Discontinuous Brillouin strain monitoring of small concrete bridges ...

Smart Structures and Materials 2005: Nondestructive Evaluation for Health Monitoring and Diagnostics Proc. SPIE, Vol. 5765, pp. 612-623 (2005).

Discontinuous Brillouin strain monitoring of small concrete bridges: comparison between near-to-surface and “smart” FRP fiber installation techniques Filippo Bastianini*, Andrea Rizzo, Nestore Galati, Ursula Deza, Antonio Nanni University of Missouri - Rolla, 1870 Miner Circle, Rolla, MO, USA 65409-0710 ABSTRACT Brillouin fiber optic sensing is a promising technology for Structural Health Monitoring (SHM) whose diffusion is however at present reduced by the unavailability of proper sensor products and established installation techniques specifically aimed at the building industry. Due to its intrinsic distributed sensing capability, Brillouin systems can individually measure the deformation of any single segment of considerable lengths of single-mode fiber. In addition, Brillouin retains all the other typical advantages of Fiber Optic Sensors (FOS), such as harsh environment durability and electro-magnetic interference rejection. These advantages, especially considering that the required sensors are really low cost, make the system particularly attractive for periodical (“discontinuous”) strain monitoring of unattended infrastructures that might be exposed to ageing and vandalism damages. Despite the high equipment cost, the technique can become economically convenient when the same initial investment can be amortized over a number of applications that can be monitored periodically using the same device. This work presents a comparison between two different Brillouin sensor installation techniques: Near-to-Surface Fiber (NSF) embedding and smart-FRP sensor bonding. Both systems have been experimented in the field on small Reinforced Concrete (RC) bridges subject to a diagnostic load test. The obtained results clearly highlight the advantages of the smart-FRP system, in terms of performance enhancements, installation cost, and time reduction. Keywords: Bridge monitoring, Brillouin, distributed sensing, fiber optic, load test, reinforced concrete, smart material.

1. INTRODUCTION The highway transportation infrastructure plays a role of primary strategic importance for the economy and therefore attracts the most part of Structure Health Monitoring (SHM) and preservation efforts. However, it has to be taken into account that a consistent part of the investments accumulated during all the development history of a country is scattered on a large number of small structures, such as country roads, small RC bridges and tunnels, which are an integral part of the ordinary road transportations network. Although the economic value of each individual structure is relatively low, the global value of the whole network is large, not only for the global investment associated to it, but also for the social costs that could arise from a partial network unavailability. These social costs are extremely difficult to be estimated and are often unknown: they may vary from increasing the travel time due for example to a bypass (with an increase of fuel consumption and stress for the drivers) to higher vehicle collision risk. Moreover, these small structures are typically dispersed over the territory. They can be placed far away from electrical power and telecom networks and they do not have homogeneous design characteristics and are very different for materials, manufacturing, age and damage history. All these considerations suggest that the management of the ordinary road infrastructure could be more difficult to optimize than that of major highway infrastructures. Thus, for smaller bridges, ordinary SHM techniques and on-line monitoring could be more expensive and less justifiable. Brillouin distributed systems can overcome several of these economic and practical problems and represent an ideal solution for this type of SHM1.

* [email protected]; phone +39 3475966878; 223 E.R.L., University of Missouri-Rolla, 1870 Miner Circle, Rolla, MO, USA

2. THEORETICAL BACKGROUND Brillouin effect is an anelastic scattering process that arises from the interaction between optical (photons) and acoustic waves (phonons) propagating in the same physical medium. The process is characterized by a partial energy transfer between the colliding photons and phonons that, when the medium is illuminated with a monochromatic light source, produces scattered photons characterized by a certain frequency shift with respect to the incident photons2. Due to entropy considerations, frequency downshifting (“Stokes”) process is found to be prevalent3, and in the ’70s the phenomenon was first observed in single-mode silica optical fibers4. More recently, the development of self-heterodyne techniques made possible to scan the spectrum of Brillouin scattered light with better resolution allowing the assessment of an empirical correlation between strain level and Brillouin frequency5 shift, suitable to be used for sensing purposes.

3. BRILLOUIN DISCONTINUOS MONITORING Brillouin distributed sensing has important exclusive peculiarities that make itself an ideal tool for SHM, but still retaining all the “classical” advantages of FOS over traditional electronic sensors, such as harsh environment withstanding capability, electromagnetic interference immunity, high durability and superior miniaturization grade. Distributed sensing makes Brillouin an ideal solution when many areas of a structure need to be monitored in order to be able to foresee “where” a certain phenomenon will be located during the life of the structure. In addition, the low cost of the FOS allows SHM engineers to install the sensor in every area of the structure to be monitored with the limited budget usually affecting non-research applications. Moreover, in comparison with other distributed sensing systems, such as Rayleigh scattering OTDR techniques6 and electrical time domain reflectometry7, Brillouin produces local quantitative evaluation of a strain level, that is a physical parameter directly related to the structural behavior, while other technologies only produce “damage” parameters that are arbitrarily scaled. The low cost of the sensor can fit the reduced budget associated with small bridges management and its durability and ruggedness allow savings in the installation procedures, since no additional water or moisture protection are required. The capability to scan the whole installation at a glance greatly simplifies the periodical testing procedure that basically consists of bringing the test equipment on the site, connecting the optical cable to the strain analyzer and doing some scans, while doing a basic diagnostic load test with a vehicle positioned on the deck. Generally, such procedure can be completed in a short timeperiod (few hours), and it can be conducted periodically on the structures under observation, thus obtaining what is currently addressed as “discontinuous” SHM. A great advantage of this approach is the possibility to share the same equipment between a large number of installations, thus spreading the investment related to the Brillouin strain analyzer and obtaining a faster and more effective amortization.

4. CONSIDERATIONS ON TEST EQUIPMENT Among the various Brillouin technologies that are commercially available8, Brillouin Optical Time Domain Reflectometry (BOTDR)9 based on spontaneous scattering seem to be the most preferable for field applications even though it is characterized by slightly lower accuracy performances. BOTDR is a reflection-based technology capable of working even in the case of some type of failures that could affect the sensor fiber installed on the structure. In fact, in case of circuit interruption, the survived fiber length can still be scanned, and if the opposite circuit end is accessible, a second scan can acquire the data from the part of the sensor that is isolated from the first circuit end. However, when multiple interruptions are experienced, only the most external segments are to be considered still working. Such difficulty can be solved by installing connection boxes along the circuit during the installation phase, and bypassing the interruption if multiple failures of the FOS are experienced. Moreover, when the fault is not an interruption but a localized optical loss, such as a sharp fiber bending, BOTDR equipments can generally continue working with reduced performances.

5. OVERVIEW OF BRILLOUIN “SMART” SENSORS FOR RC MEMBERS RC structures generally crack under service loads. The position of such cracks is not generally known at the time of installation of the sensors. If discrete punctual sensors were installed on the concrete surface close to where the cracks formed, they would measure a much smaller strain than the one corresponding to loading condition (e.g. the strain on the

concrete surface close to cracks is very low). For this reason, common resistive and FOS strain sensors that have point sensitivity are not suitable for RC SHM purposes, unless they were installed on the reinforcement at the time of the construction of the structure. This problem was solved by realizing “long” interferometric gages10 which however only give an integral estimation of the deformation over the entire length of the instrumented member. However, the use of “long” interferometric gages does not allow to distinguish between “physiological” (e.g. normal cracking of RC structures), and “pathological” (e.g. cracking near the supports due for example to load anomalies) behavior of RC structures. The capability of distributed sensing offered by Brillouin technology is an ideal solution for RC-members, since both the problem of assessing the crack pattern distribution and the problem of retrieving a specific location-independent strain quantitative evaluation are solved. In addition, the low cost of the sensing element allows monitoring all the areas of interest without substantially affecting the budget, so that safety considerations and global structural behavior can be drawn out by load testing the structure. Commercial optical fiber cables are not generally suited for strain measurements since they are designed to decouple the fibers from any applied strain, so no deformation measurement is possible with them. Pioneer Brillouin applications used bare fibers and tight polyamide coated fibers, bonded to the RC substrate using an intermediate Fiber Reinforced Polymer (FRP) layer11, this because of the high surface roughness of RC that could induce unwanted sharp bending of the fiber. Surface roughness is not the only difficulty to be faced by dealing with Brillouin applications to RC structures, other important parameters are: accuracy of the strain measurements, fiber protection during the “unattended” life, and the installation process and time. In order to enhance the accuracy performances, several precautions in bonding the fiber to the substrate have been suggested by different authors, ranging from the use of loop type installations to increase sensitivity in case of short gauge lengths12, to the use of special systems in order to avoid fiber pinching or bending in the fiber/structure attachment points, such as flexible sleeves and rounded bearings placed in strategic points13. First strain sensing fiber cables specifically designed for concrete embedding have been developed jointly by Nippon Telegraph & Telephone Corporation (NTT) and by Shimizu Corporation (Tokyo, Japan)14. The cable consists of a single mode optical fiber that is embedded in the center gap of an inner FRP core, and finally surrounded by a resin molded layer that has helical or slotted threading on the external surface for optimal adhesion to concrete (Figure 1). For installing Brillouin fiber sensor onto existing RC members, smart FRP materials having embedded fibers have been suggested. Advanced materials having multiple embedded optical fibers (Figure 2) were among the first smart FRPs to be successfully experimented and manufactured15. These materials are obtained by weaving a warp of structural fiber strands with a weft that is generally made of textile fibers. By choosing a suitable combination of fibers, i.e. Nylon®, Eglass, Aramid (Kevlar®, Spectra®), High Modulus Carbon (HM-C) or High Strength Carbon (HS-C) for the warp and Nylon®, Polyamide, E-glass, Aramid or HS-C for the weft, the desired mechanical characteristics of the final material can be easily designed in advance. Moreover, other physical properties of the final material, such as its electrical and thermal conductivity, can be controlled.

Figure 1: structure of RC embeddable sensing fiber

Figure 2: structure of woven smart FRP material

Figure 3: structure of thermoplastic smart FRP material

Figure 4: manufacturing of a woven smart-FRP

In the weaving process (Figure 4), some of the warp strands are substituted or accompanied by the optical fibers, thus obtaining a tape-like textile product that is flexible and easy to handle. The structural fibers protect the optical fibers from pulling damages and the tape-like geometry avoids sharp bending and furthermore suppresses the dangerous tendencies to jamming, pinching or self-looping of the single thin fibers. Weaving techniques however, are not simple to optimize in order to obtain a final product without damages of the optical fibers. In usual weaving equipment the optical fibers must be drawn through a complex system of rolls, pulleys and nozzles, a condition that easily leads to micro-bending and squeezing damages. Present proprietary technology can greatly reduce the problem and smart FRP sensor can be manufactured with losses lower than 0.29 dB/km and spool length up to several kilometers. This smart FRP sensor is simply installed by unrolling the textile on the RC member surface and then applying a thermosetting resin, such as epoxy or polyester that acts as bonding medium to the RC substrate and as saturating polymer to make the final self-protecting FRP. RC surface has however to be prepared in advance by grinding and cleaning in order to accommodate the smart-FRP. In addition, using a material with multiple optical fibers, it is possible to install at the same time parallel tight and loose coupled fibers in order to obtain a distributed thermal compensation, and also to add more strain sensing fibers in order either to obtain redundancy (spare circuits) or to increase the system accuracy by correlating data from different sensing segments placed in the same position. This smart FRP sensor has been successfully experimented also for crack detection purposes on masonry substrates16. Other smart-FRP materials that seem to be commercially available consist of a rigid laminated product with strain sensing and compensation fibers in barycentric position (Figure 3). The material is based on thermoplastic saturant and has been recently introduced with lengths up to 400m.

6. APPLICATION TO BRIDGE #1330005, PHELPS COUNTY, MO (USA) 6.1. Structure description Phelps County Bridge #1330005 bridge (Figure 5) is located in the District 09 over a branch of Meramec River on County Road 3560, Missouri, USA. The bridge was built in 1900 with under-dimensioned steel reinforcement and is subject to considerable traffic of trucks transporting country products towards the town. The bridge was a candidate for retrofit with Mechanically Fastened FRP (MF-FRP) laminates. The bridge is a girder type, with four RC girders, over a span of 7.93 m. Height of the bridge is 1.68 m. Width of deck is 6.71 m (25.4 cm length guardrail included). The deck is constituted by four RC tee beams spaced 1.8 m on center. The thickness of the slab is 15.2 cm. Deck cross section is provided in Figure 6. Some spalls of concrete in the longitudinal edges of the bridge were observed. Concrete in the beams and deck is in good condition but traces of the steel rebar corrosion can be observed on the deck. In the middle of the bottom span of every beam, bending cracking is also observed, while a longitudinal crack extends in the middle of the deck. The abutments are in good conditions even if there are few vertical cracks through the entire height. 6.2. Brillouin SHM near-to-surface mounted fibers The fact that the bridge was to be retrofitted with MF-FRP laminates, the external strengthening left limited available space on the bottom surface of the girders, which is where the maximum tensile strain are experienced, for installing the Brillouin sensing fibers.

Figure 5: side view of bridge #1330005

Figure 6: deck cross-section (quotes in mm)

For this reason, a novel installation technique suited to fit in the small area at the bottom edge of the girders was used as schematically described in Figure 7. A 7.5 mm square groove was cut at the bottom edge of each girder that had to be instrumented (Figure 9) and on the abutments in order to connect the grooves of the different girders. Then two fibers, one 9/125 µm silica single mode fiber with ∅ 900 µm tight PA buffer coating and one 9/125 µm silica single mode fiber with ∅ 900 µm gel decoupled PVC buffer coating, were installed on the bottom of the groove using acryl cyanide fast acting glue (Figure 10). The groove was then filled with epoxy putty (Figure 11) obtaining a good fiber protection (Figure 12). A total of 64 m of optical fiber, comprised of 32 m of strain sensing fiber and 32 m of thermal compensation fiber, was installed on all the four girders. Four connection boxes were installed at the final ends of the groove and at some intermediate locations, in each box a 5 m free length of each fiber was left stored in a small ∅ 20 cm loop for future bypass failure recovery. In the box at the far end of the circuit the two fibers were joined by fusion splicing, while in the box closest to the bridge access point was stored the FC-PC equipment connection port followed by the 15 m service cable were stored. Schematic survey of the SHM installation is provided in Figure 8.

Figure 7: schematic section and detail of the sensor location.

Figure 9: groove cutting

Figure 8: schematic survey of the SHM installation.

Figure 10: fiber positioning

Figure 11: groove filling with epoxy putty

Figure 12: final overview of Near to Surface Fiber (NSF) installation

6.3. Diagnostic load test The bridge was tested for service performance under static loads using a H15 calibrated truckload with three axles (Figure 13). In addition to Brillouin SHM, resistive strain gages and LVDT displacement sensors were temporarily installed at midspan of each girder. Five paths, each with three stations (truck stops), were marked along the longitudinal direction of the bridge to give critical loading cases.

Figure 13: execution of the diagnostic load test

Figure 14: detail of one of the cracks crossing the NSF sensor

6.4. Experimental results The strain distribution was measured using an AQ8603 BOTDR (Yokogawa Electric Corp., Tokyo, Japan) and is presented in Figure 15, Figure 16, Figure 17 and Figure 18, respectively for girder 1, 2 3 and 4. A tri-dimensional rendering of the strain distribution of the whole structure is presented in Figure 19 (stop 4A) and Figure 20 (stop 4B).

Figure 15: strain distribution along girder 1




Figure 19: 3D rendering of the strain distributions at stop 4A

Figure 20: 3D rendering of the strain distributions at stop 4B

Although the measured deformation behavior matches the expected one both qualitatively and for the order of magnitude of the peak strain levels, the accuracy of the data is to be considered rather poor. In fact the test was done at night while snowing, and the extremely low temperature was responsible for an increased density of the strain decoupling gel of the thermal compensation fiber. For this reason, the compensation fiber exhibited unusual strain sensitivity that made impossible distributed compensation. Furthermore, during the first step of the test several new cracks were found in

particular on girder 1, mostly in the area ±1.5 m around midspan. The cracks are visible both on bottom and sides and they are strongly believed to cross the NSF sensor being clearly detectable on the epoxy free surface (Figure 14). These cracks are among the symptoms of a permanent deformation of girder 1, whose strain distribution, in fact, remains practically unchanged during the various load steps (Figure 15).

7. APPLICATION TO BRIDGE ON WALTERS STREET, ST. JAMES COUNTY, MO (USA) 7.1. Structure description The Walters Street bridge (Figure 21) is located in the residential area of St. James, Missouri (USA). Among four bridges built with FRP technologies as replacement of deteriorated concrete structures, this bridge consists of nine precast concrete panels reinforced with FRP bars, interconnected with shear keys and spanning in the direction of the vehicular traffic. The short-span bridge is 7.3 m long and 7.8 m wide. For the CFRP bars, a guaranteed design tensile strength of 1860 MPa and a tensile elastic modulus of 104.7 GPa were given by the manufacturer. For the GFRP bars these values were 723.4 MPa and 41.3 GPa, respectively. The panels were designed according to ACI Committee 440 guidelines for reinforcing concrete with FRP bars and were manufactured and installed by Oden Enterprises, Inc. in June 2001. The bridge was designed to carry a standard HS20-44 (approximately 180- kN) truck load with deflections within the requirements of the American Association of State Highway and Transportation Officials (AASHTO). The dead load of the bridge panels was considered of 7.2 kN/m2.

Figure 21: side view of Walters Street bridge

Figure 22: surface preparation

7.2. Brillouin SHM with smart-FRP In December 2003 a Brillouin SHM system was installed on three of the nine RC slabs, using a smart-FRP material. A 30 cm wide strip in the middle of the bottom surface of the concrete panels was first prepared with a rotary steel brush (Figure 22) and cleaned with compressed air. Then a first layer of epoxy resin (Wabo MBrace) was spread with a painting roller and the smart-FRP tape was installed over the resin and held in place with adhesive tape at the ends. Finally, the tape was fully encapsulated with the same epoxy in order to obtain suitable fiber protection (Figure 23). A total of 99 m of single mode optical fiber were installed on three slabs. The installation technique was found to be fast and effective even in difficult working conditions. The smart-FRP has three embedded optical fibers, two intended for strain detection that are a ∅ 450 µm and a ∅ 900 µm tight buffer coating single-mode fiber, respectively, and one, intended for distributed thermal compensation, that is a 900 µm gel-coupled loose buffer coating single mode fiber. The fiber are respectively placed 25 mm, 17.5 mm and 10 mm from the tape edge (Figure 24). The tape has a total width 80mm and is woven using a 68 TEX E-grass warp (Young modulus = 6.9 GPa) a weft of 3,000 filaments of HS-Carbon (Young modulus = 230 GPa). The individual sensing arms were joined by fusion splicing (Figure 25), thus obtaining a single optical circuit. Some access points (connection boxes) were provided along the optical circuit in order to allow future fault recovery capability: in each box a free length of 5 m of free fiber has been left wounded on a ∅ 100 mm spool. The results of a survey of the SHM installation are provided in Figure 26. It is important to notice that, since high optical losses localized at access points the were experienced during preliminary OTDR testing, all the boxes were open and all the fibers were unrolled and left hanging out from the boxes during strain measurements. In addition to Brillouin SHM, resistive strain gages and LVDT displacement sensors were temporarily installed at midspan of each girder.

Figure 23: smart-FRP encapsulation

Figure 24: structure of the smart-FRP installed (quotes in mm)

7.3. Diagnostic load test The bridge was tested for service performance under static loads using a H25 truckload with three axles (Figure 27). Seven paths, each with 5 stations, were marked along the longitudinal direction of the bridge to give critical loading cases.

Figure 25: field splicing operations

Figure 27: execution of the diagnostic load test

Figure 26: schematic survey of the SHM installation

Figure 28: detail of one of the cracks crossing the smart-FRP

7.4. Experimental results The strain distribution was measured as described for section 6.4 and is presented in Figure 29, Figure 30 and Figure 31, respectively, for slab 2, 3 and 4 together with the data retrieved from resistive strain gages. A tri-dimensional strain rendering of the whole structure is presented in Figure 32 (stop 1A) Figure 33 (stop 1C) and Figure 34 (stop 3D).

Figure 29: strain distributions recorded for panel 2



Figure 32: 3D rendering of the strain distributions at stop 1A

Figure 33: 3D rendering of the strain distributions at stop 1C

Figure 34: 3D rendering of the strain distributions at stop 3D

The measured deformation behavior matches the expected one both qualitatively and for the order of magnitude of the peak strain levels, while the accuracy of the data is in accordance with the declared performance of the BOTDR (±35µε error). As reported for the experience on bridge #1330005, the test was undertaken at night with extremely low temperature (-4°C) which was responsible of an increased density of the strain decoupling gel of the thermal compensation fiber. For this reason, it was not possible to apply the distributed thermal compensation. No specific strainBrillouin shift calibration was performed for the used fibers, and default values of the equipment were assumed as valid.

Similarly to what happened in the other test, several cracks were noted opening in the fiber optic sensor area (Figure 28), but the smart-FRP structure, thanks to the presence of structural fibers, was not affected by the cracks.

8. CONCLUSIONS Two bridges with similar dimensions were instrumented and tested under similar environmental conditions. They are an interesting benchmark for comparison of two different fiber optic installation techniques used, near-to-surface mounted fibers (NSF) and smart-FRP. First of all, it has to be noted that NSF took about 28 man-hours to be installed on four girders, while smart-FRP took only 9 man-hours to be installed on three slabs. Second, NSF installation resulted to be much more problematic for the amount of dust and noise produced during groove cutting. From a comparison between the results obtained on the two bridges, it appears that smart-FRP ensures a higher strain sensitivity and overall better performances. This can be explained considering the large number of cracks noted on the concrete during the load tests, and the local behavior of the two different installation techniques around the crack edges. For NSF sensor, acryl cyanide and epoxy putty were used respectively as bonding and encapsulating media. Being both fragile materials, the crack could easily propagate across the NSF section inducing a strain distribution on the sensing fiber that is mostly concentrated in the small distance between the crack edges (Figure 35). This very short step-like condition in the strain distribution is extremely demanding for the performance of the BOTDR equipment, since the declared accuracy could be obtained only with a strain step length of 1m. The situation is different for smart-FRP sensor (Figure 36), where the bridging effect due to structural fibers “spreads” the strain peak over a certain shear stress transfer length17. Both the bigger fiber length that is interested by the phenomenon and the smoother transition in the strain level contribute to enhance the strain sensitivity of smart-FRP in comparison with other fiber installation techniques.

Figure 35: schematic shear distribution in the crack area for NSF sensor

Figure 36: schematic crack distribution in the crack area for smart-FRP sensor

In conclusion, Brillouin distributed sensing has been proven to be an effective technique for SHM of small RC bridges, and its strain sensitivity has been found to be adequate to detect the deformations induced by an ordinary diagnostic load test. Moreover, the smart-FRP fiber installation technique has to be preferred over NSF, for the better performances and the lower installation cost and time.

9. ACKNOWLEDGEMENTS The project was made possible with the financial support received from UMR University Transportation Center (UTC) on Advanced Materials and NDT Technologies. The precious contribution from: Yokogawa Corporation of America/ANDO Product Group, in particular in the person of Dr. Nobuyuki Morita, is gratefully acknowledged.

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