Distributed Long-Gauge Optical Fiber Sensors Based Self-Sensing FRP Bar for Concrete Structure.

sensors Article

Distributed Long-Gauge Optical Fiber Sensors Based Self-Sensing FRP Bar for Concrete Structure Yongsheng Tang 1 and Zhishen Wu 2, * 1 2

*

School of Urban Rail Transportation, Soochow University, Suzhou 215137, China; [email protected] International Institute for Urban System Engineering, Southeast University, Nanjing 210096, China Correspondence: [email protected]; Tel.: +86-512-6760-1052

Academic Editor: Jose Luis Santos Received: 5 January 2016; Accepted: 19 February 2016; Published: 25 February 2016

Abstract: Brillouin scattering-based distributed optical fiber (OF) sensing technique presents advantages for concrete structure monitoring. However, the existence of spatial resolution greatly decreases strain measurement accuracy especially around cracks. Meanwhile, the brittle feature of OF also hinders its further application. In this paper, the distributed OF sensor was firstly proposed as long-gauge sensor to improve strain measurement accuracy. Then, a new type of self-sensing fiber reinforced polymer (FRP) bar was developed by embedding the packaged long-gauge OF sensors into FRP bar, followed by experimental studies on strain sensing, temperature sensing and basic mechanical properties. The results confirmed the superior strain sensing properties, namely satisfied accuracy, repeatability and linearity, as well as excellent mechanical performance. At the same time, the temperature sensing property was not influenced by the long-gauge package, making temperature compensation easy. Furthermore, the bonding performance between self-sensing FRP bar and concrete was investigated to study its influence on the sensing. Lastly, the sensing performance was further verified with static experiments of concrete beam reinforced with the proposed self-sensing FRP bar. Therefore, the self-sensing FRP bar has potential applications for long-term structural health monitoring (SHM) as embedded sensors as well as reinforcing materials for concrete structures. Keywords: self-sensing FRP bar; Brillouin scattering-based distributed optical fiber sensing technology; long-gauge sensors; macro-strain; concrete structure

1. Introduction Concrete structure is the main type of infrastructures due especially to its low cost. However, concrete cracks often develop under service loads and environmental and accidental actions. In addition to decreasing structural stiffness, these cracks will speed up the corrosion of steel rebar, influencing the structure durability or safety. Therefore, damage identification, including damage detection, localization and quantification, has received considerable attention during the past two decades [1]. In order to accurately determine the damage location and severity, a suitable parameter should be chosen firstly, which must be sensitive to local damage and be a function of space coordinates. Strain field distribution can be a good indicator to interpret the local structural damage [2]. However, previous measurement incapability has prevented it from serving as an effective indicator. For large-scale concrete structures especially, the cracks present a distribution with great randomness. Thus, it is difficult to detect the cracks using the traditional “point” strain gauge. In 1989, a new strain sensing technique was proposed based on Brillouin scattering [3], with which the strain distribution along the optical fiber (OF) can be obtained easily, capable of identifying random concrete cracks. Since then, many developments have been achieved from both lab and Sensors 2016, 16, 286; doi:10.3390/s16030286

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field applications. The optical fiber sensors based on Brillouin optical time domain reflectometer (BOTDR) were bonded on the beam surface to measure the distributed tensile strain and detect concrete cracks [4,5]. These sensors were also bonded on the steel rebar surface to monitor the inner strain [6]. Additionally, the technique was applied to get the strain field for damage identification in the destructive loading test of a pre-stressed concrete bridge [7]. Besides the beam structures, it was also used to monitor other types of structures, such as tunnel [8] and river levee [9]. The most popular BOTDR system is AQ8603 made by Company ANDO, with a maximum sensing distance of 80 km and a best strain measurement accuracy of 30 µε. Meanwhile, the spatial resolution, namely the least distance for satisfying measurement needs, is 1 m, which means that the obtained strain does not reflect the structural information for some special point but for some zone about 1 m long. For detecting cracks at early age, the performance of measurement system should be improved, such as strain measurement accuracy and spatial resolution. Therefore, the techniques based on Brillouin optical time domain analysis (BOTDA) were developed due to higher strain measurement accuracy and spatial resolution [10,11]. For most BOTDA systems, the spatial resolution can be up to 0.1 m, while the strain measurement accuracy can be better than 10 µε. The BOTDA shows great advantages in structural health monitoring (SHM), especially for large-scale concrete structures. However, the strain sensing accuracy around concrete crack will still be greatly influenced by the spatial resolution even it is only 0.1 m, as the complicated strain distribution within the spatial resolution will decrease the strain measurement accuracy greatly [12]. Another problem should also be solved when the optical sensors are applied with BOTDA. It is well known that the OF is so fragile that it may be easily broken during the installation. Hence, the OF sensors often need special packaging, making the installation very complicated. The sensor durability is one more key factor to be considered when long-term SHM is conducted by bonding the OF sensors on the structure surface or steel rebar surface. Therefore, some improvement should be implemented upon the OF sensor. In this paper, some methods were proposed to improve the comprehensive performance of the distributed optical fiber sensors. The OF sensor was designed as long-gauge OF sensor at first. Within the sensing gauge, the strain distributes uniformly along the OF to ensure satisfied strain measurement accuracy even around cracks. Then the proposed improved OF sensor was advised to be embedded into fiber reinforced polymer (FRP) bar to make a new type of self-sensing FRP bar due to some obvious advantages [13–15]. First, FRP will provide OF sensors with reliable and durable protection due to the excellent mechanical and chemical properties. Second, the OF sensors will make FRP self-monitoring avoid some unsatisfied failure. Lastly, it is easy to install the self-sensing FRP bar just as steel rebar. The work in references [13–15] has actually solved some problems, such as providing a good sensor package method with FRP. However, the OF is just bonded together with the outside FRP, named overall bonding style, which will not obviously increase the strain sensing accuracy around the concrete crack when the above distributed sensing technology is applied into concrete structure monitoring. However, in this paper, the OF sensor is not directly overall bonded with the outside FRP, but with some fixed point, named long-gauge style, which will greatly improve the strain sensing accuracy around cracks. The research was developed around the proposed new sensor, self-sensing FRP bar, in this paper. Firstly, the measurement principle of BOTDA was illustrated as well as the key parameter, spatial resolution. Secondly, the improved distributed optical sensor was proposed using long-gauge structure, of which two key parameters, namely sensing gauge length and bonding length, were investigated and optimized. Then, the manufacturing process was introduced with two main steps. After that, the strain and temperature sensing performance of the proposed self-sensing FRP bars were verified with experiments as well as basic mechanical properties. Furthermore, the bonding performance between self-sensing FRP bar and concrete was investigated to study its influence on strain sensing. Lastly, the strain sensing performance was testified by embedding the self-sensing FRP bars into concrete beam.

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2. BOTDA-Based Distributed OF Sensing Principle 2. BOTDA-Based Distributed OF Sensing Principle

2.1. Measurement Principle of BOTDA

2.1. Measurement Principle of BOTDA

The BOTDA technique is based on the stimulated Brillouin back scattering, and two laser sources. The BOTDA technique is source based on thethe stimulated back scattering, andwhich two laser One is a pulse laser (pump laser) and other isBrillouin a continuous laser source, aresources. introduced OneOF is from a pulse laser (pump laser) When source the andfrequency the other is a continuous laser the source, areequal into the different fiber ends. difference between two which lasers is introduced into the OF from different fiber ends. When the frequency difference between the two to the Brillouin frequency shift, the back Brillouin scattering will be stimulated, and energy transfer lasers is equal to the Brillouin frequency shift, the back Brillouin scattering will be stimulated, and will be generated between the two lasers as well. The center frequency of Brillouin back scattering energy transfer will be generated between the two lasers as well. The center frequency of Brillouin will move to the location where the strain or temperature [16] change happens within the OF. Thus, back scattering will move to the location where the strain or temperature [16] change happens within the continuous strain or temperature along the OFalong can be by measuring the OF. Thus, the continuous strain measurement or temperature measurement theimplemented OF can be implemented the Brillouin frequency shift (Figure 1). by measuring the Brillouin frequency shift (Figure 1).

Figure 1. Measurement principle of BOTDA.

Figure 1. Measurement principle of BOTDA.

The Brillouin frequency shift νВ of OF changes in proportion to the variation of strain or temperature along the OF. The linear between the Brillouin frequency shift andof strain ε or The Brillouin frequency shift ν B relationships of OF changes in proportion to the variation strain or temperature T are given as: temperature along the OF. The linear relationships between the Brillouin frequency shift and strain ε or temperature T are given as:  (T ,  )  C        T ,   (1) B

0



0

B0

0

0

 B0(,Tεq ,  0“) CCε TpεT´εT00q `  νBB0 νB pT pT 0 T 0 , 0 ,0 ε 0 q

(2)

where Cε and CT are the strain and temperature coefficients, respectively, and T0 and ε0 are the strain νB pT, ε 0 q “ CT pT ´ T0 q ` νB0 pT0 , ε 0 q and temperature that correspond to a reference Brillouin frequency νВ0.

(1) (2)

where Cε and CT are the strain and temperature coefficients, respectively, and T0 and ε0 are the strain 2.2. Spatial Resolution and temperature that correspond to a reference Brillouin frequency ν B0 . For BOTDA, the spatial resolution is a key parameter influencing application effects of strain

2.2. Spatial Resolution distribution measurement around cracks especially. Due to existence of pulse width, the received back Brillouin scattering does not reflect the information for some point of the OF but a small zone.

For BOTDA, the spatial resolution is a key parameter influencing application effects of strain The zone length is named as the spatial resolution, calculated as: distribution measurement around cracks especially. Due to existence of pulse width, the received (3) zone. S= /2 back Brillouin scattering does not reflect the information for some point of the OF but a small The zone length islight named as in theOF, spatial as: where ν is the speed and τresolution, is the pulse calculated width. To accurately measure local strain, the spatial resolution should be improved. From Equation (3), to S “isντ{2 obtain the higher spatial resolution, the pulse with the only parameter to be decreased. However, there (3) will be some problems. The first is that since the phonon-field strength does not have time to reach a level, the scattering increasing resolution. The weaker signal strength wheremaximum ν is the light speed in OF,amplitude and τ is decreases the pulsewith width. and consequently poorer signal-to-noise ratios (SNR) lead to lower measurement accuracyFrom and increased To accurately measure local strain, the spatial resolution should be improved. Equation (3), acquisition times. In addition, weak signal levels limit maximum sensing lengths due to the effects ofHowever, fiber to obtain the higher spatial resolution, the pulse with is the only parameter to be decreased. attenuation. The second is that the narrow optical pulses required to obtain high resolution will have large there will be some problems. The first is that since the phonon-field strength does not have time to spectral widths, far in excess of the Brillouin resonance line width. This will result in severe broadening reach a maximum level, the scattering amplitude decreases with increasing resolution. The weaker of the measured Brillouin spectrum, causing greatly reduced accuracy in determining the Brillouin signalfrequency. strengthThus, and for consequently poorer signal-to-noise ratios (SNR) lead to lower measurement the normal Brillouin scattering based time-domain system, the pulse width is 10 ns. accuracy and increased acquisition In addition, In other words, the spatial resolutiontimes. is not higher than 1 m.weak signal levels limit maximum sensing lengths due to the effects of fiber attenuation. The second that the narrow optical pulses required To overcome this difficulty, a pre-pump technique has is been developed, where two laser sources to obtain high resolution spectral in source, excess of the actually Brillouin resonance are introduced into OF.will Thehave main large difference is in widths, the pumpfar laser which includes two line types of pulse. One is a pre-pump laser (PL) that is applied to fully stimulate phonons, and the other width. This will result in severe broadening of the measured Brillouin spectrum, causing greatly

reduced accuracy in determining the Brillouin frequency. Thus, for the normal Brillouin scattering

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based time-domain system, the pulse width is 10 ns. In other words, the spatial resolution is not higher than 1 m. To overcome this difficulty, a pre-pump technique has been developed, where two laser sources are introduced into OF. The main difference is in the pump laser source, which actually includes two types of pulse. One is a pre-pump laser (PL) that is applied to fully stimulate phonons, and the other is a detection can be Sensors 2016, pump 16, 286 (DP) used as detecting laser. Due to the existence of PL, the phonons 4 of 18 fully stimulated before the arrival of DP. As a result, the pulse width of DP can be decreased to 1 ns is a influence detection pump used as detecting laser.scattering Due to theand existence of PL, the phonons can Accordingly, be fully without on the(DP) stimulation of Brillouin stimulated Brillouin gain. stimulated before the arrival of DP. As a result, the pulse width of DP can be decreased to 1 ns without a spatial resolution of approximately cm order has been realized [17]. Based on the above technique, influence on the stimulation of Brillouin scattering and stimulated Brillouin gain. Accordingly, a named as PPP-BOTDA, the Neubrex Company presents a system, NBX-6050, with a spatial resolution spatial resolution of approximately cm order has been realized [17]. Based on the above technique, of 10 cm, the strain accuracy of 7.5 µε and sensing distance of 2 km. named as PPP-BOTDA, the Neubrex Company presents a system, NBX-6050, with a spatial resolution of 10 cm, the strain accuracy of 7.5 με and sensing distance of 2 km.

3. BOTDA Based Distributed Long-Gauge OF Sensor 3. BOTDA Based Distributed Long-Gauge OF Sensor

3.1. Long-Gauge OF Sensor

3.1.actual Long-Gauge OF Sensorthere are mainly two bonding styles for distributed OF sensors (Figure 2): In applications, overall bonding fixed point bonding. Fortwo thebonding former,styles the strain along OF be the same2):as the In actualand applications, there are mainly for distributed OFwill sensors (Figure structural meaning that it can reflect structural deformation accurately. However, overallstrain, bonding and fixed point bonding. For the former, the strain along OF will be the same due as theto the structural strain,resolution, meaning that can reflect structural accuracy deformation accurately. However, dueiftothe thestrain existence of spatial theitstrain measurement will be greatly decreased existence of spatial resolution, the strain measurement accuracy will be greatly decreased if the strain distributes complicatedly within the spatial resolution. For the latter, the strain along the OF between distributes complicatedly withinuniformly, the spatial resolution. For the latter, the strain along OF between the the two fixed points distributes which means that it can reflect thethe average deformation two fixed points distributes uniformly, which means that it can reflect the average deformation information of the monitored zone. This long-gauge style might decrease the sensitivity a little for information of the monitored zone. This long-gauge style might decrease the sensitivity a little for the the small local strain change, but the basic strain measurement accuracy can be ensured due to the small local strain change, but the basic strain measurement accuracy can be ensured due to the simple simple strain distribution within spatial resolution. Thus, distributed long-gauge OF sensor is strain distribution within the the spatial resolution. Thus, the the distributed long-gauge OF sensor is proposed for improving thethe sensing performance concrete structures especially. proposed for improving sensing performance of of monitoring monitoring concrete structures especially.

(a)

(b)

Figure 2. Different bonding styles for distributed optical sensor: (a) overall; and (b) fixed point.

Figure 2. Different bonding styles for distributed optical sensor: (a) overall; and (b) fixed point.

3.2. Key Sensor Parameters

3.2. Key Sensor Parameters

From Figure 2b, there are two key parameters for the distributed long-gauge OF sensor, namely

From 2b, bonding there arelength, two key parameters the distributed long-gauge OF sensor, namely gauge Figure length and to be investigatedfor and optimized by static tension experiments. In thelength experiments, the BOTDA system, NBX-6050, is applied a spatialby resolution of 10 cm. gauge and bonding length, to be investigated andwith optimized static tension experiments. In the experiments, the BOTDA system, NBX-6050, is applied with a spatial resolution of 10 cm. 3.2.1. Gauge Length

3.2.1. Gauge Length In Figure 3, the OFs were bonded on the surface of two steel plates with an initial crack for simulating concrete crack. Thebonded OF applied bare optical fibersteel with aplates diameter of 0.25 mm and its for In Figure 3, the OFs were on were the surface of two with an initial crack structure can be seen in Figure 3b. These OFs would be applied with different strain through moving simulating concrete crack. The OF applied were bare optical fiber with a diameter of 0.25 mm and its the plates. For investigating the influence of gauge length, the length was set as 5 cm, 10 cm, 15 cm, structure can be seen in Figure 3b. These OFs would be applied with different strain through moving 20 cm and 30 cm for the five sensing parts, respectively, while the bonding length was 5 cm for all the plates. For investigating influence of gauge length, thewas length wasasset 5 cm, 10 cm, 15 cm, the gauges, long enough tothe avoid slip failure. The epoxy resin selected theasbonding material 20 cmdue andto30 cm for the five sensing parts, respectively, while the bonding length was 5 cm for all the its excellent property.

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gauges, long enough to avoid slip failure. The epoxy resin was selected as the bonding material due to its excellent property. Sensors 2016, 16, 286 5 of 18

(a)

(b)

(c) Figure 3. Tension experiments for distributed optical sensor with different gauge length: (a) sensor

Figure 3. Tension experiments for distributed optical sensor with different gauge length: (a) sensor distribution; (b) OF structure; and (c) OF sensor and experimental setup. distribution; (b) OF structure; and (c) OF sensor and experimental setup. During the experiments, the loading and unloading were conducted five times. At the same time, the sampling distance was set as 5 cm along the OF. The typical strain distribution is as shown in During the experiments, the loading and unloading were conducted five times. At the same time, Figure 4a, from which the sensing parts can be recognized from the free segments easily except the the sampling distance was set as 5 cm along the OF. The typical strain distribution is as shown in part with 5 cm gauge length. Actually, the strain value is the largest for the 5 cm gauge length; Figure 4a, from the sensing parts can be failed recognized fromthe the free The segments easily however, which the BOTDA system has obviously to measure strain. only reason to except explain the part with 5 cm length. Actually, the strain valuethan is the for the 5 cm length; however, the gauge failure is that the sensing length is smaller thelargest spatial resolution and gauge the measurement accuracy will not promised failed in this condition. Forthe thestrain. other four parts, the present the BOTDA system hasbeobviously to measure Thesensing only reason to results explain the failure is some obvious trends. The larger strain change will be obtained when the gauge length is smaller. In not be that the sensing length is smaller than the spatial resolution and the measurement accuracy will other words, increasing gauge length will decrease strain sensitivity to some extent. However, the number promised in this condition. For the other four sensing parts, the results present some obvious trends. of the points within the sensing part shows an inverse trend for extracting the strain average. In this paper, The larger change obtained the gauge lengthstrain is smaller. In other increasing therestrain are 1, 1, 2 and 4 will valuebe points appliedwhen for obtaining the average for each sensor of 10words, cm, 15 cm, gauge length will decrease strain sensitivity to some extent. However, the number of the points 20 cm and 30 cm, respectively. The two value points at the sides of each small “step” are excluded from within the averaging process. statistics, accuracythe willstrain be obtained with In larger the sensing part shows an From inverse trendthe forhigher extracting average. thissample paper,number. there are 1, 1, consideration the two for factors above, thethe integrated performance assessed. 2 and 4In value points of applied obtaining average strain forshould each be sensor of 10 cm, 15 cm, 20 cm In Figure 4b–f, the average strain from OF is compared with the theoretical value for all the and 30 cm, respectively. The two value points at the sides of each small “step” are excluded from the sensing parts. Here, the theoretical strain is calculated by the measured crack width. In Figure 4b, the averaging process. From statistics, the higher accuracy will be obtained with larger sample number. results present an obvious failure for the sensor with a 5 cm gauge length. However, from the results In consideration thesensors, two factors above,strain the integrated performance should be assessed. of the otheroffour the excellent sensing properties are present, namely good linearity In and Figure 4b–f, the average strain from OF is compared with the theoretical value repeatability. To investigate the sensing performance in detail, the slope of the fitting linefor is all the between thetheoretical measured strain by is OFcalculated and the theoretical and thencrack listed width. in Table 1. sensingextracted parts. Here, the strain by thevalue measured InThe Figure 4b, valuepresent of the slope be closer to 1 when the measured strainais5obtained withlength. higher accuracy. Thus, the results an will obvious failure for the sensor with cm gauge However, from the the slope can reflect the measurement accuracy. The average values are 1.101, 1.128, 1.118 and 0.995 results of the other four sensors, the excellent strain sensing properties are present, namely good for these 4 sensors of gauge length 10 cm, 15 cm, 20 cm and 30 cm, respectively; in other words, the linearityrelative and repeatability. To investigate the sensing performance in detail, the slope of the fitting line errors are 10.1%, 12.8%, 11.8% and 0.5%. Meanwhile, the relative standard deviation ranges is extracted between the measured strain by OF and the value and then in Table 1. from 0.6% to 1.8%, meaning a perfect sensing stability andtheoretical no actual difference for all fourlisted different The value of the slope beitcloser tofound 1 when is obtained higher accuracy. gauges. From the will results, is easily thatthe themeasured sensor withstrain 30 cm gauge length with obtains the best integrated strain sensing Thus,accuracy. 30 cm is suggested as the values optimized length here. Thus, the slope can reflect theperformance. measurement The average aregauge 1.101, 1.128, 1.118 and

0.995 for these 4 sensors of gauge length 10 cm, 15 cm, 20 cm and 30 cm, respectively; in other words, the relative errors are 10.1%, 12.8%, 11.8% and 0.5%. Meanwhile, the relative standard deviation ranges from 0.6% to 1.8%, meaning a perfect sensing stability and no actual difference for all four different gauges. From the results, it is easily found that the sensor with 30 cm gauge length obtains the best integrated strain sensing performance. Thus, 30 cm is suggested as the optimized gauge length here.

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400

Strain ()

Strain measured by OF ()

500

15cm

300 20cm

200 30cm 100

5cm 0 13.0

13.5

14.0

14.5

15.0

15.5

40 20 0 -20 Equation: y = a*(x-b) R^2 = 0.78257 a -0.06869 ±0.0128 b 354.78771 ±58.25691

-40 -60 -80

16.0

Test 1 Test 2 Test 3 Test 4 Test 5 Average  Average fitting

Distance (m)

0

200

400

(a)

400



600

Equation: y = a*(x-b) R^2 = 0.99988 a 1.10085 ±0.00433 b -0.4985 ±1.48548

200

0 200

500 400 300 200

Equation: y = a*(x-b) R^2 = 0.99993 a 1.12789 ±0.00332 b -1.28569 ±0.82699

100

400

600

800

0

Theoretical strain ()

100

200

300



300 200

Equation: y = a*(x-b) R^2 = 0.99806 a 1.11618 ±0.01738 b 11.09257±3.35008

100 0 0

100

200

400

500

600

(d)


400

300


(c) 500

1000


0 0

800

(b) 600


800

600


300


400


250 200 150 100

Equation: y = a*(x-b) R^2 = 0.99907 a 0.99471 ±0.0107 b 2.21028 ±1.60337

50 0 0

500

50

100

150

200

250

300


(e)

(f)

Figure length: (a)(a) typical strain distribution; (b) (b) 5 cm; (c) Figure 4. 4. Experimental Experimentalresults resultsfor fordifferent differentgauge gauge length: typical strain distribution; 5 cm; 10 (d) (d) 15 cm; (e) (e) 20 cm; andand (f) 30 (c) cm; 10 cm; 15 cm; 20 cm; (f) cm. 30 cm. Table Table 1. 1. Experimental Experimental results results of of strain strain measurement measurement accuracy. accuracy.

Gauge Test 1 Test 2 LengthGauge Test 1 Test 2 Length 5 cm −0.084 −0.079 ´0.079 10 cm 5 cm 1.099´0.084 1.094 10 cm 1.099 1.094 15 cm 15 cm 1.117 1.117 1.119 1.119 20 cm 20 cm 1.125 1.125 1.107 1.107 0.996 30 cm 30 cm 1.005 1.005 0.996

Test 3

Test 3

−0.050 ´0.050 1.099 1.099 1.142 1.142 1.138 1.138 1.020 1.020

Test 4

Test 5

Test 4

Test 5

−0.076 ´0.076 1.099 1.099 1.115 1.115 1.094 1.094 0.976 0.976

´0.052 1.114 1.114 1.150 1.150 1.125 1.125 0.979 0.979

−0.052

Standard Average Standard Deviation Average Deviation −0.068 0.016 ´0.068 0.016 1.101 0.007 1.101 0.007 1.128 1.128 0.0160.016 1.118 1.118 0.0170.017 0.995 0.0180.018 0.995

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3.2.2. Bonding Based on the Length results above, another key parameter, bonding length, is studied further. In Figure 5, 3.2.2. Bonding Length 10 sensing segments in five groups were onbonding two steel plates with further. same sensing Based on the results above, another keybonding parameter, length, is studied In Figuregauge Based on the results above, another key parameter, bonding length, is studied further. Figure length, cm, with different bonding namely 5 mm, mm,with 15 mm, mmIn and 30 mm. 5, 1030sensing segments in five groups lengths, were bonding on two steel10plates same20 sensing gauge 5, 10 sensing segments in five groups were bonding on two steel plates with same sensing 30 cm, with different lengths, namelyto5 confirm mm, 10 mm, mm, 20 mm and 30 mm.gauge The the The length, monotonous tension testsbonding were implemented the15 bonding performance. From length, 30 cm, with tests different bonding lengths, namely 5the mm, 10 mm, 15 mm, 20 mm andthe 30 results, mm. The monotonous were confirm bonding From results, as showntension in Figure 6, theimplemented trend can betoeasily found that theperformance. debonding strain is larger for the monotonous tension tests were implemented to confirm the bonding performance. From the results, as shown in Figure 6, the trend can be easily found that the debonding strain is larger for the sensors sensors with bigger bonding length. In detail, the results are as follows: 2140larger µε and 2787sensors µε for the as shown Figure 6,length. the trend can be the easily found that debonding strain biggerinbonding In detail, results are as the follows: 2140 με andis2787 μεfor forthe the two two with specimens with 5 cm bonding length; 5434 µε and 5298 µε for 10 cm bonding length; 6333 µε and with bigger bonding length. length; In detail, theμε results are as 2140 με andlength; 2787 με forμε theand two specimens with 5 cm bonding 5434 and 5298 μεfollows: for 10 cm bonding 6333 6239 µε for 15 cm bonding length; and 7289 µε and 7395 µε for 20 cm bonding length. However, at the specimens with cm bonding length; 5434με μεand and7395 5298με μεfor for2010cm cmbonding bondinglength. length;However, 6333 με and 6239 με for 15 cm 5bonding length; and 7289 at end the of6239 the experiments, the bond is still effective for the two specimens with length. 30 bonding cm bonding length, cm bonding and με and με for 20 cm bonding However, endμε of for the 15 experiments, thelength; bond is still7289 effective for7395 the two specimens with 30 cm length,at the end of the experiments, the bond is still effective for the two specimens with 30 cm bonding length, while the strain is near 8000 µε. As is well known, the applied strain will be under 1000 µε at while the strain is near 8000 με. As is well known, the applied strain will be under 1000 με at most most while the lifetime strainof isof near 8000infrastructures. με. As is well known, applied strain length, willlength, be 5under 1000satisfy με atsatisfy most during the the lifetime concrete Thus,the theleast least bonding 5 mm, can during concrete infrastructures. Thus, the bonding mm, can the the during the lifetime of concrete infrastructures. Thus, the least bonding length, 5 mm, can satisfy the demands. However, in in consideration long-termperformance, performance, such as fatigue and degradation, demands. However, consideration of of the the long-term such as fatigue and degradation, demands. However, in consideration of the long-term performance, such as fatigue and degradation, the bonding length is advisedtotonot notbe be smaller smaller than the bonding length is advised than2020mm. mm. the bonding length is advised to not be smaller than 20 mm.

Figure 5. Tension experiments for for distributed distributed optical sensor with different bonding length. Figure 5. Tension experiments optical sensor with different bonding length. Figure 5. Tension experiments for distributed optical sensor with different bonding length.

Measured strain by optical fifer() Measured strain by optical fifer()

5mm-1# 5mm-2# 5mm-1# 10mm-1# 5mm-2# 10mm-2# 10mm-1# 15mm-1# 10mm-2# 15mm-2# 15mm-1# 20mm-1# 15mm-2# 20mm-2# 20mm-1# 30mm-1# 20mm-2# 30mm-2# 30mm-1# 30mm-2#

8000 8000 7000 7000 6000 6000 5000 5000 4000 4000 3000 3000 2000 2000 1000 1000 0 0 0 0

1000 2000 3000 4000 5000 6000 7000 8000 1000 2000 3000 4000 5000 7000 8000 Theoretical strain (6000 )


Figure 6. Experimental results for different bonding length.

Figure 6. Experimental bondinglength. length. Figure 6. Experimentalresults resultsfor for different different bonding

4. Fabrication of Self-Sensing FRP Bar with Improved Distributed OF Sensor

4. Fabrication of Self-Sensing FRPBar Barwith with Improved Improved Distributed OF Sensor 4. Fabrication of Self-Sensing FRP Distributed OF Sensor

There are two main steps for fabricating the proposed self-sensing FRP bar, namely preparation There are twomanufacture main steps for the proposed self-sensing FRP bar, namely preparation of sensing of fabricating self-sensing bar. There arecore twoand main steps for fabricating theFRP proposed self-sensing FRP bar, namely preparation of sensing core and manufacture of self-sensing FRP bar. In the first step, the common bare OF is first wrapped with many segments of plastic tube at, the of sensing core and manufacture of self-sensing FRP bar. In the firstequals step, the common bare OF is firstaswrapped with many segments of plastic tube at, the length of which the sensing gauge length, shown in Figure 7a.many At the same time, the plastic In the first step, the common bare OF is first wrapped with segments ofthe plastic tube length of which equals the sensing gauge length, as shown in Figure 7a. At the same time, plastic tube should keep a good mechanical performance under the high temperature, 200 °C, which is the at, thetube length of which equals the sensing gauge length, as shown in Figure 7a. °C, At the same should a good performance under the high temperature, is thetime, temperature ofkeep making FRPmechanical bar. This is critical for long-gauge sensors, as the strain200 will notwhich distribute the plastic tube should keep a bar. good mechanical performance underasthe high temperature, 200 ˝ C, temperature of making FRP This is critical for long-gauge sensors, the strain will not distribute uniformly along OF if the tube is pressed together with the inner OF. Second, the fiber is braided uniformly along OFOF. ifofthe tubethis isFRP pressed withbetween thefor inner OF. Second, the fiber isshould braided which is thethe temperature making bar.together This is critical long-gauge sensors, as the strain will around wrapped During process, the distance the plastic tube segments around the wrapped OF. During this process, the distance between the plastic tube segments should not distribute OF if the tube islength, pressed together OF. Second, be fixed as uniformly the same asalong the designed bonding about 2 cm–3with cm inthe thisinner paper. Then, somethe pre-fiber is be fixed as the as theOF. designed length, about 2 cm–3 cm in this paper. some prebraided around the wrapped During the distance between the tube segments strain should besame conducted upon the bonding OF,this theprocess, amount of which is determined by plastic theThen, actual needs. strain should be conducted upon the OF, the amount of which is determined by the actual needs. should be fixed as the same as the designed bonding length, about 2 cm–3 cm in this paper. Then,

some pre-strain should be conducted upon the OF, the amount of which is determined by the actual

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needs. Several hundreds of micro-strains are usually enough. If the pre-strain is too large, there may be some problems with the OF sensor, as enough. fatigue If fracture and debonding. Aftermay the be pre-tension, Several hundreds of micro-strains aresuch usually the pre-strain is too large, there some the epoxy resin is applied to make long-gauge sensor formed and keep the pre-strain. Then, the problems with the OF sensor, suchthe as fatigue fracture and debonding. After the pre-tension, the epoxy sensing core can beto well prepared with a sensor diameter notand larger than 1 mm. Then, the sensing core resin is applied make the long-gauge formed keep the pre-strain. can bebeing well prepared with a diameter not larger than enough 1 mm. to survive the extrusion process of FRP After packaged, the sensing core is strong After being packaged, the sensing core is strong enough to survive the extrusion process of FRP bar the bar manufacture. Thus, in the second step it is easy to make the self-sensing FRP bar by putting manufacture. Thus, in the second step it is easy to make the self-sensing FRP bar by putting the sensing sensing core together with the other fibers and passing the FRP bar manufacture process to be formed. core together with the other fibers and passing the FRP bar manufacture process to be formed. However, However, there are still some key points to be paid attention to. First, some additional strain should be there are still some key points to be paid attention to. First, some additional strain should be conducted conducted upon the sensing core to overcome the contraction due to FRP bar’s cooling down. Second, upon the sensing core to overcome the contraction due to FRP bar’s cooling down. Second, the free OF the free OF should taken from bar to make sensor connections. Luckily, there is between nearly no should be takenbe from the bar to the make sensor connections. Luckily, there is nearly no bond thebond between the plastic applied plastic tube andFRP. theThus, outeritFRP. Thus, it is easy to remove outerinFRP, as7c. shown applied tube and the outer is easy to remove the outer FRP, asthe shown Figure in Figure 7c. The typicaland structure product of thein bar are shown The typical structure productand of the bar are shown Figure 7b,c. in Figure 7b,c.

(a)

(b)

(c) Figure 7. Fabrication of self-sensing FRP bar: (a) the unit of the long-gauge OF sensing core; (b)

Figure 7. Fabrication of self-sensing FRP bar: (a) the unit of the long-gauge OF sensing core; (b) typical typical structure of the self-sensing FRP bar; and (c) the product. structure of the self-sensing FRP bar; and (c) the product.

5. Properties of Self-Sensing FRP Bar

5. Properties of Self-Sensing FRP Bar

Because of the following advantages, high ultimate strain, good environment resistance and low cost as in another paper by the author [15], the basaltgood fiberenvironment is applied to package the and OF low Becauseillustrated of the following advantages, high ultimate strain, resistance and manufacture the self-sensing basalt FRP (BFRP) bar. Thus, the properties studied in this paper cost as illustrated in another paper by the author [15], the basalt fiber is applied to package the OF are based on the bar. The(BFRP) influence thethe properties duestudied to different bar paper sizes are and manufacture the self-sensing self-sensingBFRP basalt FRP bar.upon Thus, properties in this has been already investigated in that paper [15], so the bar specimens are applied with the same based on the self-sensing BFRP bar. The influence upon the properties due to different bar sizes has diameter of 10 mm. The BOTDA system applied in the experiments is NBX-6050, with a spatial been already investigated in that paper [15], so the bar specimens are applied with the same diameter resolution of 10 cm and a strain accuracy of 7.5 με.

of 10 mm. The BOTDA system applied in the experiments is NBX-6050, with a spatial resolution of 10 cm5.1. and a strain accuracy Strain Sensing Propertyof 7.5 µε. 5.1. Strain Sensing Property 5.1.1. Experimental Description As shown in Figure 8, the self-sensing FRP bar was 1.4 m long totally, while the sensing gauge 5.1.1. Experimental Description was 30 cm and located in the middle of the bar with two 30 mm long bonding areas. To implement

As in Figure the self-sensing FRP mbar long totally, thesteel sensing gauge the shown tensile test, two steel8,anchors were installed at bar eachwas side 1.4 of the with epoxy while resin. The anchor was 30 cm and located in the middle of the bar with two 30 mm long bonding areas. To implement the tensile test, two steel anchors were installed at each side of the bar with epoxy resin. The steel anchor

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was about 25 cm long. For comparison, one FBG (Fiber Bragg Grating) sensor was fixed on the surface Sensors 2016, 16, 286 9 of 18 of the specimen at the same position as the inner sensing gauge. Meanwhile, the gauge length and was about 25were cm long. comparison, one FBG Grating) sensor was fixed on sensing the surface of bonding length alsoFor the same for the two (Fiber typesBragg of sensors. Due to high strain accuracy, Sensors 2016, 16, 286 9 of 18 the specimen at the same position asFBG the inner gauge. as Meanwhile, the in gauge and bonding the strain measurement results from weresensing considered true values the length experiments. The FBG were also the samesystem for the SM130 two types of sensors. Due to high strain accuracy, theaccuracy strain data length were obtained by the by MOI Company withsensing a strain was about 25 cm long. For comparison, onemade FBG (Fiber Bragg Grating) sensor was fixedsensing on the surface of of measurement results from FBG were considered as true values in the experiments. The FBG data were aboutthe 1 µε. specimen at the same position as the inner sensing gauge. Meanwhile, the gauge length and bonding obtained by the system SM130 made by MOI Company with a strain sensing accuracy of about 1 με. length were the same for the loading two typescases of sensors. Due to high strain sensingstrain accuracy, strain During the also experiments, two were applied: Case I, small case;the and Case II, During the experiments, two loading cases were applied: Case I, small strain case; and Case II, measurement results from FBG were considered as true values in the experiments. The FBG data were largelarge strain case. In Case I, the largest applied strain was about 200 µε with a loading step of about strain case. In Case I, the largest applied strain was about 200 με with a loading step of about obtained by the SM130 made the by MOI Company with a strain sensingcracking accuracy of about 1 με. 20 µε.20 This case wassystem used to simulate structural concrete the early με. This case was used to simulate the structuralstate state before before concrete cracking ororinin the early ageage of During the experiments, two loading cases were applied: Case I, small strain case; and Case II, cracking. In Case II, the largest applied strain was about 2000 µε with a loading step of about 200 µε. of cracking. In Case II, the largest applied strain was about 2000 με with a loading step of about 200 large strain case. In Case I, the largest applied strain was about 200 με with a loading step of about This με. caseThis is used toused simulate the structural statestate afterafter serious damages, such case is to simulate the structural serious damages, suchasaslarge largecracks cracks and and steel 20 με. This case was used to simulate the structural state before concrete cracking or in the early age rebar yielding. each case, testrepeated was repeated five times. rebarsteel yielding. For eachFor case, the testthe was five times. of cracking. In Case II, the largest applied strain was about 2000 με with a loading step of about 200 με. This case is used to simulate the structural state after serious damages, such as large cracks and steel rebar yielding. For each case, the test was repeated five times.

Figure8.8.Experimental Experimental specimen setup. Figure specimenand and setup.

5.1.2. Experimental Results and Analysis

Figure 8. Experimental specimen and setup. 5.1.2. Experimental Results and Analysis

Test 3 Test 4 Test Test 51 Average Test 2 - Average Test 3 fitting Test 4 Test 5 Average - Average fitting

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The typical strain-sensing results are shown in Figure 9. From the results, some distinct The strain-sensing are shown inbetween FigureBrillouin 9. From the results, some characteristics can beResults found:and (1) results the linear relationship frequency shift and straindistinct is 5.1.2. typical Experimental Analysis characteristics canallbethe found: (1) thecoefficients linear relationship between Brillouin frequency shift confirmed as correlation of the fitting lines are larger than 0.99; and (2) and goodstrain The typical strain-sensing results are shown in Figure 9. From the results, some distinct repeatability present between the different cycles. is confirmed as is allobviously the correlation coefficients of thetest fitting lines are larger than 0.99; and (2) good characteristics can be found: (1) the linear relationship between Brillouin frequency shift and strain is For further understanding ofbetween sensing performance, thetest slopes, namely the strain-sensing coefficient repeatability is obviously present the different cycles. confirmed as all the correlation coefficients of the fitting lines are larger than 0.99; and (2) good as illustrated in Equation (1), were extracted and listed in Tables 2 and 3namely for Case the I andstrain-sensing Case II, respectively. For further understanding of sensing performance, the slopes, coefficient repeatability is obviously present between the different test cycles. In Case I, the average values of five tests are 45.5 MHz/0.1%, 46 MHz/0.1% and 46.9 MHz/0.1% for the as illustrated Equation (1), were of extracted and listed in the Tables 2 and 3 forthe Case I and Case coefficient II, respectively. For in further understanding sensing performance, slopes, namely strain-sensing three specimens. The largest relative deviation among these values is only 2.9%, which means that the as illustrated in Equation (1),of were extracted and listed in Tables 2 and for Case I andand Case46.9 II, respectively. In Case I, the average values five tests are 45.5 MHz/0.1%, 46 3MHz/0.1% MHz/0.1% for strain sensing performance is stable for the different specimens. Meanwhile, the relative standard In Case I, the average five testsdeviation are 45.5 MHz/0.1%, 46 MHz/0.1% MHz/0.1% for the that the three specimens. The values largestofrelative among these values is and only46.9 2.9%, which means deviation of the five test cycles is not larger than 3% for each specimen, verifying a good stability and three specimens. The largest relative deviation among these values is only 2.9%, which means that the the strain sensingThe performance stable the different the relative standard repeatability. characteristiciscan also for be found in Case II.specimens. The averageMeanwhile, values are 49.8 MHz/0.1%, 48.7 strain sensing performance is stable for the different specimens. Meanwhile, the relative standard deviation of the five test cycles isfor not than 3% for eachwhich specimen, verifying good stability MHz/0.1% and 49.7 MHz/0.1% thelarger three specimens, among the largest relativeadeviation is less and deviation of the five test cycles is not larger than 3% for each specimen, verifying a good stability and repeatability. The candeviation also be of found intest Case II. The values are 49.8 MHz/0.1%, than 3% and thecharacteristic relative standard the five cycles is notaverage larger than 1% for each specimen. repeatability. The characteristic can also be found in Case II. The average values are 49.8 MHz/0.1%, 48.7 These resultsand have49.7 proved the excellent property for large strain.which the largest relative deviation 48.7 MHz/0.1% MHz/0.1% for sensing the three specimens, among MHz/0.1% and 49.7 MHz/0.1% for the three specimens, among which the largest relative deviation is less is lessthan than and the relative standard deviation oftest the five test cycles is not larger than 1% for each 3%3% relative standard deviation of the five 100 cycles is not larger than 1% for each specimen. 10and theTest 1 Test 1 specimen. results proved thesensing excellent sensing property for large strain. Test proved 2 havethe These These results have excellent property for large Teststrain. 2 Test 3 Test 4 Test 51 Test Test 2 Average Test 3 fitting  Average Test 4 Test 5 Average  Average fitting

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Strain of FBG () Strain of FBG (and ) (b) large strain. Figure 9. Typical experimental results of strain sensing-Specimen 2#: (a) small strain;

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Figure 9. Typical experimental results of strain sensing-Specimen 2#: (a) small strain; and (b) large strain.

Figure 9. Typical experimental results of strain sensing-Specimen 2#: (a) small strain; and (b) large strain.

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Table 2. Experimental results of small strain sensing linearity property (MHz/0.1%). Specimen No.

Test 1

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1# 45.6 45.9 46.1 46.4 44.5 45.5 0.7 2# 45.32. Experimental 47.8 46.3of small44.9 45 linearity property 46 Table results strain sensing (MHz/0.1%). 1.2 3# 46.3 46.1 47.4 47.1 46.7 46.9 0.5 Specimen No. Test 1 Test 2 Test 3 Test 4 Test 5 Average Standard Deviation 1# 45.6 45.9 46.1 46.4 44.5 45.5 0.7 Table 3. Experimental results of large strain sensing linearity property (MHz/0.1%). 2# 45.3 47.8 46.3 44.9 45 46 1.2 3# 46.3 46.1 47.4 47.1 46.7 46.9 0.5 Specimen No. Test 1 Test 2 Test 3 Test 4 Test 5 Average Standard Deviation 1# 49.63. Experimental 49.9 49.7 of large49.8 49.9linearity property 49.8 Table results strain sensing (MHz/0.1%). 0.1 2# 49.1 48.6 48.7 48.6 48.6 48.7 0.2 1 Test 2 49.7 Test 3 49.8 Test 4 Test Standard Deviation 3#Specimen No. 49.3 Test 49.7 49.95 Average 49.7 0.2 1# 49.6 49.9 49.7 49.8 49.9 49.8 0.1 2# 49.1 48.6 48.7 48.6 48.6 48.7 0.2 However,3#compared49.3 with the theoretical value, 49.7 MHz/0.1%, the coefficients 49.7 49.7 49.8 49.9 49.7 0.2obtained in Case

II present a closer result. All the values obtained in Case I are smaller than the theoretical value the is theoretical value, MHz/0.1%, coefficients obtained in Case by about However, 6%~8%. compared The samewith trend found for the49.7 results of thethe specimens with overall bonded II present closer result. All thefor values obtained in Case I are smaller theoretical value cases by is OF [15], as the aaverage coefficient specimens with a diameter of 10 than mm the in the small strain about 6%~8%. The same trend is found for the results of the specimens with overall bonded OF [15], 47.0 MHz/0.1% and 49.5 MHz/0.1% for the large strain cases. The explanation for the difference is not as the average coefficient for specimens with a diameter of 10 mm in the small strain cases is 47.0 clear. The difference of relative strain sensing accuracy between small strain and large strain might be MHz/0.1% and 49.5 MHz/0.1% for the large strain cases. The explanation for the difference is not clear. one main reason. Another possible reason might be the actual strain sensing properties were actually The difference of relative strain sensing accuracy between small strain and large strain might be one different. actualAnother application, thereason theoretical MHz/0.1% will be appliedwere for convenience. mainIn reason. possible mightvalue be the49.7 actual strain sensing properties actually Thus,different. it is necessary to assess the strain measurement accuracy. In actual application, the theoretical value 49.7 MHz/0.1% will be applied for convenience. In Figures 10 and 11tothe errors to evaluate Thus, it is necessary assess thewere straincalculated measurement accuracy.the strain sensing accuracy by applying In Figures and 11, theThe errors were calculated to evaluate the strain sensing the coefficient of 49.710MHz/0.1%. absolute error mainly ranges from ´15 µε to 10 accuracy µε for thebysmall the coefficient of 49.7increase MHz/0.1%. Thethe absolute error ranges strain. from −15 με the to 10large με for strainapplying case, showing no obvious with increase ofmainly the applied For strain the small strain case, showing no obvious increase with the increase of the applied strain. For the case, the range of absolute error is from ´50 µε to 10 µε. At the same time, the absolute error seems large strain case, the range absolutestrain error for is from −50 με 2# to 10 με. At theHowever, same time,itthe absolute case to present an increase with theofapplied Specimen especially. is another error seems to present an increase with the applied strain for Specimen 2# especially. However, it is about the relative error. There is a trend that the relative error will decrease with the increase of applied another case about the relative error. There is a trend that the relative error will decrease with the strain. Mainly the range of relative error is as follows: ´15%~10% for the strain smaller than 200 µε; increase of applied strain. Mainly the range of relative error is as follows: −15%~10% for the strain ´10%~0% forthan the200 strain smaller than 1000 µε smaller but larger µε; larger and ´5%~0% forand the −5%~0% strain larger smaller με; −10%~0% for the strain thanthan 1000200 με but than 200 με; than 1000 µε. for the strain larger than 1000 με. 50

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Figure 10. Measurement error of small strain sensing: (a) absolute error; and (b) relative error.

Figure 10. Measurement error of small strain sensing: (a) absolute error; and (b) relative error.

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Figure 11. Measurement error of large strain sensing: (a) absolute error; and (b) relative error.

(a) error of large strain sensing: (a) absolute error; (b) and (b) relative error. Figure 11. Measurement Figure 11.Sensing Measurement error of large strain sensing: (a) absolute error; and (b) relative error. 5.2. Temperature Property

5.2. Temperature Sensing Property

5.2. Temperature Sensing Property 5.2.1. Experimental Description

5.2.1. Experimental Description

The Brillouin scattering will also be sensitive to temperature change along the OF, so the 5.2.1. Experimental Description

The Brillouin scattering will alsobebestudied sensitive to temperature change along the OF, so the temperature sensing property should for temperature compensation. The Brillouinproperty scattering will also betemperature sensitive to sensing temperature change along thesize OF,and so the Thesensing self-sensing FRP bar applied in tests were of the same OF temperature should be studied for temperature compensation. temperature sensing property should be studied for temperature compensation. sensor installation as that shown in Figure 8. To conduct the tests, the specimens were put into an OF The self-sensing FRP bar applied in temperature sensing tests were of the same size and The self-sensing FRP bar applied inwere temperature sensing tests were of the same temperature. size and OF insulation box (Figure 12), where there four heaters to control the surrounding sensor installation as that shown in Figure 8. To conduct the tests, the specimens were put into an sensor installationwas as that shown in Figure 8. To conduct thethe tests, the specimens were put into the an The temperature measured thermometer, while Brillouin frequency shifts along insulation box (Figure 12), where with therea were four heaters to control the surrounding temperature. insulation box (Figure 12),with where there were heaters to control the surrounding temperature. specimen were measured NBX-6050. Thefour range of applied temperature was from 20 °C to 55 °C The temperature was measured with whilethe theBrillouin Brillouin frequency shifts along The witha athermometer, thermometer, while frequency the the withtemperature a step of 5 °C.was Themeasured test was repeated five times. Besides the three self-sensing FRPshifts bars, aalong segment ˝ C to 55 ˝ C specimen were measured with NBX-6050. The range of applied temperature was from 20 specimen were withtoNBX-6050. The range of applied temperature was from 20 °C to 55 °C of bare free OF measured was also used measure Brillouin frequency shifts for comparison. ˝ C. The test was repeated five times. Besides the three self-sensing FRP bars, a segment with awith stepa step of 5 of 5 °C. The test was repeated five times. Besides the three self-sensing FRP bars, a segment of bare free OF alsoalso used to to measure shiftsfor forcomparison. comparison. of bare free was OF was used measureBrillouin Brillouin frequency frequency shifts

(a)

(b)

Figure 12. Experimental setup for temperature sensing: (a) sketch map; and (b) field picture.

(a)

(b)

Figure 12. Experimental for temperature sensing: (a) sketch map; and (b) field picture. 5.2.2. Experimental Results andsetup Analysis Figure 12. Experimental setup for temperature sensing: (a) sketch map; and (b) field picture. Figure 13 shows the typical results of the temperature sensing tests. From these results, the 5.2.2. Experimental Results and Analysis characteristics can be also found: perfect linear relationship between Brillouin frequency 5.2.2. following Experimental Results and Analysis 13 shows and the typical results of the temperature tests. From these results, the shift Figure and temperature good repeatability. The slope of thesensing fitting line is the temperature sensing Figure 13characteristics shows the typical of the temperature tests. From these results, the following can be results also (2), found: perfect linear relationship Brillouin frequency coefficient as illustrated in Equation the results of which aresensing listed inbetween Table 4. following characteristics can good be also found: linear relationship between Brillouin shift The and temperature and repeatability. The tests slope the fitting linefor is the sensing relative standard deviation of theperfect five isofless than 5% thetemperature coefficients offrequency each coefficient as illustrated in Equation (2), sensing the results of which areproposed listed inline Table 4. temperature shift and temperature and good repeatability. The slope of the the fitting is the sensing specimen, proving a good temperature stability of self-sensing BFRP bar. The The relative deviation the fiveand tests is less than 5% the forTable the coefficients of each coefficient asvalues illustrated in Equation (2), of the results of 1.537 which are listed in 4. average arestandard 1.488 MHz/°C, 1.439 MHz/°C MHz/°C for three specimens, close to specimen, proving a good temperature sensing stability of the proposed self-sensing BFRP bar. The the average value, 1.403 MHz/°C, from the specimens with overall bonded OF [15]. The results have The relative standard deviation of the five tests is less than 5% for the coefficients of each specimen, average values 1.488 MHz/°C, 1.439 MHz/°C 1.537 MHz/°C for long-gauge the three specimens, close to verified that temperature theare temperature sensing property is and not the package. proving a good sensing stability of the influenced proposedby self-sensing BFRP bar. The average the average value, frequency 1.403 MHz/°C, from the specimens with overall bonded OF [15]. The results have The Brillouin shifts here are influenced by two aspects. One is the thermal expansion, ˝ ˝ ˝ values are 1.488 MHz/ C, 1.439 MHz/ C and 1.537 MHz/ C for the three specimens, close to the verified theistemperature sensing property is not influenced by the long-gauge and the that other that the temperature change effects the propagation of light package. wave. In actual average value, 1.403 MHz/˝ C, from the specimens with overall bonded OF [15]. The results have The Brillouin frequency shifts here are influenced by two aspects. One is the thermal expansion, verified temperature property is not influenced by the long-gauge package. andthat the the other is that the sensing temperature change effects the propagation of light wave. In actual

The Brillouin frequency shifts here are influenced by two aspects. One is the thermal expansion, and the other is that the temperature change effects the propagation of light wave. In actual application,

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Sensors 2016, 16,FRP 286 bar will bond together with the monitored concrete structures, meaning 12 ofthat 18 the the self-sensing bar will present the same deformation as the outer concrete due to the thermal expansion. The strain application, the self-sensing FRP bar will bond together with the monitored concrete structures, of thismeaning part is the actual structural deformation, so it does not need to be compensated. Thus, the other that the bar will present the same deformation as the outer concrete due to the thermal part isexpansion. the only part be compensated. thermal expansion of OF issovery small, µε/˝ C, The to strain of this part is As thethe actual structural deformation, it does notabout need 0.5 to be it is reasonable to consider Brillouin frequency shifts obtained As from free OF are all caused compensated. Thus, the that otherthe part is the only part to be compensated. thethe thermal expansion of by theOF change light propagation. Therefore, the temperature can frequency be implemented is veryofsmall, about 0.5 με/°C, it is reasonable to considercompensation that the Brillouin shifts by obtained fromOFthe OF are all caused by the change of light propagation. Therefore, the applying the free as free reference. temperature compensation be implemented byof applying free as reference. The average temperaturecan sensing coefficient the freetheOF is OF 1.128 MHz/˝ C with a standard ˝ The average temperature sensing coefficient of the free OF is 1.128 MHz/°C with˝ C. a standard deviation of 0.055 MHz/ C, only 5.4% larger than the theoretical value, 1.07 MHz/ Considering deviation of 0.055 MHz/°C, only 5.4% larger than the theoretical value, 1.07 MHz/°C. Considering the the variance of the test itself, the theoretical value can be applied in actual monitoring application. variance of the test itself, the theoretical value can be applied in actual monitoring application. Furthermore, the thermal expansion coefficient can be calculated for this new type of self-sensing Furthermore, the thermal expansion coefficient can be calculated for this new type of selfBFRPsensing bar by BFRP removing the part caused by the change of light propagation. Therefore, the coefficient bar by removing the part caused by the change of light propagation. Therefore, the ˝ C–8 µε/˝ C. It is close to the thermal expansion coefficient of concrete, about can becoefficient obtainedcan as 6beµε/ obtained as 6 με/°C–8 με/°C. It is close to the thermal expansion coefficient of ˝ 10 µε/ C, so the additional to temperature change will bechange decreased when the self-sensing concrete, about 10 με/°C,stress so the due additional stress due to temperature will be decreased when BFRPthe barself-sensing is embedded into structures. BFRP barconcrete is embedded into concrete structures.

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Figure 13. Typical results of temperature sensing: (a) Specimen 2#; and (b) Free OF.

Figure 13. Typical results of temperature sensing: (a) Specimen 2#; and (b) Free OF. Table 4. Experimental results of temperature sensing linearity property (MHz/°C).

Table 4. Experimental results of temperature sensing linearity property (MHz/˝ C). Specimen No. Test 1 Test 2 Test 3 Test 4 Test 5 Average Standard Deviation 1.439 1.479 1.488 0.040 Deviation Specimen No. 1# Test 1 1.541 Test 2 1.468Test 1.513 3 Test 4 Test 5 Average Standard 2# 1.508 1.373 1.454 1.446 1.416 1.439 0.050 1# 1.541 1.468 1.513 1.439 1.479 1.488 0.040 3# 1.660 1.493 1.515 1.517 1.501 1.537 0.069 2# 1.508 1.373 1.454 1.446 1.416 1.439 0.050 Free OF 1.206 1.501 1.103 1.128 0.055 0.069 3# 1.660 1.088 1.493 1.0801.5151.166 1.517 1.537 Free OF 1.088 1.080 5.3. Basic Mechanical Property

1.166

1.206

1.103

1.128

0.055

The basic mechanical 5.3. Basic Mechanical Property properties were investigated as well. For elastic modulus measurement, the load was obtained with load sensors, while the strain was applied with the results from the OFs

The properties were investigated asbe well. For elasticwith modulus measurement, only basic beforemechanical 2000 με. However, the ultimate strength could easily assessed the load and the the load was obtained with 5, load whileresults the strain was applied the results from the specimen size. In Table thesensors, experimental of elastic modulus with and ultimate strength are OFs present 2000 for comparison. Fromthe theultimate results, the average could elastic be modulus 45.6 GPa,with whilethe theload average only before µε. However, strength easilyisassessed and the ultimate MPa. The relative standard deviations 0.3% and 3.1% for the specimen size.strength In Tableis5,1019 the experimental results of elastic modulusare and ultimate strength aretwo present parameters, respectively, manifesting a good stability. Compared with the results of the similar for comparison. From the results, the average elastic modulus is 45.6 GPa, while the average ultimate specimens with overall bonded OF [15], the two mechanical parameters are decreased to some extent, strength is 1019 MPa. The relative standard deviations are 0.3% and 3.1% for the two parameters, about 1.5% and 6.9%, respectively. In other words, the mechanical performance is not remarkably respectively, manifesting a good stability. Compared with the results of the similar specimens with influenced by the embedded sensing core. This can be well understood as the sensing core is of a overall bonded OF [15], the two mechanical parameters are decreased to some extent, about 1.5% and diameter of about 1 mm, taking only 1% of the whole bar area. 6.9%, respectively. In other words, the mechanical performance is not remarkably influenced by the embedded sensing core. This can be well understood as the sensing core is of a diameter of about 1 mm, taking only 1% of the whole bar area.

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Table 5. Experimental results of basic mechanical property. Sensors 2016, 16, 286

Specimen No.

1#

2#

3#

Average

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Elastic modulus (GPa) 45.5 45.7 45.5 45.6 property. 0.1 Table 5. Experimental results of basic mechanical Ultimate strength (MPa) 1056 997 1005 1019 32 Specimen No. 1# 2# 3# Average Standard Deviation Elastic modulus (GPa) 45.5 45.7 45.5 45.6 0.1 5.4. Bonding Performance between Self-Sensing FRP Bar and Concrete Ultimate strength (MPa) 1056 997 1005 1019 32

The self-sensing FRP bar is designed to be embedded into concrete structures to implement 5.4. Bonding Performance between Self-Sensing FRP Barthe and bonding Concrete performance between self-sensing the monitoring and structural reinforcing. Thus, FRP bar The andself-sensing concrete is considered an important parameter influencing thetoeffects. Asthe guided FRP bar is designed to be embedded into concrete structures implement monitoring and structural reinforcing. Thus, bonding performance between self-sensing FRP and by the ACI440.3R-04, the specimens werethe prepared as shown in Figure 14a, with a bar diameter of concrete is consideredBFRP an important parameter effects. guided 10 mm of self-sensing bar. The size ofinfluencing concrete the block wasAs200 mmby ˆ the 200ACI440.3R-04, mm ˆ 200 mm, specimens were prepared shownfive in Figure 14a, with a diameter 10 mm self-sensing bar. here. whilethethe bonding length wasasabout times the diameter ofofthe FRPofbar, namelyBFRP 50 mm The size of concrete block was 200 mm × 200 mm × 200 mm, while the bonding length was about five The average concrete compressive strength was 37.5 MPa. There are many parameters influencing times the diameter of the FRP bar, namely 50 mm here. The average concrete compressive strength was the bonding performance, such as concrete strength, FRP bar size and FRP type. However, the simple 37.5 MPa. There are many parameters influencing the bonding performance, such as concrete strength, investigation is and implemented in this the paper to investigation illustrate how the bonding performance effects the FRP bar size FRP type. However, simple is implemented in this paper to illustrate sensing performance. how the bonding performance effects the sensing performance.

Stress (MPa)

15

10

5

0

(a)

1# 2# 3#

0

5

10

Slip (mm)

15

20

(b) Figure 14. Bond test: (a) specimen; and (b) results at the free end.

Figure 14. Bond test: (a) specimen; and (b) results at the free end.

For the three specimens, the failure mode is the same as if the bar were pulled out from the concrete. From the three resultsspecimens, in Figure 14b,the the failure bondingmode interface three stages, no slip with For the is experiences the same as if the barnamely were pulled out load from the increasing, slip increasing with load increasing and load decreasing with slip increasing. For the three concrete. From the results in Figure 14b, the bonding interface experiences three stages, namely no specimens, the trend is the while there some obvious differences. strain is 1607 1383 με slip with load increasing, slipsame, increasing withare load increasing and load The decreasing withμε, slip increasing. and 1065με for the 1#, 2# and 3# specimen, respectively, when the slip starts. This means that the bonding For the three specimens, the trend is the same, while there are some obvious differences. The strain interface can transfer the strain from the concrete to the self-sensing FRP bar perfectly before 1000 με. The is 1607 µε, 1383 µε and 1065 µε for the 1#, 2# and 3# specimen, respectively, when the slip starts. strain at the ultimate strength is 5325 με, 6315 με and 5381 με for the three specimens, respectively. This means that the bonding interface can transfer the strain from the concrete to the self-sensing FRP Considering the bond test results above, the bonding performance will not be a problem most time when bar perfectly before 1000 µε. TheFRP strain atmonitor the ultimate strength µε, 6315 µε and 5381 µε for using the proposed self-sensing bar to concrete structureisas5325 the strain of concrete structure the three specimens, respectively. Considering the bond test results above, the bonding performance is smaller than 1000 με under its service loading. However, the bonding performance should be taken into will not be a problem mostfortime usingcases, the proposed self-sensing FRP bar to monitor concrete consideration, especially somewhen large strain such as earthquakes.

structure as the strain of concrete structure is smaller than 1000 µε under its service loading. However, the bonding performance should be taken into consideration, especially for some large strain cases, such as earthquakes.

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Sensors 2016, 16, 286 6. Sensing Performance Verification with Concrete Beam

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6. Sensing Performance Verification with Concrete Beam

6.1. Experimental Description

6.1. Experimental Description To further verify the performance of the proposed self-sensing FRP bar, one concrete beam was

preparedTo and reinforced with both steel and self-sensing barsone asconcrete shown beam in Figure further verify the performance of rebar the proposed self-sensing FRP FRP bar, was 15a. One prepared self-sensing FRP bar was installed at the upside of beam section to measure compressive and reinforced with both steel rebar and self-sensing FRP bars as shown in Figure 15a. Onestrain self-sensing FRP bar was installed the upside of beam section to measure compressive strain andtotally and the other one at the downside to at measure tensile strain. As shown in Figure 15b, there were the other one atsensing the downside to inside measure tensile strain. Asbar shown thereof were 10 BOTDA-based gauges the self-sensing withinaFigure gauge15b, length 275totally mm and a 10 BOTDA-based gauges inside thesix self-sensing barFBG withsensors, a gauge namely length offrom 275 mm and bonding length of 25 sensing mm. For comparison, long-gauge F3 to F8,a were bonding length of 25 mm. For comparison, six long-gauge FBG sensors, namely from F3 to F8, were also embedded into the bar with same gauge parameter as the BOTDA-based sensors. In this paper, also embedded into the bar with same gauge parameter as the BOTDA-based sensors. In this paper, the fiber applied to make the self-sensing FRP bar is basalt fiber. the fiber applied to make the self-sensing FRP bar is basalt fiber. The typical loading pattern of four-points bending was applied in the static loading experiments The typical loading pattern of four-points bending was applied in the static loading experiments as shown in Figure 15c.15c. Meanwhile, transducers were installed to measure as shown in Figure Meanwhile,three three displacement displacement transducers were installed to measure the the displacement at middle span and the one-third span. Corresponding to the sensor distribution as displacement at middle span and the one-third span. Corresponding to the sensor distribution as shown in Figure 15d, thethe beam specimen into1212elements. elements. shown in Figure 15d, beam specimenwas was divided divided into

(a)

(b)

(c)

(d) Figure 15. Experimental setup (Unit: mm): (a) beam specimen; (b) the self-sensing FRP bar; (c) loading

Figure 15. Experimental setup (Unit: mm): (a) beam specimen; (b) the self-sensing FRP bar; (c) loading position and displacement transducer arrangement; and (d) distribution of inner OF sensors along the position and displacement transducer arrangement; and (d) distribution of inner OF sensors along beam. the beam.

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Sensors 2016, 16, Results 286 Experimental

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6.2. Experimental Results Due to the long-gauge 6.2. Experimental Results sensing pattern, the measured strain from each sensor is macro-strain, Due to the long-gauge sensing pattern, the measured strain from each sensor is macro-strain, namely the average strain of the monitored In Figure 16, the macro-strain results of all Duethe to average the long-gauge pattern, element. the measured strain16, from sensor is macro-strain, namely strain ofsensing the monitored element. In Figure theeach macro-strain results of all monitored areare present cases before concrete cracking (Figure 16a) and namelyelements theelements average strain of for the monitored In Figure 16, cracking the macro-strain results all after monitored present fortypical typicalload loadelement. cases before concrete (Figure 16a) and of after concrete cracking (Figure 16b). Moreover, all the macro-strain results are as shown in Figure monitored elements are present for typical load cases before concrete cracking (Figure 16a) and after concrete cracking (Figure 16b). Moreover, all the macro-strain results are as shown in Figure 17. From 17. Fromthe theresults, results, thekey key structural information obtained each monitored element, such concrete cracking (Figure 16b). Moreover, allcan thecan macro-strain results are as shown in Figure 17. From the structural information be be obtained for for each monitored element, such as as the results, the key structural information can be obtained for each monitored element, such as concrete cracking steel yielding. concrete cracking andand steel yielding. concrete cracking and steel yielding. 110

600

110 90

600 500 500 400

70 50 50 30

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100 0 -1000 -100 -200

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E9 E10

-30 -50

Element No.

-200 -300

Element No.

-50

Element No.

-300

Element No.

(a) (b) (a) (b) Figure 16. Typical macro-strain distribution at: (a) Load 5 kN; and (b) Load 10 kN. Figure 16. Typical macro-strain distribution at: (a) Load 5 kN; and (b) Load 10 kN. Figure 16. Typical macro-strain distribution at: (a) Load 5 kN; and (b) Load 10 kN.

-1200

-900

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E1 E2 E1 E3 E2 E4 E3 E5 E4 E6 E5 E7 E6 E8 E7 E9 E8 E10 E9 E10

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40 30 30 20

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0

500

1000

2000

2500

3000

Strain 1500 ( ) 2000

1500

2500

3000

(b) Strain ( ) (b)

Figure 17. Macro-strain results: (a) compressive strain; and (b) tensile strain. Figure 17. Macro-strain results: (a) compressive strain; and (b) tensile strain.

Figure 17. Macro-strain results: (a) compressive strain; and (b) tensile strain. As a key indicator to assess the sensing performance of self-sensing FRP bar, the strain As a keyaccuracy indicatorwas to studied assess the sensing performance of self-sensing the strain measurement through comparing the results from FBGFRP andbar, BOTDA. The measurement accuracy through results from and The As a key indicator towas assess thein sensing of the self-sensing FRPBOTDA. bar, the strain typical comparison results arestudied shown Figure comparing 18.performance From thethe results, high FBG measurement accuracy typical comparison results are shown in Figure 18. From the results, the high measurement accuracy is verifiedaccuracy for both compressive and tensile strain the measurement, as FBG the difference between measurement was studiedstrain through comparing results from and BOTDA. Thethe typical is verified for both compressive and tensile measurement, as the difference between results from FBG and BOTDA is strain small.18. comparison results are shown in Figure From thestrain results, the high measurement accuracy is the verified results from FBG and BOTDA iserrors small. In detail, the measurement were calculated for theas BOTDA-based sensors, considering the from for both compressive strain and tensile strain measurement, the difference between the results In detail, the measurement errors were calculated for the BOTDA-based sensors, considering the results from the FBG as the true value. The results of measurement error are as shown in Figures 19 FBG and BOTDA is small. results from the FBG as the true value. The results of measurement error are as shown in Figures and 20 for the compressive strain and tensile strain, respectively. For the absolute error, the trend 19 is In detail, the measurement errors were calculated for the BOTDA-based sensors, considering and 20 forthe theerror compressive tensile strain, respectively. Forthe the relative absoluteerror error,presents the trendan is obvious, increasesstrain withand increasing applied strain, while the results from the FBG as the true value. The results of measurement error are as shown in obvious, the error increases with increasing applied strain, while the relative error presents an opposite trend. In detail, the absolute error ranges from −60 με to 60 με for compressive strain Figures 19 and 20 for the compressive strain and tensile strain, respectively. For the absolute error, the opposite trend. In detail, the absolute error ranges from −60 με to 60 με for compressive strain measurement, and it is from −120 με to 100 με for tensile strain. However, the relative errors show trendno is obvious obvious, the increases with increasing applied strain, while therelative relative error presents measurement, anderror it isbetween from −120 to 100 με for tensile However, the errors show difference the με compressive strain andstrain. tensile strain measurement, and most of no obvious difference between the compressive strainthe and tensile strain measurement, and most of an opposite trend. In detail, the absolute error ranges from ´60strain µε to is 60larger µε for compressive strain them distribute from −10% to 10%. Moreover, when applied than 1000 με, the them distribute −10% to to 10%. thestrain. appliedHowever, strain is larger than 1000 με,show the no relative error befrom decreased about ±5%. measurement, andcan itfrom is ´120 µε toMoreover, 100 µε forwhen tensile the relative errors relative error can be decreased to about ±5%. obvious difference between the compressive strain and tensile strain measurement, and most of them

distribute from ´10% to 10%. Moreover, when the applied strain is larger than 1000 µε, the relative error can be decreased to about ˘5%.

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Sensors 2016, 16, 286 Sensors 2016, 16, 286 Sensors 2016, 16, 286

FBG FBG BOTDA BOTDA FBG BOTDA

-1200 -1200 -1200

-900 -600 -300 -900 -600( ) -300 Strain Strain (  ) -900 -600 -300 (a) ( ) Strain

0 0

60 60 50 60 50 40 50 40 30 40 30 20 30 20 10 20 10 0 10 0 0 00

Load (kN) Load (kN) Load (kN)

Load (kN) Load (kN) Load (kN)

60 60 50 60 50 40 50 40 30 40 30 20 30 20 10 20 10 0 10 0 0

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500 1000 1500 2000 2500 3000 500 1000 Strain 1500( ) 2000 2500 3000 ( ) 2000 500 1000 Strain 1500 2500 3000 (b) Strain ( )

(a) (b) (a)strain comparison (E5): (a) compressive strain; and (b) Figure 18. Typical (b) tensile strain. Figure 18. Typical strain comparison(E5): (E5): (a) (a) compressive and (b)(b) tensile strain. Figure 18. Typical strain comparison compressivestrain; strain; and tensile strain. Figure 18. Typical strain comparison (E5): (a) compressive strain; and (b) tensile strain.

-900 -600 -300 -900 -600( ) -300 Strain ( ) -900 Strain -600 -300 (a) ( ) Strain

0 0 0

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60 60 50 60 50 40 50 40 30 40 30 20 30 20 10 20 10 0 10 0 -10 0 -10 -20-10 -20 -30-20 -30 -30

Relative error (%) Relative error (%) Relative error (%)

E3 E3 E4 E4E3 E5 E5E4 E6 E6E5 E7 E7E6 E8 E8E7 E8

Error (  Error (  ) ) Error ( )


-1200 -1200 -1200

60 60 60 40 40 40 20 20 20 0 0 0 -20 -20 -20 -40 -40 -40 -60 -60 -60

0 0 0

(a) (b) (a) error for compressive strain: (a) absolute value;(b) Figure 19. Strain measurement and (b) relative value.

Figure 19. Strain measurement error for compressive strain: (a) absolute value; and (b) relative value.

FigureFigure 19. Strain measurement error for for compressive strain: value;and and(b) (b)relative relative value. 19. Strain measurement error compressive strain:(a) (a)absolute absolute value; value.

Error Error ( ( )) Error ( )


50 0 50

0 0 -50 -50 -100-50 -100 -100 -150 -150 0 0 -150

0

500 1000 1500 2000 2500 3000 500 1000 Strain 1500( ) 2000 2500 3000 ( ) 2000 500 1000 Strain 1500 2500 3000 (a) Strain ( ) (a)

50 50 40 4050 30 3040 20 2030 10 1020 0 010 -10 -10 0 -20 -20-10 -30 -30-20 -40 -40-30 -50 -50-400 0 -50


Relative error (%) Relative error (%) Relative error (%)

150 150 150 100 100 50100

0

500 1000 1500 2000 2500 3000 500 1000 Strain 1500( ) 2000 2500 3000 Strain (  ) 500 1000 (b) 1500 2000 2500 3000 (b) Strain ( )

(a) Figure 20. Strain measurement error for tensile strain: (a) absolute value;(b) and (b) relative value. Figure 20. Strain measurement error for tensile strain: (a) absolute value; and (b) relative value. Figure 20. Strain measurement error for tensile strain: (a) absolute value; and (b) relative value.

Figure 20. Strain measurement for tensileanother strain: (a) absoluteisvalue; and (b) relative value. To further assess the sensingerror performance, parameter e compared. With the strain To further assess the sensing performance, another parameter is e compared. With the strain distribution, the displacement distribution can be another calculated with the basic structural mechanics To further assess the sensing performance, parameter is e compared. With the strain distribution, the displacement distribution can be calculated with the basic structural mechanics knowledge. Here, thedisplacement strain of E0 and E11 were constructed with linear comparison with the results of E1strain To further assess the sensing performance, another parameter is e compared. With the distribution, the distribution can be calculated with the basic structural mechanics knowledge. Here, the strain of E0 and E11 were constructed with linear comparison with the results of E1 and E10, the respectively. typical displacement results are with shown in comparison Figure 21a. with The the displacement knowledge. Here, the The strain ofdistribution E0 and E11 were linear results of E1 distribution, displacement canconstructed be calculated with the basic mechanics and E10, respectively. The typical displacement results are shown in Figure 21a.structural The displacement assessed by BOTDA-based self-sensing FRP bar is close to the are results fromin displacement transducer, while and E10, respectively. The typical displacement results shown Figure 21a. The displacement knowledge. the strain of E0 and E11 with from linear comparison with thewhile results of assessedHere, by BOTDA-based self-sensing FRPwere bar isconstructed close to the results displacement transducer, there is stillby some difference. self-sensing The relativeFRP measurement error isresults from −10% to 0, as showntransducer, in Figure 21b. assessed BOTDA-based bar is close to the from while there is still some difference. The relative measurement error is from −10% to 0, as shown in Figure 21b. E1 and E10, respectively. The typical displacement results are shown indisplacement Figure 21a. The displacement The error is caused partially by strain sensing accuracy, and partially by the bonding performance and there is still some difference. relative measurement errorpartially is from by −10% to 0, as shown in Figureand 21b. The error is caused partially byThe strain sensing accuracy, the bonding performance assessed by BOTDA-based self-sensing FRP bar is closeand to the results from displacement transducer, The error is caused partially by strain sensing accuracy, and partially by the bonding performance and

while there is still some difference. The relative measurement error is from ´10% to 0, as shown in Figure 21b. The error is caused partially by strain sensing accuracy, and partially by the bonding performance and the analysis model especially after the steel rebar yielding. With the results here, the good sensing accuracy is verified for the proposed self-sensing FRP bar from the other side.

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Sensorsthe 2016, 16, 286 model especially after the steel rebar yielding. With the results here, the good sensing 17 of 18 analysis

accuracy is verified for the proposed self-sensing FRP bar from the other side. 10

60

Relative error (%)

Load (kN)

50 40 D2 BOTDA

30 20

0 -10 N4 N6 N8

-20 -30

10 0

-40

0

5

10

15

20

Displacement ( mm)

(a)

25

30

0

5

10

15

20

25

30

Displacement (mm)

(b)

Figure 21. Displacement measurement: (a) typical results at N6; and (b) measurement error.

Figure 21. Displacement measurement: (a) typical results at N6; and (b) measurement error.

7. Conclusions and Remarks

7. Conclusions and Remarks

In this paper, a new type of self-sensing FRP bar is proposed for concrete structures with

In this paper, distributed a new type of self-sensing FRPtobar is proposed for concrete structures with BOTDA-based long-gauge OF sensors be used as both sensing and strengthening materials. For preparing the proposed self-sensing bar,used the distributed long-gauge sensor is BOTDA-based distributed long-gauge OF sensorsFRP to be as both sensing andOFstrengthening firstly introduced as well as the industrial manufacturing process. Then, the packaged sensors are materials. For preparing the proposed self-sensing FRP bar, the distributed long-gauge OF sensor is embedded into FRP to make the self-sensing FRP bar. After that, the sensing and basic mechanical firstly introduced as well as the industrial manufacturing process. Then, the packaged sensors are properties are studied by experimentation. Moreover, the sensing performance is verified with static embedded into FRP to make the self-sensing FRP bar. After that, the sensing and basic mechanical experiments of one concrete beam reinforced with the proposed self-sensing FRP bar. Thus, some properties are studied by experimentation. Moreover, the sensing performance is verified with static conclusions can be drawn as follows. experiments of one concrete beam reinforced with the proposed self-sensing FRP bar. Thus, some (1) The BOTDA-based long-gauge OF sensor can supply reliable strain measurement accuracy, conclusions canwith be drawn as length follows. especially the gauge of 30 cm when the spatial resolution applied is 10 cm. Meanwhile, (1) BOTDA-based long-gauge sensor can supply reliable strain measurement accuracy, the The bonding length is advised to not beOF smaller than 2 cm. especially(2) with thelong-gauge gauge length of 30 cm when the spatial applied iswith 10 cm. The distributed OF sensor can resolution be well prepared theMeanwhile, proposed the manufacturing process based on fiber package. Meanwhile, it makes the manufacture of self-sensing bonding length is advised to not be smaller than 2 cm. FRPThe barlong-gauge much more convenient. (2) distributed OF sensor can be well prepared with the proposed manufacturing (3) The performance of strain sensing temperatureofsensing is verified withmuch process based on excellent fiber package. Meanwhile, it makes theand manufacture self-sensing FRP bar experiments, namely good linearity, repeatability and stability. The absolute strain measurement more convenient. error mainly ranges from −15 με to 10 με for the small strain case, while it is up to −50 με~10 με for (3) The excellent performance of strain sensing and temperature sensing is verified with the large strain case. However, the relative error will decrease with the increase of applied strain. It experiments, namely linearity, repeatability and stability. The absolute strain measurement is also proven thatgood the temperature sensing property is not influenced by the improved sensor error package. mainly ranges from ´15 µε to 10 µε for the small strain case, while it is up to ´50 µε~10 µε for Thus, the temperature effects can be compensated using the free OF sensor. the large strain However, the relativeoferror will decrease with theFRP increase applied strain. It is (4) Thecase. mechanical performance the proposed self-sensing bar isofnot remarkably also proven thatbythe influenced thetemperature embedded OFsensing sensors.property is not influenced by the improved sensor package. The bonding performance the self-sensing and concrete shows no influence Thus, the (5) temperature effects can be between compensated using theFRP freebar OF sensor. upon sensing performance when is smaller than 1000 με.bar However, it should beinfluenced taken (4) Thethe mechanical performance of the the strain proposed self-sensing FRP is not remarkably into consideration to explain the sensing accuracy when the strain is larger than 1000 με. by the embedded OF sensors. (6) The advantage of sensing performance is further verified with static experiments of one (5) The bonding performance between the self-sensing FRP bar and concrete shows no influence concrete beam reinforced with the proposed self-sensing FRP bar. The distributed compressive strain upon the sensing performance when the strain is smaller than 1000 µε. However, it should be taken and tensile strain can be obtained with an accuracy of about 10%. When the applied strain is larger into consideration to explain the sensing accuracy whentothe strain larger than 1000distribution, µε. than 1000 με, the measurement error can be decreased about 5%.isBesides the strain (6) The advantage of sensing performance is further verified with static experiments of one the self-sensing FRP bar can monitor the displacement with an accuracy of about 10%. concrete beam reinforced the proposed FRPfuture bar. to The distributed compressive However, there is with still some work to be self-sensing conducted in the make the proposed self-sensingstrain and tensile can be obtained with an accuracy of about 10%. When the applied is larger FRP barstrain more useful and applicable. Firstly, the long-term strain sensing performance should strain be studied, such as the sensing accuracy and stability under different temperature, moisture and strain cases. than 1000 µε, the measurement error can be decreased to about 5%. Besides the strain distribution, the self-sensing FRP bar can monitor the displacement with an accuracy of about 10%. However, there is still some work to be conducted in the future to make the proposed self-sensing FRP bar more useful and applicable. Firstly, the long-term strain sensing performance should be studied, such as the sensing accuracy and stability under different temperature, moisture and strain cases. Secondly, it is of great importance to ensure the long-term effective bond, especially under the fatigue loading. Thirdly, the actual performance should be investigated with filed experiments.

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There will be a lot of research to be implemented if the proposed self-sensing bar is used to make long-term SHM. Therefore, the work in this paper is just a good start. Due to the excellent properties, a good future can be expected for the application of the proposed self-sensing FRP bar in large-scale concrete infrastructure. Acknowledgments: This paper is financially supported by National Natural Science Foundation of China (Grant No. 51508364), Natural Science Foundation of Jiangsu Province (Grant No. BK20150333) and University Natural Science Foundation of Jiangsu Province (Grant No. 14KJB580009). Author Contributions: Yongsheng Tang organized the entire experimental program and drafted the manuscript. Zhishen Wu proposed the research. Conflicts of Interest: The authors declare no conflict of interest.

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Optical Fiber Grating Hydrogen Sensors: A Review.

Strain Sharing Assessment in Woven Fiber Reinforced Concrete Beams Using Fiber Bragg Grating Sensors.

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Spatially continuous distributed fiber optic sensing using optical carrier based microwave interferometry.

Fire Source Localization Based on Distributed Temperature Sensing by a Dual-Line Optical Fiber System.

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Bimetal coated optical fiber sensors based on surface plasmon resonance induced change in birefringence and intensity.

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Evanescent field characteristics of eccentric core optical fiber for distributed sensing.

Distributed Optical Fiber Sensors with Ultrafast Laser Enhanced Rayleigh Backscattering Profiles for Real-Time Monitoring of Solid Oxide Fuel Cell Operations.

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