . 2C "(t) = [" (t) " (t)], (3) i t where " (t) and " (t) are the strain of the incident and transmission bars, respectively; A is the i t Carbon FRP 70 × 38 0.165 15% 4423 1 10.9 18.5 3 cross-sectional area of bars; A and L are the initial cross-sectional area and the height of the s s (CFRP) SHP specimen, respectively. Polyethylene 70 × 38 terephthalate 0.841 31% 751 1 11.4 18.5 3 test 2.3. Drop-Weight Test (PET) FRP Twelve concrete cylinders with a diameter of 95 mm, a height of 285 mm, and an average 28-day 70 × 38 PET FRP 0.841 31% 751 2 11.1 18.5 3 compressive strength of 58.5 MPa were prepared. The preparation of the concrete column and the fabrication 2.3. Drop-Wei of ght the Te FRP-confined st concrete column were the same as those presented in Section 2.2. Among them, 8 cylinders were wrapped with one layer of the PET FRP, 2 cylinders were confined with Twelve concrete cylinders with a diameter of 95 mm, a height of 285 mm, and an average one layer of the AFRP, of which the nominal thickness, the tensile strength, and the elastic modulus 28-day compressive strength of 58.5 MPa were prepared. The preparation of the concrete column are 0.169 mm, 3732 MPa, and 115 GPa, respectively, and 2 remaining unconfined concrete cylinders and the fabrication of the FRP-confined concrete column were the same as those presented in Section were tested as control specimens (Table 2). All specimens were denoted as “A-B-C”, where A, B, and C 2.2. Among them, 8 cylinders were wrapped with one layer of the PET FRP, 2 cylinders were stand for the type of FRP, the number of FRP layers, and the drop height, respectively. Two nominally confined with one layer of the AFRP, of which the nominal thickness, the tensile strength, and the identical specimens were tested at each drop height to reduce the data scatter. elastic modulus are 0.169 mm, 3732 MPa, and 115 GPa, respectively, and 2 remaining unconfined concrete cylinders were tested as control specimens (Table 2). All specimens were denoted as “A-B-C”, where A, B, and C stand for the type of FRP, the number of FRP layers, and the drop Appl. Sci. 2019, 9, 4987 5 of 11 Table 2. Summary of specimens for the drop-weight impact test. Nominal Tensile Unconfined Volume Number Impact Number of Type of D  H Thickness Strength Concrete Type of FRP Fraction of Height Identical Test (mm) of FRP of FRP Strength of Fiber Layers (m) Specimen (mm) (MPa) (MPa) 95  285 - - - - - 2 58.5 2 95  285 AFRP 0.169 19% 3732 1 2 58.5 2 Drop-weight 95  285 PET FRP 0.841 31% 751 1 2 58.5 2 Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 11 test 95  285 PET FRP 0.841 31% 751 1 3 58.5 2 95  285 PET FRP 0.841 31% 751 1 4 58.5 2 95  285 PET FRP 0.841 31% 751 1 5 58.5 2 height, respectively. Two nominally identical specimens were tested at each drop height to reduce the data scatter. A large-capacity drop-weight setup was used for the impact test [23], consisting of a hammerhead A large-capacity drop-weight setup was used for the impact test [23], consisting of a with a weight of 188 kg, 14 balancing weights each with a weight of 68 kg, a steel frame, a track, a hammerhead with a weight of 188 kg, 14 balancing weights each with a weight of 68 kg, a steel control system, and a lifting device (Figure 4). The maximum drop height was 16 m. Di erent impact frame, a track, a control system, and a lifting device (Figure 4). The maximum drop height was 16 m. energies can be obtained by adjusting the mass of the drop weight and the drop height. In this test, the Different impact energies can be obtained by adjusting the mass of the drop weight and the drop drop weight was held the same at 256 kg (the hammerhead + one balancing weight), and di erent height. In this test, the drop weight was held the same at 256 kg (the hammerhead + one balancing impact energies were achieved by varying the impact height (i.e., 2, 3, 4, and 5 m). weight), and different impact energies were achieved by varying the impact height (i.e., 2, 3, 4, and 5 m). Figure 4. Drop-weight test setup. Figure 4. Drop-weight test setup. 3. Results and Discussion Table 2. Summary of specimens for the drop-weight impact test. Figure 5 shows the typical stress–strain curves of the PET fiber bundle specimens at di erent Nominal Tensile Unconfined Volume Impact Number of strain rates. The curves exhibit a bilinear stress–strain relationship and a rapid drop from the peak D × H Type of thickness strength Number concrete Type of test fraction height identical stress to zero, which indicates a brittle fracture of the PET fiber bundle. The tensile strength, the elastic (mm) FRP of FRP of FRP of layers strength of fiber (m) specimen modulus, the failure strain, and the toughness are sensitive to the strain rate. When the strain rate (mm) (MPa) (MPa) increased from 1/600 to 160 s , there was an increase in the tensile strength, which can be attributed 95 × 285 - - - - - 2 58.5 2 to the fact that there was not enough time for the defects of filaments in the fiber bundle to develop 95 × 285 AFRP 0.169 19% 3732 1 2 58.5 2 at a high strain rate [45]. The initial elastic modulus and the second elastic modulus (i.e., slopes of Drop-weight 95 × 285 PET FRP 0.841 31% 751 1 2 58.5 2 the two portions of the stress–strain curve) increased with an increase in the strain rate from 1/600 to test 95 × 285 PET FRP 0.841 31% 751 1 3 58.5 2 160 s . Conversely, there was a decrease in the failure strain and the toughness with the increasing 95 × 285 PET FRP 0.841 31% 751 1 4 58.5 2 strain rate from 1/600 to 160 s . Figure 6 shows the failure modes of the PET fiber bundles at di erent 95 × 285 PET FRP 0.841 31% 751 1 5 58.5 2 strain rates. At a low strain rate (e.g., 1/600 s ), the fracture surface of the fiber bundle had a chaotic appearance with di erent fracture positions of filaments. At a high strain rate (e.g., 160 s ), the fiber 3. Results and Discussion bundle ruptured with a relatively trim fractography. Figure 5 shows the typical stress–strain curves of the PET fiber bundle specimens at different strain rates. The curves exhibit a bilinear stress–strain relationship and a rapid drop from the peak stress to zero, which indicates a brittle fracture of the PET fiber bundle. The tensile strength, the elastic modulus, the failure strain, and the toughness are sensitive to the strain rate. When the strain −1 rate increased from 1/600 to 160 s , there was an increase in the tensile strength, which can be attributed to the fact that there was not enough time for the defects of filaments in the fiber bundle to develop at a high strain rate [45]. The initial elastic modulus and the second elastic modulus (i.e., slopes of the two portions of the stress–strain curve) increased with an increase in the strain rate −1 from 1/600 to 160 s . Conversely, there was a decrease in the failure strain and the toughness with −1 the increasing strain rate from 1/600 to 160 s . Figure 6 shows the failure modes of the PET fiber −1 bundles at different strain rates. At a low strain rate (e.g., 1/600 s ), the fracture surface of the fiber bundle had a chaotic appearance with different fracture positions of filaments. At a high strain rate −1 (e.g., 160 s ), the fiber bundle ruptured with a relatively trim fractography. Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 11 Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 11 Appl. Sci. 2019, 9, 4987 6 of 11 PET fiber bundle -1 PET fiber bundle 1/600 s -1 -1 1/600 s 40 s -1 800 -1 40 s 80 s -1 -1 80 s 120 s -1 -1 120 s 160 s -1 160 s 0 3 6 9 12 15 0 3 6 9 12 15 Strain (%) Strain (%) Figure 5. Tensile stress–strain curves of the PET fiber bundle at different strain rates. Figure 5. Tensile stress–strain curves of the PET fiber bundle at di erent strain rates. Figure 5. Tensile stress–strain curves of the PET fiber bundle at different strain rates. (a) (b) (a) (b) (c) (d) (c) (d) (e) (e) 1−1 1 −1 −1 1 −1 1 Figure Figure 6. 6. S SEM EM images images of of failur failur ee modes modes at atthe the strain strain rates rateof s of 1 /1 600 /600 s s (a (a );); 40 40 s s ((b b ); );80 80 ss ( (c c); ); 120 120 s s ((d d)); ; −1 −1 −1 −1 Figure −1 6. SEM images of failure modes at the strain rates of 1/600 s (a); 40 s (b); 80 s (c); 120 s (d); 160 160 s s (( ee )). . −1 160 s (e). Figure 7a shows the failure mode of one layer of the CFRP-confined concrete at a strain rate of Figure 7a shows the failure mode of one layer of the CFRP-confined concrete at a strain rate of 1 1 218 s fr Fiom gure SHPB 7a sho tests. ws th Atea failu strain re rate mode of of about one 220 layer s of , one the layer CFRP of -con the fined CFRP-confined concrete atconcr a strai ete n was rate of −1 −1 218 s from SHPB tests. At a strain rate of about 220 s , one layer of the CFRP-confined concrete was −1 −1 smashed 218 s from together SHPB with test the s. At external a strain FRP rate ro uptur f aboe, ut wher 220 seas, , one due lay to er its of very the CFRP large-con rupt fiur ned e strain, concret two e was smashed together with the external FRP rupture, whereas, due to its very large rupture strain, two layers smashe of the d PET togeth FRP-confined er with the concr externa etel could FRP rupture not be destr , whoyed ereas, (Figur due to e 7 its b). very Ther efor large, e to rupture investigate strain, the two layers of the PET FRP-confined concrete could not be destroyed (Figure 7b). Therefore, to investigate failur laye ers mechanism of the PET of FRP the-con concr fined ete confined concrete could with the not PET be destroy FRP, the ed specimen (Figure 7b was ). Th repeatedly erefore, to impacted investigate the failure mechanism of the concrete confined with the PET FRP, the specimen was repeatedly under the the failure same mimpact echanism velocity of the until confailur crete e con (Figur fined e 7wit b). h As thshown e PET in FRP Figur , the e 7speci b, only men a small was r piece epeatedl of y Stress (MPa) Stress (MPa) Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 11 impacted under the same impact velocity until failure (Figure 7b). As shown in Figure 7b, only a small piece of the concrete was peeled off the surface of concrete confined with the PET FRP after the −1 −1 second impact with a strain rate of 272 s . Under the third impact at a strain rate of 280 s , partial concrete was crushed with no rupture of the external PET FRP. Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 11 Appl. Sci. 2019, 9, 4987 7 of 11 impacted under the same impact velocity until failure (Figure 7b). As shown in Figure 7b, only a small piece of the concrete was peeled off the surface of concrete confined with the PET FRP after the the concrete was peeled o the surface of concrete confined with the PET FRP after the second impact −1 −1 second impact with a strain rate of 272 s . Under the third impact at a strain rate of 280 s , partial 1 1 with a strain rate of 272 s . Under the third impact at a strain rate of 280 s , partial concrete was concrete was crushed with no rupture of the external PET FRP. crushed with no rupture of the external PET FRP. −1 st −1 nd −1 rd −1 218 s 1 impact (222 s ) 2 impact (272 s ) 3 impact (280 s ) (a) (b) Figure 7. Failure modes of the concrete confined with the CFRP (a) and the PET FRP (b). Figure 8 shows the dynamic stress–strain curves of specimens confined with the CFRP and the −1 st −1−1 nd −1 rd −1 218 s 1 impact (222 s) 2 impact (272 s) 3 impact (280 s ) PET FRP at a strain rate of around 220 s . The PET and CFRPs were designed for a comparison based on ultimate jacket strengths (i.e., the product of tensile strength and thickness of FRP jacket). (a) (b) One layer of the CFRP provides an ultimate jacket strength of 729.8 N/mm that falls between the Figure Figure 7. 7. Failur Failure modes of th e modes of thee concrete concrete confined confined with with the the CFRP CFRP ( (aa )) and t and the he PET PET FRP FRP ( (bb ).). values provided by one layer and two layers of the PET FRPs (631.6 N/mm and 1263.2 N/mm, respectively). The dynamic compressive strength of the specimen confined with one layer of the Figure 8 shows the dynamic stress–strain curves of specimens confined with the CFRP and the PET Figure 8 shows the dynamic stress–strain curves of specimens confined with the CFRP and the CFRP was 89.4 MPa, whereas the corresponding value for one layer of the PET FRP-confined −1 FRP at a strain rate of around 220 s . The PET and CFRPs were designed for a comparison based on PET FRP at a strain rate of around 220 s . The PET and CFRPs were designed for a comparison concrete was 82.9 MPa with a larger compressive strain of 4.58%. In addition, the dynamic ultimate jacket strengths (i.e., the product of tensile strength and thickness of FRP jacket). One layer of based on ultimate jacket strengths (i.e., the product of tensile strength and thickness of FRP jacket). compressive strength and the corresponding strain of two layers of the PET FRP confined-concrete the CFRP provides an ultimate jacket strength of 729.8 N/mm that falls between the values provided by One layer of the CFRP provides an ultimate jacket strength of 729.8 N/mm that falls between the were remarkably higher than those of the CFRP-confined counterpart, indicating the superior one layer and two layers of the PET FRPs (631.6 N/mm and 1263.2 N/mm, respectively). The dynamic values provided by one layer and two layers of the PET FRPs (631.6 N/mm and 1263.2 N/mm, impact performance of the PET FRP-confined concrete due to the large rupture strain characteristic. compressive strength of the specimen confined with one layer of the CFRP was 89.4 MPa, whereas respectively). The dynamic compressive strength of the specimen confined with one layer of the Figure 9 shows the dynamic compressive stress–strain curves of the specimen confined with the corresponding value for one layer of the PET FRP-confined concrete was 82.9 MPa with a larger CFRP was 89.4 MPa, whereas the corresponding value for one layer of the PET FRP-confined two layers of the PET FRP under multiple impacts. The compressive strength of two layers of the compressive strain of 4.58%. In addition, the dynamic compressive strength and the corresponding concrete was 82.9 MPa with a larger compressive strain of 4.58%. In addition, the dynamic PET FRP-confined concrete decreased after the second impact due to the progressive concrete strain of two layers of the PET FRP confined-concrete were remarkably higher than those of the compressive strength and the corresponding strain of two layers of the PET FRP confined-concrete damage, which reveals superior impact-resistant behavior as far as deformability is concerned due CFRP-confined counterpart, indicating the superior impact performance of the PET FRP-confined were remarkably higher than those of the CFRP-confined counterpart, indicating the superior to the large rupture strain characteristic. concrete due to the large rupture strain characteristic. impact performance of the PET FRP-confined concrete due to the large rupture strain characteristic. Figure 9 shows the dynamic compressive stress–strain curves of the specimen confined with two layers of the PET FRP under multiple impact -1s. The compressive strength of two layers of the CFRP 218 s -1 PET FRP-confined concrete decreased after the second impact due to the progressive concrete 1-layer PET FRP 231 s -1 2-layer PET FRP 222 s damage, which reveals superior 1 impa 00 ct-resistant behavior as far as deformability is concerned due to the large rupture strain characteristic. 40 -1 CFRP 218 s 120 -1 1-layer PET FRP 231 s -1 2-layer PET FRP 222 s 0 1 2 3 4 5 6 7 8 Strain (%) Figure 8. Comparison of the dynamic compressive stress–strain curves of the specimen at a strain rate Figure 8. Comparison of the dynamic compressive stress–strain curves of the specimen at a strain of around 220 s . −1 rate of around 220 s . Figure 9 shows the dynamic compressive stress–strain curves of the specimen confined with two 01 23 45678 layers of the PET FRP under multiple impacts. The compressive strength of two layers of the PET Strain (%) FRP-confined concrete decreased after the second impact due to the progressive concrete damage, which reveals superior impact-resistant behavior as far as deformability is concerned due to the large Figure 8. Comparison of the dynamic compressive stress–strain curves of the specimen at a strain rupture strain characteristic. −1 rate of around 220 s . Stress (MPa) Stress (MPa) Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 11 Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 11 Appl. Sci. 2019, 9, 4987 8 of 11 st -1 1 222 s 2-layer PET600 FRP nd -1 100 2 272 s st -1 rd -1 1 222 s 3 280 s 2-layer PET600 FRP nd -1 100 2 272 s rd -1 3 280 s 0 1 2 3 4 5 6 7 8 Strain (%) 0 1 2 3 4 5 6 7 8 Strain (%) Figure 9. Dynamic compressive stress–strain curves of the specimen confined with two layers of the PET FRP under multiple impacts. Figure 9. Dynamic compressive stress–strain curves of the specimen confined with two layers of the Figure 9. Dynamic compressive stress–strain curves of the specimen confined with two layers of the PET FRP under multiple impacts. PET FRP under multiple impacts. Figure 10a shows the time histories of specimens with three different confining conditions (i.e., no Figur FRP,e one 10a lshows ayer of the AFtime RP, and histories PET F of Rspecimens P) under the with same thr ee drop di he erent ight confining of 2 m frconditions om the drop (i.e., -weight no Figure 10a shows the time histories of specimens with three different confining conditions (i.e., FRP test , one s. It layer can b of e AFRP seen ,th and at, c PET ompared FRP) under to the the uncon same findr ed op spec height imen, of th 2e m appli from catio the n drof op-weight AFRP led tests. to an no FRP, one layer of AFRP, and PET FRP) under the same drop height of 2 m from the drop-weight It can incre be asseen e in th that, e peak compar impact ed to force. the unconfined It can also be specimen, seen in Fithe gure application 10a that thof e appli AFRP caled tion to of an PET incr FRP ease not tests. It can be seen that, compared to the unconfined specimen, the application of AFRP led to an in only the peak impro impact ved th for e peak ce. Itimp canact also force, be seen but in also Figur proe longed 10a that the the imapplication pact process, ofleading PET FRP to not a superior only increase in the peak impact force. It can also be seen in Figure 10a that the application of PET FRP not impr impact oved res the ispeak tance. impact Figure for 10 ce, b show but also s the prtim olonged e histori the es impact of the pr idocess, enticalleading PET FRP to-a con super fined ior spe impact cimens only improved the peak impact force, but also prolonged the impact process, leading to a superior resistance. tested at Figur different e 10 b drop shows heithe ghts. time As histories can be seen of the in F identical igure 10b PET , the FRP-confined peak impact specimens force of the tested specimen at impact resistance. Figure 10b shows the time histories of the identical PET FRP-confined specimens di sler ightly ent dr incre op heights. ased, an As d th can e impact be seen dur in Figur ation egra 10b, duthe ally peak decre impact ased with force th of e the incr specimen easing drop slightly height tested at different drop heights. As can be seen in Figure 10b, the peak impact force of the specimen incr from eased, 2 to and 5 m. the It iis mpact wortduration h noting th gradually at the dur decr ation eased of th with e specimen the incr easing impacted drop und height er a 4 frm om drop 2 to height 5 m. slightly increased, and the impact duration gradually decreased with the increasing drop height It is was worth longe noting r than that that theoduration f the counterp of thea specimen rt under impacted a 3 m drop under heig aht 4, m which drop height can bewas cons longer idered than a test from 2 to 5 m. It is worth noting that the duration of the specimen impacted under a 4 m drop height that sca of tter the in this counterpart study. under a 3 m drop height, which can be considered a test scatter in this study. was longer than that of the counterpart under a 3 m drop height, which can be considered a test scatter in this study. 1200 1200 PET-1-2 Control 900 900 PET-1-3 AFRP-1-2 1200 1200 PET-1-4 PET-1-2 PET-1-2 Control PET-1-5 900 900 PET-1-3 AFRP-1-2 PET-1-4 PET-1-2 300 300 PET-1-5 600 600 0 0 300 300 -300 -300 0 0 -1 0 1 2 3 4 -1 0 1 2 3 4 Time (ms) Time (ms) -300 -300 -1 0 1 2 3 4 -1 0 1 2 3 4 (a) (b) Time (ms) Time (ms) Figure 10. Impact force time histories of the FRP-confined concrete with di erent confining conditions Figure 10. Impact force time histories of the FRP-confined concrete with different confining (a) (b) (a) and di erent drop heights (b). conditions (a) and different drop heights (b). Figure 10. Impact force time histories of the FRP-confined concrete with different confining 4. Conclusions conditions (a) and different drop heights (b). 4. Conclusions Based on the dynamic tensile test of PET fiber bundles, the SHPB test, and the drop weight test of Based on the dynamic tensile test of PET fiber bundles, the SHPB test, and the drop weight test 4. Conclusions the PET FRP-confined concrete, the following conclusions can be drawn: of the PET FRP-confined concrete, the following conclusions can be drawn: 1. PET fiber bundle is a strain rate-sensitive material. The tensile strength and the elastic modulus Based on the dynamic tensile test of PET fiber bundles, the SHPB test, and the drop weight test 1. PET fiber bundle is a strain rate-sensitive material. The tensile strength and the elastic increased, whereas the failure strain and the toughness decreased with the increasing strain rate from of the PET FRP-confined concrete, the following conclusions can be drawn: modulus increased, whereas the failure strain and the toughness decreased with the increasing 1/600 to 160 s . 1. PET fiber bundle is a strain rate-sensitive material. The tensile strength and the elastic −1 strain rate from 1/600 to 160 s . 2. Two layers of the PET FRP-confined concrete survived after a single impact compared to its modulus increased, whereas the failure strain and the toughness decreased with the increasing 2. Two layers of the PET FRP-confined concrete survived after a single impact compared to its −1 counterpart confined with one layer of the CFRP that can be crushed into pieces under a similar strain strain rate from 1/600 to 160 s . counterpart confined with one layer of the CFRP that can be crushed into pieces under a similar rate of about 220 s . 2. Two layers of the PET FRP-confined concrete survived after a single impact compared to its −1 strain rate of about 220 s . 3. The dynamic compressive strength of the PET FRP-confined concrete experienced a decrease counterpart confined with one layer of the CFRP that can be crushed into pieces under a similar −1 under multiple impacts with the same impact energy due to the progressive damage of concrete. strain rate of about 220 s . Impact fo Im rp ce ac (k t fo N) rce (kN) Stress S (MPa tress) (MPa) Impact force (kN) Impact force (kN) Appl. Sci. 2019, 9, 4987 9 of 11 4. The application of the PET FRP for the concrete column not only increased the peak impact-resistance force of columns, but also led to a prolonged impact duration. 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The e ects of gage length and strain rate on tensile behavior of Kevlar (R) 29 single filament and yarn. J. Compos. Mater. 2017, 51, 109–123. [CrossRef] © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/). http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Applied Sciences Multidisciplinary Digital Publishing Institute

Dynamic Behavior of PET FRP and Its Preliminary Application in Impact Strengthening of Concrete Columns

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Abstract

applied sciences Communication Dynamic Behavior of PET FRP and Its Preliminary Application in Impact Strengthening of Concrete Columns 1 1 2 3 1 , Yu-Lei Bai , Zhi-Wei Yan , Togay Ozbakkaloglu , Jian-Guo Dai , Jun-Feng Jia * and Jun-Bo Jia Key Laboratory of Urban Security and Disaster Engineering of Ministry of Education, Beijing University of Technology, Beijing 100124, China; baiyulei@bjut.edu.cn (Y.-L.B.); yanzhw@126.com (Z.-W.Y.); junbojia2001@yahoo.com (J.-B.J.) Ingram School of Engineering, Texas State University, San Marcos, TX 78667, USA; togay.oz@txstate.edu Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong 999077, China; cejgdai@polyu.edu.hk * Correspondence: jiajunfeng@bjut.edu.cn Received: 15 October 2019; Accepted: 15 November 2019; Published: 20 November 2019 Abstract: Polyethylene terephthalate (PET) fiber has attracted significant attention for reinforced concrete (RC) structure rehabilitation due to its large rupture strain (LRS; more than 7%) characteristic and recyclability from waste plastic bottles. This study presents a dynamic tensile test of PET fiber bundles performed using a drop-weight impact system. Results showed that the tensile strength and the elastic modulus of the PET fiber bundles increased, whereas the failure strain and the toughness decreased with the increasing strain rate from 1/600 to 160 s . In addition, the performance of concrete confined with the PET fiber-reinforced polymer (FRP) under impact loading was investigated based on a 75 mm-diameter split Hopkinson pressure bar (SHPB) device and a drop-weight apparatus. For the SHPB test, owing to the large rupture strain property of PET FRP, the PET FRP-confined concrete exhibited significantly better performance under impact loading compared to its counterpart confined with carbon FRPs (CFRPs). During the drop-weight test, the confinement of the PET FRP composites to the concrete columns as external jackets not only improved the peak impact force, but also prolonged the impact process. Keywords: composite materials; polymeric composites; PET FRP; dynamic behavior 1. Introduction Fiber-reinforced polymers (FRPs) have been widely used for seismic strengthening of existing reinforced concrete (RC) structures, owing to their advantages of high strength-to-weight ratios, corrosion resistance, and superior durability [1–14]. Considering the fact that the existing RC columns may be subjected to not only seismic loading but also dynamic loading such as vehicle/boat impact during their service life [15], the investigations of impact behaviors of RC structures strengthened with FRP composites are of vital importance for the application of FRPs in the impact-resistant design of RC structures. Although extensive studies of impact-resistant behaviors of conventional concrete and RC structures have been conducted [16–21], only a few studies of impact-resistant behaviors of FRP-confined concrete were conducted [22–27]. Xiao and Shen [23] conducted the drop-weight test to investigate the impact-resistant behaviors of the concrete-filled steel tube column confined with carbon FRPs (CFRPs) and found that the damage of the specimen could be reduced with the application of CFRPs as external jackets. Huang et al. [26] investigated the impact-resistant behaviors of the glass FRP (GFRP)–SR (spiral reinforcement)-confined Appl. Sci. 2019, 9, 4987; doi:10.3390/app9234987 www.mdpi.com/journal/applsci Appl. Sci. 2019, 9, x FOR PEER REVIEW 2 of 11 mm-diameter split Hopkinson pressure bar (SHPB) apparatus to investigate the dynamic mec Appl. hani Sci. cal 2019 behavio , 9, 4987rs of the concrete confined with aramid FRPs (AFRPs). The experimental result 2 of s 11 showed that the dynamic compressive strength, ultimate strain, and toughness of the concrete were remarkably increased with the application of the external AFRP jacket to confine the core concrete. concrete columns and found that an increase in the thickness of the GFRP tube led to an increase Considering the fact that the FRP mainly bears tensile force in the strengthening of the in the impact-resistant behaviors of the specimen. Yang et al. [27] used a 100 mm-diameter split structures and the fiber bundles are the main load-bearing element in FRP composites, the dynamic Hopkinson pressure bar (SHPB) apparatus to investigate the dynamic mechanical behaviors of the tensile mechanical properties of fiber bundles are necessary for studying the failure mechanism concrete confined with aramid FRPs (AFRPs). The experimental results showed that the dynamic between the FRP and the concrete under impact loading. To address this issue, some studies have compressive strength, ultimate strain, and toughness of the concrete were remarkably increased with been reported to date. Ou et al. [28] investigated the strain rate effect on the dynamic tensile the application of the external AFRP jacket to confine the core concrete. mechanical properties of the glass-fiber bundle specimen and found that the tensile strength Considering the fact that the FRP mainly bears tensile force in the strengthening of the structures −1 increased with the increasing strain rate from 1/600 to 160 s . Zhu et al. [29] investigated the effect of and the fiber bundles are the main load-bearing element in FRP composites, the dynamic tensile −1 the strain rate (20 to 100 s ) on the dynamic tensile mechanical properties of the Kevlar 49 fiber mechanical properties of fiber bundles are necessary for studying the failure mechanism between the bundle specimen. The experimental results showed that the tensile strength, the elastic modulus, FRP and the concrete under impact loading. To address this issue, some studies have been reported to and the toughness increased with the increasing strain rate. date. Ou et al. [28] investigated the strain rate e ect on the dynamic tensile mechanical properties of The past decade has witnessed the emergence of a new type of FRP composite, polyethylene the glass-fiber bundle specimen and found that the tensile strength increased with the increasing strain terephthalate (PET) [30–42]. Research has shown that its very large rupture strain (i.e., more than 1 1 rate from 1/600 to 160 s . Zhu et al. [29] investigated the e ect of the strain rate (20 to 100 s ) on 7%) may lead to enhanced structural ductility and impact energy dissipation in the impact-resistant the dynamic tensile mechanical properties of the Kevlar 49 fiber bundle specimen. The experimental retrofitting/strengthening of RC structures [43]. This paper briefly presents the authors’ recent results showed that the tensile strength, the elastic modulus, and the toughness increased with the studies on the dynamic tensile mechanical properties of PET fiber bundles and the dynamic increasing strain rate. compressive behavior of concrete confined by the PET FRP. The preliminary results illustrated the The past decade has witnessed the emergence of a new type of FRP composite, polyethylene great potential of the use of PET FRP in impact strengthening applications of RC structures at the terephthalate (PET) [30–42]. Research has shown that its very large rupture strain (i.e., more than material (i.e., PET fiber bundle and PET FRP-confined concrete) and component (i.e., PET 7%) may lead to enhanced structural ductility and impact energy dissipation in the impact-resistant FRP-confined concrete column) levels. It is expected that the results presented here will facilitate retrofitting/strengthening of RC structures [43]. This paper briefly presents the authors’ recent studies further investigation of the impact-resistant behavior of PET FRP-strengthened RC structures. on the dynamic tensile mechanical properties of PET fiber bundles and the dynamic compressive behavior of concrete confined by the PET FRP. The preliminary results illustrated the great potential 2. Materials and Methods of the use of PET FRP in impact strengthening applications of RC structures at the material (i.e., PET fiber bundle and PET FRP-confined concrete) and component (i.e., PET FRP-confined concrete 2.1. Dynamic Tensile Test of PET Fiber Bundles column) levels. It is expected that the results presented here will facilitate further investigation of the A typical specimen was fabricated with a gauge length of 25 mm (Figure 1) according to Ou et impact-resistant behavior of PET FRP-strengthened RC structures. al. [28]. A long fiber bundle with a 1800 mm length was extracted from the unidirectional PET fiber −3 fabric and weighed by an electronic balance to obtain a mass of 0.307 g. The linear density (1.7  10 2. Materials and Methods g/cm) could be calculated by dividing the mass by the length, and the area of the transverse section −3 2 2.1. Dynamic Tensile Test of PET Fiber Bundles of the bundle (1.23  10 cm ) could be obtained by dividing the linear density by the bulk density (1.38 g/cm ) that was provided by the manufacturer (Maeda Kosen Co., Sakai-shi, Japan). A fiber A typical specimen was fabricated with a gauge length of 25 mm (Figure 1) according to bundle with a length of 60 mm was cut from the extracted long fiber bundle. Two aluminum sheets Ou et al. [28]. A long fiber bundle with a 1800 mm length was extracted from the unidirectional with a length of 20 mm, a width of 15 mm, and a thickness of 0.2 mm were roughhewed by a PET fiber fabric and weighed by an electronic balance to obtain a mass of 0.307 g. The linear density serrated steel 3 plate and folded along the long side to catch the fiber bundle using the epoxy resin. (1.7  10 g/cm) could be calculated by dividing the mass by the length, and the area of the transverse After the resin was fully cured an 3 d the 2 redundant fiber bundle at both sides was cut off, a fiber section of the bundle (1.23  10 cm ) could be obtained by dividing the linear density by the bulk bundle specimen with 3 a gauge length of 25 mm was completed. density (1.38 g/cm ) that was provided by the manufacturer (Maeda Kosen Co., Sakai-shi, Japan). A Five PET fiber bundle specimens were tested for each displacement rate of 1, 2, 3, and 4 m/s fiber bundle with a length of 60 mm was cut from the extracted long fiber bundle. Two aluminum −1 with the respective strain rates of 40, 80, 120, and 160 s , which is the ratio of the displacement rate sheets with a length of 20 mm, a width of 15 mm, and a thickness of 0.2 mm were roughhewed by to the corresponding gauge length of the specimen, using an Instron drop-weight impact system a serrated steel plate and folded along the long side to catch the fiber bundle using the epoxy resin. (Figure 2). The impact height of the system ranged from 0.03 to 1.10 m, and the maximum mass of a After the resin was fully cured and the redundant fiber bundle at both sides was cut o , a fiber bundle drop hammer was 37.5 kg. The drop hammer dropped freely along the guide rail and impacted the specimen with a gauge length of 25 mm was completed. specimen after the release of the drop hammer. Figure 1. Schematic diagram of a fiber bundle specimen. Figure 1. Schematic diagram of a fiber bundle specimen. Five PET fiber bundle specimens were tested for each displacement rate of 1, 2, 3, and 4 m/s with the respective strain rates of 40, 80, 120, and 160 s , which is the ratio of the displacement rate to the corresponding gauge length of the specimen, using an Instron drop-weight impact system (Figure 2). Appl. Sci. 2019, 9, 4987 3 of 11 The impact height of the system ranged from 0.03 to 1.10 m, and the maximum mass of a drop hammer was 37.5 kg. The drop hammer dropped freely along the guide rail and impacted the specimen after the Appl. Sc release i. 2019of , 9, x FOR the dr PEER op hammer REVIEW . 3 of 11 Figure 2. Instron drop-weight impact system. Figure 2. Instron drop-weight impact system. For comparison, five PET fiber bundle specimens were also tested under quasi-static conditions at For comparison, five PET fiber bundle specimens were also tested under quasi-static conditions a crosshead speed of 2.5 mm/min (strain rate of 1/600 s ) using an MTS universal testing machine. The −1 at a crosshead speed of 2.5 mm/min (strain rate of 1/600 s ) using an MTS universal testing machine. loading capacity of the MTS system ranges from 0.1 to 30 kN with a data sampling rate of 1 kHz. The The loading capacity of the MTS system ranges from 0.1 to 30 kN with a data sampling rate of 1 kHz. tension of the PET fiber bundle specimen was measured with a force sensor, of which the maximum The tension of the PET fiber bundle specimen was measured with a force sensor, of which the range and the data acquisition rate are 1 kN and 20 Hz, respectively. The deformation of the PET fiber maximum range and the data acquisition rate are 1 kN and 20 Hz, respectively. The deformation of bundle specimen was considered as the displacement of the upper beam of the system, due to the fact the PET fiber bundle specimen was considered as the displacement of the upper beam of the system, that the sti ness of the specimen was far lower than that of the loading system [44]. due to the fact that the stiffness of the specimen was far lower than that of the loading system [44]. 2.2. Split Hopkinson Pressure Bar Test 2.2. Split Hopkinson Pressure Bar Test Concrete cylinders with a diameter of 150 mm and a height of 305 mm were cast with ordinary Concrete cylinders with a diameter of 150 mm and a height of 305 mm were cast with ordinary Portland cement, river sand, and crushed granitic rocks with a particle size ranging from 5 to 10 mm. Portland cement, river sand, and crushed granitic rocks with a particle size ranging from 5 to 10 mm. The average 28-day compressive strength of the concrete was 18.5 MPa. The average 28-day compressive strength of the concrete was 18.5 MPa. A total of 9 concrete columns with a 70 mm diameter and a 38 mm height were prepared. Among A total of 9 concrete columns with a 70 mm diameter and a 38 mm height were prepared. them, 3 concrete cylinders were wrapped with one layer of the PET FRP, of which the nominal thickness, Among them, 3 concrete cylinders were wrapped with one layer of the PET FRP, of which the the tensile strength, and the elastic modulus were 0.841 mm, 751 MPa, and 17.9 GPa, respectively, 3 nominal thickness, the tensile strength, and the elastic modulus were 0.841 mm, 751 MPa, and 17.9 concrete cylinders were wrapped with two layers of the PET FRP, and 3 specimens were wrapped with GPa, respectively, 3 concrete cylinders were wrapped with two layers of the PET FRP, and 3 one layer of the CFRP, of which the nominal thickness, the tensile strength, and the elastic modulus specimens were wrapped with one layer of the CFRP, of which the nominal thickness, the tensile were 0.165 mm, 4423 MPa, and 240 GPa, respectively, for the purpose of the comparison (Table 1), strength, and the elastic modulus were 0.165 mm, 4423 MPa, and 240 GPa, respectively, for the according to the manual wet layup way [30]. A continuous fiber sheet saturated with the epoxy resin, purpose of the comparison (Table 1), according to the manual wet layup way [30]. A continuous mixed at a ratio of the main resin component L-500AS to the hardener L-500BS of 2:1 (provided by fiber sheet saturated with the epoxy resin, mixed at a ratio of the main resin component L-500AS to SANYU Shanghai Company), was wrapped around the concrete cylinder with the fibers orientated the hardener L-500BS of 2:1 (provided by SANYU Shanghai Company), was wrapped around the in the hoop direction of the cylinder. To prevent the debonding failure, a fiber sheet with a length concrete cylinder with the fibers orientated in the hoop direction of the cylinder. To prevent the of 350 mm (i.e., one circumference + half circumference of the overlapping zone) was used for the debonding failure, a fiber sheet with a length of 350 mm (i.e., one circumference + half circumference fabrication of one-layer FRP-confined concrete specimens. To reduce the test scatter, three identical of the overlapping zone) was used for the fabrication of one-layer FRP-confined concrete specimens. specimens were tested at each impact velocity using a 75 mm-diameter SHPB device to investigate the To reduce the test scatter, three identical specimens were tested at each impact velocity using a 75 dynamic compressive behavior of the FRP-confined concrete (Figure 3). The apparatus comprised an mm-diameter SHPB device to investigate the dynamic compressive behavior of the FRP-confined energy source, a 600 mm-long projectile, a 5000 mm-long incident bar, a 3000 mm-long transmission concrete (Figure 3). The apparatus comprised an energy source, a 600 mm-long projectile, a 5000 bar, a digital oscilloscope, and a velocity testing system that was used to record the initial velocity of mm-long incident bar, a 3000 mm-long transmission bar, a digital oscilloscope, and a velocity the projectile. testing system that was used to record the initial velocity of the projectile. The specimen was sandwiched between the incident and transmission bars. The stress wave was reshaped with a rubber shaper (1 mm in thickness and 20 mm in diameter) installed on the impact end of the incident bar. The time histories of the strain ε(t), stress σ(t), and strain rate ε̇ (t) of the specimen can be obtained by Equations (1)–(3), according to the two-wave theory [27]: 2C  (t) [ (t) (t)]dt ,  (1) 0 i t s Appl. Sci. 2019, 9, 4987 4 of 11 Appl. Sci. 2019, 9, x FOR PEER REVIEW 4 of 11 Table 1. Summary of specimens for the SHPB test. Nominal  (t) E Tensile  (t) , Unconfined (2) Volume Number Initial Number of Type of D  H Thickness Strength Concrete Type of FRP Fraction of Velocity Identical Test (mm) of FRP of FRP Strength of Fiber Layers (m/s) Specimen (mm) (MPa) (MPa) 2C Carbon FRP   (t) [ (t) (t)] , (3) i t 70  38 0.165 15% 4423 1 10.9 18.5 3 (CFRP) SHPB Polyethylene test 70  38 terephthalate 0.841 31% 751 1 11.4 18.5 3 where εi(t) and εt(t) are the strain of the incident and transmission bars, respectively; A is the (PET) FRP cross-sectional area of bars; As and Ls are the initial cross-sectional area and the height of the 70  38 PET FRP 0.841 31% 751 2 11.1 18.5 3 specimen, respectively. Figure 3. Split Hopkinson pressure bar (SHPB) apparatus. Figure 3. Split Hopkinson pressure bar (SHPB) apparatus. The specimen was sandwiched between the incident and transmission bars. The stress wave was Table 1. Summary of specimens for the SHPB test. reshaped with a rubber shaper (1 mm in thickness and 20 mm in diameter) installed on the impact end ofT the incident bar. The time histories of the strain "(t), stress (t), and strain rate " (t) of the specimen can be obtained by Equations (1)–(3), according to the two-wave theory [27]: 2C e Nominal Tensile Unconfined Number "(t) = [" (t) " (t)]dt, (1) Volume i Initial o D × H Type of thickness 0 strength Number concrete of fraction velocity f (mm) FRP of FRP of FRP of layers strength identical of fiber (m/s) (t) = E " (t), (2) t (mm) (MPa) (MPa) specimen . 2C "(t) = [" (t) " (t)], (3) i t where " (t) and " (t) are the strain of the incident and transmission bars, respectively; A is the i t Carbon FRP 70 × 38 0.165 15% 4423 1 10.9 18.5 3 cross-sectional area of bars; A and L are the initial cross-sectional area and the height of the s s (CFRP) SHP specimen, respectively. Polyethylene 70 × 38 terephthalate 0.841 31% 751 1 11.4 18.5 3 test 2.3. Drop-Weight Test (PET) FRP Twelve concrete cylinders with a diameter of 95 mm, a height of 285 mm, and an average 28-day 70 × 38 PET FRP 0.841 31% 751 2 11.1 18.5 3 compressive strength of 58.5 MPa were prepared. The preparation of the concrete column and the fabrication 2.3. Drop-Wei of ght the Te FRP-confined st concrete column were the same as those presented in Section 2.2. Among them, 8 cylinders were wrapped with one layer of the PET FRP, 2 cylinders were confined with Twelve concrete cylinders with a diameter of 95 mm, a height of 285 mm, and an average one layer of the AFRP, of which the nominal thickness, the tensile strength, and the elastic modulus 28-day compressive strength of 58.5 MPa were prepared. The preparation of the concrete column are 0.169 mm, 3732 MPa, and 115 GPa, respectively, and 2 remaining unconfined concrete cylinders and the fabrication of the FRP-confined concrete column were the same as those presented in Section were tested as control specimens (Table 2). All specimens were denoted as “A-B-C”, where A, B, and C 2.2. Among them, 8 cylinders were wrapped with one layer of the PET FRP, 2 cylinders were stand for the type of FRP, the number of FRP layers, and the drop height, respectively. Two nominally confined with one layer of the AFRP, of which the nominal thickness, the tensile strength, and the identical specimens were tested at each drop height to reduce the data scatter. elastic modulus are 0.169 mm, 3732 MPa, and 115 GPa, respectively, and 2 remaining unconfined concrete cylinders were tested as control specimens (Table 2). All specimens were denoted as “A-B-C”, where A, B, and C stand for the type of FRP, the number of FRP layers, and the drop Appl. Sci. 2019, 9, 4987 5 of 11 Table 2. Summary of specimens for the drop-weight impact test. Nominal Tensile Unconfined Volume Number Impact Number of Type of D  H Thickness Strength Concrete Type of FRP Fraction of Height Identical Test (mm) of FRP of FRP Strength of Fiber Layers (m) Specimen (mm) (MPa) (MPa) 95  285 - - - - - 2 58.5 2 95  285 AFRP 0.169 19% 3732 1 2 58.5 2 Drop-weight 95  285 PET FRP 0.841 31% 751 1 2 58.5 2 Appl. Sci. 2019, 9, x FOR PEER REVIEW 5 of 11 test 95  285 PET FRP 0.841 31% 751 1 3 58.5 2 95  285 PET FRP 0.841 31% 751 1 4 58.5 2 95  285 PET FRP 0.841 31% 751 1 5 58.5 2 height, respectively. Two nominally identical specimens were tested at each drop height to reduce the data scatter. A large-capacity drop-weight setup was used for the impact test [23], consisting of a hammerhead A large-capacity drop-weight setup was used for the impact test [23], consisting of a with a weight of 188 kg, 14 balancing weights each with a weight of 68 kg, a steel frame, a track, a hammerhead with a weight of 188 kg, 14 balancing weights each with a weight of 68 kg, a steel control system, and a lifting device (Figure 4). The maximum drop height was 16 m. Di erent impact frame, a track, a control system, and a lifting device (Figure 4). The maximum drop height was 16 m. energies can be obtained by adjusting the mass of the drop weight and the drop height. In this test, the Different impact energies can be obtained by adjusting the mass of the drop weight and the drop drop weight was held the same at 256 kg (the hammerhead + one balancing weight), and di erent height. In this test, the drop weight was held the same at 256 kg (the hammerhead + one balancing impact energies were achieved by varying the impact height (i.e., 2, 3, 4, and 5 m). weight), and different impact energies were achieved by varying the impact height (i.e., 2, 3, 4, and 5 m). Figure 4. Drop-weight test setup. Figure 4. Drop-weight test setup. 3. Results and Discussion Table 2. Summary of specimens for the drop-weight impact test. Figure 5 shows the typical stress–strain curves of the PET fiber bundle specimens at di erent Nominal Tensile Unconfined Volume Impact Number of strain rates. The curves exhibit a bilinear stress–strain relationship and a rapid drop from the peak D × H Type of thickness strength Number concrete Type of test fraction height identical stress to zero, which indicates a brittle fracture of the PET fiber bundle. The tensile strength, the elastic (mm) FRP of FRP of FRP of layers strength of fiber (m) specimen modulus, the failure strain, and the toughness are sensitive to the strain rate. When the strain rate (mm) (MPa) (MPa) increased from 1/600 to 160 s , there was an increase in the tensile strength, which can be attributed 95 × 285 - - - - - 2 58.5 2 to the fact that there was not enough time for the defects of filaments in the fiber bundle to develop 95 × 285 AFRP 0.169 19% 3732 1 2 58.5 2 at a high strain rate [45]. The initial elastic modulus and the second elastic modulus (i.e., slopes of Drop-weight 95 × 285 PET FRP 0.841 31% 751 1 2 58.5 2 the two portions of the stress–strain curve) increased with an increase in the strain rate from 1/600 to test 95 × 285 PET FRP 0.841 31% 751 1 3 58.5 2 160 s . Conversely, there was a decrease in the failure strain and the toughness with the increasing 95 × 285 PET FRP 0.841 31% 751 1 4 58.5 2 strain rate from 1/600 to 160 s . Figure 6 shows the failure modes of the PET fiber bundles at di erent 95 × 285 PET FRP 0.841 31% 751 1 5 58.5 2 strain rates. At a low strain rate (e.g., 1/600 s ), the fracture surface of the fiber bundle had a chaotic appearance with di erent fracture positions of filaments. At a high strain rate (e.g., 160 s ), the fiber 3. Results and Discussion bundle ruptured with a relatively trim fractography. Figure 5 shows the typical stress–strain curves of the PET fiber bundle specimens at different strain rates. The curves exhibit a bilinear stress–strain relationship and a rapid drop from the peak stress to zero, which indicates a brittle fracture of the PET fiber bundle. The tensile strength, the elastic modulus, the failure strain, and the toughness are sensitive to the strain rate. When the strain −1 rate increased from 1/600 to 160 s , there was an increase in the tensile strength, which can be attributed to the fact that there was not enough time for the defects of filaments in the fiber bundle to develop at a high strain rate [45]. The initial elastic modulus and the second elastic modulus (i.e., slopes of the two portions of the stress–strain curve) increased with an increase in the strain rate −1 from 1/600 to 160 s . Conversely, there was a decrease in the failure strain and the toughness with −1 the increasing strain rate from 1/600 to 160 s . Figure 6 shows the failure modes of the PET fiber −1 bundles at different strain rates. At a low strain rate (e.g., 1/600 s ), the fracture surface of the fiber bundle had a chaotic appearance with different fracture positions of filaments. At a high strain rate −1 (e.g., 160 s ), the fiber bundle ruptured with a relatively trim fractography. Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 11 Appl. Sci. 2019, 9, x FOR PEER REVIEW 6 of 11 Appl. Sci. 2019, 9, 4987 6 of 11 PET fiber bundle -1 PET fiber bundle 1/600 s -1 -1 1/600 s 40 s -1 800 -1 40 s 80 s -1 -1 80 s 120 s -1 -1 120 s 160 s -1 160 s 0 3 6 9 12 15 0 3 6 9 12 15 Strain (%) Strain (%) Figure 5. Tensile stress–strain curves of the PET fiber bundle at different strain rates. Figure 5. Tensile stress–strain curves of the PET fiber bundle at di erent strain rates. Figure 5. Tensile stress–strain curves of the PET fiber bundle at different strain rates. (a) (b) (a) (b) (c) (d) (c) (d) (e) (e) 1−1 1 −1 −1 1 −1 1 Figure Figure 6. 6. S SEM EM images images of of failur failur ee modes modes at atthe the strain strain rates rateof s of 1 /1 600 /600 s s (a (a );); 40 40 s s ((b b ); );80 80 ss ( (c c); ); 120 120 s s ((d d)); ; −1 −1 −1 −1 Figure −1 6. SEM images of failure modes at the strain rates of 1/600 s (a); 40 s (b); 80 s (c); 120 s (d); 160 160 s s (( ee )). . −1 160 s (e). Figure 7a shows the failure mode of one layer of the CFRP-confined concrete at a strain rate of Figure 7a shows the failure mode of one layer of the CFRP-confined concrete at a strain rate of 1 1 218 s fr Fiom gure SHPB 7a sho tests. ws th Atea failu strain re rate mode of of about one 220 layer s of , one the layer CFRP of -con the fined CFRP-confined concrete atconcr a strai ete n was rate of −1 −1 218 s from SHPB tests. At a strain rate of about 220 s , one layer of the CFRP-confined concrete was −1 −1 smashed 218 s from together SHPB with test the s. At external a strain FRP rate ro uptur f aboe, ut wher 220 seas, , one due lay to er its of very the CFRP large-con rupt fiur ned e strain, concret two e was smashed together with the external FRP rupture, whereas, due to its very large rupture strain, two layers smashe of the d PET togeth FRP-confined er with the concr externa etel could FRP rupture not be destr , whoyed ereas, (Figur due to e 7 its b). very Ther efor large, e to rupture investigate strain, the two layers of the PET FRP-confined concrete could not be destroyed (Figure 7b). Therefore, to investigate failur laye ers mechanism of the PET of FRP the-con concr fined ete confined concrete could with the not PET be destroy FRP, the ed specimen (Figure 7b was ). Th repeatedly erefore, to impacted investigate the failure mechanism of the concrete confined with the PET FRP, the specimen was repeatedly under the the failure same mimpact echanism velocity of the until confailur crete e con (Figur fined e 7wit b). h As thshown e PET in FRP Figur , the e 7speci b, only men a small was r piece epeatedl of y Stress (MPa) Stress (MPa) Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 11 impacted under the same impact velocity until failure (Figure 7b). As shown in Figure 7b, only a small piece of the concrete was peeled off the surface of concrete confined with the PET FRP after the −1 −1 second impact with a strain rate of 272 s . Under the third impact at a strain rate of 280 s , partial concrete was crushed with no rupture of the external PET FRP. Appl. Sci. 2019, 9, x FOR PEER REVIEW 7 of 11 Appl. Sci. 2019, 9, 4987 7 of 11 impacted under the same impact velocity until failure (Figure 7b). As shown in Figure 7b, only a small piece of the concrete was peeled off the surface of concrete confined with the PET FRP after the the concrete was peeled o the surface of concrete confined with the PET FRP after the second impact −1 −1 second impact with a strain rate of 272 s . Under the third impact at a strain rate of 280 s , partial 1 1 with a strain rate of 272 s . Under the third impact at a strain rate of 280 s , partial concrete was concrete was crushed with no rupture of the external PET FRP. crushed with no rupture of the external PET FRP. −1 st −1 nd −1 rd −1 218 s 1 impact (222 s ) 2 impact (272 s ) 3 impact (280 s ) (a) (b) Figure 7. Failure modes of the concrete confined with the CFRP (a) and the PET FRP (b). Figure 8 shows the dynamic stress–strain curves of specimens confined with the CFRP and the −1 st −1−1 nd −1 rd −1 218 s 1 impact (222 s) 2 impact (272 s) 3 impact (280 s ) PET FRP at a strain rate of around 220 s . The PET and CFRPs were designed for a comparison based on ultimate jacket strengths (i.e., the product of tensile strength and thickness of FRP jacket). (a) (b) One layer of the CFRP provides an ultimate jacket strength of 729.8 N/mm that falls between the Figure Figure 7. 7. Failur Failure modes of th e modes of thee concrete concrete confined confined with with the the CFRP CFRP ( (aa )) and t and the he PET PET FRP FRP ( (bb ).). values provided by one layer and two layers of the PET FRPs (631.6 N/mm and 1263.2 N/mm, respectively). The dynamic compressive strength of the specimen confined with one layer of the Figure 8 shows the dynamic stress–strain curves of specimens confined with the CFRP and the PET Figure 8 shows the dynamic stress–strain curves of specimens confined with the CFRP and the CFRP was 89.4 MPa, whereas the corresponding value for one layer of the PET FRP-confined −1 FRP at a strain rate of around 220 s . The PET and CFRPs were designed for a comparison based on PET FRP at a strain rate of around 220 s . The PET and CFRPs were designed for a comparison concrete was 82.9 MPa with a larger compressive strain of 4.58%. In addition, the dynamic ultimate jacket strengths (i.e., the product of tensile strength and thickness of FRP jacket). One layer of based on ultimate jacket strengths (i.e., the product of tensile strength and thickness of FRP jacket). compressive strength and the corresponding strain of two layers of the PET FRP confined-concrete the CFRP provides an ultimate jacket strength of 729.8 N/mm that falls between the values provided by One layer of the CFRP provides an ultimate jacket strength of 729.8 N/mm that falls between the were remarkably higher than those of the CFRP-confined counterpart, indicating the superior one layer and two layers of the PET FRPs (631.6 N/mm and 1263.2 N/mm, respectively). The dynamic values provided by one layer and two layers of the PET FRPs (631.6 N/mm and 1263.2 N/mm, impact performance of the PET FRP-confined concrete due to the large rupture strain characteristic. compressive strength of the specimen confined with one layer of the CFRP was 89.4 MPa, whereas respectively). The dynamic compressive strength of the specimen confined with one layer of the Figure 9 shows the dynamic compressive stress–strain curves of the specimen confined with the corresponding value for one layer of the PET FRP-confined concrete was 82.9 MPa with a larger CFRP was 89.4 MPa, whereas the corresponding value for one layer of the PET FRP-confined two layers of the PET FRP under multiple impacts. The compressive strength of two layers of the compressive strain of 4.58%. In addition, the dynamic compressive strength and the corresponding concrete was 82.9 MPa with a larger compressive strain of 4.58%. In addition, the dynamic PET FRP-confined concrete decreased after the second impact due to the progressive concrete strain of two layers of the PET FRP confined-concrete were remarkably higher than those of the compressive strength and the corresponding strain of two layers of the PET FRP confined-concrete damage, which reveals superior impact-resistant behavior as far as deformability is concerned due CFRP-confined counterpart, indicating the superior impact performance of the PET FRP-confined were remarkably higher than those of the CFRP-confined counterpart, indicating the superior to the large rupture strain characteristic. concrete due to the large rupture strain characteristic. impact performance of the PET FRP-confined concrete due to the large rupture strain characteristic. Figure 9 shows the dynamic compressive stress–strain curves of the specimen confined with two layers of the PET FRP under multiple impact -1s. The compressive strength of two layers of the CFRP 218 s -1 PET FRP-confined concrete decreased after the second impact due to the progressive concrete 1-layer PET FRP 231 s -1 2-layer PET FRP 222 s damage, which reveals superior 1 impa 00 ct-resistant behavior as far as deformability is concerned due to the large rupture strain characteristic. 40 -1 CFRP 218 s 120 -1 1-layer PET FRP 231 s -1 2-layer PET FRP 222 s 0 1 2 3 4 5 6 7 8 Strain (%) Figure 8. Comparison of the dynamic compressive stress–strain curves of the specimen at a strain rate Figure 8. Comparison of the dynamic compressive stress–strain curves of the specimen at a strain of around 220 s . −1 rate of around 220 s . Figure 9 shows the dynamic compressive stress–strain curves of the specimen confined with two 01 23 45678 layers of the PET FRP under multiple impacts. The compressive strength of two layers of the PET Strain (%) FRP-confined concrete decreased after the second impact due to the progressive concrete damage, which reveals superior impact-resistant behavior as far as deformability is concerned due to the large Figure 8. Comparison of the dynamic compressive stress–strain curves of the specimen at a strain rupture strain characteristic. −1 rate of around 220 s . Stress (MPa) Stress (MPa) Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 11 Appl. Sci. 2019, 9, x FOR PEER REVIEW 8 of 11 Appl. Sci. 2019, 9, 4987 8 of 11 st -1 1 222 s 2-layer PET600 FRP nd -1 100 2 272 s st -1 rd -1 1 222 s 3 280 s 2-layer PET600 FRP nd -1 100 2 272 s rd -1 3 280 s 0 1 2 3 4 5 6 7 8 Strain (%) 0 1 2 3 4 5 6 7 8 Strain (%) Figure 9. Dynamic compressive stress–strain curves of the specimen confined with two layers of the PET FRP under multiple impacts. Figure 9. Dynamic compressive stress–strain curves of the specimen confined with two layers of the Figure 9. Dynamic compressive stress–strain curves of the specimen confined with two layers of the PET FRP under multiple impacts. PET FRP under multiple impacts. Figure 10a shows the time histories of specimens with three different confining conditions (i.e., no Figur FRP,e one 10a lshows ayer of the AFtime RP, and histories PET F of Rspecimens P) under the with same thr ee drop di he erent ight confining of 2 m frconditions om the drop (i.e., -weight no Figure 10a shows the time histories of specimens with three different confining conditions (i.e., FRP test , one s. It layer can b of e AFRP seen ,th and at, c PET ompared FRP) under to the the uncon same findr ed op spec height imen, of th 2e m appli from catio the n drof op-weight AFRP led tests. to an no FRP, one layer of AFRP, and PET FRP) under the same drop height of 2 m from the drop-weight It can incre be asseen e in th that, e peak compar impact ed to force. the unconfined It can also be specimen, seen in Fithe gure application 10a that thof e appli AFRP caled tion to of an PET incr FRP ease not tests. It can be seen that, compared to the unconfined specimen, the application of AFRP led to an in only the peak impro impact ved th for e peak ce. Itimp canact also force, be seen but in also Figur proe longed 10a that the the imapplication pact process, ofleading PET FRP to not a superior only increase in the peak impact force. It can also be seen in Figure 10a that the application of PET FRP not impr impact oved res the ispeak tance. impact Figure for 10 ce, b show but also s the prtim olonged e histori the es impact of the pr idocess, enticalleading PET FRP to-a con super fined ior spe impact cimens only improved the peak impact force, but also prolonged the impact process, leading to a superior resistance. tested at Figur different e 10 b drop shows heithe ghts. time As histories can be seen of the in F identical igure 10b PET , the FRP-confined peak impact specimens force of the tested specimen at impact resistance. Figure 10b shows the time histories of the identical PET FRP-confined specimens di sler ightly ent dr incre op heights. ased, an As d th can e impact be seen dur in Figur ation egra 10b, duthe ally peak decre impact ased with force th of e the incr specimen easing drop slightly height tested at different drop heights. As can be seen in Figure 10b, the peak impact force of the specimen incr from eased, 2 to and 5 m. the It iis mpact wortduration h noting th gradually at the dur decr ation eased of th with e specimen the incr easing impacted drop und height er a 4 frm om drop 2 to height 5 m. slightly increased, and the impact duration gradually decreased with the increasing drop height It is was worth longe noting r than that that theoduration f the counterp of thea specimen rt under impacted a 3 m drop under heig aht 4, m which drop height can bewas cons longer idered than a test from 2 to 5 m. It is worth noting that the duration of the specimen impacted under a 4 m drop height that sca of tter the in this counterpart study. under a 3 m drop height, which can be considered a test scatter in this study. was longer than that of the counterpart under a 3 m drop height, which can be considered a test scatter in this study. 1200 1200 PET-1-2 Control 900 900 PET-1-3 AFRP-1-2 1200 1200 PET-1-4 PET-1-2 PET-1-2 Control PET-1-5 900 900 PET-1-3 AFRP-1-2 PET-1-4 PET-1-2 300 300 PET-1-5 600 600 0 0 300 300 -300 -300 0 0 -1 0 1 2 3 4 -1 0 1 2 3 4 Time (ms) Time (ms) -300 -300 -1 0 1 2 3 4 -1 0 1 2 3 4 (a) (b) Time (ms) Time (ms) Figure 10. Impact force time histories of the FRP-confined concrete with di erent confining conditions Figure 10. Impact force time histories of the FRP-confined concrete with different confining (a) (b) (a) and di erent drop heights (b). conditions (a) and different drop heights (b). Figure 10. Impact force time histories of the FRP-confined concrete with different confining 4. Conclusions conditions (a) and different drop heights (b). 4. Conclusions Based on the dynamic tensile test of PET fiber bundles, the SHPB test, and the drop weight test of Based on the dynamic tensile test of PET fiber bundles, the SHPB test, and the drop weight test 4. Conclusions the PET FRP-confined concrete, the following conclusions can be drawn: of the PET FRP-confined concrete, the following conclusions can be drawn: 1. PET fiber bundle is a strain rate-sensitive material. The tensile strength and the elastic modulus Based on the dynamic tensile test of PET fiber bundles, the SHPB test, and the drop weight test 1. PET fiber bundle is a strain rate-sensitive material. The tensile strength and the elastic increased, whereas the failure strain and the toughness decreased with the increasing strain rate from of the PET FRP-confined concrete, the following conclusions can be drawn: modulus increased, whereas the failure strain and the toughness decreased with the increasing 1/600 to 160 s . 1. PET fiber bundle is a strain rate-sensitive material. The tensile strength and the elastic −1 strain rate from 1/600 to 160 s . 2. Two layers of the PET FRP-confined concrete survived after a single impact compared to its modulus increased, whereas the failure strain and the toughness decreased with the increasing 2. Two layers of the PET FRP-confined concrete survived after a single impact compared to its −1 counterpart confined with one layer of the CFRP that can be crushed into pieces under a similar strain strain rate from 1/600 to 160 s . counterpart confined with one layer of the CFRP that can be crushed into pieces under a similar rate of about 220 s . 2. Two layers of the PET FRP-confined concrete survived after a single impact compared to its −1 strain rate of about 220 s . 3. The dynamic compressive strength of the PET FRP-confined concrete experienced a decrease counterpart confined with one layer of the CFRP that can be crushed into pieces under a similar −1 under multiple impacts with the same impact energy due to the progressive damage of concrete. strain rate of about 220 s . Impact fo Im rp ce ac (k t fo N) rce (kN) Stress S (MPa tress) (MPa) Impact force (kN) Impact force (kN) Appl. Sci. 2019, 9, 4987 9 of 11 4. The application of the PET FRP for the concrete column not only increased the peak impact-resistance force of columns, but also led to a prolonged impact duration. 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Journal

Applied Sciences – Multidisciplinary Digital Publishing Institute

Published: Nov 20, 2019

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