This paper presents our recent application of Brillouin-based optical fiber sensors for geotechnical monitoring. We embed an optical fiber into a prestressing steel strand (hereafter ‘strand’) during the manufacturing process, where the epoxy-coated strand is fully integrated with the optical fiber without sacrificing long-life durability. We install the strand to prestressed ground anchors for the stabilization of steep slopes. The experimental results indicate that the sensorembedded strand can detect changes in the loaded force on the anchor and can determine the cause of such changes. As a field application, we insert the single strand into a drilling hole and grout it to the anchor. After setting a hydraulic jack, we remove the epoxy coating at the end of the strand and pull out the embedded optical fiber. Then, an optical connector is attached using a fusion splicer for connecting an optical measurement instrument. The strain distribution along the anchor is measured during load testing and after anchoring. As a result, the stress distribution along the anchor is accurately measured in situ during construction and thus, these sensors have the potential to monitor soil conditions.
As an optical fiber is able to act as a sensing medium, a Brillouin-based sensor provides continuous strain information along an optical fiber. The sensor has been used in a wide range of civil engineering applications because no other tool can satisfactorily detect discontinuity such as a crack. Cracking generates a local strain change on the embedded optical fiber, thus Brillouin optical correlation domain analysis (BOCDA), which offers a high spatial resolution by stimulated Brillouin scattering, is expected to detect a fine crack on concrete structures. The author installed the surface-mounted optical fiber on a concrete deck and periodically monitored strain distribution for seven years. This paper demonstrates how a BOCDA-based strain sensor can be employed to monitor cracks in a concrete surface. Additionally, focusing on another advantage of the sensor, the natural frequency of the deck is successfully measured by dynamic strain history.
A significant technical advancement in distributed fiber optic strain sensors has been accomplished: Brillouin optical
correlation domain analysis (BOCDA) provides a high spatial resolution and the smallest measurement interval due to
Brillouin scattering stimulated by the correlation of two counter-propagating lightwaves. In a BOCDA-based system, the
measurement position can be varied continuously by changing the modulation frequency, whereas other systems require
a sophisticated A/D board for localizing the measurement position. In fact, 50 mm is the current limit of the
measurement interval in conventional time-domain-based systems, because higher sampling rates are required to process
information traveling at the speed of lightwaves. This paper presents an experimental study on cracked concrete
specimen retrofitted with a ply of smart fabric; a fiber optic sensor (FOS) is woven into the fabric. The strain distribution
along the sensing fiber is measured to detect the debonding of the smart fabric from the concrete specimen under loading,
and the measured highly dense strain information obtained using BOCDA is found to potentially facilitate a better
understanding of structural behavior.
To develop a simple method for detecting and monitoring FRP-concrete debonding with the use of distributed Brillouinbased
fiber optic strain sensor, this study proposes a model that takes into consideration both the steady and the transient
Brillouin interaction states. Assuming that the transient term has an analogous effect on the steady state term, two
parameters, the effective transient length and the intensity reduction ratio, are introduced. The proposed model shows
that the stimulated Brillouin signal intensity distribution at the specific frequency, which corresponds to the maximum
strain at the debonded region, is sensitive to the occurrence of debonding. For evaluation of the model, experiments are
carried out on a reinforced concrete beam retrofitted with glass FRP sheets on which sensing fibers are mounted, and the
results agree with the observation. This numerical and experimental study demonstrates the effectiveness of the proposed
model that incorporates not only the steady Brillouin interaction state. The model enables debonding detection without
baseline measurement, leveraging the stimulated Brillouin scattering principle with high spatial resolution and high