Modeling the cracking of cover concrete due to non-uniform corrosion of reinforcement

In situations when external chloride penetration is the cause of depassivation, the corrosion process may start from the outer region of a rebar, which might expand non-uniformly. Therefore, the main objective of the present work is to explore the effect of non-uniform corrosion on cracking behavior of cover concrete. The influences of concrete heterogeneities and the porous layer generated at the rebar/concrete interface on the failure patterns and the corrosion level of cover concrete are considered. The random aggregate structures of concrete are built, and the concrete is regarded as a composite composed of three phases, i.e. the aggregate, mortar matrix, and the interfacial transition zones (ITZs). The plasticity damaged model is employed to describe the mechanical properties of the mortar matrix and the ITZs, and it is assumed that the aggregate is elastic. Non-uniform radial displacement with a half ellipse shape is adopted to describe the expansion distribution of the corrosion products. The failure pattern and the corrosion pressure of cover concrete, and the critical corrosion level when the cover concrete cracks due to non-uniform corrosion expansion are studied based on the meso-scale numerical method. The comparison of the simulation results with the available test results on the failure pattern of cover concrete shows fairly good agreement. Moreover, the influence of meso-structural heterogeneities is explored, and the cracking behavior obtained under non-uniform and uniform expansion conditions are compared. Finally, the influences of cover thickness, rebar diameter and the location of rebar (namely side-located rebar and corner-located rebar), on the failure pattern and the corrosion level are examined.     

Distribution of millscale on corroded steel bars and penetration of steel corrosion products in concrete

This study investigated a reinforced concrete specimen that had deteriorated in an artificial environment for 2 years. The steel/concrete interface and corrosion-induced cracks were observed by SEM to investigate the millscale on the rebar surface and the distribution of rust. The millscale was not further oxidised before the surface cracking of the concrete cover. The penetration of corrosion products into concrete and the formation of a corrosion layer proceeded simultaneously. The rust did not fill the corrosion-induced cracks in concrete. Instead of the three-stage model, a two-stage model is proposed to describe the concrete cracking process induced by steel corrosion.     

A mathematical model for prediction of time from corrosion initiation to corrosion cracking in RC structures

Prediction of time from corrosion initiation to corrosion cracking is a crucial factor in evaluating concrete structures' durability and integrity deteriorated by reinforcement corrosion. This paper presents a mathematical model to predict the time from corrosion initiation to corrosion cracking. A dynamic corrosion rate model based on Bulter–Volmer kinetics and a rust expansion model based on Faraday's law are developed. An analytical model is developed to demonstrate the mechanical consequences of rust buildup around the rebar. The time to corrosion cracking is found to be a function of surrounding concrete material properties, chemical composition of rust, ratio of concrete cover thickness to rebar diameter, and corrosion current density. Comparisons with available experimental results are conducted to show the accuracy and reliability of the present model. Further parametric analysis is carried out to show the influences of various parameters included in the model on the time to corrosion cracking.     

Comparison of uniform and non-uniform corrosion induced damage in reinforced concrete based on a Gaussian description of the corrosion layer

A Gaussian function is proposed to describe the non-uniform spatial distribution of corrosion products as measured from chloride-exposed reinforced concrete specimens using backscattered electron imaging and image analysis. The model provides a good fit to the observed data and limited data sets reported in the literature. Subsequently, it is used as an input into a finite element analysis to provide a qualitative comparison of the damage induced on concrete from idealized uniform corrosion and more realistic non-uniform corrosion. The spatial distribution of corrosion is shown to have a major impact on the propagation phase, directly influencing residual service life.     

混凝土锈胀开裂寿命预测模型及其应用研究

采用圆孔扩张理论对混凝土钢筋锈胀过程进行分析,推导了不同钢筋锈胀量对应的塑性区边界混凝土应力及塑性区半径计算公式,在此基础上建立了混凝土锈胀开裂寿命准则及寿命预测模型。锈胀开裂寿命模型分析表明,预测寿命值与钢筋相对保护层厚度m和混凝土等级有直接关系。随着相对厚度m增加,锈胀开裂预测寿命值增加,并且前期增加较快,后期增加较缓慢。混凝土强度等级对预测寿命值影响显著,随混凝土强度增大,混凝土锈胀开裂寿命增大。工程应用表明,该模型预测寿命可为工程的中度维护提供参考。     

Composition and expansion coefficient of rust based on X-ray diffraction and thermal analysis

Corrosion production can make cracks in the concrete cover. Because their volumes are higher than that of the original metal. Therefore, the rust expansion coefficient is one of the key parameters in concrete cracking model caused by steel corrosion. X-ray diffraction and the thermal analysis were used to characterize eight corrosion products collected from various different environments. The results show that the compositions and the expansion coefficients for rust samples are different due to their different servicing environments. The rust expansion coefficients corresponding to the different environments were proposed, which can be applied to steel corrosion induced concrete cracking model.     

盐水条件下不同混凝土中钢筋的快速腐蚀行为的研究

 通过盐水条件下钢筋的快速腐蚀试验,研究了钢筋在不同水灰比、不同掺料合料掺量以及掺入RI—IC2型阻锈剂情况下钢筋的锈蚀行为.结果表明,水灰比是影响钢筋腐蚀的重要因素之一,相同条件下水灰比越大,钢筋锈蚀越严重;掺入粉煤灰和硅粉后,可有效降低钢筋的腐蚀率;RI—IC2型阻锈剂有良好的阻锈效果,相同掺量下,水灰比越大,其阻锈效果越明显.     

Effect of surface condition on the initial corrosion of galvanized reinforcing steel embedded in concrete

The present work was aimed at determining the effect of coating surface condition on the initial corrosion of hot-dip galvanized reinforcing steel bar (HDG rebar) in ordinary Portland cement (OPC) concrete. During zinc corrosion in OPC concrete, calcium hydroxyzincate (CHZ) formed on untreated HDG steel provided sufficient protection against corrosion. Therefore, it is concluded that treating HDG rebar with dilute chromic acid is unnecessary as a method of passivating zinc. A layer of zinc oxide and zinc carbonate formed, through weathering, on HDG bars increased the initial corrosion rate and passivation time compared with the non-weathered rebar exposed to concrete. HDG steel with an alloyed coating, i.e. containing only of Fe-Zn intermetallic phases, required a longer time to passivate than those with a pure zinc surface layer. The lower zinc content of the surface limited the rate of CHZ formation; hence, delayed passivation. Regardless of the surface condition, the coating depth loss after two days of embedment in ordinary Portland cement concrete was insignificant.     

Corrosion Evolution of Steel Reinforced Concrete Under Simulated Tidal and Immersion Zones of Marine Environment

The corrosion evolution processes of steel reinforced concrete under simulated tidal and immersion zones of marine environment were investigated by using electrochemical measurements and corrosion morphology observations.The results indicate that the corrosion of rebar in concrete under both environments experiences the deterioration from passivation to pitting corrosion and then to general corrosion.Specially,the pitting plays the major role only in the early stage of corrosion,and the general corrosion replaces the dominate role of pitting during the long-term corrosion.In addition,both the pitting depth on local surface and the rust thickness on the overall surface of rebar in the tidal condition are larger than those in immersion condition,which is attributed to the faster corrosion rate in tidal zone caused by the concentrated chloride ions and sufficient oxygen supply.     

Determination of corrosion rate of reinforcement with a modulated guard ring electrode; analysis of errors due to lateral current distribution

The main source of errors in measuring the corrosion rate of rebars on site is a non-uniform current distribution between the small counter electrode (CE) on the concrete surface and the large rebar network. Guard ring electrodes (GEs) are used in an attempt to confine the excitation current within a defined area. In order to better understand the functioning of modulated guard ring electrode and to assess its effectiveness in eliminating errors due to lateral spread of current signal from the small CE, measurements of the polarisation resistance performed on a concrete beam have been numerically simulated. Effect of parameters such as rebar corrosion activity, concrete resistivity, concrete cover depth and size of the corroding area on errors in the estimation of polarisation resistance of a single rebar has been examined. The results indicate that modulated GE arrangement fails to confine the lateral spread of the CE current within a constant area. Using the constant diameter of confinement for the calculation of corrosion rate may lead to serious errors when test conditions change. When high corrosion activity of rebar and/or local corrosion occur, the use of the modulated GE confinement may lead to significant underestimation of the corrosion rate.     

Corrosion mechanism of hot-dip galvanised reinforcement bar in cracked concrete

In this paper, probable corrosion mechanisms of hot-dip galvanised reinforcement bar in cracked concrete based on long-term durability tests and other studies are determined. The fist mechanism consists of a local dissolution of eta (η) and zeta (ζ) phase. In the second mechanism due to non-uniform zinc coating local dissolution of eta (η) and zeta (ζ) phase together with longitudinal and perpendicular cracking in zinc layer may lead to separation of zinc layer. In the third mechanism due to non-uniform zinc coating full dissolution of eta (η) and partial dissolution of zeta (ζ) phase together with longitudinal and perpendicular cracking in ferrite may lead to separation of zinc layer and ferrite. Corrosion rate with used cement type was decreasing. That is not dependent on concrete quality. However, concrete type has an effect on corrosion rate and moisture conditions.     

混凝土中钢筋锈蚀的电化学阻抗谱特征研究

关于混凝土钢筋锈蚀等效电路模型中各元件的物理意义理解不尽相同.为了对模型元件的物理意义给出合理解释,设计了由氯盐侵蚀和混凝土碳化导致钢筋锈蚀的两种加速试验.通过对钢筋锈蚀试块阻抗谱特征的分析研究,对模型元件的物理意义给出了合理解释.研究发现,氯盐锈蚀试块具有三段容抗弧,即三个时间常数;而碳化锈蚀试块阻抗谱与钝化钢筋试块...     

The effect of temperature on the corrosion of steel in concrete. Part 2: Model verification and parametric study

A comprehensive model for predicting the corrosion rate of steel in concrete has been developed using the concept of simulated polarization resistance experiments. This model is developed by carrying out a nonlinear regression analysis on data obtained from numerical experiments that are based on the solution of Laplace’s equation in a domain determined by the polarized length of the rebar. This part of the paper provides a comprehensive verification of the developed model and illustrates the application of the model to investigate the coupled effects of parameters affecting corrosion of steel in concrete. The results of the verification study show that the model predictions are in good agreement with the experimental data.     

Steel corrosion in concrete: Determinist modeling of cathodic reaction as a function of water saturation degree

The prediction of the long-term behavior of reinforced concrete structures involved in the nuclear waste storage requires the assessment and the modeling of the corrosion processes of steel reinforcement. This paper deals with the modeling of the cathodic reaction that is one of the main mechanisms of steel rebar corrosion. This model takes into account oxygen reduction and oxygen diffusion through a diffusion barrier (iron oxide and/or carbonated concrete) as a function of water saturation degree. It is demonstrated that corrosion rate of reinforcement embedded in concrete with water saturation degree as low as 0.9 could be under oxygen diffusion control. Thus, transport properties of concrete (aqueous and gaseous phase, dissolved species) are key parameters that must be taken into account to model electrochemical processes on the reinforcement.     

The influence of cracks on chloride-induced corrosion of steel in ordinary Portland cement and high performance concretes subjected to different loading conditions

Many reinforced concrete structures are subjected to variable loads, but little information is available on the effect of such loading, and the accompanying opening and closing of cracks, on the initiation and propagation of chloride-induced rebar corrosion. The purpose of this paper is to provide that information for reinforced ordinary Portland cement concrete and high performance concrete exposed to static or cyclic three-point bending and exposed to salt solution.It was observed that: (i) corrosion occurred only at intersections of the rebar with cracks in the concrete; (ii) high performance concrete was more protective than ordinary Portland cement concrete and (iii) the type of loading had less impact on corrosion than the type of concrete and exposure conditions.     

Influence of outer rust layers on corrosion of carbon steel and weathering steel during wet–dry cycles

The influence of the rust layers of carbon steel and weathering steel on the corrosion were investigated. It was found that corrosion of carbon steel slows down when its outer rust layer is removed. This phenomenon might be attributed to the shortening of the wetting time in wet–dry cycles when the outer rust layers are removed. What is more, growth time of the corrosion products is shortened as well, which results in the formation of the fine corrosion products. However, the behavior of corrosion of weathering steel is not obviously influenced by the outer rust layer and the wetting time.     

Application of harmonic analysis in measuring the corrosion rate of rebar in concrete

The corrosion rate (CR) of rebar embedded in cement mortar, concrete and cement extract is determined using harmonic analysis technique (HA). Simultaneously using other electrochemical techniques such as impedance spectroscopy (EIS) and Tafel extrapolation (TET), the CR was determined and compared with the weight loss method. CR obtained from HA is comparable to that of EIS provided that the Stern–Geary constant (B value) obtained from HA is used in the calculation. In concrete, comparable corrosion rates are obtained between TET and HA only under active condition of the rebar whereas under passive state, the corrosion current (i_corr) by TET is 10 times lower than that of HA. A good agreement is obtained between the HA and weight loss method. The outcome of the result suggests that HA is capable of providing a higher degree of accuracy than that of EIS and TET in the determination of i_corr in the medium like rebar in concrete having very low rate of corrosion.     

基于固结试验和接触理论分析的铁锈力学性能

选取自然锈蚀、电解锈蚀两种锈蚀产物颗粒作为研究对象,采用X射线衍射(XRD)、显微镜观测等方法研究两者物理性能上的差异。通过改进的固结试验方法,同时引入Hertz弹性接触理论,对不同粒径的不同锈蚀产物进行不同荷载作用下的模量分析计算,得到了不同荷载与铁锈模量之间的关系,表明铁锈不是线弹性物质,铁锈模量随荷载呈线性增加趋势。在试验结果基础上,还考虑尺寸效应影响,提出了自然锈蚀产物颗粒的内部损失表达式,并对自然锈蚀产物颗粒的内部损失进行定量分析,可描述理想无损自然锈蚀产物的力学性能。     

丙烯酸系乳胶用于钢筋混凝土的防腐蚀研究

用电化学阻抗谱研究了丙烯酸系乳胶作为混凝土添加剂或钢筋表面涂层时对钢筋腐蚀行为的影响,加速腐蚀试验结果表明混凝土中添加乳胶后能够延缓钢筋表面钝化层的破坏,而对混凝土的渗透性能影响不大,乳胶涂层能够显著减小钢筋腐蚀速率,涂层的存在改变了钢筋表面的腐蚀状态,在此基础上提出了改善钢筋混凝土抗蚀能力的措施。