Quasi Distributed OTDR Crack Sensor for Reinforced Concrete Structures

A quasi-distributed fiber optic sensor is developed for embedment in concrete structures. The sensor principles are simple, and therefore, practical for crack detection and deformation measurement in civil structural applications. The distributed sensor developed herein operates based on the intensity measurements of optical power. The sensor consists of a number of individual segments on one line, with gauge lengths designed according to the structural and materials requirements. An optical time domain reflectometers (OTDR) was employed for interrogation of the sensor signal. The study reported herein is aimed at demonstrating the applicability of this sensor in monitoring performance of concrete structures. Crack detection methodologies were established through experiments with plain concrete beams. Structural monitoring capability of the sensor was evaluated through experiments with reinforced concrete beams. INTRODUCTION Full-scale monitoring of structures requires sensing at multiple points and within large volumes. Therefore, many sensors are normally required. For this reason, various multiplexing technologies have been considered in civil structural applications (Sirkis, 1998). The Bragg grating has been the dominant sensor of choice in most cases (Morey, et. al., 1989, Kersey, 1993). Bragg grating sensor architectures have been successfully employed in concrete elements in order to monitor strains in reinforcing bars and prestressing tendons (Nawy, 1992). Distributed sensors are most suited for large structural applications, since all the segments of the optical fiber act as sensor, and therefore, the perturbations within various segments of the structure can be sensed. However, distributed sensors have not found widespread usage in civil structural applications. There is a need for development of optimum distributed sensing technologies for civil structures. Two of the most widely employed distributed sensor methodologies correspond to optical time domain reflectometry (Ansari, 1997) and Brillouin scattering (Brown, et. al.,1998). In optical time domain reflectometry, Rayleigh or Fresnel scattering is used for transduction of structural perturbations. On the other hand, in Brillouin scattering the Doppler shift in the frequency of light is related to the measurands. It is also possible to develop sensors that provide average values of measurands over specific gauge lengths. A multi-gauge-distributed sensor is comprised of the assemblage of individual sensors in series, each measuring an average value of measurand over distinct segments. One such sensor is developed here for application in concrete structures. The basic principle of operation for the crack sensor is based on intensity variation of the optical power within the fiber due to the initiation and opening of cracks. This article reports on the functional characteristics of the sensor in terms of resolution, hysteresis, and sensitivity. Moreover, it demonstrates the applicability of the sensor in condition monitoring of reinforced concrete elements under monotonic and fatigue loading conditions. METHODOLOGY Optical Time Domain Reflectometers (OTDR) have been developed for characterization of anomalies in the telecommunication links. It is an important tool for detection of transmission irregularities due to splice losses, and local damage along the fiber length. OTDR also provides capability for one-port operation at the fiber input with no need to access the fiber output. In a basic OTDR measurement, a laser transmitter launches an optical pulse into the fiber under test. The optical signal travels through the optical fiber and then it is reflected back through the fiber into the OTDR. Signal reflection occurs due to Rayleigh and Fresnel back scattering events. Microscopic density fluctuations within the core of the fiber material give rise to refractive index impurities which in turn are responsible 2 for Rayleigh back scattering. Fresnel reflections originate from many points along the fiber where abrupt and discrete discontinuities occur in the index of refraction. One example is poorly spliced and or connectorized regions along the length of the fiber. Straining of the fiber within a segment bounded by the spliced regions result in intensity fluctuations of the reflected signal, which is due to the loss of optical power at the air-to-fiber interface in the spliced point. Single Gauge Sensor The multi-gauge sensor developed here requires optical segmentation of the fiber at several points along its length in order to create Fresnel reflection points. In this way, an optical fiber is divided into several gauge lengths through which monitoring of cracking and deformations are accomplished. The back-reflected Fresnel signal is employed to pin point, L, the location of the disturbance along the total length of the fiber. Moreover, the deformation is sensed through the intensity variations of the Fresnel-peaks in the back-scattered signal. Fresnel points can be created by using a special precision optical fiber cleaver in order to slice the fiber into a number of smaller segments, each representing a gauge length. The cleaved fibers are spliced along one line in order to create the distributed sensor. In the formulations to follow, both transmission and reflection losses are used in the transduction of the measurand. A typical segment of a sensor between two Fresnel points is shown in Fig.1. The Fresnel signals are also given in the schematics representing the OTDR screen. It is assumed that the optical fiber consists only of one gauge length between the two Fresnel points representing the reference and reflector planes R0 and R1, respectively. The pulse of laser enters the fiber from the reference plane at R0 with intensity I0. It is partially transmitted and reflected at the reflector plane R1. The reflected signals re-enter the OTDR and create the Fresnel peaks. The incident, transmitted and reflected signals are represented by I0, IT and IR, respectively. Accordingly only a portion of the incident signal is transmitted, whereas, the rest is reflected. The insertion loss, lo I , that represents the ratio of incident and the actual transmitted signal is given in logarithmic scale in decibels (db) as: I I I lo T = 10 0 log (1) In a similar manner the return loss of the incident light is: R I I lo R = 10 0 log (2) In the absence of strain, Ilo and Rlo remain constant. However, upon straining, Ilo and Rlo vary with strain. Variations in Ilo and Rlo are manifested in the amplitude of the Fresnel reflection peaks. Accordingly, the insertion and return losses can be employed for the determination of strain or deformation through the following relationships: ∆I I I lo lol lo = − (3) ∆R R R lo lol lo = − (4) δ or ) ( lo lo R or I ∆ = ∆ = β ε α ε (5) Where, ∆Il0 = insertion loss after strain ∆Rlo = return loss after strain ε = average strain within the gauge length, d, of the optical fiber δ = deformation of the optical fiber along the gauge length, d α, β = proportionality constant relating deformations and strains to the optical loss factors. 3 As noted in Eq.(6), both of the loss parameters, 0 l I ∆ , and 0 l R ∆ can be employed for the determination of strain. However, at the same strain level, more sensitivity can be achieved with the reflection loss, since upon reflection the optical signal travels through twice as many Fresnel points. Fig. 1 A typical segment of the sensor between two Fresnel points and their peaks in the OTDR screen Multi-Gauge Distributed Sensor A distributed sensor consisting of n-segments in series is schematically depicted in Fig. 2. The insertion and return losses for the i-th reflector plane (Ri) with respect to the reference plane (R0) is expressed as: ( ) n i I I I Ti i lo , . 2 , 1 log 10 0 L = = (6) ( ) R I I i n lo i Ri = = 10 1 2 0 log , , , L (7) where, Ilo i and Rlo i pertain to the insertion and return losses of the i-th segment at the reference plane. The reference plane, R0, corresponds to the point of laser pulse entry into the first segment (i=1). ITi and IRi are the transmitted and the return light intensities of the i-th fiber optic segment, respectively. ITi and IRi represent accumulated insertion and reflection losses from sensor 1 all the way to sensor i. To monitor and measure strain and deformation in the i-th sensor alone, it is necessary to obtain the insertion and return loss of the i-th fiber optic segment according to the following relationships: ( ) I I I i n lo i Ti Ti ( ) log , , , = = − 10 1 2 1 L (8) Fiber core Fiber cladding d I0 Reflecting plane (R1) IR IT d Reference plane (R0) OTDR Screen Ilo (Rlo)