Characterization Of The Diffusion And Distribution Behaviors Of Chloride Ions In Concrete And Their Influencing Factors

The corrosion of steel in concrete is primarily due to the ingress of the chloride ions.Therefore,studies on the diffusion and distribution of chlorides can facilitate the evaluation of the usage conditions of reinforced concrete structures,and provide a theoretical basis for the design of the concrete structure with excellent durability in a chloride salt environment.The natural immersion and electric field were adopted to realize the diffusion of chloride ions into concrete,this thesis also carefully investigates the influence of different diffusion modes on the diffusion behavior and distribution of chloride ions in concrete.Besides,the relationship between different forms of chloride ions along with the influence of different conditions(water-cement ratio,mineral mixture and dosage,age,carbonization)on the diffusion and distribution of chloride ions present in concrete was studied.It is followed by evaluating the effects of pore-structure and the characteristic of interfacial transition zone(ITZ)on chloride diffusion,respectively based on the Rosin-Rammler-Bennet(RRB)distribution principle of powder particles and the artificial ITZ formed around the frustum of a square pyramid aggregate.This indeed provides a theoretical basis for the mix design,production,construction and maintenance of highly durable concrete.Based on the work conducted in this thesis,a few specific conclusions can be drawn as follows:(1)In comparison to the natural immersion condition,electric field was adopted as well to examine its effect on the diffusion and distribution behaviors of chloride ions in concrete systems made with various mineral admixtures.The results show that the electric field could hardly change the distribution of chloride,especially,the correlation among the total,free and bound chloride ions,whereas it was able to influence the diffusion rate and concentration.In addition,the total chloride is the most important factor that directly controls the free and bound chloride ions,whereas other factors such as the water-binder ratio,amount and type of mineral admixtures,and curing age typically affect the total penetrating chloride ions through changing the pore-structure of concrete,which in turn influenced the free and bound chloride ions.Besides,as per the principles of chemical reaction equilibrium and the linear isothermal absorption,the relationship between different forms of chloride ions present in concrete was established,a bunch of models are established to respectively describe the relationship between a single branch of chloride ions(free chloride,bound chloride,physical adsorption chloride,chemical bonding chloride and harmful chloride)and the total chloride ions.The physical absorption and chemical bonding chloride ratio of bound chloride can be calculated,it is 29% and 71% respectively.Given that,taking into account the total chloride content,the concentration of those branches of chloride ions can be calculated respectively.This therefore provides a convenient way for determining their contents in concrete.(2)The chloride concentration in carbonated concretes was measured after rapid chloride permeability test,so as to investigate the influence of carbonation degree on chloride ions diffusion and distribution.It is found that the carbonization,at early stage,contributed to the densification of pores inside concrete and increased the binding ability of chloride ions,therefore improving the chloride resistance and decreasing the chloride concentration.However,when it comes to the late stage,the carbonization led to the coarsening of pores along with the reduced binding ability of chloride ions.Which thus weakened the chloride resistance instead,and increased the chloride concentration.Three zones were identified in the late but not completely carbonated concrete: the completely carbonated zone,the early carbonated zone,and the uncarbonated zone.As a result,an interface with a distinct pore structure was formed between the completely carbonated zone and the early carbonated zone.The pore-structure on the side of the early carbonated zone was denser than that of the completely carbonated zone,hindering the diffusion of chloride ions at the interface.Furthermore,due to the capillary absorption and the enhanced chloride-binding ability led by carbonization,a peak in the chloride concentration profile appeared on the side of the early carbonated zone at the interface.(3)The influence of pore-structure on diffusion behavior of chloride ions was investigated through controlling the water-to-binder ratio,the employed mineral admixture and curing duration.The main results illustrate that the mineral admixture with a higher reactivity led to a smaller pore size,which in turn helped improve the chloride resistance of concrete.The improved RRB model is applicable to describe the practical distribution of pore size inside concrete with different mix ratios.The pore structure parameter model,established based on the porosity,outputted parameters from the improved RRB model and the fractal dimensions of the pore surface associated with the pore axis,has a good exponential relationship with the chloride ion diffusion coefficient.This reveals the quantitative correlation between the considered pore-structure parameters and the chloride diffusion coefficient.(4)The varying ITZ characteristics associated with the pore-structure were essentially achieved through introducing the mineral admixture with different reactivity in order to control the hydration degree of binder materials.Besides,the frustum of a square pyramid aggregates were introduced to artificially create the requested ITZ,yielding a diffusion channel to accelerate the chloride diffusion.This study quantitatively investigates the characteristic of ITZ as well as the chloride diffusion behavior in varying concrete systems,and the main results show that the characteristics of ITZ were strongly dependent on the internal pore-structure.Introducing the mineral admixture with varying reactivity into the ordinary concrete system was noted to improve the reactivity,and subsequently enhanced the hydration degree,leading to a denser pore-structure as well as reducing the portlandite crystal orientation index and thickness of ITZ.This also made the pore-structure and chloride diffusion coefficient at ITZ close to that seen in the interior of mortar;The thickness of ITZ ranges from 20 to 42 μm,the chloride ions diffusing through ITZ basically registers a thirtyfold to seventyfold quick rate as compared to the mortar matrix,and this means the presence of ITZ essentially provides a green way for chloride ions to diffusion in concrete.However,it should be emphasized that the volumetric proportion taken up by ITZ is usually far fewer than the mortar matrix.Thus,the latter still dominates the overall diffusion rate of chloride ions inside concrete system.Given that,refining the micro-structure of mortar matrix should be the most effective way to improve the chloride-resistance of concrete.