Study Of The Deterioration Mechanism Of Chloride-Contained Reinforced Concrete Under The Combined Effects Of Carbonation And Chloride Ions

Sand is one of the basic components in concrete material.Due to increasing scare of natural reiver sand,to relieve the contradiction between the limited supply of reiver sand and fast development of modern cities,some regions need to explore the ways to use sea sand as the construction material.However,the chloride contained in sea sand could induce serious steel corrosion in concrete.How to scientifically determine the allowable chloride content limit in sea sand has become a complicated problem.Especially when considering the combined effects of carbonation and chloride transport,the sea sand concrete structures may face great uncertainty of steel corrosion.Based on the engineering investigation,experimental study,and numerical simulation,the deterioration mechanisms of chloride-contained reinforced concrete under carbonation environment are deeply analyzed in this paper.The main results are as follows:1.For the Portland cementitious materials,there are significant differences between natural and accelerated carbonation,including the crystal allotropes of calcium carbonate,the decalcification degree of calcium silicate hydrate（C-S-H）,and the morphology of carbonation product.Compared the results of accelerated carbonation with different CO₂ concentrations,the carbonation characteristics of cement paste are consistent when CO₂ concentration is from 3%to 20%.Considering the carbonation similarity and fast acceleration,the appropriate CO₂ concentration in accelerated carbonation can be selected as no more than 20%.2.The chloride ions introduced by sea sand can affect the hydration process of cement.When the chloride content is within 1%,it can accelerate the hydration rate of cement at early age and generate two chloride-contained hydration products,namely Kuzel’s or Friedel’s salt.Meanwhile,the containing of chloride ions can improve the carbonation resistance of cement pastes.The carbonation coefficient is decreased exponentially with the increase of chloride content.The effect mechanism of blended chlorides on cement hydration and carbonation resistance of cement pastes are explained by thermodynamic simulation.3.Most of bound chlorides have been released as the free chloride after carbonation.The decompositions of Friedel’s（or Kuzel’s）salt and C-S-H are the main reasons for the transformations of chemically and physically bound chlorides as the free chlorides,respectively.If combining together the influences of chlorides on carbonation resistance and the carbonation effect on releasing bound chlorides as free chlorides,lowering the pH to neutralize the concrete,and changing the pore structure characteristic,the combined effects of carbonation and contained-chloride on corrosion behavior of reinforcement steel in sea sand concrete can be notably found.4.The initially blended chloride ions in sea sand concrete are uniformly distributed after casting.However,the chlorides are redistributed after carbonation,which shows a concentration peak around the carbonation front at carbonated area.The redistribution of chlorides after carbonation is caused by the combined effects of carbonation and chloride transport.The carbonation process has changed the concentration of free chlorides and concrete porosity,which leads to a concentration gradient of free chlorides alongside the carbonation depth to trigger the transport of chlorides from carbonated area to the uncarbonated area.5.According to the results of steel corrosion,under the corrosion condition set in this study,it can be found that single accelerated carbonation（without containing chlorides）and chloride blending（without carbonation）will not cause the corrosion of steel reinforcements in cement paste or concrete.However,under the combined effects of carbonation and chlorides,the steel reinforcements show obvious corrosion result.This indicates that combined effects of carbonation and chlorides can significantly affect the corrosion process of reinforcement steel in sea sand concrete.6.Considering the influences of carbonation degree,moisture transport,pore structure characteristics,and chloride binding capacity during the carbonation process,the mathematical equations for the unsteady transport of chloride ions in chloridecontained concrete under carbonation effect were proposed.Based on the twodimensional concrete random aggregates model,the numerical simulation and verification of the proposed model were carried out by COMSOL finite element software.The numerical simulation results show that proposed model has good applicability,and analyses of the temporal chloride redistribution in chloride-contained concrete under carbonation are realized.7.By synthesizing the combined effects of carbonation and chloride transport,the deterioration mechanism of chloride-contained reinforced concrete in atmospheric environment is explained.By using the proposed mathematical model and defining the chloride threshold as[CL^-]/[OH^-],the initiation corrosion time of reinforcement in sea sand concrete is predicted.In addition,the durability designing method for chloridecontained concrete under atmospheric carbonation and the service life evaluation method for existing suspected sea sand concrete structures are proposed based on the analyses above,which is then successfully applied to analyze the deterioration process of a practical engineering in Shenzhen.