Robert Maaskant

Fiber-optic Bragg grating sensors for bridge monitoring

Abstract Fiber-optic Bragg grating strain sensors hold a great deal of potential for structural monitoring because of their exceptional stability and demonstrated potential for long-term monitoring. This sensing technology takes advantage of a spectrally encoded signal which provides inherent immunity from signal intensity fluctuations which plague many other fiber-optic and electronic sensing techniques. This results in measurement stability and lead/interconnect insensitivity which permit longterm and intermittent monitoring with high resolution and accuracy. Fiber-optic grating sensors are intrinsic to the optical fiber, thus capitalizing on its extremely small size and inherent strength and durability. Recent results are provided from a sensor array installed in a road bridge. The strain sensors are attached to both steel and carbon-fiber-reinforced plastic prestressing tendons, which are embedded in the precast girders of the bridge. Measurements of traffic loads and the relaxation behaviour of the tendons are presented. The potential of fiber grating technology is briefly discussed including its application in long-gage strain-sensing and strain-distribution measurements.

A structurally integrated Bragg grating laser sensing system for a carbon fiber prestressed concrete highway bridge

Carbon fiber based composite material is of considerable interest for the replacement of steel in large concrete structures, such as bridges, where corrosion is a serious problem. A new two-span concrete highway bridge built in the city of Calgary in 1993 is the first in the world to use carbon fiber composite prestressing tendons in several of its precast concrete deck support girders. We have instrumented a number of these girders with an array of fiber optic intracore Bragg grating sensors in order to monitor the changes in the internal strain that take place over an extended period of time. A four-channel fiber laser demodulation system was developed for interrogating the set of Bragg grating sensors embedded within the bridge girders. This demodulation system was demonstrated to be rugged, compact and transportable to the bridge construction site where it allowed changes in the internal strain on all three types of prestressing tendon (steel and two types of carbon fiber composite) to be tracked over several months. The same set of structurally integrated Bragg grating sensors has also been used to measure the change in the internal strain within the deck girders arising from both static and dynamic loading of the bridge with a 21 ton truck. This first permanent testbed for structurally integrated Bragg grating sensors demonstrates the feasibility of building into new bridges fiber optic long-term structural monitoring sensing technology that will allow the use of these advanced composite materials to be monitored in a manner not previously practical. The strain information available from this type of monitoring system will assist engineers in their assessment of new materials and innovative design features, and has a potential role in maintenance and repair activities.

Bragg grating structural sensing system for bridge monitoring

Corrosion of steel within bridges and other large concrete structures has become a serious problem and consequently there is growing interest in replacing the steel within such structures with carbon fiber based composite materials. The first highway bridge in the world to use carbon fiber composite prestressing tendons was constructed and opened to the public in the fall of 1993. This two span bridge was also unique in another respect, it is the first highway bridge in the world to have been built with a set of fiber optic Bragg grating sensors structurally integrated into several of its precast concrete deck support girders. A four-channel fiber laser sensor demodulation system that was rugged, compact and transportable was developed for this project. This demodulation system monitored the changes in the internal strain on all three types (steel and two types of carbon fiber composite) of prestressing tendons over several months. The same structurally integrated fiber optic sensing system has also been used to measure the change in the internal strain within the deck girders arising from both static and dynamic loading of the bridge with a large truck.

Fiber optic Bragg grating sensor network installed in a concrete road bridge

The installation of a fiber optic Bragg grating strain sensor network in a new road bridge is described. These sensors are attached to prestressing tendons embedded in prefabricated concrete girders. Three types of prestressing tendons are being monitored: conventional steel strand and two types of carbon fibers reinforced plastic tendons. Sensor durability issues are reviewed and the installation is described. Initial measurements indicate that the sensors are operational and provide some early comparison of tendon performance.

Characteristics of fiber grating sensors and their relation to manufacturing techniques

This work examines the effects of different manufacturing techniques of fiber optic intracore Bragg gratings on their sensing and system parameters. Specifically, we have looked at tolerance with respect to center wavelength, strain, and temperature sensitivity. Our test results indicate a slight variation in the strain and temperature sensitivity of different gratings. In an attempt to quantify the effect of transverse loading on fiber gratings, three differently manufactured sensors were diametrically loaded and their spectrums studied. Our experimental results indicate a splitting of the grating spectrum with small loads for an optical fiber grating manufactured in either boron doped fiber or hydrogen loaded fibers. On the other hand, very small transverse sensitivity was observed for a fiber optic grating formed in a bend insensitive fiber suggesting some degree of control on the transverse sensitivity of these sensors.

Bragg grating fiber optic sensing for bridges and other structures

We have demonstrated that fiber optic intracore Bragg grating sensors are able to measure the strain relief experienced over an extended period of time by both steel and carbon composite tendons within the concrete deck support girders of a recently constructed two span highway bridge. This is the first bridge in the world to test the prospects of using carbon fiber composite tendons to replace steel tendons. This unique set of measurements was accomplished with an array of 15 Bragg grating fiber optic sensors that were embedded within the precast concrete girders during their construction. We have also demonstrated that these same sensors can measure the change in the internal strain within the girders associated with both static and dynamic loading of the bridge with a truck. We are now studying the ability of Bragg grating fiber optic sensors to measure strong strain gradients and thereby provide a warning of debonding of any Bragg grating sensor from its host structure...one of the most important failure modes for any fiber optic strain sensor.

Application and characterization of intracore grating sensors in a CFRP prestressed concrete girder

Intracore Bragg grating sensors have been bonded on to the CFRP tendons of a prestressed concrete girder. The sensors survived both the installation procedure and casting of the concrete beam. Sensor performance is characterized in terms of maximum tensile strength and fatigue behavior. The fiber sensors survived strains of greater than 8000 (mu) (epsilon) and showed no change in either center wavelength or spectral content for 2000 (mu) (epsilon) over 320,000 cycles. The intracore grating sensor was used in a static loading test of the girder to failure and showed excellent stability and durability in comparison with the conventional technology.

Multiplexed Bragg grating laser sensors for civil engineering

A portable, rugged, and compact 4-channel Bragg grating fiber laser sensor demodulation system was developed for interrogating an array of 20-intracore Bragg grating sensors that we embedded within several of the concrete girders used to support the deck of a new two span road in Calgary, Alberta. Another unique feature of this bridge is that two types of Carbon fiber based composite prestressing tendons are being tested in a number of the concrete girders. We have instrumented five of the approximately 60 ft long concrete girders with fiber optic Bragg grating strain and temperature sensors in order to monitor the loads on these girders and their response during the construction of the bridge and subsequently over an extended period of time.

Phase-based Bragg intra-grating sensing of strain gradients

Light reflected from chirped fiber Bragg gratings possess both phase and amplitude information which is unique. In this paper we report for the first time the use of the relative phase to determine the strain-induced linear chirp. By measuring this intramodal (chromatic) dispersion induced on the incident wave, the wavelength dependent penetration depth and therefore the strain gradient can be found by using a closed-form approximation. Moreover, the error can be reduced by selecting the appropriate grating properties like the writing induced chirp and the index variation parameters.

Distributed fiber optic strain sensing based on spectral integration of Bragg grating reflection

A spectral integration method of extracting the monotonic strain distribution along a fiber optic Bragg grating sensor has been developed and tested on a number of applications. Pre-chirping the Bragg grating has been shown to alleviate some of the restrictions on the spectral integration method of acquiring strain profile information.