Impact of Spectral Shift Quality (SSQ) on Fibre Optic Sensor Readings in Reinforced Concrete Beams

Abstract

Efficient maintenance and health monitoring programs prevent irreversible damage to critical civil infrastructures and save millions of dollars of taxpayer’s money. The precision of the sensors and the volume of the experimental data collected are extremely important factors in the actual testing of structural health monitoring systems. Distributed fibre optic sensors (DOFS), currently being used extensively in civil structures for structural response measurements. However, DOFS suffers from exceeding its measurable range of strain when applied to concrete structures after initiation of cracks which cause erroneous data in the data stream. This phenomenon can be identified by using spectral shift quality (SSQ) levels of the measurements. Therefore, an experimental plan was proposed with representative size reinforced concrete (RC) beam to study the effect of SSQ on DOFS (SMF 28) readings. A RC beam with a dimension of 4000 mm × 200 mm × 400 mm was cast with embedded DOFS rebar sensor for the tension side. After the curing period, DOFS surface sensor was attached to concrete bottom surface. The beam was tested under flexural loading up to a maximum load of 160 kN until the beam failure. Four-point bending arrangement was used to test the beam and the measurements were taken in 10 kN intervals by using an optical backscattered reflectometer (OBR 4600). The SSQ should theoretically have a value between zero and one. The manufacturer of OBR 4600 recommended to remove any data with an SSQ of equal or less than 0.15, and the same threshold was applied in the study. Due to the crack initiation on bottom surface, SSQ reached 0.15 threshold after 40 kN load and for subsequent loadings for bottom surface sensor. This means that the sensor can be utilized until a load of 40 kN, which is 25% of the maximum load. However, experimental findings reveal that rebar attached DOFS SSQ values are within an acceptable range until 80% elastic limit strain. The rebar sensors functioned until 120 kN of loading, or 75 % of the maximum loading. Consequently, it can be noted that DOFS operate well with rebars and that SSQ had no effect on the results until 80% of elastic limit. However, the influence of SSQ on the concrete surface sensor was notably substantial. Therefore, it is warranted to pay a closer attention to the DOFs sensor readings after formation of hairline cracks on the attached concrete surfaces.