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"On the bonding performance of distributed optical fiber sensors (DOFS) in structural concrete" presented at IALCCE2018 by Antonio Barrias

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Distributed optical fiber sensors (DOFS) is one of the most promising and exciting technologies under research to be applied in the structural health monitoring (SHM) of civil engineering infrastructures. Therefore, in this paper, the authors present a laboratory experiment where a reinforced concrete beam was instrumented with a 5-meter-long polyimide DOFS in a way that four equal segments were bonded to the bottom surface of the beam using for each segment a different type of adhesive. Three strain gauges were also used to compare the results. The beam was then loaded, generating expected equal levels of strain for each of the segments allowing for a direct comparison between them. In this exercise, additionally to the comparison with the other instrumented sensors, it is also important the consideration and analysis of the associated Spectral Shift Quality (SSQ) values of the DOFS measurements.

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"On the bonding performance of distributed optical fiber sensors (DOFS) in structural concrete" presented at IALCCE2018 by Antonio Barrias

  1. 1. Reducing Uncertainty in Structural Safety Special Session SS6 Ghent, Belgium 28-31 October 2018
  2. 2. António Barrias, Joan R. Casas and Sergi Villalba On the bonding performance of distributed optical fiber sensors (DOFS) in structural concrete
  3. 3. ▪ Introduction – Distributed Optical Fiber Sensors (DOFS) ▪ Motivation ▪ Experimental test ▪ Test results and discussion ▪ Conclusion Outline 2
  4. 4. Introduction – Optical Fiber Sensors Cylindrical symmetric structure that is composed by a central “core” with a diameter between 4 and 600 µm and a uniform refractive index Advantages: ▪ Immunity from electromagnetic interferences ▪ Small size and lightweight ▪ High sensitivity ▪ Withstand high temperatures ▪ Chemically inert – free from corrosion 4
  5. 5. Introduction – Optical Fiber Sensors All possible crack openings are covered by the extension of the sensor 5
  6. 6. Optical Backscatter Reflectometry - OBR 1. OBR system measures the Rayleigh backscatter as a function of length in an optical fiber with high spatial resolution (1mm) 2. External stimulus 3. Temporal and spectral shifts in the local backscattered pattern 4. Shifts can be measured and scaled to give distributed strain or temperature measurements Swept wavelength interferometry (fiber is divided in small windows) ▪ Sensing range – up to 70 m ▪ Spatial resolution – up to 1 mm ▪ Strain resolution – ± 2 µε ▪ Measurement range – ± 13000 µε 6
  7. 7. Motivation 7 ▪ Lack of knowledge is still present regarding the choice of the optimal bonding adhesive, especially in its use when deploying these sensors in reinforced concrete structures ▪ This is even a more critical issue when applying DOFS without any protective thick coating, such as the case of the deployed fiber in this research ▪ An additional point of interest in this study was the assessment of the influence of different spatial resolution used in the DOFS system
  8. 8. Experimental Test 8 • Reinforced concrete beam with 60x15x15 [cm] dimensions; • 5,2m polyimide DOFS was instrumented to the beam • Four different adhesives were used: Silicone, Polyester, Epoxy and Cyanoacrylate.
  9. 9. Experimental Test 9 View of loading arrangement Test cycle number Spatial Resolution Sampling Rate 1 1 cm 0.2 Hz 2 3 cm 3 1 mm 4-rupture 1 cm It was decided to initially perform three separate but identical load cycles where a different spatial resolution was used; A final load was applied inducing cracking and continuing until failure of either the specimen or the DOFS in test cycle 4.
  10. 10. 10 Applied load in test cycles 1 to 3 Applied load in test cycle 4 Properties 𝑓𝑐𝑚 𝑓𝑐𝑡𝑚 𝐸𝑐 𝜀 𝑐𝑡 [MPa] [MPa] [MPa] [µε] Specimen 48.027 3.944 37886.64 104.10 Experimental Test
  11. 11. 11 Test results and discussion - Comparison of spatial resolution input
  12. 12. 12 Test results and discussion - Comparison of spatial resolution input Spatial Resolution Comparison Statistical Properties Segments average Δ(1mm-3cm) mean (µ) -1.07 std (σ) 19.19 Δ(1mm-1cm) mean (µ) -1.23 std (σ) 19.34 Δ(1cm-3cm) mean (µ) 0.29 std (σ) 3.38
  13. 13. 13 Test results and discussion - Comparison of spatial resolution input
  14. 14. 14 Test results and discussion - Comparison of spatial resolution input Spatial Resolution Comparison Statistical Properties Segments average Δ(MA_1mm- 3cm) mean (µ) -1.05 std (σ) 4,74 Δ(MA_1cm- 3cm) mean (µ) 0.22 std (σ) 2.31 Δ(1cm-3cm) mean (µ) 0.29 std (σ) 3.38
  15. 15. 15 Test results and discussion - Comparison of bonding adhesives under elastic behavior
  16. 16. 16 Testresultsand discussion- Comparisonof bondingadhesives underelasticbehavior
  17. 17. 17 Testresultsand discussion- Comparisonof bondingadhesives underelasticbehavior
  18. 18. 18 Testresultsand discussion- Comparisonof bondingadhesives wheninducingcracking ▪ Cracking initiated at 24 kN load level; ▪ All bonded segments detect and locate the developed crack ; ▪ All segments, except the silicone bonded one, vary its readings between positive and negative values after cracking at its location; ▪ Fiber broke completely at minute 62 which corresponded to a load of 58.2 kN but in practical terms the data from the fiber was deemed unusable from minute 50 (Load of 31 kN)
  19. 19. 19 Testresultsand discussion- Comparisonof bondingadhesives wheninducingcracking
  20. 20. 20 Testresultsand discussion- Comparisonof bondingadhesives wheninducingcracking Spectral Shift Quality - SSQ Spectral Shift Quality = max 𝑈𝑗 𝜐 𝑈𝑗 𝜐 − Δ𝜐𝑗 Σ𝑈𝑗 𝜐 2 ▪ 𝑈𝑗(ν) is the baseline spectrum for a given segment of data; ▪ 𝑈𝑗(𝜈 − Δ𝜈𝑗) is the measurement spectrum under a strain or temperature change; ▪ symbol is used to represent the cross-correlation operator. SSQ is a qualitative measure of the strength of the correlation between the conducted measurement (at any point and time) and the original baseline reflected spectra. The value of the SSQ should theoretically be between 0 and 1, where 1 is obtained when a perfect correlation is achieved and 0 when it is uncorrelated. The manufacturer advises to disregard any measurements with a SSQ below 0.15.
  21. 21. 21 Testresultsand discussion- Comparisonof bondingadhesives wheninducingcracking
  22. 22. 22 Testresultsand discussion- Comparisonof bondingadhesives wheninducingcracking ▪ Remove strain data with corresponding SSQ below 0.15; ▪ Remove strain data outside of acquisition system measurement range ±13000 µε; ▪ Remove data with negative strain data (all segments were under tension); ▪ Interpolate surface with remaining data.
  23. 23. 22 Conclusions ▪ The results show how the use of a spatial resolution of 1 mm presented an undesired high spatial variability. This does not occur with a spatial resolution of 1 cm, which presented a good correlation with the data measured by the strain gauges. Therefore, the use of a spatial resolution of 1cm is deemed suitable and recommended for this type of applications ▪ It was observed how in un-cracked condition, all segments compared fairly well with the data measured by strain gauges. Nevertheless, the silicone bonded segment presented a smoother and more uniform data than the other bonded segments ▪ When the tensile capacity of the concrete was exceeded, it was verified how all the segments were able to detect and locate the crack formation. Notwithstanding, the associated SSQ values of the measurements quickly dropped below an acceptable threshold ▪ The use of silicone adhesive provides measurements, which seem to be less influenced by the decrease of the associated SSQ values and therefore provide coherent information for further stages of the load but presenting less spatially accurate measurements. The other researched adhesives, especially the cyanoacrylate, provide more precise spatially data but are limited earlier to the effect of the decrease of the SSQ values
  24. 24. The TRUSS ITN project (http://trussitn.eu) has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 642453 Thanks for your attention

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