Effects Of Coarse Aggregate On Chloride Ion Transport In Concrete By Meso-scale Simulation

The durability of reinforced concrete has always been a threat to the service life of building structures.The most serious corrosion of reinforced concrete is caused by chloride,which has caused a large number of reinforced concrete structures in the world to be decommissioned before reaching the designed service life.In the past few decades,scholars have done a lot of experiments and theoretical simulations on the transport properties of chloride ions in concrete.It is very difficult to monitor the durability and predict the service life of concrete structures because of the complexity of the internal microstructure changes and environmental conditions of concrete structures.However,computer modeling can track the changing environmental conditions of concrete structures and evaluate the transport performance of cement-based materials at the same time.At present,many models of chloride ion transport in cement-based materials have been proposed.Most of the existing studies do not consider the effect of coarse aggregate on the chloride ion transport in concrete.However,coarse aggregate is a phase in concrete with a large volume proportion,and the shape of aggregate is also different.The change of volume fraction and shape of coarse aggregates will lead to the change of pore structure in concrete,which will affect the transport performance of chloride ion in concrete.Therefore,coarse aggregates are significant for studying the transport law of chloride ion in concrete,the structural characteristics and durability of concrete.In addition,most of the concrete in service is in the unsaturated state,so the chloride ion transport property of concrete under the condition of drying-wetting cycle also needs to be explored.At present,many experimental and simulation results fit the empirical equation of chloride ion transport coefficient in concrete,but this macroscopic one-dimensional mathematical model is not enough to distinguish the influence of volume fraction and shape of aggregates on the chloride ion transport property of concrete.In this paper,the finite element software Comsol Multiphysics was used to build a three-dimensional mesoscopic concrete model with random distribution of irregular shape aggregate.And the influence of coarse aggregate content,shape and interfacial transition zone(ITZ)in concrete was discussed and studied.Then,on the basis of three-dimensional mesoscopic aggregate concrete model with irregular shapes,the water and chloride ion transport equations in concrete were established respectively under the condition of drying-wetting cycle,and the effects of simulation dimension and drying-wetting time ratio of the model were explored.Finally,a three-dimensional random distribution of aggregate-like concrete model was used to simulate the transport process of water and chloride ions in concrete mixed with supplementary cementitious materials(SCMs)under the condition of drying-wetting cycle.The influence of the number of drying-wettingting cycles,water-cement ratio and SCMs on the transport property of chloride ions in concrete mixed with SCMs were discussed.The main results of this study are as follows:(1)The more irregular the aggregate is,the more obvious the tortuosity effect is,and the lower the chloride ion transport coefficient of concrete is.The aggregate shape also has an obvious effect on the chloride ion transport in concrete,and the aggregate shape also has an effect on the chloride ion transport streamline distribution.When chloride ions pass through narrow paths,the distribution of transport streamlines becomes dense,thus impeding the transport of chloride ions.The chloride ion concentration contours of the aggregate with smooth surface are sparse,while the chloride ion concentration contours of the aggregate with convex edges and corners are closely distributed.(2)As for unsaturated concrete,when the drying-wetting time ratio is less than or equal to 1,the value of chloride ion transport coefficient presents a certain rule.However,when the drying-wetting time ratio is greater than 1,the data of chloride ion transport coefficient show no regularity.On the other hand,for the model simulation dimension,the three dimensions of the model can be used to simulate the transport property of chloride ion in concrete.The results of the one-dimensional analytical solution model and the two-dimensional finite element model are somewhat discrete,and the convective zone of the two-dimensional finite element model is in between0.5 mm and 2.5 mm,where more chlorine ions gathered.While the three-dimensional finite element model restored the mesoscopic structure inside concrete and distinguish multiple phases of concrete.At the same time,the simulation results change more gently with the diffusion depth.(3)For SCMs-blended concrete,with the increase of drying-wetting cycles,concrete chloride ion transport property is improved.The higher the number of drying-wetting cycles at each transport depth,the higher the content of chloride ions will basically be.At the same time,the range of chloride ion convection zone basically showed a lengthened trend,and the transport depth corresponding to the peak of chloride ion content gradually deepened.For ordinary concrete,this rule is more obvious,and the concrete blended with slag and fly ash also shows a certain trend,while the concrete blended with single slag or single fly ash does not show obvious hysteresis of chloride ion content peak.On the other hand,with the increase of water-cement ratio,the transport depth corresponding to the peak value of chloride ion content gradually deepens,and the chloride ion content in the depth shows an increasing trend.With the increase of water-cement ratio,the apparent chloride ion transport coefficient of concrete increases under each wetting and drying cycles.The addition of SCMs significantly reduces the content of free chloride ions in the deep layer of concrete,but there is a tendency of accumulation of free chloride ions in the surface layer.