Structural health monitoring of composite structures for durability

19 Structural health monitoring of composite structures for durability S. Alampalli, New York State Department of Transportation, USA Abstract: During the last decade, fiber-reinforced polymer composite materials have been increasingly considered by infrastructure owners for extending the life of existing structures and increasing the durability of new structures due to the advantages they offer over conventional materials. Considering the wide variability of application, this chapter focuses on application of composites in a relatively new arena, i.e., bridge structure applications. As owners start considering the life-cycle costs for effective bridge management, these materials have potential for increased use in the future. At the same time, structural health monitoring (SHM) is emerging as another tool that bridge owners are more and more comfortable using to make decisions related to bridge management activities. Given that composites are relatively new to the bridge industry, SHM has great potential for use in a complementary fashion with composite materials. This chapter briefly describes this complementary relationship with case studies. Key words: structural health monitoring (SHM), composites, bridge engineering, fiber-reinforced polymers (FRP), bridge durability.

19.1 Introduction With increased use and constrained resources, the transportation industry is facing severe challenges due to rapidly deteriorating infrastructure. Due to an emphasis on uninterrupted mobility and high reliability from the transportation infrastructure, reduction in the travel time delays and service interruptions due to reconstruction and maintenance are emphasized by bridge owners to meet customer expectations. Hence, to meet customer expectations with constrained resources, advanced materials with improved durability and maintainability, innovative and costeffective construction methodologies, and new design procedures are under consideration by bridge owners. Composite materials are increasingly becoming popular due to the advantages they offer that include their light weight, better corrosion resistance, shop fabrication capabilities, and high strength. During the last ten years, several bridge superstructures and decks were built around the world (Alampalli et al. 2002; Triandafilou and O’Connor 2009). There has also been an increased use of external wrapping to protect bridge components for strengthening and protection from adverse environments (Hag-Elsafi et al. 2002; Sen 2003). Considering these materials are still new to the industry and the harsh in-service environments bridge structures are subjected to, more data on their in-service structural behavior, durability, maintainability, and serviceability are 543 © Woodhead Publishing Limited, 2011

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required to use them appropriately and cost-effectively. This chapter will explore the use of structural health monitoring of composite materials in bridge structural applications. The next two sections of this chapter will discuss the use of FRP structures in bridge applications and structural health monitoring (SHM) issues. The fourth section discusses how SHM can benefit understanding of FRP structures, help improve durability, and assist in implementation issues. This section will be followed by case studies to further illustrate the use of SHM in FRP applications. Finally, the last section will present some recommendations for future research and practice.

19.2 FRP structures in the bridge industry Fiber-reinforced polymer (FRP) materials have been widely used for several decades in the aerospace industry due to their light weight and high strength. They have also become popular in the automotive industry due to their light weight and non-corrosive properties. Compared to these industries, application of composites to infrastructure applications is relatively new. Most of the bridge applications utilizing FRP materials were started in the early 1990s, on an experimental basis, primarily to increase the service life of existing structures by taking advantage of their light weight characteristics. Thus, early applications included bridge decks for old deteriorated truss bridges to replace the heavy concrete decks with asphalt overlays to cost-effectively extend the service life and avoid complete replacement. They were also used for wrapping deteriorated bridge piers and beams to protect them from salt water ingress and to arrest/decrease further deterioration. With these experimental applications, engineers also started paying attention to other potential advantages these materials could offer besides lightweight characteristics. These included higher strengths, non-corrosive properties, engineerable characteristics, water-resistance, easy transportation, shop fabrication, ease of erection, and perceived long-term durability. Capabilities to shop fabricate these components, coupled with short erection times compared to conventional materials, are an attractive feature to prevent long traffic interruptions and costly work zone control required during the construction. In general, use of FRP materials in bridge applications can be broadly divided into four areas: superstructures/decks (see Fig. 19.1–19.5), external reinforcement/wrapping for strengthening (bond-critical) applications (see Fig. 19.6), maintenance/temporary (non-bond-critical) applications (see Fig. 19.7 and 19.8), and internal reinforcement (see Fig. 19.9). Each application differs significantly in both structural and non-structural requirements and thus requires very different structural and material characteristics, workmanship, quality control and quality assurance methods, and inspection requirements. For example, fatigue properties are extremely important for a composite superstructure or deck, whereas creep is more important when used as an internal reinforcement in prestressed

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19.1  An FRP slab bridge during construction: Bennetts Creek Bridge in the USA.

19.2  Bennetts Creek Bridge after construction during the proof load testing.

applications or strengthening applications for concrete piers. More than 100 superstructures and bridge decks have been built around the world, several on an experimental basis or using research funding (Alampalli et al. 2002; Triandafilou and O’Connor 2009). There have also been hundreds of column/beam wrappings,

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19.3  An FRP slab bridge after construction: Troups Creek Bridge in the USA.

19.4  An FRP bridge deck during construction: Bentley Creek Bridge in the USA.

mostly for maintenance applications with a few for strengthening (Hag-Elsafi 2002; Sen 2003). Internal reinforcement is also used in several applications, but is still not yet used widely (e.g. Chen et al. 2008). Standard specifications are emerging in recent years, but are still in their infancy due to the limited long-term

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19.5  An FRP bridge deck after construction: Schroon River Bridge in the USA.

data available on their behavior in the harsh in-service conditions in which these bridge structures operate.

19.3 Structural health monitoring All structures are built for a purpose and it is the responsibility of owners to make sure the intended purpose is served at minimal or optimal costs while ensuring safety. This requires knowing the condition (or health) of the structure and taking appropriate actions, in a cost-effective manner, just in time to make sure that the condition or actions proposed have minimal adverse impact on the structure fulfilling its intended purpose. In the case of a building, it could be the time of occupation, occupants’ comfort, and ability to do the work for which the building was intended. In the case of a bridge, it is making sure that the bridge can carry the

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19.6  External FRP reinforcement for strengthening application.

19.7  External reinforcement for maintenance application: during construction.

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19.8  Columns and pier-cap wrapped with CFRP: Everett Road, New York in the USA.

19.9  Bridge deck with FRP rebars (courtesy of Dr GangaRao, West Virginia University).

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loads it is intended to carry with uninterrupted mobility during operational hours. In order to ensure required safety and mobility, bridge owners use several tools to assess structural condition and capacity. This is loosely defined in the literature as Structural Health Monitoring (SHM). SHM can be accomplished in several ways, depending on the decision(s) to be made – periodically or continuously, visually or using sensors and instrumentation, and manually or remotely. Appropriate decision making depends on two factors: structural capacity that accounts for the condition of the structure at any given time and corresponding loading. In most cases, SHM examples presented in the literature deal with the structural capacity and seldom with loading. In most cases, loading is defined by the codes and specifications effective during the original construction or reconstruction. Ensuring safety is the predominant reason for SHM in infrastructure applications. But in the last decade, besides safety, there has been more emphasis on uninterrupted service and reliability of the service. While security was taken for granted before, this has emerged as another challenging item to consider in design and maintenance of structures. All of these reasons had several implications in the infrastructure arena and thus, to meet stakeholder expectations in the face of multiple hazard environments, new designs, innovative construction and maintenance procedures, and new materials are being explored and increasingly used. These are making the field of SHM more popular and thus, have increased its use in recent years. This trend is expected to continue in the coming years. It is argued by Ettouney and Alampalli (2011a, 2011b) that SHM contains three distinct phases: measurements, structural identification, and damage detection. They introduced the term structural health in civil engineering by adding another phase, ‘Decision-Making,’ with an argument that any SHM project that does not integrate decision-making (or cost-benefit) ideas in all tasks cannot be a successful project (see Fig. 19.10). Full treatment of the subject with detailed applications can be found in Ettouney and Alampalli (2011a, 2011b).

19.10  Structural health in civil engineering concept.

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19.4 FRP structures and SHM As noted earlier, even though there have been numerous applications of FRP materials in infrastructure and bridge applications, a majority of these applications were funded by special grants or research funding on an experimental basis and are still considered by many as relatively new to civil engineering. There has been considerable research on their behavior through analysis, load testing, and monitoring (Alampalli 2006; Reising et al. 2004; Farhey 2005; Reay and Pantelides 2006). Most monitoring has been for relatively short periods when compared to the expected service life, due to budgetary considerations and change in personnel involved with these projects. Owners are still hesitant to widely adopt these materials for civil applications due to the following factors: • limited knowledge and understanding of long-term behavior and durability of FRP materials, • high initial costs compared to conventional materials, • highly restrained resource environment, • inadequate understanding and unavailability of maintenance and inspection procedures, • unavailability of documented repair procedures, • lack of specifications, • lack of design software, • lack of training materials, • lack of quick analysis to determine capacity in case of accidental damage while in-service for unforeseen conditions such as fire, impact, or snow-plow damage. The behavior of FRP materials for mechanical and environmental demands has been extensively researched in the aerospace and automotive industries and is relatively well understood. But the knowledge gained cannot be used directly in civil applications due to the following differences: • In-service conditions for civil structures are quite different due to geographical location. For example, within the United States, a structure built in California needs high-seismic resistance whereas in the Northeastern states they face corrosive road salts due to their use in winter for traction. • The service lives are long compared to aerospace and automotive applications. At present, the expected design life of a new bridge is 75 years in the United States and there are efforts to extend this to 100 or more years. • Inspection efforts are quite different for bridge applications. In the United States, by federal mandate, all bridges require an inspection at least once in two years. Most inspections are still visual based with nondestructive test methods used on a limited basis, as needed, based on visual inspection findings (Alampalli and Jalinoos 2009). Most FRP applications do not lend themselves well to visual inspections and thus need more advanced methodologies.

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• Corrective and preventive maintenance plans are also very different. In civil applications, most of these plans are reactive not pro-active. Deterioration and failure mechanisms of structural components made using conventional materials are well understood by civil engineers and thus details are designed to offer time for reactive maintenance. But deterioration/failure mechanisms associated with FRP components tend to accelerate faster than conventional materials and thus require pro-active maintenance. • Civil FRP structures such as bridge decks are sandwich structures with several laminates, joints, connections, etc. Often these are integrated (or connected) with conventional materials such as steel and concrete. Even though there is considerable data available on individual components, there is not much data available on the entire system. There have been very few or no long-term studies reported in the literature that monitored the behavior of the entire system rather than individual components. Due to the above complexities and differences with other applications that have been well studied, more data is required for improving the existing knowledge of FRP materials and to better maintain and manage them once they are built. SHM has great potential to bridge this gap and is an essential ingredient to promote the use of FRP materials in civil engineering applications and enhance their costeffective management. Along with conventional sensors and instrumentation, fiber-optic sensors and other instrumentation methodologies are under investigation as they can be better integrated into FRP structures during their construction (e.g. Amano et al. 2007). The next section gives three case studies, one in each of the following areas for bridge superstructure: bridge decks, column wrapping for strengthening, and column wrapping for temporary repairs. These studies were supported by the New York State Department of Transportation, where SHM was used to better understand the behavior and durability of FRP bridge applications.

19.5 Case studies The New York State Department of Transportation has used FRP materials for several applications in the last two decades. Realizing that FRP structures and the structures retrofitted with these materials should be monitored to ensure their adequate in-service performance and to gain more knowledge on their behavior and durability, they have monitored several of these applications and evaluated further with advanced analyses (Halstead et al. 2000; Hag-Elsafi et al. 2000; Hag-Elsafi et al. 2002; Alampalli et al. 2002; Alampalli and Kunin 2002; Chiewanichakorn et al. 2003; Aref et al. 2005a, 2005b; Alampalli 2005, 2006; Alnahhal et al. 2007). This section briefly describes three bridge applications the author was directly involved with using FRP materials in New York and discusses the SHM used to evaluate their in-service performance.

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19.5.1 Bridge deck One of the primary applications of the FRP decks has been its use as an alternative to replace old deteriorated heavy concrete bridge decks to increase the live load capacity of old steel superstructures with minimal repairs. The behavior of a few of the bridges fitted with FRP decks has been studied under static loads in the literature. Even though FRP decks have been tested for fatigue by several researchers (Dutta et al. 2007; Kitane et al. 2004; Brown and Berman 2010), there have been limited studies available on how the entire bridge system performs due to the lighter deck and thus this needs careful evaluation. This case study gives one such example where SHM that includes field testing followed by experimentally validated finite element (FE) models was used to make appropriate recommendations. More details on this case study can be found in Alampalli and Kunin (2002, 2003) and Chiewanichakorn et al. (2006). Reason for SHM The heavy concrete deck of an old deteriorated truss bridge was replaced with a lighter FRP deck. Verification of design assumptions, such as no composite action between the deck and the floor-beams it is attached to, effectiveness of field joints in transferring the loads between FRP panels, and an evaluation of the effects of the rehabilitation process on the remaining fatigue life of the structure was needed. Structure Bentley Creek Bridge, 42.7 m long and 7.3 m wide, is a highway bridge located on State Route 367 in Chemung County, New York State in the United States. The floor system was made up of steel transverse floor-beams at 4.27 m center-tocenter spacing with longitudinal steel stringers. It was originally built as a single simple-span, steel truss bridge with a reinforced concrete slab. Repairs In 1997, based on a capacity analysis, due to additional dead load from asphalt overlays and the deterioration of the steel trusses and floor system due to corrosion, the New York State Department of Transportation rehabilitated this bridge by replacing the reinforced concrete slab with a fiber-reinforced polymer (FRP) deck to prolong the structure’s service life as well as satisfying new load rating requirements (see Fig. 19.11 and 19.12). The FRP deck consists of top and bottom face skins and a web core. The face skins are composed of two plies of QM6408 and six plies of Q9100 E-glass stitched fiber fabric for a total thickness of 15 mm. The web core structure is made of two plies (3.7 mm) of QM6408 E-glass stitched fiber fabric wrapped around

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19.11  FRP bridge deck on a truss bridge that replaced an old concrete deck with asphalt overlays (elevation view).

19.12  FRP bridge deck on a truss bridge that replaced an old concrete deck with asphalt overlays (plan view).

150 mm × 300 mm × 350 mm isocycrinate foam blocks used as stay-in-place forms. The deck was designed using finite element analysis. Orthotropic in-plane properties were used in the analysis. Stresses in the composite materials were limited to 20% of their ultimate strength and deflection was limited to span/800. © Woodhead Publishing Limited, 2011

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The deck panels were designed to span between the floor-beams. The steel stringers were left in place to provide bracing to the structure, although they no longer function in carrying live load. A total of six FRP panels were used to replace the roadway. Bearing pads made of 6 mm thick neoprene pads were placed across the full length of the floor-beams to provide uniform bearing between the structural steel and the FRP deck. A polymer concrete haunch was placed on top of the bearing pads to provide a cross slope to the bridge deck. Three-inch-diameter holes were drilled in through the top face skin and foam core of the deck panels. A one-inch-diameter hole was then drilled through the bottom face of the composite deck, haunch material, and the top floor-beam flange. A structural bolt secured with a locking nut attached the deck to the superstructure. The drilled holes were then filled with a non-shrink grout. The panels were connected to each other using epoxy and splice plates. The joints consist of a longitudinal joint that runs the entire length of the bridge and four transverse joints that each span one lane. Vertical surface joints between panel sections were glued together with epoxy. Top and bottom splice plates were bonded using an acrylic adhesive. A 10 mm thick epoxy thin polymer overlay was used as the wearing surface of both the deck and sidewalk. Most of the wearing surface was applied to the panels during fabrication. Portions of the wearing surface covering panel joints and bolt lines were applied on-site after the FRP surface was lightly sandblasted and cleaned. SHM instrumentation Sensors and instrumentation were designed appropriately to suit the objectives of the SHM. Conventional, general purpose, uniaxial 350 ohm, self-temperature compensating, constantan foil strain gages were used to measure strains during the testing (see Fig 19.13). The strain gages were bonded to steel and the FRP deck with adhesive and then waterproofed. A total of 18 strain gages were used, six placed on a steel floor-beam and 12 placed on the FRP deck. The data was collected using a computerized data acquisition system. Testing and analysis Two fully loaded trucks of required configuration were used to load the bridge. The loads were positioned on the deck in such a way that the SHM objectives could be accomplished and enough data could be collected for the calibration of the finite element models that would be developed for further analysis (see Fig 19.14). Truck configuration and weights used in the testing can be found in Alampalli and Kunin (2002, 2003). Loaded trucks were also driven across the bridge at crawl speeds to create influence lines for calibration of a detailed finite element model. A finite element model of the entire bridge was developed, according to the construction drawings, using commercial finite element modeling software and

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19.13  Strain gage locations on FRP deck during load testing for measuring joint effectiveness and data for calibration of finite element models.

required analysis was performed using a general purpose finite element analysis package. This model with the FRP deck system was validated against load test results obtained from the field testing. The FRP deck was also replaced by a generic reinforced concrete deck in a model to simulate a pre-rehabilitated deck system. Implicit dynamic time-history analyses were conducted with appropriate loading configuration for a moving design fatigue truck. Fatigue life of all truss members, floor-beams, and stringers were determined based on a fatigue resistance

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19.14  Load testing of the FRP deck.

formula in the appropriate specifications used for bridge design. The modeling method used in this study is described in detail in Chiewanichakorn et al. (2006). Conclusions The field test results indicated that the FRP deck was designed and fabricated conservatively. As assumed in the design, no composite action between the deck and the superstructure was verified. But the study showed, in contrary to assumptions made, that the joints are only partially effective in load transfer between different panels. Thus, it was recommended that a future load test should be considered to determine if the combination of in-service loads and environmental exposure weakens the joints. Based on the finite element analyses, it was found that this bridge would expect to have 354 years or, presumably, infinite fatigue life based on anticipated average daily truck traffic (ADTT) and new construction assumption. The results indicated that the fatigue life of the FRP deck system almost doubles when compared with the pre-rehabilitated reinforced concrete deck system. Based on the estimated truck traffic that the bridge carries, stress ranges of the FRP deck system lie in an infinite fatigue life regime and thus imply that fatigue failure of the trusses and floor system would not be expected during its service life (Chiewanichakorn et al. 2006). Fatigue life of critical members in one of the trusses of the FRP deck was found to be more than 1000 years as illustrated in Fig. 19.15.

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19.15  Estimated fatigue life comparison (in years) of critical truss members after deck replacement. (Symbol 8 = infinity)

19.5.2 Bridge wrapping (non-bond-critical) FRP materials have been widely used for increasing structural durability against environmental (mostly corrosion) damage through wrapping substructure components, such as columns and pier caps, cost effectively when compared with conventional concrete repairs. Lightweight characteristics, resistance to expansive tendencies of the corrosion products, relatively easy assessment with visual and simple non-destructive evaluation (NDE) methods, and relatively minor changes to the original structural geometry and dimensions makes these wrappings attractive besides their cost-effectiveness. Rajan Sen (2003) provides a good overview of application of FRP materials for repairing corrosion-damaged structures by external wrapping. The primary conclusions from this study included that the wrapping does not stop corrosion but reduces the corrosion rate; better

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performance is achieved when the component is fully wrapped than when partially wrapped; and effectiveness is increased when used with epoxies that offer a better barrier to chloride penetration. The New York State Department of Transportation has used FRP wrapping for maintenance applications since 1998 on numerous applications. Both glass and carbon FRP materials were used with a combination of surface preparation methods and labor skills (Halstead et al. 2000; Alampalli 2005). Application of FRP materials to sound concrete may be considered a long-term repair as these may provide protection against the environment – especially to chloride penetration and moisture ingress. Thus, design life can be extended to match the remaining life of the bridge elements. The short-term performance of these materials has been generally satisfactory. Long-term monitoring is in progress. This case study gives a general overview of one such application where the SHM has been very useful to evaluate the surface preparation options, available to bridge owners on the durability of FRP repairs for short-term applications. More details on this case study can be found in Alampalli (2005). Reason for SHM Different surface preparation (or concrete removal methods) and the number of FRP layers for short-term repairs can have significant cost differences. Hence, effectiveness of one layer of FRP wrapping along with various concrete repair strategies in reducing the corrosion rate in the reinforcing bars was investigated. Three concrete repair strategies considered were: (1) Removal of unsound concrete to a depth of no less than 25 mm from the rear-most point of reinforcement to sound concrete at an estimated cost of about $750/m2; (2) removal of unsound concrete to rebar depth at an estimated cost of about $270/m2; and (3) no removal of concrete except for minor patching of the depressions and uneven areas at a minimal cost. Note that all the above costs do not include costs for FRP wrapping (material or labor), pressure washing the concrete, and sand blasting the concrete surface for a good bond between the concrete and FRP materials. The cost for FRP repairs was estimated, in 2002, at $125/m2 per layer of E-Glass and $175/m2 per layer of carbon. Structure The 430 m long and 23 m wide bridge carrying Route 2 over the Hudson River in Troy, NY in the United States, built in 1969 with eight spans of steel stringers and a concrete deck, was chosen for this experimental project (see Fig. 19.16). The columns (three in total) in one of the spans were deteriorated, partly due to leaking deck joints above. These columns were rectangular and tapered from bottom to top. The deterioration was non-structural and was repaired using concrete patch work in 1991–92. These repairs failed quickly and hence,

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19.16  The bridge intended for FRP wrapping for investigation of surface preparation effects on durability.

further non-structural (cosmetic) repairs were again needed in 1999 to slow or avoid future deterioration. Hence, FRP materials were used as a cost-effective way to repair the concrete. At the same time, it was also decided to evaluate the three possible concrete repair strategies described in the section above with one layer of FRP materials to see their long-term influence on the durability of the repair in terms of rebar corrosion rate and bond between the FRP materials and concrete surface. Repairs The North column was repaired using repair strategy 1, South column was repaired using strategy 2, and center column was repaired using repair strategy 3, except for minor patching. Once the repairs were done, concrete surfaces were pressure-washed and sandblasted to obtain a good bond between the concrete and FRP wrapping. One layer of Sika Wrap Hex 106G, which is a bidirectional E-glass fabric, was used to wrap all the columns. Sikadur 330, a high modulus, high strength, impregnating resin was used. It was covered with Sikadur 670W, which is a water-dispersed, acrylic protective, anti-carbonation coating. The repair work was conducted in August and September of 1999.

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SHM instrumentation The durability was evaluated using the rate of corrosion and bond between the concrete and FRP materials. The corrosion rates of the longitudinal rebar in the column were measured using the corrosion probes from Concorr, Inc., which were installed inside the column during the concrete repair (see Fig. 19.17). PR500 data acquisition equipment was used to measure the corrosion rate from the probes. A total of nine corrosion probes (three for each column) were installed based on the measured half-cell potentials, which are an indication of the probability of corrosion activity. Probes were embedded at locations which showed the maximum corrosion activity. Vaisala HMP44 humidity/temperature probes, three per column, next to the corrosion probes, with an HM141 indicator were used to measure humidity and temperature inside the columns (see Fig. 19.18).

19.17  Corrosion sensor installation during the concrete repairs.

Monitoring Corrosion rates, humidity, and temperature were collected periodically from August 1999 through 2008. Humidity levels inside the columns were found to be around 90%, indicating constant moisture levels, and this was attributed to water ingress from the unsealed top of columns. The data also indicated no correlation between the concrete temperature inside the column and the rebar corrosion rates. A typical time history plot of rebar corrosion rates for the center column is shown in Fig. 19.19.

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19.18  Humidity sensor after installation.

19.19  Typical time history of corrosion rates in column.

Conclusions The corrosion rates initially went up, then gradually slowed down and decreased with time. After about two years, they converged to values of about (or less than) 2 mils/year and stayed constant after that, indicating that the FRP wrapping is effective in controlling the corrosion rates irrespective of the concrete repair strategy used. Visual inspections and thermographic inspections indicate that, in general, the bond quality did not significantly deteriorate compared to the time of construction in 1999 (see Fig. 19.20). Thus, results indicate that FRP wrapping

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19.20  The columns after seven years in-service.

was effective, for temporary (five to seven years) repairs, in confining the repaired/ delaminated concrete columns and that concrete removal strategies did not influence the durability during the four-year monitoring duration.

19.5.3  External reinforcement (bond-critical) FRP laminates were used to strengthen a T-Beam bridge in Rensselaer County, New York in 1999 to demonstrate the application of FRP materials for costeffective rehabilitation of deteriorated reinforced concrete bridges to improve capacity and extend service life. This case study briefly describes this project and use of SHM to evaluate the durability of the FRP strengthening system after two years in service. More details on this case study can be found in Hag-Elsafi et al. (2001, 2004). Reason for SHM In strengthening applications, the bond between the FRP laminates and the concrete surface is very crucial. Hence, appropriate surface preparation and proper application are very important for the long-term durability of the retrofit strengthening system and NDE-based SHM techniques are often required to ensure desired quality of installation and assess bond effectiveness. Coin tapping and thermographic imaging are generally used respectively for local and global assessment of bond quality and

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effectiveness of the FRP retrofit system. But SHM using load testing gives a better picture of the durability of the strengthening application. This case study briefly describes such an application. Structure The 12.19 m long and 36.58 m wide reinforced-concrete structure with 26 simplysupported T-beams was built in 1932 and carries Route 378 over the Wynantskill Creek in the City of South Troy, New York (see Fig. 19.21). The bridge carries five lanes of traffic with annual daily traffic of about 30 000 vehicles. Concerns over section loss of the reinforcing steel to corrosion and the overall safety of the structure prompted the bridge strengthening using FRP laminates to improve flexural and shear capacities.

19.21  T-beam bridge before strengthening.

Repairs The Replark® laminate system consisting of Replark 30® unidirectional carbon fibers and three types of Epotherm materials (primer, putty, and resin), all manufactured exclusively by Mitsubishi Chemical Corporation of Japan, was used. The ultimate strength of the laminate system is 3400 MPa corresponding to a guaranteed ultimate strain of 1.5%. Details of the laminate system are described in Hag-Elsafi et al. (2001). Laminates were located at the bottom of the webs and between beams oriented parallel to the beams. Those at the flange soffits, spanning

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19.22  T-beam bridge strengthened with FRP laminates for additional flexure and shear strength.

between the beams, are oriented at a right angle to the beams. The U-jacket laminates, applied on the bottom and sides of the beams, are oriented parallel to the legs of the U-jackets (see Fig. 19.22). SHM instrumentation The initial instrumentation and loading was to collect data to ascertain the effectiveness of the FRP retrofit system in reducing the steel rebar stresses, ensuring the bond between the laminate and the concrete, and the effect of the retrofit system on transverse load distribution, effective flange width, and neutral axis location. Nine beams were instrumented to provide information on transverse load distribution on the bridge. Foil strain gages mounted directly on the reinforcing steel and FRP laminates, and concrete strain gages with large measuring grids were bonded using an epoxy resin (see Fig. 19.23). All gages were made watertight and protected from the environment for long-term monitoring purposes. System 6000, a general purpose data acquisition system, manufactured by the Measurements Group®, was used for data collection. Monitoring and testing The bridge was instrumented and load tested before and after installation of the FRP laminates to evaluate effectiveness of the strengthening system. Four trucks

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19.23  Typical strain gage scheme for load testing and data for calibration of finite element models.

with known loads were used for the load testing (see Fig. 19.24). The load tests were repeated after two years to monitor in-service performance of the installed system. The analysis includes general flexural behavior of the most heavily stressed beam during the testing, bond between the FRP laminates and concrete, effective flange width, and neutral axis location. Conclusions The load tests generally indicated lower strains than those measured during the test immediately after the construction, good quality of the bond between the FRP laminates and concrete, and no change in the effectiveness of the retrofit system after two years in service (see Fig. 19.25).

19.6 Summary FRP materials are relatively new to bridge applications. Hence, to overcome the knowledge gap and to widely use these materials in infrastructure and bridge applications, more in-service long-term performance data is required. Thus, FRP structures and the structures retrofitted with FRP materials should be monitored to ensure their adequate in-service performance and to collect in-service performance data. SHM is not only useful in evaluating the performance of these materials, but is also helpful in improving our knowledge to develop rational, cost-effective design and construction procedures. This chapter briefly described

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19.24  Load testing of the T-beam bridge.

19.25  Effectiveness of repairs, using rebar strain, after two years in service.

some bridge applications using FRP materials in New York State and discussed the test methods used to evaluate their in-service performance. Alampalli and Ettouney (2006) reviewed the long-term issues related to structural health of bridge decks. The following summarizes the potential use of SHM in various stages of the FRP materials use.

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Design and analysis FRP decks are still designed mostly by manufacturers using finite element analysis as these are yet to be standardized. Many properties used in the analysis are not openly revealed due to their proprietary nature. At the same time, as noted earlier, not much performance data is available and hence the designs are overconservative. Integrating SHM into these applications can help verify the design assumptions made on various performance characteristics. This data can also lead to development of simplified approaches to verify the designs by owners for quality assurance purposes, new design development, and to ascertain capacity once they are applied in the field (e.g. Alnahhal and Aref 2008). The data can then be used for calibration of the standards and specifications that can lead to design procedures that can be easily adopted by owners’ engineers. Planning At present, very limited data is available on the long-term performance of these bridges (e.g. Alampalli 2006). Integration and incorporation of SHM can fill this gap by understanding the deterioration rate and cycle of the FRP applications such that maintenance, rehabilitation, and replacement activities can be planned appropriately. Construction Considering most FRP components are shop fabricated and transported to site, quality control and assurance are required to make sure that they are not damaged on-route or during construction. Since visual methods are not very suitable for inspection of these components, integrated SHM offers utilizing sensors such as fiber-optic sensors that can accommodate quality assurance during the transportation and construction process. In-service issues As noted earlier, durability of FRP materials is not yet well documented and these materials are also not as forgiving as conventional materials. Thus, they require pro-active maintenance and SHM (either passive or active depending on the decisions required) can assist immensely. One of the big drawbacks faced by owners is the lack of available standardized procedures to make quick decisions when these structures suffer in-service damage due to conditions such as truck impact, snow-plow, vandalism, fire, etc. In such situations, owners have to make a quick decision on what actions, such as closing the lane or entire bridge or not to close, should be taken. Effectiveness of wearing surfaces has been a big issue. There has been little study done in this area (Kalny et al. 2004; Wattanadechachan

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et al. 2006; Alnahhal et al. 2006) and integrating SHM with these applications could help develop such procedures. Durability Durability issues have been investigated by several researchers in the literature (Singhvi and Mirmiran 2002; Harries 2005; Aref and Alampalli 2001; El-Ragaby et al. 2007). There is limited data on FRP civil system performance, especially on fatigue, creep, moisture, temperature, UV, etc. Thus, integrated SHM considering all these issues can help advance this knowledge that can further lead to better durable systems.

19.7 References Alampalli, S. (2005) ‘Effectiveness of FRP Materials with Alternative Concrete Removal Strategies for Reinforced Concrete Bridge Column Wrapping.’ International Journal of Materials and Product Technology, Inderscience Publishers, 23(3/4), 338–347. Alampalli, S. (2006) ‘Field Performance of an FRP Slab Bridge.’ Journal of Composite Structures, Elsevier Science, 72(4), 494–502. Alampalli, S., and Ettouney, M.M. (2006) ‘Long-Term Issues Related to Structural Health of FRP Bridge Decks.’ Journal of Bridge Structures: Assessment, Design and Construction, Taylor and Francis, 2(1), 1–11. Alampalli, S., and Jalinoos, F. (2009) ‘Use of NDT Technologies in US Bridge Inspection Practice,’ Materials Evaluation, Journal in Nondestructive Testing/Evaluation/ Inspection, 67(11), 1236–1246. Alampalli, S., and Kunin, J. (2002) ‘Rehabilitation and Field Testing of an FRP Bridge Deck on a Truss Bridge.’ Journal of Composite Structures, Elsevier Science, 57(1–4), 373–375. Alampalli, S., and Kunin, J. (2003) ‘Load Testing of an FRP Bridge Deck on a Truss Bridge.’ Journal of Applied Composite Materials, Kluwer Academic Publishers, 10(2), 85–102. Alampalli, S., O’Connor, J., and Yannotti, A. (2002) ‘Fiber-Reinforced Composites for the Superstructure of a Short-Span Rural Bridge.’ Journal of Composite Structures, Elsevier Science, Vol. 58, No. 1, pp. 21–27, September 2002. Alnahhal, W.I. and Aref, A.J. (2008) ‘Structural Performance of Hybrid Fiber Reinforced Polymer-Concrete Bridge Superstructure Systems.’ Composite Structures, Elsevier Science, 84, 319–336. Alnahhal, W.I., Chiewanichakorn, M., Aref, A.J., and Alampalli, A. (2006) ‘Temporal Thermal Behavior and Damage Simulations of FRP Deck.’ Journal of Bridge Engineering, 11(4), 452–464. Alnahhal, W.I., Chiewanichakorn, M., Aref, A.J., Kitane, Y., and Alampalli, S. (2007) ‘Simulations of Structural Behavior of Fiber-Reinforced Polymer Bridge Deck Under Thermal Effects.’ International Journal of Materials and Product Technology, Interscience Publishers, 28(1/2), 122–140. Amano, M., Okabe, Y.O., Takeda, N., and Ozaki, T. (2007) ‘Structural Health Monitoring of an Advanced Grid Structure with Embedded Fiber Bragg Grating Sensors.’ Structural Health Monitoring, Sage Publications, 6(4), 309–316.

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Aref, A., and Alampalli, S. (2001) ‘Vibration Characteristics of a Fiber-Reinforced Polymer Bridge Superstructure.’ Journal of Composite Structures, Elsevier Science, 52(3–4), 467–474, 2001. Aref, A.J., Alampalli, S., and He, Y. (2001) ‘A Ritz-Based Static Analysis Method for Fiber-Reinforced Plastic Rib Core Skew Bridge Superstructure.’ Journal of Engineering Mechanics, 127(5), 450–458. Aref, A.J., Alampalli, S., and He, Y. (2005a) ‘Performance of a Fiber-Reinforced Polymer Web Core Skew Bridge Superstructure: Field Testing and Finite Element Simulations.’ Journal of Composite Structures, Elsevier Science, 69(4), 491–499. Aref, A.J., Alampalli, S., and He, Y. (2005b) ‘Performance of a Fiber-Reinforced Polymer Web Core Skew Bridge Superstructure: Failure Modes and Parametric Study.’ Journal of Composite Structures, Elsevier Science, 69(4), 500–509. Brown, D.L. and Berman, J.W. (2010) ‘Fatigue and Strength Evaluation of Two Glass FiberReinforced Polymer Bridge Decks.’ Journal of Bridge Engineering, 15(3), 290–301. Chiewanichakorn, M., Aref, A., and Alampalli, S. (2003) ‘Failure Analysis of Fiber-Reinforced Polymer Bridge Deck System.’ Journal of Composites Technology and Research, 25(2), 119–128. Chiewanichakorn, M., Aref, A.J., and Alampalli, S. (2006) ‘Dynamic and Fatigue Response of a Truss Bridge with Fiber-Reinforced Polymer Deck.’ International Journal of Fatigue, Elsevier Science, 29(8), 1475–1489. Chen, R.H.L., Choi, J-H., GangaRao, H.V., and Kopac, P.A. (2008) ‘Steel Versus GFRP Rebars?’ Public Roads, 72(2). Dutta, P.K., Lopez-Anido, R., and Kwon, S.C. (2007) ‘Fatigue Durability of FRP Composite Bridge Decks at Extreme Temperatures.’ International Journal of Materials and Product Technology, 28(1/2), 198–216. El-Ragaby, A., El-Salakawy, E., and Benmokrane, B. (2007) ‘Fatigue Life Evaluation of Concrete Bridge Deck Slabs Reinforced with Glass FRP Composite Bars.’ Journal of Composites for Construction, 1193, 258–268. Ettouney, M., and Alampalli, S. (2011a) ‘Infrastructure Health in Civil Engineering: Theory and Components.’ CRC Press (to be published) Ettouney, M., and Alampalli, S. (2011b) ‘Infrastructure Health in Civil Engineering: Applications and Management.’ CRC Press (to be published) Farhey, D.N. (2005) ‘Long-Term Performance of Tech 21 All-Composite Bridge.’ Journal of Bridge Engineering, 9(3), 255–262. Hag-Elsafi, O., Alampalli, S., and Kunin, J. (2001) ‘Applications of FRP Laminates for Strengthening a Reinforced Concrete T-Beam Bridge Structure.’ Journal of Composite Structures, Elsevier Science, 52(3–4), 453–466. Hag-Elsafi, O., Alampalli, S., and Kunin, J. (2004) ‘In-Service Evaluation of a Reinforced Concrete T-Beam Bridge FRP Strengthening System.’ Journal of Composite Structures, Elsevier Science, 64(2), 179–188. Hag-Elsafi, O., Alampalli, S., Kunin, J., and Lund, R. (2000) ‘Application of FRP Materials in Bridge Retrofit,’ Seventh Annual International Conference on composites Engineering, Denver, CO, 305–306. Hag-Elsafi, O., Lund, R., and Alampalli, S. (2002) ‘Strengthening of a Bridge Pier Capbeam Using Bonded FRP Composite Plates.’ Journal of Composite Structures, Elsevier Science, 57(1–4), 393–403. Halstead, J.P., O’Connor, J.S., Luu, K.T., Alampalli, S., and Minser, A. (2000) ‘FiberReinforced Polymer Wrapping of Deteriorated Concrete Columns.’ Transportation Research Record 1696, National Research Council, Washington, D.C., 2, 124–130.

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Harries, K. (2005) ‘Fatigue Behavior of Bonded FRP Used for Flexural Reinforcement,’ International Symposium on Bond Behavior of FRP in Structures, International Institute for FRP in Construction, 547–552. Kalny, O., Peterman, R.J., and Ramirez, G. (2004) ‘Performance Evaluation of Repair Technique for Damaged Fiber-Reinforced Polymer Honeycomb Bridge Deck Panels.’ Journal of Bridge Engineering, 9(1), 75–86. Kitane, Y., Aref, A.J., and Lee, G.C. (2004) ‘Static and Fatigue Testing of Hybrid FiberReinforced Polymer-Concrete Bridge Superstructure.’ Journal of Composites for Construction, 8(2), 182–190. Reay, J.T. and Pantelides, C.P. (2006) ‘Long-Term Durability of State Bridge on Interstate 80.’ Journal of Bridge Engineering, 11(2), 205–216. Reising, R.M., Shahrooz, B.M., Hunt, V.J., Neumann, A.R., and Helmicki, A.J. (2004) ‘Performance Comparison of Four Fiber-Reinforced Polymer Deck Panels.’ Journal of Composites for Construction, 8(3), 265–274. Sen, R. (2003) ‘Advances in the Application of FRP for Repairing Corrosion Damage,’ Progress in Structural Engineering and Materials, John Wiley Publications, 5(2), 99–113. Singhvi, A., and Mirmiran, A. (2002) ‘Creep and Durability of Environmentally Conditioned FRP-RC Beams Using Fiber Optic Sensors.’ Journal of Reinforced Plastics and Composites, 21(4), 2002. Triandafilou, L., and O’Connor, J. (2009) ‘FRP Composites for Bridge Decks and Superstructures: State of the Practice in the U.S.’ International Conference on Fiber Reinforced Polymer (FRP) Composites for Infrastructure Applications, University of Pacific, Stockton, CA. Wattanadechachan, P., Aboutaha, R., Hag-Elsafi, O., and Alampalli, S. (2006) ‘Thermal Compatibility and Durability of Wearing Surfaces on GFRP Bridge Decks.’ Journal of Bridge Engineering, 11(4), 465–473.

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