Durability of cement concrete material is always a major issues and hotspot 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 alt. 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 impacts of transport performance of chloride ion in concrete concludes water binder,ratio,mineral admixture,aggregate which establish organic connections between micro structure and macro properties of cement concrete. The typical representative volume elements are selected in different scale to predict the transport performance of chloride ion using upscaling method and homogenization theory. 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 compressive strength and diffusion coefficient of chloride ion of cement paste, mortar,concrete is tested in experiments. The changes of hydration process of cement pastes are studied by experiments of isothermal calorimetry,non-contact resistivity experiment and mercury porosimetry experiment. 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 explored with 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, second scale for cement paste scale,third scale for cement mortar scale, fourth scale for cement concrete scale. The transport performance of chloride ion is predicted using upscaling method and homogenization theory from microscopic to macroscopic scale. Changes of chloride ion diffusion coefficient of concrete are tested under tensile load by loading equipment specially designed. The main research conclusions and innovative results are as follows.
(1)Experimental study of the hydration process
An exothermic process of different water binder ratio, different dosage of fly ash, 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 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 in hydration process
The 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 volume fraction of hydration product of cement paste mixed with fly ash is calculated using the two component system hydration model proposed by Papadakis. 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 calculation
The 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.
(4) Changes of chloride transport performance under tensile load
Tensile 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.