The overall deterioration of the national civil infrastructure due to aging and usage beyond the anticipated loads and lifetimes for which it was designed, combined with the increasing cost of maintenance and repair, has resulted in the need for improved techniques for non-destructive evaluation of the structural health of reinforced concrete. A recent review of the available statistics reveals that almost 40% of United States bridges are ‘structurally deficient’ or ‘functionally obsolete’ [1]. New reinforced concrete constructions would also benefit from in situ structural monitors which could detect a decrease in performance or imminent failure, thereby optimizing lifetimes without compromising safety. Finally, although modeling the behavior of some structures made from well-characterized materials is fairly accurate, the use of new materials, unusually complex designs, or variability in strength-related factors such as void fraction or moisture content can lead to unexpected structural weakening, damage or failure. The inadequacy of the nation’s highways, bridges, etc. prompted the initiation in 1993 of a National Science Foundation program, with the goal of developing new technologies aimed at ‘prolonging the life and enhancing the capacity of our existing and future civil infrastructure systems’ [2]. In response to the increased need, various techniques are being developed, and some of the most promising are based on the use of fiber optic sensors (FOS).

https://iopscience.iop.org/article/10.1088/0964-1726/5/2/008/pdf

Fiber optic sensors in concrete structures: a review

A review on five key sensors for monitoring of concrete structures

Improving the durability of the optical fiber sensor based on strain transfer analysis

Optical fiber sensors are the most promising technique in monitoring physical and chemical variables of civil structures. For the brittle material characteristics, a bare sensing fiber is prone to breakage under the shear or torsional action existed in the construction and operation. To guarantee the survival and long-term service of the sensors, the packaging measure is particularly significant. This treatment generates an intermedium layer between the sensing fiber and the monitored structure, which leads to the strain of the host material not entirely transferred to the sensing fiber for a portion of strain loss in the transferring path. To correct the error and improve the measurement accuracy, strain transfer theory is developed to establish the quantitative strain relationship between the sensing fiber and the host material. A state-of-the-art review on strain transfer theory of optical fiber based sensors developed for civil structures is addressed. It aims to demonstrate the advance, the application and the challenge of strain transfer theory and provide scientific guidance for the better understanding of the multi-layered sensing model and the theoretical instruction for the optimum design, calibration and measurement accuracy enhancement of optical fiber sensors.

Strain transfer theory of industrialized optical fiber-based sensors in civil engineering: A review on measurement accuracy, design and calibration

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.

State-of-the-art in the applications of fiber-optic sensors to cementitious composites

The paper presents a nondestructive testing method based on fiber optics for measuring strain and stress in concrete structures in situ. When an optical fiber is embedded in concrete, and its refractive index, dimension, and/or shape are changed as a result of the surrounding matrix deformation, the characteristics of the light signal transmitted along the fiber are modified. Optical techniques capable of determining the changes in light characteristics are described. The two most promising techniques for concrete-structures diagnostics are demonstrated in the laboratory using concrete cylinders subjected to unconfined uniaxial compression with optical fibers embedded in a direction parallel and perpendicular to the aplied strain (load). Encouraging experimental results indicate that fiber-optic sensors can be a feasible tool to determine strain/stress distribution in buildings and infrastructures.

Kun Pan

Papers

Fiber-Optic Sensors for Concrete Strain-Stress Measurement