Research And Application Of Influence Of Surface Carbonation On The Chloride Diffusion In Marine Environment Concrete

Concrete structure of long service life in marine environments usually are subject to both the surface carbonation and ingress of chloride ions.In the past,these two processes were considered separately,ignoring the interaction of the carbonation and chloride ingress.With the increase of the service life of engineering,the interaction of carbonation and chloride ingress becomes more and more important.In this paper,the effect of concrete surface carbonation on chloride ion migration was studied theoretically and experimentally.Taking the composite beam of Shenzhong sea link project as an example,the influence of surface carbonation on the design values of concrete cover thickness to steel bars is calculated.In this study,concretes with different water/binder ratios(0.5 and 0.6),and different admixtures(OPC+8% fly ash+14% slag and OPC)were prepared.Cement paste specimens with OPC binders were prepared in parallel.The concretes and cement paste specimens were divided into carbonation and non-carbonation groups.After accelerated carbonation or naturally placed for 3 months,the specimens were soaked in sodium chloride solution for 35 days.The total chloride ion content,free chloride ion content and pH value were measured from the specimens afterwards.The density and porosity of the concrete were measured by the specific gravity method.The Fick's model and an extended diffusion model were selected to regress the chloride diffusion coefficients of the carbonated and uncarbonated specimens respectively.The thermogravimetric test and the mercury intrusion porosimetry(MIP)were performed on the OPC cement paste samples.Based on the experimental data,the effect of carbonation on the diffusion coefficient of chlorides was analyzed from three aspects: change of pore structure,change of ion environment of pore solution,and effect of carbonation on adsorption of chloride ions.Finally,the influence factors that affecting chloride diffusivity due to carbonation were quantified.Using the extended diffusion model,the design values of concrete cover thickness to steel bars in composite beams in Shenzhong sea link project were determined.The results show that carbonation significantly improves the pore structure of concrete and cement paste,which will reduce the diffusion coefficient of chloride ions.The decrease of ionic strength and electrical potential of the pore solution after carbonation will accelerate the chloride diffusion.After the carbonation,the content of calcium hydroxide and calcium silicate hydrates in concrete drops significantly,resulting in a decrease in the adsorption capacity of chlorides.Overall,the carbonation increases the diffusion coefficient of chloride ions and accelerates the ingress of chloride ions into concrete.The total chloride and the free chloride contents after carbonation significantly increase in concrete,thereby increasing the risk of steel corrosion.For concrete structure of long service life in marine atmospheric environments,the effect of concrete surface carbonation on chloride ingress should be considered.