Study On Chloride Ion Transmission Properties Of Cement-based Materials Under Tensile Stress

In the field of civil engineering,concrete durability research is always a traditional experimental science.In the current major projects,many of them are in extremely harsh environments such as sulfate attack,chloride attack,freeze-thaw cycle,and drywet cycle,which thus leads to higher service requirements for concrete.This makes the laboratory research more difficult.Therefore,there is an urgent need to use numerical simulation methods to reveal the mechanism of damage caused by durability.This paper mainly uses the multi-scale numerical modelling,to study the damage evolution process of mortar at the micro-and micro-level subsequently.Then analyze the transmission properties of cement-based materials after damage The main achievements of the paper are listed as follows:(1)Analysis of damage evolution processTwo interfaces for finite element analysis of the micro-mechanical properties of cement-based materials have been developed.They are divided into solid element models and lattice models according to the nature of the model elements.From the modeling mechanism,the analysis of solid element models has more advantages.The damage process simulated using these two methods is similar: the final damage is caused by a main crack,and there are multiple micro crack zones at the same time.The verification of the mechanical results is consistent with the results of the relevant literature,which proves the reliability of the simulation method.(2)Cement-based materials microstructure diffusion propertiesIn the damaged state,the ion diffusion process of cement stone can be divided into four stages,and the relative strain = 2 is the critical point.It is calculated that the volume content of cracks(number of crack units / total number of model units)does not exceed 1.4%,and when the ratio of cracks / total pores does not exceed 4%,the diffusion performance in microstructure will not change much.With the development of damage,there are three processes in the cement-based materials: cracks,crack penetration,and crack width increase.In the crack penetration stage,the diffusion performance increased the fastest.As the saturation increases,the chloride ion diffusion coefficient gradually increases,and the increase conforms to a certain trend.Before the saturation is 60%,the growth rate is slower,and afterwards,the growth rate is fast.The reason is that when the saturation is higher,the connectivity of water increases,and ion transmission channels begin to penetrate.The water connectivity depends on the porosity and pore connectivity.When the degree of hydration is high,the pores are not connected or the connectivity is not high.The unsaturated microstructures have low water connectivity,but once under stress if the pores change,then the connectivity of the water will change accordingly.In addition to water connectivity,transmission performance is also affected by the overall water content.The higher the water content,the more ion transmission channels caused by water connectivity.(3)transport performance in Damaged mortarIn saturated damage mortar,when the pores are all closed pores,the diffusion is mainly affected by the diffusion properties of the matrix.When there are semiconnected pores,the diffusion performance will be accelerated,and the connected pores have the greatest impact on the diffusion performance.In unsaturated damage mortar,the lower saturation results in a lower the water content,so that the ion diffusion in the mortar is dominated by the clean mortar.with the same degree of hydration,the lower the saturation,the lower the ion diffusion performance.In unsaturated damage mortar,the lower the saturation,the lower the water content,so that the ion diffusion in the mortar is dominated by the clean mortar.At the same degree of hydration,the lower the saturation,the lower the ion diffusion performance,so keeping the saturation low can significantly reduce the ion diffusion performance.The result of the non-steady-state transportconcentration of mortar shows that the diffusion of ions in the damaged mortar is not simply unidirectionally transported along the preset direction of high and low concentration,but is transported along the crack to the surrounding trees.At the same time,the presence of aggregates hinders the transmission of ions.The paper has 71 figures,10 tables,133 references.