| Due to its superiority in performance and price,the reinforced concrete structure has become the main structural form of existing engineering,and it is also widely used.However,the reinforced concrete structure is seriously damaged with the increase of the service time,and the main influencing factor is steel corrosion.The corrosion of steel bars leads to the deterioration of the mechanical properties of buildings.The bonding force between the steel bars and the concrete decreases rapidly,which greatly reduces the bearing capacity of the whole building.At present,steel corrosion has become a hot research topic in the field of civil engineering.Although many methods which are studied by the researchers from the home and abroad for monitoring steel corrosion have been developed,these methods have many disadvantages,such as high cost,weak feasibility and low sensitivity.Ultrasonic guided wave monitoring technology is a dynamic,online,real-time and continuous non-destructive monitoring technology,which can devide the different stages of steel corrosion and realize the real and significative health monitoring.In this paper,ultrasonic guided wave monitoring technology is used to monitor the whole process of steel corrosion in concrete.The specific research aspects are as follows:1.Piezoelectric ceramic plates were used as sensors to stimulate and receive ultrasonic guided wave signals.Electrochemical method was used to accelerate corrosion.Ultrasonic guided wave was used to monitor the whole process of steel corrosion in wrapped concrete.Then the amplitude and peak frequency trend of ultrasonic guided wave were obtained.The results showed that the amplitude and peak frequency of ultrasonic guided waves had the same trend,which could be divided into three stages: descending stage,ascending stage and redescending stage.The reason for the descending stage was that a small amount of corrosion products were filled between concrete and steel at the beginning of corrosion.In the ascending stage,micro-cracks appeared in the concrete,and the cracks constantly extended outward,and the bonding stress between concrete and reinforcement decreased.In the redescending stage,macroscopic cracks appeared in the surface of concrete,and vertical reinforcement was damaged,and electrolyte solution entered into internal concrete.2.According to the micro-topography meansurement between steel bar and concrete scanned by SEM and the energy spectrum obtained by EDS,it can be seen that when micro cracks appeared in the specimen,the width of crack was small and there was a small amount of Fe element in the internal concrete and the interface of the steel and concrete,which proved that the steel bar had been corroded and the cracks were spreading outwardly.When visible cracks appeared in specimen,the width of crack was large,and Cl element was found in the reinforcement,which proved that cracks had been generated in the surface of the specimen,and electrolyte solution entered into the concrete.3.Corrosion of steel bar under different influence factors was monitored by using ultrasonic guided wave,and these factors included different protective layer thickness(23mm,43 mm,63mm),different water-cement ratio(0.4,0.5,0.6),and different types of reinforcement(plain round bar and ribbed bar).In the same conditions,the value of a single variable was changed,and the results under different factors were summarized and compared:Protective layer thickness: the amplitude of ultrasonic guide wave is inversely proportional to the thickness of concrete protective layer,and the fitting formula is y=492.31-5.354 x.At the same time,when the protective layer thickness is higher than 83 mm,the ultrasonic guided wave technology can't monitor it.In the test of three types of protective layer thickness of 23 mm,43mm and 63 mm,the ultrasonic guide wave amplitude of the protective layer thickness of 63 cm was the minimum,and the time of micro-cracks and visible cracks were the longest too.What's more,the ultrasonic guided wave amplitude varied less in the same period.Water-cement ratio: according to the compressive test,the water-cement ratio of concrete is inversely proportional to the strength of concrete.The larger the water-cement ratio is,the lower the strength of concrete is.In the tests of water-cement ratio of 0.4,0.5 and 0.6,and the samples with water-cement ratio of 0.6 has the shortest time of micro-cracks and visible cracks in the ultrasonic guide-wave amplitude diagram,and its ultrasonic guide wave amplitude was the largest.In addition,the theoretical model of the rebar corrosion time was compared with the test results,and the error is within 20%.Therefore,the test results were reasonable.The type of rebar: first of all,it could be seen that the time of appearance of micro-cracks of the ribbed rebar and the plain bar were 42 h in the ultrasonic guide-wave amplitude diagram.The visible cracks of the plain rebar was114 h,the visible cracks of the ribbed rebar was 132 h.Because of the existence of the rib,the time of appearance of visible cracks of the plain bar was earlier than the ribbed steel bar.But the rib's area is very small,and the influence was small enough to be neglected.Secondly,the amplitude range of the plain bar was 160mV-242 mV,and the amplitude range of the ribbed bar was 168mV-212 mV.Therefore,it could be seen that the ultrasonic guided wave amplitude of the ribbed and plain bar were very close.Finally,according to Faraday's law,the theoretical mass loss of plain bar and ribbed bar was not much different from the actual loss,the error was 7%.And the actual corrosion mass loss of plain bars and ribbed bars was also very close,the different value was 0.3g,which also indicated that the rib has little effect on the corrosion of steel bars.4.The acoustic emission technique was compared with the ultrasonic guided wave method.The acoustic emission and ultrasonic guided wave monitoring steel corrosion were carried out simultaneously for the specimens with different influencing factors,and the values were contrasted and verified.Compared with cumulative hits-time diagram and energy-time diagram,the time of micro-cracks and visible cracks by ultrasonic guide wave amplitude-time diagram were found to be roughly the same.Therefore,the measured data by acoustic emission technology can better verify the feasibility and effectiveness of ultrasonic guided wave technology.The change trend of AE signal number-time diagram of different protective layer thickness was first rising and then falling,and the corresponding time of its maximum peak point was basically the same as the visible crack.It could be seen that the AE signal numbers of the ribbed bar were less than the plain bar at the initial stage of corrosion,but the AE signal numbers of the ribbed bar were more than the plain bar at the later stage of corrosion.According to the energy-peak frequency variation diagram of different water-cement ratios,the larger the water-cement ratio is,the greater the peak frequency is.And the more the corresponding energy is,the more intense the crack is.Therefore,it could better explain the results of reinforcement corrosion development stage under different influencing factors by using ultrasonic guided wave.The results of this paper showed that the ultrasonic guided wave monitoring technology could monitor the different stages of steel corrosion in real time and online under different influencing factors,and it could reflect the degree of corrosion effectively. |
PDF Full Text Request