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Conventional methods for displacement based condition assessment of bridges solely rely on the maximum level of displacements experienced by the piers, and do not take into account the accumulated damage that result from cyclic loading. More advanced approaches take this into account by considering the structural damage as a linear combination of the normalized maximum displacements and hysteretic energy. Computation of the dissipated hysteretic energy requires monitoring of the lateral forces during the seismic events, which are not as practical as monitoring bridge pier deformations. This article reports on the development of a simple damage assessment method that considers the effect of cyclic loading on the state of damage and it is merely based on monitoring the bridge pier deformations. A fiber optic displacement serial array was designed for measuring the crack opening displacement reversals at the plastic hinge areas.

This article describes a simplified technique to measure the vertical deflection of masonry arches in the presence of horizontal movements. One-dimensional laser Doppler vibrometer was utilized to measure the vertical displacement of the arch. A flat steel plate was mounted on the top surface of the arch crown to eliminate the measurement error due to the curvature of the arch. A reflective mirror was employed to guide the emitted laser beam normal to the surface of the steel plate. Vertical displacements recorded by laser Doppler vibrometer were compared to the readings of the mechanical dial gauge.

The study presented in this article concentrated on investigating the ductility and characterization of damage in concrete beams post tensioned with hybrid carbon-glass fiber-reinforced polymer (HFRP) composites. The investigation included an approach for design of flexural members with HFRP tendons and characterization of damage, load deformation response, ultimate strength, and failure modes. Direct tensile tests of hybrid FRP rods in a previous study had indicated elastoplastic response, enhanced ductility, and increased strain capacity. In this context, the current study focused on design and fabrication of post tensioned beams using glass or steel rebars for partial prestressing. All the beams were tested in flexure under four-point bending configuration. Results of the study are presented in terms of ductility index and enhanced load-deflection response in comparison with the conventional FRP materials. Damage characterization involved evaluating the specific features of the acoustic emissions for d...

The objective of this paper is to study the flexural performance of reinforced concrete (RC) beams strengthened with externally bonded carbon fibre reinforced polymer (CFRP) fabrics under fatigue loadings, including load, which will produce high stress levels in the materials. The crack development and the stress change in the steel and the CFRP were monitored by strain gauges and fibre optic sensors in the process of the fatigue tests, and the failure mechanisms were compared with that of statically loaded similar beams. The behaviour of the S–N (cyclic stress versus numbers of cycle to failure) data from the current tests and the available technical literature is analysed and compared with existing fatigue design rules for un-strengthened RC beams using ACI 215 guidelines and AASHTO specifications.

This article describes the development of a fiber optic accelerometer based on Fiber Bragg Gratings (FBG). The accelerometer utilizes the stiffness of the optical fiber and a lumped mass in the design. Acceleration is measured by the FBG in response to the vibration of the fiber optic mass system. The wavelength shift of FBG is proportional to the change in acceleration, and the gauge factor pertains to the shift in wavelength as a function of acceleration. Low frequency version of the accelerometer was developed for applications in monitoring bridges. The accelerometer was first evaluated in laboratory settings and then employed in a demonstration project for condition assessment of a bridge. Laboratory experiments involved evaluation of the sensitivity and resolution of measurements under a series of low frequency low amplitude conditions. The main feature of this accelerometer is single channel multiplexing capability rendering the system highly practical for application in condition assessment of bridges. This feature of the accelerometer was evaluated by using the system during ambient vibration tests of a bridge. The Frequency Domain Decomposition method was employed to identify the mode shapes and natural frequencies of the bridge. Results were compared with the data acquired from the conventional accelerometers.

Bridges form crucial links in the transportation network especially in high seismic risk regions. This research aims to provide a quantitative methodology for post-earthquake performance evaluation of the bridges. The experimental portion of the research involved shake table tests of a 4-span bridge which was subjected to progressively increasing amplitudes of seismic motions recorded from the Northridge earthquake. As part of this project, a high resolution long gauge fiber optic displacement sensor was developed for post-seismic evaluation of damage in the columns of the bridge. The nonlinear finite element model was developed using Opensees program to simulate the response of the bridge and the abutments to the seismic loads. The model was modified to predict the bent displacements of the bridge commensurate with the measured bent displacements obtained from experimental analysis results. Following seismic events, the tangential stiffness matrix of the whole structure is reduced due to reduction in structural strength. The nonlinear static push over analysis using current damaged stiffness matrix provides the longitudinal and transverse ultimate capacities of the bridge. Capacity loss in the transverse and longitudinal directions following the seismic events was correlated to the maximum displacements of the deck recorded during the events.

Optical fiber sensors have been successfully implemented in aeronautics, mechanical systems, and medical applications. Civil structures pose further challenges in monitoring mainly due to their large dimensions, diversity and heterogeneity of materials involved, and hostile construction environment. This article provides a summary of basic principles pertaining to practical health monitoring of civil engineering structures with optical fiber sensors. The issues discussed include basic sensor principles, strain transfer mechanism, sensor packaging, sensor placement in construction environment, and reliability and survivability of the sensors.

The creep properties of optical fiber used in fiber optical sensors were studied in this paper. A low coherent white light double interferometer system was designed and calibrated and the creep deformations of optical fibers under static and cyclic loadings were measured with this device. The research results showed that polymer coated optical fibers crept at the beginning when they were under static or cyclic load. As the number of the cyclic loading or the static loading times increased the creep tended to stop. Thus to ensure that the optical fiber keeps pre-stress for long time in pressure transducer, it is recommended that the optical fiber should be tensioned cyclically before being fixed into the sensor device.

Development and testing of a new type of bridge weigh-in-motion system (BWIM) based on the measurement of shear forces near the supports of the bridges is described. The proposed system is applicable to both determinate as well as indeterminate bridges. Fiber-optic Bragg grating rosette sensors were used in this approach. Formulations are based on the establishment of shear influence lines in terms of axle weights and spacings. The system further involves calibration for the estimation of a bridge parameter α, which is a function of the cross-sectional and material properties of the bridge. Three different bridges, one box-girder prestressed concrete bridge and two concrete-slab-on-steel-girder bridges with different span lengths, were instrumented for the evaluation of the proposed BWIM system. Field implementation involved a series of truck runs for calibration and evaluation of the BWIM system. Measured and actual truck-axle weights, spacings, and axle speeds as well as the gross vehicle weight...

This study reports on the results acquired from shaking table tests of a column subjected to progressively increasing amplitudes of seismic motions recorded during the Northridge earthquake. Scope of the investigation includes application and evaluation of deterministic structural health monitoring methods to assess the post-seismic conditions following application of the various amplitudes of earthquake motions. Results from this study demonstrate the applicability of specially designed surface-adhered fiber optic sensors for detecting the formation and the progression of cracks subjected to reverse dynamic loads, as well as application of various SHM techniques for evaluation of damage. The range of damage identification methods evaluated comprises of simple frequency and damping analysis of the column under free vibrations, and modal analysis techniques including curvature mode shapes. The concept of strain mode shape analysis, which was previously established for dynamic analysis of structures, is also introduced here for damage identification. Evaluation of the SHM techniques involves efficiency in terms of the number of sensors required to investigate the progression of damage, and resolution in terms of damage identification both in terms of location as well as the state of the damage.

Implementation of successful civil structural health monitoring strategies requires selection and placement of sensors suitable for measurement of key parameters that influence the performance and health of the structural system. Optical fiber sensors have been successfully implemented in aeronautics, mechanical systems, and medical applications. Civil structures pose further challenges in monitoring mainly due to their large dimensions, diversity as well as heterogeneity of materials involved, and hostile construction environment. This article provides a summary of basic principles pertaining to practical health monitoring of civil engineering structures with optical fiber sensors. The issues discussed include basic sensor principles, strain transfer mechanism, sensor packaging, sensor placement in construction environment, and reliability and survivability of the sensors.

Debonding failure of the concrete cover in reinforced concrete beams retrofitted with fiber reinforced polymer (FRP) composites is a brittle phenomenon which in most cases occurs in an abrupt manner. A complete understanding of bond requires information about the relationship between the local bond stress and slip. Bond-slip defines the constitutive relationship of the interface, and it provides means for computation of ultimate strength and distribution of the bond stress. For FRP composites interface slip is very small even at the ultimate stage prior to failure. For this reason, the study of local bond in the earlier studies did not include slip. This article presents a fiber optic based method for measurement of local slip at the interface between concrete and FRP as well as for prediction of bond failure in reinforced concrete members. Since bond failure in FRP retrofitted concrete is a brittle phenomenon, development of effective means to predict the failure prior to occurrence plays an important role in structural health monitoring of such structures. Therefore, the scope of the study includes two types of tests, namely pull out tests and beam flexure tests. In general, the measured interface slip between concrete substrate and FRP at ultimate load stage is

Imaging of rebars inside a concrete block is investigated using the quadratic forward model of RF Tomography for three different antenna configurations. Reconstructed images are obtained using the method of moments.

Air voids are entrained in concrete for protection of constructed elements, especially highway pavements, against freeze–thaw damage. Entrained air void systems inadvertently reduce the compressive strength of the concrete. The present study describes development of an analytical model for evaluation of the effects of entrained air void system on the compressive strength of concrete. The model developed here will assist in predicting the compressive strength of concrete for specified mix designs. The constitutive relationships for air-entrained concrete were established by considering a micro cracked porous material with randomly distributed circular air voids and uniformly oriented cracks from the air voids. Linear elastic fracture mechanics was employed to explain the evolution of damage due to the individual voids and cracks that emanate from such voids. The damage model considers the interactions among the voids and cracks during various stages of loading. The analytical results from this study were evaluated through an experimental program for comparison of the computed and measured compressive strengths. A wide range of samples were examined that included concretes with air contents ranging from 2% to 13% air by volume of concrete. The experiments involved microscopic determination of air content and spacing factors as well as compressive strength tests for all the concrete samples.

The development of a fiber-optic displacement/strain sensor using the intensity variation of laser speckle pattern has been demonstrated. This sensor can be embedded in concrete for the detection and measurement of crack opening displacements. The transduction mechanism is based on the correlation between the external axial perturbation to the optical fiber and the integrated light intensity variation of speckle patterns due to mode redistribution in a multimode fiber. Calibration tests indicated that the sensor possesses sufficient sensitivity and dynamic range for application in cementitious composite. Application of the sensor has been demonstrated by embedment of the sensor in concrete beams for measurement of the crack tip opening displacements. Experimental results have been validated with analytical formulations. The proposed sensor is simplistic in design and offers a method for the measurement of internal strains and displacements in concrete.

Crack tip opening displacement (CTOD) is an important parameter that is often employed in characterization of fracture in engineering materials. Due to inherent difficulties in direct determination of CTOD, the crack mouth opening displacement (CMOD) has been measured during fracture tests of concrete beams and then related to CTOD. Analytical schemes are used for the determination of CTOD from the measured crack mouth opening displacements, which leads to the determination of other fracture parameters of importance. The research presented here describes development of an experimental technique for direct determination of CTOD based on embedded fiber optic sensors. The transduction mechanism is based on the correlation between the applied strain and the light intensity variation of speckle patterns generated at the output end of the multimode optical fiber due to mode redistribution.The measured displacements, CTOD, and CMOD, are compared and the validity of analytical schemes in estimation of CTOD is evaluated.

Structural health monitoring with optical fibers provides practical sensing capabilities in many applications including in aeronautics and mechanical structures. A variety of optical fiber sensors have been used including Bragg gratings, intensity or amplitude sensors, and Fabry–Perot ones. Civil structures pose further challenges in monitoring mainly due to their large dimensions, diversity as well as heterogeneity of materials involved, and hostile construction environment. Monitoring of strains, deformations, and deflections provides clues essential for evaluation of design parameters and behavior under service loads. Long gage distributed or multiplexed sensors are excellent candidates for such applications. On the other hand, detection of structural damage and anomalies such as cracking in concrete, splintering of fibers in composites, and fracturing of welds and connections are best accomplished by acoustic sensors. This paper describes principles involved in serial multiplexing of two kinds of optical fibers, namely long gage and acoustic sensors. Both sensor types offer promise in structural health monitoring of large civil structural systems. Representative examples are introduced and described in detail.

Development of a serially multiplexed fiber-optic acoustic emission (AE) sensor is described. The sensor detects AE events in real time at two different points along the length of an optical fiber. The time-domain waveform contains signals from the two locations, and a method is introduced for identification of the signals. Detection and identification of signals along a single data stream reduces the data-acquisition rigor and provides for rapid real-time damage location detection in materials. The sensor consists of two optical fiber coils along an arbitrary length in a Michelson interferometer arrangement. The formulations involve the methodology for tagging of the time-domain signals. The process leads to matching of the arrival times in the waveform to the individual coils along the length of the optical fiber. The waveform is detected at the quadrature point for maximum sensitivity. A bias is created through differentiation and superposition of the waveform to accomplish signal identification. The experimental program involved proof of concept tests by way of damage location detection along the lengths of individual composite rods. Conventional piezoelectric transducers were also employed for comparison of results.

The development and testing of a pseudoductile hybrid fiber-reinforced polymer rod consisting of glass and carbon fibers is described. Two different hybrid forms were considered for the development of the hybrid rods, including (1) random dispersion of the glass and carbon fibers; and (2) a core-shell design with a glass fiber core protected by the carbon fiber shells. The mechanical properties of the hybrids were evaluated through uniaxial tension tests. The hybrid rods developed by way of the core-shell approach exhibited pseudoductile properties that included a yield point of 1,153 MPa and an ultimate strength of 1,191 MPa, with a strain of 3.5% at ultimate. The experimental program also included damage assessment by way of a single-channel fiber optic acoustic emission (AE) sensor. The acoustic emission technique was employed for real-time determination of the progressive damage due to rupture of fibers. Moreover, the spectral energies of the frequency spectrum from the AE signals were employed for as...

Let me begin by wishing the Journal of Civil Structural Health Monitoring (JCSHM) a very happy birthday. JCSHM is now 5 years old! The journal has come a long way since its humble beginnings in 2011. The first volume of JCSHM had two issues with a total of 124 pages, whereas volume number five in 2015 was comprised of 5 issues and 765 pages. We will readily surpass 1000 pages in 2016, and increase the number of issues in 2017. JCSHM is dedicated to publishing practice-oriented articles and specifically aims at providing technical information about the state-of-practice and technologies of importance for maintenance and safety of all types of civil infrastructure systems. This is a niche that the journal has consistently filled and no other journals have been able to publish articles specific to best practices in technologies impacting the discipline of Civil Structural Health Monitoring. We owe this success to the authors, reviewers, and the editorial board consisting of 17 members from 10 different countries that are internationally recognized as experts in the field of civil structural health monitoring. The board members strive to deliver a journal that includes high-quality, peerreviewed articles. JCSHM is the brainchild of ISHMII, which is an international society with global perspective to advance the understanding and the application of civil structural health monitoring. Membership covers a broad range of engineers and practitioners, members of the academic community, and infrastructure managers and students. The high standards set by ISHMII have been essential and will sed range of engineers and practitioners, members of the academic community, and infrastructure managers and students. The high standards set by ISHMII have been essential and will serve as a guiding light for the expansion of journal contents. JCSHM recognizes excellence, and awards the authors of the best paper from each volume with the Aftab Mufti Medal. The medal has been awarded four times since its inception in December 2011. The Aftab Mufti Medal honors the service, dedication, and lifetime achievements of Dr. Aftab Mufti, cofounder and the first president of ISHMII. To celebrate JCSHM’s 5 years of existence, we will be publishing state-of-the-art articles in several issues of volume six in 2016. Due to the interdisciplinary expertise and interests of the journal’s readers, we anticipate that the sixth volume will provide an invaluable resource for researchers and practitioners in civil structural monitoring. Six of the state-of-the-art articles are published in this issue of the journal, covering a wide spectrum of topics of interest to our readers. These topics include monitoring of complex civil infrastructure systems, wireless smart sensors, cable-stayed bridges, bridge weigh-in-motion systems, experiences in monitoring of heritage structures in Italy, and inverse problems in vehicle–bridge interaction dynamics. In addition to the state-of-the-art articles, the regular papers in this first issue cover topics such as piezoelectric sensors for detection of cracks in concrete & Farhad Ansari fansari@uic.edu

Development and testing of a serially multiplexed fiber optic sensor system is described. The sensor differs from conventional fiber optic acoustic systems, as it is capable of sensing AE emissions at several points along the length of a single fiber. Multiplexing provides for single channel detection of cracks and their locations in large structural systems. An algorithm was developed for signal recognition and tagging of the AE waveforms for detection of crack locations. Laboratory experiments on plain concrete beams and post-tensioned FRP tendons were performed to evaluate the crack detection capability of the sensor system. The acoustic emission sensor was able to detect initiation, growth and location of the cracks in concrete as well as in the FRP tendons. The AE system is potentially suitable for applications involving health monitoring of structures following an earthquake

Fiber reinforced polymer composites (FRP) have found widespread usage in repair and strengthening of concrete structures. FRP composites exhibit high strength to weight ratio, corrosion resistance, and convenient to use in repair applications. Several methods have been devised for repair of concrete beams with FRP fabrics, including wrapping of the cracked members, or adhesion of the fabric to the tension face of the members. A common cause of failure in such members is associated with debonding of the FRP substrate from the concrete in an abrupt manner. The mechanism of debonding is investigated through embedment of a distributed optical fiber sensor at the interface between the cracked concrete and the FRP fabric during repair of reinforced concrete beams under load. The fiber optic system consists of segmented long gauge length sensors along the length of an embedded optical fiber. This arrangement allows for complete interrogation of the interface deformations and debonding phenomenon. Experimental results pertaining to the load testing of FRP-repaired reinforced concrete beams are reported.

A constitutive relationship for high strength concrete in triaxial monotonic and cyclic compressions is developed based on the continuum damage mechanics. The bounding surface concept is employed in the formulation of the theoretical model. An experimental program was undertaken in order to establish databases for high strength concrete under triaxial monotonic and cyclic compressions. The stress-strain responses of high strength concrete subjected to triaxial monotonic and cyclic compressions were acquired through an experimental program. Comparison of the stress strain results indicates good agreement between the theoretical model and the experimental data.

The existence of a coating on an optical fiber results in a difference between the strain of the matrix concrete and that sensed by the embedded fiber optic sensor. This paper deals with the theoretical model for measurement of thermal expansion coefficient of concrete by embedded fiber optic sensor. The fiber core, the coating of the optical fiber, and the matrix concrete were all supposed to be elastic. And the bonding surfaces were supposed to be intact, that is to say, there is no relative slip deformation at the interface between the matrix concrete and the coating of the optical fiber, and that between the coating and the fiber core. Based on these assumptions, a theoretical model was developed for measurement of thermal expansion coefficient of concrete. Some experiments were done to verify the validation of the theoretical model. And the comparison indicates a good agreement between the experimental and theoretical results.

Fiberoptic sensors are increasingly employed for sensing and measurement of strains in structural materials. The glass core of the optical fiber senses the strain through intensity fluctuations, interference, or frequency modulation. Brittleness of the glass core limits practical usage, and therefore, the glass core of optical fibers is coated with low modulus softer protective coatings. The protective coating alters the strain transduction capabilities of the sensor. It absorbs a portion of the strain, and hence only a segment of structural strain is sensed. The study reported here corrects for this error through development of a theoretical model to account for the loss of strain in the protective coating of the optical fiber. The model considers the coating as an elasto-plastic material and formulates strain transfer coefficients for elastic, elasto-plastic, and plastic phases of coating deformation. The theoretical findings were verified through laboratory experimentation. The experimental program involved fabrication of interferometric optical fiber sensors, embedment within mortar samples, and tensile tests in a closed-loop servo-hydraulic testing machine. The elasto-plastic strain transfer coefficients developed in this study were employed for correction of optical fiber sensor data and results were compared with conventional extensometers.

This paper deals with circumferential strain measurement for concrete in uniaxial compression by using a fiber optic sensing system. A fiber optic sensing system was employed to measure the elongation of the optical fiber for a cylindrical concrete specimen in uniaxial compression by using white light interferometry. A theoretical model describing the relationship between the elongation of the fiber glass and the circumferential strain of cylindrical concrete specimen in uniaxial compression is proposed. Some tests were performed to verify the measurement method. The results indicate that the proposed method is valid.

Performance assessment of bridge structures instrumented with structural health monitoring systems requires development of deterministic methodologies producing quantitative information on different damage states. This aim can be achieved through defining engineering limit states that may be expressed by limiting values of quantities such as damage indexes. Displacement-based damage indexes for performance assessment of bridges do not consider the accumulated damage that results from plastic cycling. In more advanced approaches, the parameter that takes into consideration the effect of cyclic loading history and accumulation of damage is the dissipated energy. However, computation of dissipated energy for the purpose of structural health monitoring requires real-time measurement of lateral forces, which is beyond the capability of conventional sensing methodologies. This paper reports on the development of a methodology to evaluate the energy dissipation in reinforced concrete columns with circular cross sections based on the curvatures measured in the plastic hinge area. Integrating the introduced method in a normalized dissipated energy index provides the possibility of detecting and quantifying minor and moderate damage that is barely visible during visual inspection. The scope of the study included the evaluation of the proposed damage assessment method through hybrid simulation tests on 1/10-, 1/8-, and 1/6-scale hybrid models of a two-span bridge subjected to various amplitudes of near source ground motions of the 1994 Northridge earthquake. Fiber-optic sensors were employed for monitoring the deformation response of the columns in the plastic hinge area.

Brillouin scattering–based fiber-optic sensors have been used in a number of applications, including monitoring of pipelines and well-casing damage in the oil and gas industry. The major attribute of these sensors is their capability for distributed sensing of strains along large sections of structures. For these reasons, Brillouin-based fiber-optic sensors have great potential for civil structural applications, such as in bridges. This study reports the investigation of the Brillouin optical time-domain reflectometer for assessing damage in structural members. The primary advantage in using the Brillouin optical time-domain reflectometer is the single-fiber installation and multidamage-detection capability for the entire structure. The challenge in using a Brillouin optical time-domain reflectometer for detection of localized damage is to overcome the effects of system noise and Brillouin frequency-peak shift distortions. The compound effects of noise and frequency-peak distortions result in disp...

This paper deals with circumferential strain measurement for concrete in triaxial compression by using fiber optic sensing system. A fiber optic sensing system was developed to measure the elongation of the optical fiber for cylindrical concrete specimen in triaxial compression by using white light interfering. And a theoretical model describing the relationship between the elongation of the fiberglass and the circumferential strain of cylindrical concrete specimen in triaxial compression was proposed, too. Some tests were done to verify the measurement method. The results indicate that the proposed method is valid.

Development and characteristics of a quasi-distributed intrinsic fiber optic strain sensor based on white light interferometry is described. The research presented here describes the development of a new optical fiber sensor system for measurement of structural strains based on white light interferometry. Individual segments of single mode optical fibers are linked in series and a scanning white light interferometer provides for distributed sensing of strain signals from various locations in the structure. The system can be configured for automatic compensation of drift due to environmental effects, i.e. temperature. The experimental results demonstrate the linearity of the system and the capability for distributed sensing of strains.

Condition Monitoring of civil structures is best accomplished by way of distributed sensors. Sensors capable of making distributed measurements allow for monitoring the entire structure. Optical fiber sensors are especially very attractive for this purpose, since they are geometrically versatile and they can be readily integrated within various types of structures and materials. The research presented here describes the development of a new optical fiber sensor system for measurement of structural strains based on white light interferometry. Individual sensors are linked in series and an optical switch provides for multiplexing of strain signals from various locations in the structure. Bragg grating type fiber optic sensors as well as strain gauges were employed for comparison and verification of strain signals as measured by the new system. The system provides capability for distributed sensing of strains in large structures.

Internal cracks are frequently initiated in the form of delaminations at the interface between the reinforcement and the concrete in bridges. The most probable cause for the generation of internal cracks and delaminations is the pressure generated by the accumulation of corrosion byproducts at the interface between the reinforcement and the concrete. Failure to detect these defects at the early stages of development results in time consuming and costly repair and replacement operations. This article describes development of a fiber optic ultrasonic sensor for distributed detection of flaws and defects in concrete bridge elements. Distributed sensing provides the opportunity for the optical fiber sensor to detect anomalies throughout the entire structure. An ultrasonic transmitter source or an impact device is used for point-by-point generation of stress waves within the bridge deck. An optical fiber is adhered to the surface of the structure, for sensing of the echoed ultrasonic signals. The optical fiber senses the stress waves, and a one-ended measurement of the optical signal by an interferometer provides information about the integrity of the concrete. This configuration eliminates the need for using a conventional piezoelectric transducer (PZT) that can only make localized measurements. The delamination detection capability of the sensor is demonstrated through experimentation with concrete beams that contain intentionally induced delaminations.

A distributed sensor is developed for detection of cracks and measurement of crack widths in composite materials. The sensor consists of a number of individual segments on one line with gauge lengths designed according to the structural and materials requirements. Return loss measurements at the interfaces between adjoining segments provide the basis for crack detection and measurement. An optical time domain reflectometer (OTDR) is employed for interrogation of the sensor signal. The quasi-distributed sensors were embedded in reinforced concrete beams in order to evaluate their crack detection capability. Experiments involved both monotonic as well as fatigue loading. The sensors were embedded in the tension zone of the concrete beams in order to facilitate comparison of sensor response with visual crack observations. Experimental results in terms of crack detection capability and resolution are presented and discussed.

A constitutive relationship for high-strength concrete (6 to 15 ksi) is developed based on the continuum damage mechanics. The bounding surface concept was employed in the formulations of the theoretical model. A comprehensive experimental program was undertaken in order to establish a database for high-strength concrete under triaxial compression. The stress-strain response of high-strength concrete subjected to triaxial compression was acquired through laboratory experiments. The testing program included three strength categories—6, 10, and 15 ksi (42, 70, and 105 MPa) concretes. The confining pressures (σ1) employed in the experiments ranged from 1.2 to 12 ksi (8.3–82 MPa). A series of uniaxial tension and compression tests were performed in order to develop the necessary data for the establishment of the failure criterion. Comparison of the stress-strain results indicated good agreement between the theoretical model and the experimental data.

Optical fiber sensors are emerging as a superior nondestructive means for condition evaluation of civil structures. The ability of an optical fiber sensor to monitor strain distribution in a structural material depends on the bonding characteristics between the material and the optical fiber. The strain field transferred from the structure to the optical fiber sensor generates changes in the characteristics of the light signal transmitted by the glass core of the optical fiber. Transduction of this signal will provide a means for measurement of strain. However, the mechanical properties of the protective coatings employed in conjunction with optical fibers alter the strain transduction capabilities of the sensor. A portion of the strain is absorbed by the protective coating of the optical fiber, and hence, only a segment of structural strain is sensed. In the study reported here, a model is introduced and tested through which it is possible to interpret the actual level of structural strains from the values measured by an optical fiber sensor. A number of realistic assumptions were introduced to simplify the development of the mathematical rigor. It was determined that the strain transfer characteristics of optical fibers depend on the mechanical properties of the glass core, the protective coating, and the gauge length of the optical fiber. Mathematical expressions are developed through which it is possible to describe the level of strain loss within the protective coating, and the amount transferred to the optical fiber core. The theoretical findings were verified through a series of experiments involving white light interferometry. The investigation encompassed repeated experiments with a range of fiber sensor gauge lengths. The experimental program included evaluation of strain transfer capabilities of coated as well as bare fibers.

A novel technique of using different lengths of optical fiber as strain sensors in a white-light interferometer to measure the strain distribution is presented. The technique for connecting the fiber sensors and the method of calculating the intensity of the transmitted and reflected light are given. The measuring principle and a four-sensor distributed fiber-optic strain-measuring system are demonstrated. Calibration experiments results show that the relationship of the strain between the embedded fiber and the matrix is linear and the sensitivity of the fiber sensor is a function of the sensing length of the embedded fiber.

The development of a fiber-optic sensor for embedment in cementitious composites and measurement of displacements associated with the opening of microcracks are described. The sensor can be employed as a transducer for measurement of crack-tip opening displacements during fracture tests. A polarization-maintaining optical fiber is used as the sensing element, and the transduction mechanism is similar to those of other sensors based on polarimetric-type based fibers. However, the deformation resolving power of the sensor is increased by way of increasing the effective length of the optical fiber. This is done by winding the optical fiber into a coil. A direct relationship between the number of loops in the coil and sensitivity of measurements is obtained. A calibration procedure is developed by which the optical signal is converted to displacements. The sensor was employed in a series of fracture tests. Experiments involved embedment of the optical fiber in concrete beams. Specimens were center-edge-notched, and the fiber coil sensors were embedded at the tip of the notch. This arrangement allowed for direct measurement of displacements associated with the opening of microcracks at the crack tip. Experimental results are presented, and crack-tip opening displacement (CTOD) results are compared with crack-opening displacements measured by conventional transducers at the crack mouth (COD).

Optical-fiber sensors are emerging as a superior non-destructive means for evaluating the condition of concrete structures. In contrast to existing non-destructive evaluation techniques, optical fibers are able to detect minute variations in structural conditions through remote measurements. Structures fully integrated with optical fibers will be able to monitor the initiation and progress of various mechanical or environmentally-induced degradations in concrete elements. Recent advances in fiber-optic sensor technology and the possibility of their use in concrete structures have instigated the development of a number of research activities. Owing to inherent interdisciplinary nature of the field of fiber optics, the expertise of the researchers active in the research and development of fiber-optic sensors covers a wide spectrum of disciplines including concrete engineering, as well as opto-electronics and physics. This article is intended for rapid dissemination of the current state-of-the-art in this emerging technology. However, because of the interdisciplinary nature of the subject, a brief discussion on the physical nature of optical fibers is also presented.

As novel construction materials become more economically and technically available throughout the globe, the structures become more complex. Therefore, an unexpected failure of these structures causes catastrophic economical and fatal losses. Over the past decades, a number of vibration based damage detection techniques have been developed to avoid unexpected failure in structures. These damage detection methods identify presence, location and magnitude of developed damage in the structure associated to change of modal properties such as natural frequency, mode shape and modal damping. An essential limitation of these techniques is that they are majorly sensitive to high intensity of damage only and do not correctly respond to minor to intermediate damage severities. This research proposed a novel damage detection methodology based on application of Hilbert-Huang Transform (HHT) on the acceleration response of the structure. HHT consists of Empirical Mode Decomposition followed by Hilbert Transform of the signal. The method develops a Damage Index Matrix for the structure by connecting energy index of the instrumented structural points. Viability of the proposed method is demonstrated through numerical examples and laboratory experiments. The method was able to locate both single and multiple damage scenarios in numerical models as well as laboratory experiment.

A new model for the determination of fracture-mechanics parameters in concrete is developed. Application of this method requires merely knowledge of peak load and peak crack-mouth–opening displacement (CMOD) values. The interrelationship between the damaged and undamaged sections within the microcracked zone is drawn through an analogous composite beam section. The extent of damage material within the process zone is assumed to vary linearly from its maximum level at the crack tip all the way to the boundary of the undamaged section. Hence, the fracture-process zone is defined as a composite material with a linearly variable modular ratio within its boundary. By using this approach, it is possible to determine a number of important fracture parameters from the knowledge of peak load and CMOD at peak load alone. These parameters include the fracture-process–zone size ( r p ), critical fracture-energy release rate ( G IC ), fracture toughness ( K IC ), and fracture energy ( G F ). Furthermore, use of the pr...

Detection of cracks while at their early stages of evolution is important in health monitoring of civil structures. Review of technical literature reveals that single or sparsely distributed multiple cracks can be detected by Brillouin-scattering-based optical fiber sensor systems. In a recent study, a pre-pump-pulse Brillouin optical time-domain analysis (PPP-BOTDA) system was employed for detection of a single microcrack. Specific characteristics of the Brillouin gain spectrum, such as Brillouin frequency shift, and Brillouin gain spectrum width, were utilized in order to detect the formation and growth of microcracks with crack opening displacements as small as 25 μm. In most situations, formations of neighboring microcracks are not detected due to inherent limitations of Brillouin-based systems. In the study reported here, the capability of PPP-BOTDA for detection of two neighboring microcracks was investigated in terms of the proximity of the microcracks with respect to each other, i.e., crack spacing distance, crack opening displacement, and the spatial resolution of the PPP-BOTDA. The extent of the study pertained both to theoretical as well as experimental investigations. The concept of shape index is introduced in order to establish an analytical method for gauging the influence of the neighboring microcracks in detection and microcrack differentiation capabilities of Brillouin-based optical fiber sensor systems.

We report on the development of a precision-tunable, dual wavelength, optical light source suitable for high performance fiber optic Brillouin scattering distributed sensing. The design is based on an Optical Phase Locked Loop (OPLL) system using novel narrow linewidth, low frequency noise and high stability PLANEX external cavity semiconductor. The inherent wavelength stability of PLANEX lasers (typically an order of magnitude better that any DFB laser on the market) enable the OPLL to operate continuously over a wide ambient temperature range without degradation in wavelength locking performance. The OPLL architecture is implemented with polarization maintaining (PM) components and has a very low beat frequency jitter on the order of few kHz. The OPLL frequency tuning range is between 8 and 14 GHz, with fast tuning of sweep steps on the order of 100 μsec. Such a frequency tuning range covers practically all corresponding temperature and strain sensing applications based on the measurement of the frequency shift produced by spontaneous or stimulated Brillouin scattering, and thus is a versatile and enabling technology for both BOTDA/BOTDR distributed sensing systems.