Mechanism And Numerical Simulation On Non-Uniform Corrosion Of Steel Bar In Chloride Contaminated Concrete

Steel corrosion is a key factor that causes the structural performance deterioration and decreases the structural durability and service life.In actual engineering,compared with the uniform corrosion caused by carbonization,the non-uniform steel corrosion induced by chloride is more common and serious.Therefore,studying the non-uniform corrosion mechanism of steel bars is an important basis for guiding the corrosion detection,corrosion protection and prediction in engineering.At present,steel corrosion experimental research is still the most commonly used by researchers,although there exist several problems,such as long test cycle and difficult detection.Based on the results of corrosion mechanism,the corrosion prediction models(i.e.empirical model and electrochemical model)have been proposed and widely applied to studying on steel corrosion in concrete.Especially for electrochemical model,it reveals the corrosion mechanism that the steel corrosion is a complicated electrochemical reaction process,such as chemical reaction and polarization.In this paper,an electrochemical corrosion model was adopted that couples the process of mass transportation,definition of corrosion area and corrosion propagation.Then,it was achieved to describe the transverse and longitudinal corrosion process and revealed the micro-cell,macro-cell corrosion mechanism of steel bars in chloride contaminated concrete by establishing 2D and 3D models.The detailed contents are described as follows:1.The whole corrosion process included mass transportation,corrosion area determination and corrosion propagation.First,the methods for defining steel corrosion area(i.e.chloride ion concentration threshold and anode-cathode area ratio)were summarized and discussed.Second,the effect of electrochemical parameters,such as electrode equilibrium potential,Tafel slope,and exchange current density,on the accuracy of numerical model was analyzed.It can be found that the modification of anode Tafel slope is a feasible method that reflects the dynamic corrosion,so that a corrosion prediction model was constructed.In terms of corrosion rate,the effect can be ranked from large to small as anode exchange current density,logarithm of cathode exchange current density,anode Tafel slope and cathode equilibrium potential.2.A two-dimensional electrochemical model was introduced to study the transverse non-uniform corrosion process of steel bar in concrete,which was carried out with a corrosion experiment from previous research.The transverse corrosion process was investigated with respects to mass distribution,corrosion rate and especially corrosion morphology.The influence of several factors,such as concrete water-cement ratio,aggregate distribution,rebar diameter and erosion boundary,on the transverse corrosion pattern were analyzed.The study showed that an ellipse-like corrosion morphology across the entire circumference of the steel bar was appeared.Additionally,its corrosion non-uniform ratio will decrease with the water-cement ratio and increase with the rebar diameter.Moreover,the aggregate random distribution will affect the local corrosion pattern.3.The two-dimensional electrochemical model was used to study the longitudinal non-uniform corrosion process of steel bars in concrete.Then,the corrosion distribution along longitudinal steel surface under typical conditions such as concrete cracking and patch repair(Concrete cover replacement method)was discussed.Meanwhile,the parameter analysis was conducted to study the effects of crack width and crack spacing on corrosion degree and corrosion area profile under cracking concrete and the influence of the repair area length and the substrate concrete condition on the repair effect.The results showed that the corrosion appeared at cracking concrete and corrosion area became larger over time.Finally,a peak-type non-uniform corrosion pattern was performed.For patch repair,due to the electrochemical environment difference,the corrosion took place in the substrate concrete and near the repair interface.Simultaneously,the anode ring effect would appear near the interface and gradually weaken as the mass transportation.4.The differences between 2D and 3D models were illustrated,and the coupling effects of horizontal and transverse corrosion in the 3D model were emphatically explored.Then,the 3D model was applied to analyzing the corrosion process in concrete beam and concrete slab.Compared with the 2D model,the 3D model can better reflect the coupling effect of the transverse and longitudinal corrosion of the steel bar.However,the calculation efficiency of 3D model was significantly lower,so that the 2D mode,for simplification,can be widely under several certain conditions.In fact,the 3D model can better describe the steel corrosion in concrete beams and slabs under the chloride environment.