Chloride Condensation And Its Mechanism In Surface Layer Of Cementitious-based Materials Under Cyclic Wetting-drying Condition

Steel corrosion induced by chloride penetration is the major cause for the deterioration of concrete structure performances and the shrinkage of service life of concrete structures.Chloride penetration in cyclic wetting-drying condition,among all other conditions,is the most sever one.Thus,it is of great significance for concrete durability investigations to explore the chloride transport law and distribution features under cyclic wetting-drying condition.Due to the fact that multiple mechanisms,including diffusion,capillary absorption,moisture evaporation,and carbonation,influences chloride transport in surface layer of concrete under wetting-drying cycles,chloride content in that condition does not always decrease monotonously with distance from the exposed surface increasing.Mass researchers have discovered a chloride condensation phenomenon in the surface layer of concrete under cyclic wetting-drying condition,that is,chloride content first increases,reaches the maximum content at a certain depth in the surface layer,and then decreases to zero with distance from the exposed surface increasing.There are two characteristic parameters for chloride condensation phenomenon:peak chloride content or the maximum chloride content(Cmax)and the depth(?x)at which Cmax appears.In chloride profile with condensation phenomenon appearing,the chloride content increasing range does not obey Fick's second law.Consequently,the occurring of condensation phenomenon severely compromises the accuracy of the traditional way of predicting service life of concrete structures by fitting with diffusion equation.In such case,some service life prediction methods were proposed domestic and overseas,which all highlights the importance of Cmax and ?x.However,on the one hand,theevaluation law of chloride condensation phenomenon under different conditions is still unclear,that is,the change law of Cmax and ?x is undefined;on the other hand,it is necessary to find out the forming mechanism and process of condensation phenomenon if the evaluation law is wanted.Yet,there are still disputes about the fundamental forming mechanism of condensation phenomenon at present.Therefore,this study investigated the evaluation law,the fundamental forming mechanism,and the forming process of condensation phenomenon combining macro experiments and micro analysis.The detailed investigation content and results are as follows:1)Given that chloride condensation phenomenon occurs in the surface layer,the accuracy of chloride distribution in surface is highly demanded.Therefore,this study designed and employed a high-precision and automatic grinding machine which can not only grind powder layer by layer automatically,but also control the error to less than 0.02mm when the grinding thickness is less than 0.5mm.This grinding method highly improves the accuracy chloride distribution along with depth and guarantees the reliability of this study.2)The chloride distribution results measured by silver nitrate titration,ion chromatography,and electron probe micro analysis show that chloride condensation phenomenon occurs in surface layer of all specimens under four different wetting-drying regimes,no matter in pastes or mortars,what water to bind ratio(W/B),whether in total chloride or free chloride distribution,with or without slag and fly ash.It proves the existence of chloride condensation phenomenon under cyclic wetting-drying condition.3)The influence of pore structure and moisture distribution on chloride condensation phenomenon was investigated.The results show that:the increase of total porosity expands spaces for salt solution and crystal deposition and thus promotes Cmax;the enlarging of critical pore size makes it easier for salt solution to enter deep position of matrix and deepens ?x;larger water adsorption porosity not only increases capacity for salt solution but also optimizes the connectivity of matrix,elevating both Cmax and ?x;the greater the non-saturation degree the more the salt solution will be adsorbed into deeper places within the same time,increasing Cmax and ?x;the decrease of internal humidity increases both Cmax and ?x,and the moisture distribution nearer the exposed surface is more closely related to Cmax and ?x.4)The effect of carbonation under three different situations(pre-carbonating and then soaking in salt solution,soaking in salt solution and then carbonating,inner-introduced with chloride and then carbonating)on chloride binding capacity of pastes was explored,which provides experiment basis for probing the forming mechanism of chloride condensation phenomenon.The results show that after complete carbonation,no matter first carbonating then reacting with chloride or otherwise,the content of bound chloride in paste all approaches zero,indicating that carbonation can deprive the chloride binding capacity of paste completely.Besides,a method of calculating the ratio of C-S-H gel adsorbed chloride to total bound chloride was discovered.The proportion of C-S-H gel adsorbed chloride to total bound chloride is 54.56%after reaching chloride binding equilibrium in paste specimens without mineral admixture,suggesting that C-S-H gel adsorbed chloride is the principle form of bound chloride.5)The fundamental forming mechanism of chloride condensation phenomenon under cyclic wetting-drying condition was investigated based on experimental results of different exposure regimes and through parallel and vertical comparison and individualizing every influencing factor.The results show that,after completely excluding the effect of carbonation on chloride transport,mere capillary suction/moisture evaporation under cyclic wetting-drying can result in weak chloride condensation phenomenon,which is closely related to moisture evaporation during drying.In cyclic wetting-drying process,every wetting significantly increase the surface water content,and every drying condenses pore solution and drives inward chloride migration.In such cycles,the actual chloride content in surface decreases while inner chloride content increases,and this contributes to the formation of chloride peak in profiles.The coupling of capillary suction/moisture evaporation and carbonation facilitates the formation of significant condensation phenomenon,and it is particularly attributed to the fact that carbonation can release bound chloride especially those in Friedel's salt.At the same time with moisture evaporation decreases surface chloride content,carbonation released bound chloride ions enter into pore solution.It,on the one hand,promotes free chloride content in pore solution and conduces to the formation of significant chloride content peak,and on the other hand those released bound chloride will be constantly brought into deeper position and accumulate there driven by capillary suction,which leads to the significant inward migration of chloride condensation phenomenon.The experiment results show that the depth of condensation phenomenon increases with carbonation degree,and the significance degree of chloride content peak highly coincides with the content of decomposed Friedel's salt by carbonation.6)The numerical model of chloride transport under cyclic wetting-drying was established considering the coupled effect of moisture hysteresis and carbonation on chloride migration.The calculated results of numerical model coincides well with the experimental results of different conditions,and especially,the position of chloride condensation phenomenon of them is highly identical,which solves one major problem in chloride distribution prediction under wet-dry condition.