Effect Of Temperature On Concrete Chloride Diffusion And Binding In Cold Regions

Reinforced concrete(RC)durability is a common concern.In marine environments and cold regions,where deicing salts are used in winter,chloride ions from deicing salts or from seawater can diffuse into RC structures and cause corrosion of their steel reinforcement,resulting in cracking and premature failure of the structure.This study aims to investigate chloride diffusion and binding in concrete under cold temperature,with the ultimate aim of accurately establishing the corrosion initiation period and end of service life of RC structures exposed to chloride diffusion in severly cold regions.Among the many factors that affect chloride concrete chloride diffusivity,temperature is one of the main factors.Research until now has mainly focused on chloride diffusion under above-zero temperatures,while only few studies have investigated it under sub-zero temperatures.In some cold regions,the temperature is normally below 0℃for several months every year,which leads to partial freezing of concrete,and the existence of ice will increase concrete tortuosity and reduce its porosity,which will affect its diffusivity.At the same time,according to Arrhenius law,the drop in temperature will also reduce diffusivity,so it is necessary to study the combined effects of low temperature and concrete ice content on its chloride diffusivity.Furthermore,due to the increase in chloride exposure duration,the cement will continue to hydrate,which affects the concrete microstructure via changes in its porosity and tortuosity.However,the effect of exposure duration on the chloride diffusion and binding in concrete subjected to sub-zero temperatures has not been studied until now.Since the corrosion onset time is a function of the free chloride concentration,it is necessary to investigate the concrete chloride binding capacity under sub-zero temperatures.In this thesis a combination of theoretical analyses and physical tests were performed to study the effect of sub-zero temperature on chloride diffusion and binding in concrete.(1)Due to supercooling and the porous nature of concrete,the pore solution is not expected to freeze at–4℃.It is hypothesized here that below the supercooling temperature,some of the pore water will freeze and pore ice will form,which will increase the pores tortuosity and reduce their connectivity,resulting in further reduction in diffusivity.To test this hypothesis,concrete samples were subjected to natural diffusion in a 10%Na Cl solution under 5℃,0℃,–5℃,or–15℃for 3 and 6 months.Potentiometric titration was performed to determine the total and free chloride concentrations through the thickness of each test specimen.It is discovered through theoretical analysis of the experimental results that Arrhenius law can capture the effect of temperature on chloride diffusion,but when the temperature falls below the supercooling temperature,the effect of pore ice must be also considered because the apparent diffusion coefficient,D_a,will further decrease due to the formation of pore ice.The extent of the reduction depends on the pore ice content,and a theoretical method is proposed for determining this reduction based on the exposure temperature and concrete microstructure.The diffusion coefficients of the frozen specimens computed by the proposed method differ a maximum of 15%from the corresponding experimental values.(2)The effects of sub-zero temperature,T,and exposure duration,t,on the chloride diffusion decay index,m,are experimentally studied.Results of the non-steady natural diffusion tests for up to 18 months show that m varies over a wide range.It varies linearly with temperature and inversely or hyperbolically with duration.Importantly,it drastically deviates from the commonly assumed value of 0.2 in the literature.(3)Air-entraining agents are commonly used in concrete structures in cold regions,but there is little information about the effects of air entrainment on chloride diffusivity in partially frozen concrete.To investigate this phenomenon,two kinds of ordinary Portland cement(OPC)concrete mixes(6%air-entrained and non-air entrained)were made and their chloride diffusivities under different cold temperatures were compared based on the gathered experimental data.6%air entrainment reduces the chloride diffusion coefficient by about 18%in unfrozen concrete and in concrete in which its evaporable water is almost totally frozen,but when the evaporable water is partially frozen,air entrainment seems to reduce chloride diffusivity by only 3%.After freezing,the effect of temperature on the diffusivity of concrete cannot be described by Arrhenius law alone.A theoretical method is proposed that can explain and capture this deviation,and its predictions closely agree with the corresponding experimental results.It is also shown that irrespective of the test temperature,the results of the current diffusion tests closely obey the Nernst-Einstein relation as they correlate well with the results of the electric conductivity/resistivity tests reported in the literature.(4)It is found that chloride binding in concretes subjected to natural diffusion under5℃,0℃,–5℃and–15℃obeys the Freundlich isotherm,regardless of whether the concrete is air-entrained or not.Furthermore,under all the test temperatures,the binding capacity of non-air entrained concrete is higher than that of air entrained concrete.During the first three months of exposure,chloride binding increases with decreasing temperature,but over longer exposure durations,no systematic relationship is observed.