Microstructure And Chloride Salt Transport Characterization Of Cement-based Materials Based On The Non-contact Resistivity Method

The microstructure parameters of cement-based materials are the most important parameter affecting the durability of reinforced concrete structures,and the related chloride ion transportation is the main cause of steel corrosion.Traditionally,the measurement of microstructure of cement-based materials has the disadvantages of high cost,troublesome pretreatment of samples,and large damage to microstructures.The conductivity of cement-based materials can be attributed to two aspects:on the one hand,the conductivity of free ions in the pore solution,depending on the concentration of ions,temperature,and ion type;on the other hand,the conductivity of the solid phase,such as aggregate,unhydrated cement particles and hydration products.Since the conductivity of the solid phase is generally much smaller than that of the pore solution,and the connectivity of the microstructure of the cement-based material also determines the conductivity of the pore solution,the macroscopic conductivity of the saturated cement-based material can be a well indirect method used to describe the connectivity of its microstructure.However,the conventional resistivity measurement method may cause polarization reaction and poor contact due to the presence of the metal electrode,and the resistivity cannot be accurately determined.Based on the non-contact resistivity meter invented by the Hong Kong University of Science and Technology,this paper proposes a modified non-contact electrical resistivity measurement which could be used to test the resistivity of hardened cement-based materials.Moreover,the chloride diffusion coefficient was determined based on the modified non-contact measurement.The microstructure of cement mortar and paste was determined by mercury intrusion porosimetry(MIP)and low field nuclear magnetic resonance(LF-NMR).The main research contents of this paper are as follows:1.The resistivity of mortar specimens with different mixing proportion was tested by the modified non-contact resistivity measurement.The microstructure of the corresponding specimens was tested by MIP,and the multi-phase phenomenological model was used to calculate the tortuosity of different samples.The drawbacks of the multi-phase phenomenological model,Archie's law and general effective model were eliminated by introducing the tortuosity into these models,and the physical meaning of the parameters in these model are explained and carried out.2.The chloride diffusion coefficient of cement-based materials was determined by the modified non-contact electrical resistivity measurement and the results were verified by RCM.In addition,in order to understand the influence of pore structure parameters on the chloride diffusion coefficient,the relationship between the pore structure parameters obtained by LF-NMR and chloride diffusion coefficient was established.3.The sample pretreatment methods for different microstructure measurements were summarized.The "ink bottle" effect in MIP is described and the different determinations of surface relaxivity in LF-NMR were discussed.For the microstructure test results of this paper,the results from MIP and LF-NMR are compared.The causes of errors in different test results are discussed The influence of different test conditions on the results of microstructure test is discussed.