Qualification of a distributed optical fiber sensor bonded to the surface of a concrete structure: a methodology to obtain quantitative strain measurements

Abstract

Distributed optical fiber systems (DOFSs) are an emerging and innovative technology that allows long-range and continuous strain/temperature monitoring with a high resolution. Sensing cables are either surface-mounted or embedded into civil engineering structures to ensure long-term structural monitoring and early crack detection. However, strain profiles measured in the optical fiber (OF) may differ from the actual strain in the structure due to the shear transfer through the intermediate material layers between the OF and the host material (i.e., in the protective coating of the sensing cable and in the adhesive). Therefore, OF sensors need to be qualified to provide accurate quantitative strain measurements. This study presents a methodology for the qualification of a DOFS. This qualification is achieved through the calculation of the so-called mechanical transfer function (MTF), which relates the strain profile in the OF to the actual strain profile in the structure. It is proposed to establish a numerical modeling of the system, in which the mechanical parameters are calibrated from experiments. A specific surface-mounted sensing cable connected to an optical frequency domain reflectometry interrogator is considered as a case study. It was found that (i) tensile and pull-out tests can provide detailed information about materials and interfaces of the numerical model; (ii) the calibrated model made it possible to compute strain profiles along the OF and therefore to calculate the MTF of the system; (iii) the results proved to be consistent with experimental data collected on a cracked concrete beam during a four-point bending test. This paper is organized as follows: first, the technical background related to DOFSs and interrogators is briefly recalled, the MTF is defined and the above-mentioned methodology is presented. In the second part, the methodology is applied to a specific cable. Finally, a comparison with experimental evidence validates the proposed approach.

Distributed Fiber Optic Sensors for Multiple Crack Monitoring in Reinforced Concrete Structures

“…Due to the difficulty in finding crack meters that can be embedded inside the concrete material and are small enough to be accurate in term of position, it is hard to validate cracks openings estimations from FO Line (2,3,5,6). However, by assuming a linear opening variation based on the front and back side DFOS estimations, it is possible to calculate a theoretical crack opening values at each FO line position over the thickness of the beam.…”

“…Different interrogation units are available nowadays based on the analysis of the Brillouin and Rayleigh backscattered light all over the silica optical fiber. While it is now well established that these systems have the ability to perform accurate strain measurements and detect the presence of cracks [1][2][3][4], the possibility to quantify the crack openings through the distributed strain measurements has been recently studied [5][6][7][8][9][10].…”

Experimental and Numerical Investigation on the Strain Response of Distributed Optical Fiber Sensors Bonded to Concrete: Influence of the Adhesive Stiffness on Crack Monitoring Performance

“…The dependence of the strain transfer process on the adhesive stiffness, as previously highlighted by theoretical approaches, suggests that it might be beneficial to use a soft polymer adhesive in the framework of crack monitoring applications. Indeed, crack opening generates a strain peak in the host material [ 7 , 29 ], which could be distributed over a broader length of the fiber core when a soft adhesive is used to bond the DOFS to the concrete surface. This solution may reduce the risk of DOFS breakage due to crack development, hence ensuring better durability of the SHM instrumentation under service condition.…”

“…Optical fiber (OF) sensor systems are now considered as one of the most promising technologies for the monitoring of civil engineering structures as this technology is capable of measuring various types of measurands such as pH, temperature, moisture, corrosion, strain, cracks, and parameters related to the chemical composition of the host medium [ 1 , 2 , 3 , 4 , 5 , 6 ]. However, before selecting an instrumentation for SHM, both the performance of the measurement chain (including the sensing element, waveguide, and interrogation device), the reliability of measurements and the durability of the sensor must be evaluated [ 5 , 7 , 8 , 9 ]. In addition, in the particular case of reinforced concrete (RC) structures, the efficiency of the sensing system to detect and localize cracks and to assess crack openings is an important issue for practitioners and infrastructure owners who need to establish real-time diagnosis on the state of wear and damage of the monitored structures [ 10 , 11 , 12 ].…”

Crack monitoring in reinforced concrete beams by distributed optical fiber sensors

“…Feng, Zhou, Sun, Zhang, and Ansari (2013), formulated a mechanical model based on shear lag theory to evaluate the relationship between strain discontinuities, such as cracks, and the measured strain distribution in optical fiber sensors. This model has been later applied by others to predict the surface crack width of concrete structures by DOFS bonded to the surface of the concrete (Billon et al, 2015) as well as embedded in the concrete (Bassil et al, 2019). While promising results have been reported, the proposed model requires the calibration of the shear lag factor, which depends on the material and geometrical properties of the cables, and it is not directly applicable to fiber sensors bonded to the reinforcement, where the assumption of perfect bond between all layers is not fulfilled.…”