Research On Monitoring Techniques Of Rebars Corrosion In Concrete Under Marine Environment Based On Theory Of Electromagnetic Field Response

Corrosion of reinforced bars in concrete is a significant issue of durability performance evaluation and service life prediction under marine environment.Currently,a large number of researches have been carried out worldwide on the mechanism and related apparatuses against reinforcement corrosion.However,most monitoring and testing apparatuses are with the assumption of uniform corrosion which means corrosion initiates equally at the rebar surface,hence,it is impossible to accurately describe the damage type,degree and locations of reinforcement corrosion inside concrete.Furthermore,the durability performance deterioration of concrete structures will not be evaluated and predicted precisely.Based on mentioned above,this paper revealed the mechanism of electromagnetic field response induced by rebars corrosion,and also,the theoretical calculation and computer simulation were also carried out respectively.The method using the physical phenomenon of electromagnetic response decline of ferromagnetic medium induced by rebar corrosion to evaluate and monitor the damage degree was also proposed.Based on the traditional Hall components,a series of different types of electromagnetic monitoring apparatuses(EMMA)were developed,and the relevant signals calibration system was also established for practical engineering application.In addition,considering the characteristics of electromagnetic monitoring devices,the combinations of electromagnetic monitoring device and acoustic emission technology,digital image correlation technology were studied respectively,which aims to monitor the entire process of reinforcement corrosion from corrosion initiation to the later corrosiontriggered expansion cracking.The main conclusions are as follows:(1)Reinforcement corrosion will trigger electromagnetic response which contains the variation of magnetic field intensity and direction of magnetic flux lines.Non-uniform corrosion is more difficult to be detected than uniform corrosion because of its less permeability loss.Rebars corrosion will weaken the electromagnetic field intensity at the cross section of reinforcement,which provides the theoretical foundation for corrosion monitoring.Besides,the diameter of reinforcement is an important factor influencing the sensitivity of electromagnetic response.For non-uniform corrosion,the electromagnetic response is most sensitive when the direction of external magnetic field applied is consistent with that of corrosion initiation,however,the electromagnetic response is least sensitive when the direction of external magnetic field applied is perpendicular to the rust direction of the reinforcement.In addition,the simulation result of the three-dimensional model about reinforcement corrosion also demonstrates the feasibility of monitoring rebar corrosion based on the theory of electromagnetism.(2)The homemade Hall-effect sensor-based EMMA is capable of monitoring reinforcement corrosion according to Hall voltage variation,and there is a linear relationship between Hall signals increasement and mass loss of reinforcement.The linear coefficient is as high as 0.996 and the minimum resolution ratio is 0.17 m V.Moreover,this version EMMA can realize detecting the process from random occurrence of rust spots to later rust layer formation.Besides,corrosion type is also a positive factor influencing the value of monitoring signals.(3)24 Hall-effect sensors were used and linearly arranged on the upgraded monitoring probe of EMMA,which aims to monitor the electromagnetic response at concrete/steel interface inside concrete specimens.EMMA can monitor both non-uniform and uniform corrosion respectively and is also capable of judging the corrosion locations of steel bars while evaluating the damage degree with index of 4 m V and minimum resolution ration of 0.133 m V.Moisture content is also another significant impact factor on electromagnetic response while relative humidity,carbonation effect and water absorption amount will influence the electromagnetic response inside concrete.The higher moisture content inside,the more electromagnetic field intensity declined.Relative humidity influences most and water absorption amount has the least impact among three factors,meanwhile,the concerned signals calibration system was also established with taking water cement ratio w/b=0.4 as a reference.(4)Combined application of EMMA and acoustic emission analyzer is capable of monitoring the entire corrosion process from corrosion initiation to further corrosioninduced cracks.Compared with carbon steel,corrosion-resistance alloy can reduce the risk of concrete cover cracking.Besides,coupled utilization of EMMA and digital image correlation(DIC)technique can also monitor the entire corrosion process from corrosion initiation to further corrosion-induced strain distribution.Compared with strain gauges,DIC technique can monitor the strain level of a certain area.However,the signals will be influenced by something covered,correspondingly,strain gauges can realize persistent signals collection but only for a certain location.(5)In order to meet the requirements of durability assessment for practical engineering under marine environment,a variety of independently-developed equipment were invented,which includes the monitoring device of chloride ion migration in huge volume concrete,the wireless transmission device of monitoring signals,and the remote real-time assessment software of concrete durability performance(including personal computer and mobile device versions).In addition,the current service life prediction model was further modified and the signals correction procedure under acceleration corrosion mechanism in laboratory was also carried out.Furthermore,the mechanical model of rust swelling and cracking in the concrete/steel interface area was also revised.