The Research On Transport Performance Of Chloride Ion In Concrete Based On Microstructure

Durability of cement concrete material is always a major issues and hot spot of research in the field of civil engineering which includes impermeability,frost resistance,carbonation resistance,alkali aggregate reaction and corrosion resistance.In durability aspects impermeability of concrete is an important index to evaluate the durability of concrete which has a close relationship to other properties of durability.The resistance medium of concrete impermeability mainly includes pressure water,chloride ion,harmful gases and other harmful solution of acid,alkali,and salt.In chloride environment,permeability of chloride ion of concrete is a key parameter related to corrosion of reinforced concrete structure,so the study of transport performance of chloride ion in concrete is a project which has practical significance of civil engineering.In the research subject the prediction model of multi-scale transport performance of chloride ions is built using upscaling method and homogenization theory based on the macroscopic properties depending on microscopic structure with different typical representative volume elements.In the cement hydration the changes of cement pastes are studied by experiments of isothermal calorimetry,non-contact resistivity experiment and mercury porosimetry experiment.In the cement paste microstructure the volume fraction of various hydration products is calculated theoretically by Powers model and J-T(Jennings-Tennis)model.Three dimensional microstructures in hydration of cement paste are built using Matlab tool according to the results by the open procedures CEMHYD3D hydration program developed by the United States National Institute of standards and Technology(NIST).The prediction model of multi-scale transport performance of chloride ions mainly contains four scales:the first scale for the hydration product scale,the second scale for cement paste scale,the third scale for cement mortar scale,and the fourth scale for cement concrete scale.Changes of chloride ion diffusion coefficient of concrete are tested under tensile load by the loading equipment specially designed with authorized patent.According to the practical engineering the selection of water binder ratio includes 0.23,0.35,0.53,and the content of mineral admixture fly ash and silica fume includes respectively 10%,30%,50%and 4%,8%,12%.The diffusion coefficient of chloride ion of cement paste,mortar,concrete is tested in experiments with self-made equipment.The main research conclusions and innovative results are as follows.(1)Experimental study of the cement paste hydration processAn exothermic process of different water binder ratio,different dosage of fly ash or silica fume of cement paste is studied by isothermal calorimetric.The relationship between the hydration heat release rate and hydration heat release degree is established which hydration heat release of mineral admixture is divided into two parts:the hydration heat of cement and intervention of mineral admixture in later period.The critical exothermic degree is defined to describe the influence of mineral admixtures to cement hydration heat.The resistivity changes in hydration process of cement paste with different water binder ratios and different contents of mineral admixtures are tested by non contacting resistivity experiments.The relationship between the change of resistivity and the change of solid phase and pore is analyzed.The changes of pore in hydration process of cement paste with different water binder ratios and different mineral admixtures are obtained by mercury porosimetry and effective porosity is obtained through two mercury withdrawal experiments in mercury porosimetry experiment.(2)Study on the microstructure of cement paste in hydration processThe volume fraction of each phase of hydration products is obtained according to Powers model and J-T(Jennings-Tennis)model including calcium hydroxide,high density and low density hydrated calcium silicate,aluminum iron phase,pores and unhydrated cement.The quantity of each phase in hydration process of cement is studied with the open procedures CEMHYD3D hydration program developed by the United States National Institute of standards and Technology to establish three dimensional microstructures of cement.(3)Prediction model of chloride ion transport performance using multi-scale calculationThe multi-scale prediction model of chloride ion transport performance mainly contains four scales:the first scale for the hydration products including high density and low density of hydrated calcium silicate(HD C-S-H,LD C-S-H),calcium hydroxide(CH),aluminum iron phase(AF)etc,the second scale for cement paste including hydration products and capillary pores,third scale for cement mortar scale including cement paste and sand,fourth scale for cement concrete including cement mortar,stone and interfacial transition zone.On the first scale the composite property of HD C-S-H and LD C-S-H is calculated using generalized self consistent theory.The composite property of C-S-H and CH,AF is calculated using Mori-Tanaka homogenization theory.The composite property of hydration products and unhydrated cement particles is calculated by using the generalized self consistent theory.On second scale the composite property of hydration products and capillary pores is calculated by using pass parallel model considering percolation effect and tortuosity of capillary pores.On third scale the composite property of paste and fine aggregate is calculated by using Mori-Tanaka homogenization theory.On fourth scale the composite property of coarse aggregates and interface transition zone is calculated by using the generalized self consistent theory and the composite property of mortar,coarse aggregates and interface transition zone is calculated by using with the series model.In order to validate the accuracy and suitability of the prediction model,experiments were carried out to compare experimental results and model predictions.The comparison shows that the prediction model results agreed well with the experimental data.(4)Changes of chloride transport performance under tensile loadTensile loading equipment combined with steady electric accelerated chloride ion diffusion instrument is specially designed to study the changes of chloride ion diffusion coefficient of concrete in different stress level and the relationship between stress level and diffusion coefficient by using application of damage mechanics principle.The designed equipment is authorized by patent with patent number ZL200910024476.8.