Experimental Study On Electric Conductivity Characteristics And Influential Factors Of Concrete

Improving the ballast bed to earth resistance of metro can be an effective way to enhance the electric resistivity of concrete,because the concrete ballast bed is the only way for metro stray current.It's one of the methods for reducing the stray current.For plentiful influential factors of concrete resistivity,there are three problems in the correlational research:?1?the level of improving the electric resistivity of concrete can't meet the demand of metro engineering at present;?2?For the exacting requirements of raw materials and technology,high insulation concrete can not be widely used;?3?Under the environment of metro running,especially under the humid environment,the ballast bed to earth resistance decreases sharply.Based on the scientific research project supported by Guangzhou Metro Design and Research Institute,theoretical and experimental methods were used to study the characteristics of high insulatinn concrete and the influential factors of concrete resistivity during the metro running.The main conclusions are as follows:?1?According to the different forming process,pores of concrete were divided into two categories:big pores and small pores.A double Gaussian distribution curve was introduced to simulate the pore size distribution.A concrete electrical conductivity model was established by considering pore size distribution and electrical conductivity of double electrode layer.The model was proved by fitting the experimental data.The model shows that the electric resistivity of concrete is not only influenced by the saturation degree,porosity,but also influenced by the pore size distribution of concrete.Small poses play the role of connecting the big pores in concrete.As a result,when the porosity is constant,more small pores means better connectivity and high conductivity.Besides,the dispersion effect of alternating current impedance spectrum of concrete is also influenced by pore size distribution.These results can be the guide for improving the resistivity of concrete.?2?The feasibility of improving the resistivity of concrete was discussed.The influential factors of concrete resistivity were studied by experiments.Two sets of high insulation concrete were achieved.The results show that the resistivity of concrete is influenced by the water-binder ratio,admixture,types of cement and so on.The water-binder ratio mainly changes the porosity;the adminxture mainly changes the pore structure and pore size distribution of concrete;the type of cement mainly changes the hydrated products and pore solution.These influence changes the electric resistivity of concrete.Finaly,according to the results from the experiments,high aluminum cement,low water-binder ratio,reasonable admixtures were considered to produce high insulation concrete.?3?The application of high insulation concrete was studied by ballast model test.The effect of rainfall was also considered.The results show that the ballast bed to earth resistance of metro can be improved by improving the resistivity of concrete.The ballast bed to earth resistance was greatly influenced by the rainfall and missing of insulating mat.There will be a conducting water film over the ballast caused by rainfall.And the conducting water film will reduce the ballast bed to earth resistance.Another result caused by rainfall is the increase of saturation degree,which will also reduce the ballast bed to earth resistance.Because of the two impact,the ballast bed to earth resistance decrease three orders of magnitude.The water can be stoped by spraying fluorocarbon paint on the surface of ballast bed,which will improve the ballast bed to earth resistance.?4?The electric resistivity of concrete influenced by underground water,stress and varying voltage was studied by experiments.The results show that the underground water enter the concrete by capillary rise,which will increase the saturation degree and electrical conductivity of concrete.According to the experiment results,an improved Terzaghi model was established to describing the capillary rise in concrete.The increasing of saturation degree above the wetting front,which is caused by the capillary condensation,was considered in the improved Terzaghi model.The error of t^0.5 law of capillary rise can be explained by this model.The microcrack extend when force acts on.The change of microstructure will affect the distribution of pore solution as well as the resistivity of concrete.The value of valtage will affect the resistivity of concrete,too.