Study On Embeddable Multi-cell Sensor For Monitoring Corrosion State In Concrete Structures

Corrosion of steel is regarded as the most important factor to cause deterioration of reinforced concrete structures when they are in the coastal environment or using de-icing salt and other chloride environments. Regarding to the assessment and prediction of service status of concrete structure, identification of maintenance cycles, the technology optimization of damage prevention and control, economic analysis of service life cycle as well as improvement of design methods of high durable concrete structure, a real, long-term multi-parameter monitoring system will be indeed needed to provide the quality of concrete cover and the various durability parameters of rebar corrosion state. Among them, the steel surface conditions parameters include the corrosion potential of reinforcing steel, dispassivation time, the macro cell current and corrosion current density after the beginning of the corrosion; The quality of concrete protection layer was primarily assessed by the thickness and resistivity of concrete cover, chloride ion content and distribution as well as the changes of concrete temperature and humidity.Based on the importance of these multiple parameters to assess the durability of concrete corrosion and eliminate the shortcomings of testing technology and existing sensor, the following sensor systems were developed in this study: (1) Long life-maintenance-free- embedded reference electrode: provide long-term and stable reference potential ,eliminate the voltage drop of concrete in electrochemical tests; (2) Embedded type concrete resistivity test unit based on a stable reference electrode potential output and impedance-based technology;(3) Embedded macro cell current testing unit based on the resistivity test cell and titanium based metal oxide auxiliary electrode; (4) Based on the reference electrode, concrete resistivity test cell, macro cell current testing unit and the developed chloride ion sensor testing principle, the assembly and test techniques of multi-parameter sensor were determined; (5) Based on the current step method, the multi-sensor was to monitor the resistance and polarization resistance of concrete at the original position. At the same time, considering the particularity of concrete media environment as well as the sensor in the applicability of chloride contaminated concrete, the reasonable electrochemical monitoring techniques and parameters were determined.First of all, the electrochemical properties results of the reference electrode prepared by constant potential anode-deposition method showed that the active substance on the deposition surface of electrode wasγ-MnO₂. The stability of electrode potential in the pore solution increased with the lower deposition potential and the electrode potential depended on the homogeneous phase composition and surface conditions ofγ-MnO₂ in alkaline solution. The carbon-based electrode substrate adsorbed oxygen groups easily, resulting in poor stable electrode potential, while the use of titanium substrate electrode showed a better potential stability, anti-polarization capability and the ability to resist chloride ion interference, although the role of surface tension as well as the differences in temperature coefficient of expansion between matrix and the deposition layers, leading to low reproducibility to prepare the same potential anode electrode. Taking into account its simple production technology and good potential stability, the packaged MnO₂ anode reference electrode can be used in linear polarization analysis due to its low demanding for the potential reproducibility. The accurate polarization resistance of rebar can also be obtained by eliminating voltage drop of the resistance of concrete through the embedded sensor in the vicinity of tested steel.Second, in order to improve the reproducibility of potential of anode reference electrod by deposition process, the powder-solid-process and package technology were deal with the reference electrode with Mn andβ-MnO₂ active material. The results show that the addition of Mn element can improve the electrode potential stability and reproducibility in the alkaline pore solution by the electrochemical reaction mechanism analysis. The reason was mainly due to electrode made of the non-uniform phase of MnOx-MnO₂-MnOOH with strong resistance to external interference. The high and low frequency performance of the reference electrode were investigated by AC impedance spectroscopy. The analysis results showed that the impedance modulus of reference electrode at high frequency was the impedance test equipment of 1/400, while the low frequency impedance modulus was much lower to meet the performance requirement better. And the temperature response coefficient to prepare reference electrode through the reference electrode potential-temperature curve was determined as -0.65mV/℃. Third, based on AC impedance technique, the packaging and testing technology of embedded concrete resistivity test unit were investigated. The impedance behavior of concrete and sensor electrode within different frequency range was discussed and the sampling frequency range of concrete resistivity was determined as 0.01kHz 20kHz. In this frequency the interface between the sensor electrode showed typical characteristics of constant phase angle element, and the values of the sampling frequency (fcutoff) based on this linear features was determined. The test results for mortar resistance with different chloride content and the polarization resistance of half-ring electrode resistance showed that the resistance of mortar increased with the chloride content decreased and the protective layer thickness increased. It is also found that the polarization resistance of half-ring electrode was corresponding to the changes of the protective layer resistance of mortar, therefore, the simplified relationship mode between Rs and Rp without chloride doped in the mortar was established.Fourth, the macro cell current sensors can be embedded into the cover-zone concrete were developed. By studying the attenuation rule of the macro cell current, the standard acquisition time of 30s was determined. The macro cell current test results in mortar and concrete different chloride content showed that the macro cell current increased with the increase of chloride content, which meant higher corrosion risk. The electrochemical nature of the macro cell current was analysed of and by the method of accelerated penetration, the response behavior of the macro cell current sensor to chloride ion in concrete was studied.The results showed that the macro cell current increased with the time of accelerated testing prolonged and later a curve plateau ouucred, at this time the steel corrosion rate depended on the oxygen concentration of the anode half-ring surface, controlled by the cathodic reduction reaction rate.Fifth, the packaging technology of the multi-parameter durability sensor was developed. Based on these sensors, the macro cell current, corrosion potential, protective layer resistance, chloride ion content of protective layer and the anode polarization resistance of can be perform the real-time monitoring. The test results based on EIS and LPR methods showed that higher chloride content and the thinner protective layer of concrete will result in lower resistance and higher risk of corrosion. A corrosion rate prediction model related to concrete layer resistance and chloride ion content was established. Based on the model, the service status and the assessment of protective layer of corrosion degradation can be implementated.Finally, the transient current method based on step-situ testing techniques was determined concrete resistance and the polarization resistance of the corresponding reinforcement in concrete protective layer. The results showed that GPT Technology avoided the interference of stray current in external environment and was suitable for on-site test in situ. The warning system for durability classification was established. The distributed data acquisition system DDAS was used as real-time data acquisition, and then transferred to a remote data processing terminal DEE. DEE was for data processing and analysis, and displayed the measured data in the terminal and made decisions and provided maintenance recommendations. According to the data of the macro cell current test cell, the early warning system was divided into blue, red and yellow three-level warning, which provided a comprehensive assessment of the service state of the concrete protective layer. The data processing and analysis was simple and feasible, and the efficiency of the test apparatus was improved.