| Concrete material is easy to crack,which affects the health of concrete structure,therefore,distributed strain measurement and crack monitoring are important issues in damage detection and performance evaluation of concrete structures or reinforced concrete structures.Fiber optic sensing has the advantages of easy implantation,good durability,low interference,low cost,etc.This technology has been widely used in structural monitoring because of its characteristics of distributed strain measurement.Due to the influence of spatial resolution,this technology is limited in the accurate location and development monitoring of concrete cracks.In recent years,optical frequency domain reflectometry(OFDR)based on Rayleigh scattering has achieved millimeter-level spatial resolution,but the research in precise positioning and quantitative analysis of cracks is still rare.Optical frequency domain reflectometer used for crack monitoring is taken as the research object in this paper,the project is studied from three aspects by experimental study and finite analysis:the optical fiber monitoring technology of concrete cracks,location of surface and internal cracks and the calculation of crack width.The main work and achievements of the paper include:(1)Finite analysis was carried out on the concrete cracking,and the strain conduction law of the optical fiber at the crack was studied.(2)The space sensitivity test of crack-monitoring by fiber optic was carried out by using glass plate.Considering the monitoring accuracy and monitoring efficiency,1cm spatial resolution was selected for crack monitoring experiments;the characteristics of four types of optical fibers was summarized,and the ability of optical fibers to identify close-range cracks was studied.(3)The model test of a simply supported concrete beams was designed,and optical fibers were laid inside and surface of the beam to monitor cracks.The results show that OFDR can accurately locate cracks with an error within 1.11%;this technology has a good ability to identify micro-cracks in the early stage of cracks;based on monitoring results of internal optical fiber,crack depth can be monitored;the neutral axis position of beam is located,which can be used for damage judgment.(4)The variation law of crack width of concrete beams was studied.The strain transfer coefficient of optical cable was calibrated to correct the monitoring data;based on the strain,the crack width was calculated and compared with the results of the crack width observation instrument,the error was small;based on crack width monitored,crack depth monitoring method is improved and depth monitoring accuracy is raise;the principle of a three-dimensional imaging software for cracks is presented. |
