| Due to its advantages of low price,convenient for materials and superior performance,concrete has become the most widely used building material in the world,and reinforced concrete structure has also become the most widely used structural form in infrastructure construction.As the service environment of building structures becomes more complex,the durability problem of structure becomes more and more prominent,among which the steel corrosion is the main reason for the lack of durability of structure.The steel corrosion will reduce the bonding force between steel bar and surrounding concrete,and reduce the cross-sectional area of steel bar,leading to the decline of structural bearing capacity.There are many safety accidents caused by the failure of construction structures earlier than their service life every year,and various countries have invested huge financial and material resources to maintain the structural damage caused by steel corrosion.Therefore,studying effective monitoring method of steel corrosion in concrete is easy to master the law of steel corrosion and development,to provide security for the safety of major projects,and has important economic value and social significance.In this paper,a method for corrosion monitoring using single mode-multi-mode-single mode(SMS)bare fiber and Fe-C coated SMS fiber is proposed.Firstly,the electrochemical and non-electrochemical methods used in steel corrosion monitoring and their advantages and disadvantages are listed.The research progress of SMS fiber is introduced in detail,and the corrosion monitoring method based on SMS fiber structure is proposed.Then the principle of steel corrosion monitoring using SMS bare fiber and Fe-C coated SMS fiber was introduced,and the numerical simulation was carried out.The deviation of optical fiber spectrum when the external environment refractive index changes was simulated in Rsoft.In the simulation process,considering the influence of the non-core fiber(NCF)length and the thickness of Au and Fe-C film on the monitoring results.The simulation results show that when the refractive index ranges from 1.34 to 1.44,the spectrum of both SMS bare fiber and Fe-C coated SMS fiber shift to right when the refractive index increases.The sensitivity of SMS bare fiber and Fe-C coated SMS fiber increases with the increase of refractive index,and the sensitivity of Fe-C coated SMS fiber is significantly lower than that of SMS bare fiber.NCF length affects the refractive index sensitivity of SMS bare fiber,but has little effect on the sensitivity of Fe-C coated SMS fiber.The sensitivity of Fe-C coated SMS fiber increases with the decrease of the thickness of Au and Fe-C films,and the effect of the thickness of Fe-C film on the sensitivity of the fiber is greater than that of the thickness of Au film.Then,SMS bare fiber was used to monitor the steel corrosion.In the experiment,three groups of non-core fiber(NCF)with length of 2cm,3cm and 4cm were considered as multi-mode fiber to prepare SMS fiber sensor.The SMS fiber sensor was attached to the surface of the polished steel bar and placed in 3.5wt.% Na Cl solution for corrosion test.The spectral change of SMS in the steel corrosion process was monitored by an Optical Spectrum Analyzer.At the same time,the corrosion potential and corrosion current density of steel were measured by open circuit potential and linear polarization resistance,and the cumulative corrosion amount of steel was calculated.The test results show that the corrosion amount of steel is a cubic function of the offset of characteristic wavelength.The sensitivity of SMS fiber corrosion monitoring increases with the increase of the length of non-core fiber,and increases initially and then decreases with the increase of corrosion time.Finally,the corrosion monitoring test of Fe-C coated SMS fiber was carried out.The length of NCF is 3cm.After SMS fiber was sputtered with gold,Fe-C films were electroplated.The gold film thickness was 100 nm and 200 nm,and the Fe-C film thickness was 7μm,17μm and 30μm,respectively.Six SMS fibers with different Au and Fe-C film thicknesses were placed in 3.5wt.% Na Cl solution for corrosion test.Spectral changes and electrochemical impedance spectroscopy(EIS)were monitored.The experimental results show that the offset of the characteristic wavelength of SMS fiber has a cubic function with the percentage of the corrosion of Fe-C film.The sensitivity of Fe-C coated SMS fiber varies with the increase of corrosion amount and is lower than that of SMS bare fiber.In this experiment,for 6 groups of SMS fibers with different thickness of Au and Fe-C film,the characteristic wavelength offset ranges from 6.3nm to 25.75 nm,the mass loss range of Fe-C film ranges from 78.21% to 99.94%,and the service life of the sensor ranges from 28.3h to80.4h.The results of this paper show that the application of SMS fiber structure to steel corrosion monitoring is reasonable and effective.The SMS bare fiber has high sensitivity and the Fe-C coated SMS fiber has good performance in sensitivity,range and service life. |
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