Distributed fiber optic sensors for measuring strains of concrete, steel, and textile reinforcement: Possible fields of application

Abstract

The article describes measurements of strains of concrete, steel and textile reinforcement with distributed fiber optic sensors (DFOS). The technology of distributed strain measurements gains currently increasing attention within the civil engineering field and indeed the DFOS can be applied in various measurement scenarios providing results and insights which were not possible before. Within this article, the fibers and adhesives that are most commonly used are compared and several measurement scenarios and their results are described, including precise strain measurements with high resolution as well as measurements on large-scale specimens. Concrete strains were measured in a multiaxial compression stress state and also during setting and hardening and in flexural tests. Strains of the steel and textile reinforcement were monitored along the bond zone and also in flexural tests. Finally, cracking patterns were observed and compared with digital image correlation methods. Validated examples of applications of DFOS in laboratory work are described.

Core versus Surface Sensors for Reinforced Concrete Structures: A Comparison of Fiber-Optic Strain Sensing to Conventional Instrumentation

“…Polyimide-coated single-mode sensing fiber was employed. Although nylon-coated fiber-optical cables have been shown, in one prior investigation, to produce more reasonable strain results in RC beams [ 18 ], the polyimide-coated fibers that were used here were chemically bonded to the fiber-optic core [ 11 ] and do not exhibit slip at the locations of sudden strain change [ 27 ], which can result in more accurate strain measurements in the areas of localized deterioration [ 12 ]. For all fiber-optic cables, the spatial resolution was set to 0.65 mm with a sampling rate of 3.125 Hz.…”

“…A small groove (i.e., slit of 1 mm width and depth) was cut along the rebar ( Figure 4 b), which has been widely practiced in previous research for attaching the DFOS to steel reinforcement and helping to prevent a premature failure of the fiber [ 11 , 17 , 29 , 30 , 31 ]. Ideally, the groove is milled along the longitudinal rib of the rebar as to not affect the cross-section or the bond from the transverse ribs.…”

“…However, this could not be practiced in the experiments herein due to laboratory equipment limitations, and instead, the groove was milled along the rebar as shown in Figure 4 b. After some initial trials, a general-purpose adhesive (“Loctite 401”, cyanoacrylate technology, or “CYN” in Figure 4 a) was used to bond the fiber into the groove of the bar, which has also been successfully used in the literature for this purpose [ 3 , 11 , 18 , 32 ]. Some researchers have concluded that applying a thin layer of protection between the optical fiber and the concrete can achieve more reasonable strain profiles and, more importantly, help to achieve measuring strains after concrete cracking has occurred [ 18 , 33 ].…”

“…To overcome the aforementioned limitations with spot sensors and to achieve the measurement of the full strain profile of reinforcement embedded in concrete, fiber-optic sensors can be used. Distributed Fiber-Optic Sensors (DFOS) allow for a high-frequency measurement of strains along the entire sensor length at high resolution [ 11 ]. When the fibers, acting as sensors, are subjected to an extension or contraction (i.e., mechanical strain), the Rayleigh scattering profile is altered due to the change in distance between the imperfections caused in the cylindrical geometry of the fibers [ 3 , 12 ].…”

“…For example, DFOS was attached to steel reinforcement in RC beams to study the bond performance in the inelastic range [ 12 ]. A recent study investigated the application of DFOS for various measurement scenarios in different structures, such as concrete cubes, RC slabs, and RC beams [ 11 ]. Another study investigated the strain profiles of beam-column connections by embedding four optical fibers with three different coatings at different locations [ 13 ].…”