Damage Assessment in a Reinforced Concrete Structure under Quasi-Static Shear Loading Using OFDR-based Fibre-Optic Distributed Strain Monitoring

Abstract

EDF and CEA have established an experimental program to improve knowledge of the behavior of wall-slab junctions under both out-of-plane bending and in-plane shear. The program involved reduced-scale mock-ups made of reinforced concrete (RC) that are representative of the structural elements of nuclear plant buildings. In parallel to vast experimental campaigns, numerical models of the junction under study are derived and calibrated in order to improve the computation of building responses under seismic excitations. The analysis of damaging process of these structures is reproduced by nonlinear numerical simulations, realized within the Finite Element framework CAST3M developed at CEA (www-cast3m.cea.fr/) and code Aster (https://code-aster.org), which account for steel plasticity and concrete damage, including crack propagation based on loading conditions and history. Of all the distributed techniques applicable to the Structural Health Monitoring (SHM) of RC structures, Optical Frequency-Domain Reflectometry (OFDR) is well suited because of its high spatial resolution. The OFDR principle relies on swept-wavelength homodyne interferometry. Light from a tunable laser source is split and sent through sensing and reference fibers, both being arms of an interferometer. The backscattered light recombines at an optical detector and an interferogram is recorded as the laser frequency is tuned. The spectral-domain signal is then Fourier-Transformed to yield the backscattering profile along the fiber. Finally, the fiber is segmented into successive centimeter-long gage lengths and a cross-correlation procedure provides the strain profile with respect to a reference state. We implemented the OBR4600 OFDR device from Luna Innovations. The OBR 4600 is a single channel device that provides static distributed strain monitoring over a range of 70 m, with an accuracy in strain of ±5 μm/m, a spatial resolution (gage length) of about 5 mm, and a readout time of typically 10 seconds. On the last mock-up of the experimental program, the jacks applied quasi-static displacements ranging from 0.69 mm to 11.49 mm, yielding progressive structural damage and eventually reaching concrete crack and steel plasticity. Strain profiles and natural frequencies were determined and compared to the modeling. Distributed Fiber Optic Sensing (DFOS) results provide access to continuous strain distributions along the instrumented rebars, localize deficiencies and local deformations around rebar crossings and also the emergence of concrete degradations in the joint. The OFDR technique enables highly reliable in-situ SHM of damage mechanisms within concrete structures, providing effective data for model verification and validation used in safety-related structures assessment.