An optical fiber sensor for cracks in concrete members: Development and applications

Title: An optical fiber sensor for cracks in concrete members: Development and applications
Author(s): Christopher K.Y. Leung, Kai Tai Wan
Paper category : conference
Book title: 2nd International RILEM Symposium on Advances in Concrete through Science and Engineering
Editor(s): J. Marchand, B. Bissonnette, R. Gagné, M. Jolin and F. Paradis
Print-ISBN: 2-35158-003-6
e-ISBN: 2351580028
Publisher: RILEM Publications SARL
Publication year: 2006
Pages: 151 - 164
Total Pages: 14
Nb references: 13
Language: English

Abstract: Concrete deteriorates through the formation of cracks. The condition of concrete members can hence be assessed from its state of cracking. Crack detection and monitoring is, however, a very challenging task, as the exact crack locations in a highly inhomogeneous material such as concrete is not known in a-priori. If conventional point sensors (e.g., strain gauges and LVDT’s) are employed, the large number of sensors required to capture all the possible major cracks will make the sensing scheme very complicated, costly and impractical. A sensor with very long gauge length can be used to span over a potential cracking region, but then it is difficult to tell between the situation with one widely opened crack that should cause alarm, or many fine cracks that are rather harmless.
When an optical fiber is deformed at any point along its length, there will be a change in the optical field. With the proper design of fiber configuration and the use of the appropriate signal demodulation technique, it is possible to develop a distributed sensor that can obtain information at each point (within a particular spatial resolution) along its length. In this paper, we’ll describe the development of an optical fiber distributed crack sensor for concrete members, based on the simple physical principle of microbending loss. An optical fiber is coupled to the concrete member in a ‘zig-zag’ manner so it will intersect potential cracks at an inclined angle. When a crack forms and opens, the fiber has to bend to stay continuous. By measuring the intensity of backscattered signal as a function of time with optical time domain reflectometry (OTDR), the crack location can be obtained from the time when a sudden drop in light intensity occurs, and the crack opening is determined from the magnitude of the drop. As long as the crack opening direction is known before hand, crack detection and monitoring can be performed without a-priori knowledge on crack locations. Moreover, it is possible to monitor a significant number of cracks with a single optical fiber.
The paper covers the development and verification of a theoretical model for the prediction of optical loss vs crack opening for the sensor. The use of the model to derive sensor design guidelines will then be demonstrated. Various applications of the sensor in static and dynamic monitoring of cracks, as well as multiple crack detection will be presented. Special attention will be paid to the application of the sensor to crack detection under restrained dimensional changes when shrinkage or differential expansion/contraction occurs.

Online publication: 2006-08-02
Classification: 2.2 Theme 2: From Fresh to Hardened Concrete
Publication type : full_text
Public price (Euros): 0.00
doi: 10.1617/2351580028.011