| Prestressed steel strand is the key stress element in prestressed structure.Whether its prestress value can meet the design requirements is directly related to the safety of the whole prestressed structure.Due to the influence of construction technology,material characteristics and various kinds of internal and external environment,the prestress in the structure is often difficult to maintain in the accurate design range,so it is necessary to monitor the prestress in the prestressed structure.At present,there are many researches on the loss of prestress,but it is still difficult to accurately obtain the true stress state because most of the prestressed steel strands are deeply embedded in the prestressed concrete members..As a new type of sensor,Fiber Bragg Grating(FBG)has been widely used in many engineering monitoring fields because of its small size,high accuracy,corrosion resistance and no electromagnetic interference.With the development of the application and research of FBG,the monitoring mode of FBG has gradually developed from the traditional single point measurement to multi-point quasi distributed measurement.In order to explore the monitoring ability of FBG in the use process,a kind of quasi distributed fiber Bragg grating steel strand(hereinafter referred to as FBG Self-sensing steel strand)is designed and applied to prestressed rectangular beam,hopeing it can be realized the internal prestressing force monitoring.The specific research contents are as follows(1)According to the characteristics of FBG which is brittle and easy to fold,a kind of groove embedded packaging method is proposed,which encapsulates the FBG sensor into the linear groove of the center wire of the prestressed steel strand to improve the survival rate of the sensor.In the packaging process,the "preloading" process is adopted to improve the monitoring range of the sensor to meet the actual monitoring needs.(2)In order to explore the stability of FBG Self-sensing steel strand in use,repeated tension tests were carried out,and the strain wavelength data of FBG sensor were analyzed.The results show that FBG Self-sensing strand has high sensitivity,and there is a good linear relationship between strain and central wavelength.During the test,the monitoring strain of all FBG sensors can reach more than 7000 με,which is far higher than the limit strain of FBG itself(about 4500 μ ε).(3)In order to explore whether FBG Self-sensing steel strand is suitable for alkaline environment in internal prestressing monitoring,an alkaline cement slurry environment simulation test was designed.Three Self-sensing center wires were put into cement slurry environment for curing for 10 days,20 days and 60 days respectively,and their corrosion was observed.The test results show that due to the fast setting time of cement paste,the Self-sensing center wire can not be corrosioned.During the test lasting for 60 days,the three groups of Self-sensing packaging structures have remained intact without colloid shedding.FBG Self-sensing steel strand can be applied to alkaline cement paste environment.(4)The causes of prestress loss in prestressed beams are analyzed,and the calculation methods of prestress loss are given.The prestress monitoring test was carried out,and four prestressed rectangular beams were prepared.The prestress loss was monitored by FBG Self-sensing steel strand.The monitoring results of FBG sensor were compared with the monitoring results of resistance strain gauge and the theoretical value calculated according to the specification.The results showed that the monitoring results of FBG sensor were closer to the theoretical value than that of resistance strain gauge,and had higher accuracy This indicates that FBG Self-sensing strand is more suitable for monitoring internal prestressing force than resistance strain gauge. |
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