Meso-scale numerical investigation on cracking of cover concrete induced by corrosion of reinforcing steel

Concrete cover cracking induced by corrosion of steel reinforcement is a major influencing factor for durability and serviceability of reinforced concrete structures. Here in this study, the influence of concrete meso-structure on the failure pattern of concrete cover is accounted for. The concrete is assumed to be a three-phase composite composed of aggregate, mortar matrix and the interfacial transition zone (ITZ). And a concrete random aggregate structure is established for the study on the mechanical behavior of radial corrosion expansion. In the present simulations, the plasticity damaged model is used to describe the mechanical behavior of the mortar matrix and the ITZ, and it is assumed that the corrosion of steel reinforcement is uniform. The cracking of concrete cover due to steel reinforcement corrosion is numerically simulated. The simulation results have a good agreement with the available test data and they are between the two analytical results. The failure patterns obtained from the macro-scale homogeneous model and the meso-scale heterogeneous model are compared. Furthermore, the influences of ratio of cover thickness and reinforcement diameter (i.e. c/d), the location of the steel reinforcement (i.e., placed at the middle and corner zones) and the concrete tensile strength on the steel corrosion rate when the concrete cover cracks are investigated. Finally, some useful conclusions are drawn.     

A model for prediction of time from corrosion initiation to corrosion cracking

Prediction of time to corrosion cracking is a key element in evaluating the service life of corroded reinforced concrete (RC) structures. This paper presents a mathematical model that predicts the time from corrosion initiation to corrosion cracking. In the present model a relationship between the steel mass loss and the internal radial pressure caused by the expansion of corrosion products is developed. The concrete around a corroding steel reinforcing bar is modeled as a thick-walled cylinder with a wall thickness equal to the thinnest concrete cover. The concrete ring is assumed to crack when the tensile stresses in the circumferential direction at every part of the ring have reached the tensile strength of concrete. The internal radial pressure at cracking is then determined and related to the steel mass loss. Faraday’s law is then utilized to predict the time from corrosion initiation to corrosion cracking. The model accounts for the time required for corrosion products to fill a porous zone before they start inducing expansive pressure on the concrete surrounding the steel reinforcing bar. The accuracy of the model is demonstrated by comparing the model’s predictions with experimental data published in the literature.     

Towards incorporating the influence of cover cracking on steel corrosion in RC design codes: the concept of performance-based crack width limits

This paper advocates for the adoption of performance-based limiting crack widths with respect to steel corrosion in reinforced concrete structures. The authors argue that, from both durability and sustainability viewpoints, the practice of adopting a universal limiting crack width for a wide range of in-service exposure conditions and concrete cover conditions and quality is not valid. As new performance-based concrete design codes are being developed and/or improved, the influence of cover cracking on steel corrosion needs to be incorporated in these codes. An experimental set-up was designed to investigate the influence of cover cracking, cover depth and concrete quality on chloride-induced corrosion. Beam specimens (120?×?130?×?375?mm) were cast using five concretes made using two w/b ratios (0.40 and 0.55) and three binders (100?% CEM I 42.5?N (PC), 50/50 PC/GGBS and 70/30 PC/FA). Other variables in the experiments included cover depth (20 and 40?mm), crack width (0, 0.4 and 0.7?mm). A total of 105 beam specimens were cast and exposed to cyclic 3-days wetting (with 5?% NaCl solution) and 4-days air-drying in the laboratory (23?°C, 50?% relative humidity). Corrosion rate was monitored bi-weekly in the specimens. The results relevant to this paper are presented and discussed. For a given concrete quality and cover depth, corrosion rate increased with increasing crack width. If crack width and cover depth are kept constant, corrosion rate increases with decreasing concrete quality, and vice versa. A model framework that can be used to objectively select cover depth, concrete quality and crack width is proposed. Such a model can be improved into, for example, a nomograph and used in the design process for RC structures prone to corrosion. Performance-based crack width limits should be adopted in the design of RC structures prone to steel corrosion. These crack width limits should be dependent on a complex interaction of, inter alia, concrete quality, cover depth, crack characteristics and prevailing exposure conditions. This study showed the inter-relationship between crack width, cover depth and concrete quality in affecting chloride-induced corrosion rate. Accurate corrosion rate prediction models incorporating the influence of cover cracking on corrosion are a pre-requisite to implementing the influence of cover cracking in future concrete design codes.     

Investigations on the influence of oxygen on corrosion of steel in concrete—Part I

The main objective of the research programme presented in the two parts of this article is determining the influence of oxygen diffusion through the concrete cover, on the corrosion process of the reinforcement in concrete structures. Altogether, 66 so-called “concrete corrosion cells”, i. e. reinforced concrete specimens with locally separated anodically and cathodically acting steel bars and different concrete compositions, have been used for the laboratory tests. Two testing methods have been used to investigate the influence of oxygen diffusion on the cathodic reaction under different environmental conditions: potentiostatic tests (presented in part I) and the determination of cathodic current density/potential curves (presented in part II). Using the results of these tests and a simplified equivalent electric circuit model for the corrosion process, it was possible to calculate the influence of oxygen diffusion on the corrosion rate of the reinforcement under defined conditions. The results of the laboratory tests and calculations show that the diffusion of oxygen is a significant limiting factor for the corrosion rate only when the concrete around the reinforcement is water saturated and most of the oxygen within the concrete near the reinforcement surface has been consumed by the cathodic reaction of the corrosion process. As a consequence, the corrosion rate is influenced by oxygen diffusion only through the concrete cover in structures which are submerged or exposed to long-term or cyclic water application that causes water saturation of the concrete for periods of several weeks. In the case of common outdoor structures being exposed to rain and not submerged or constantly water saturated due to other reasons, no reduction of the corrosion rate induced by limited oxygen diffusion is therefore to be expected.     

Investigation on the cracking behavior of concrete cover induced by corner located rebar corrosion

Non-uniform corrosion of reinforcement causes concrete cracking in chloride contaminated RC structures. Due to the special boundary conditions, the concrete cover with corner located rebar is often subjected to the attack of chloride ions in a marine environment from two directions, and thus the corresponding non-uniform corrosion distribution should be different from the one for side-located rebar. The aim of the work is to explore the effect of corner located rebar corrosion on the cracking of cover concrete. For corner located rebar, an improved non-uniform corrosion distribution model was established based on the analysis results of two-dimensional chloride diffusion in concrete. Considering the heterogeneities of concrete, a meso-scale mechanical model and method for the study on the failure behavior of concrete cover was built. In the analysis model and method, the non-uniform radial displacement distribution was adopted to simulate the corrosion expansion behavior of the rebar. The cracking of concrete cover with corner located rebar was simulated and studied. The present approach was verified by the available experimental observations. The influences of concrete heterogeneity, corrosion distribution types, rebar diameter and concrete cover thickness on the failure patterns of concrete cover and the expansive pressure were investigated. The simulation results indicate that the developed approach can well describe the cracking behavior of cover concrete and the corrosion-expansion behavior of steel rebar.     

Influence of crack width,cover depth and concrete quality on corrosion of steel in HPC containing corrosion inhibiting admixtures and fly ash

In this study the influence of crack width, cover depth and concrete quality on corrosion of steel in high performance concrete was investigated. Three mixtures, one control and two more containing corrosion inhibiting admixtures, Calcium Nitrite and Disodium Tetrapropenyl Succinate, in combination with 20% fly ash replacement with respect to the cement weight were prepared. Specimens were concrete cylinders measuring 100 mm in diameter and 65 and 105 mm in length, with a 16-mm steel bar centrally placed at two concrete covers of 25 and 45 mm. Before being exposed to a simulated marine environment, the specimens were pre-cracked under a controlled splitting test with crack widths ranging from 90 to 330 μm formed perpendicularly to the reinforcing bars. During a 16-month exposure, the corrosion risk of the reinforcing bars was evaluated by half-cell corrosion potentials and the corrosion rate by linear polarization method. Also, the total integrated corrosion current was estimated. Results show that, albeit to different degrees, cracking was found to be an influencing factor in promoting corrosion of the steel in concrete with either 25 mm or 45 mm concrete cover; nevertheless, the effectiveness of the concrete cover depended greatly on the crack thickness. Results also revealed that corrosion inhibitors and fly ash were effective in delaying corrosion even in cracked concrete.     

Influence of corrosion rate on the mechanical interaction of reinforcing steel,oxide and concrete

This study offers an experimental and numerical analysis of the influence of the current density on oxide formation and the resulting cracking of reinforced concrete subjected to accelerated corrosion. The specimens were idealized reinforced concrete prisms in which a calibrated steel tube replaced the standard ribbed bar reported in most published works. This allowed the evolution of the inner diameter and volume of the tube to be recorded, providing relevant information on the mechanical interactions of the steel–oxide–concrete system. In addition, the information recorded during the tests also included the evolution of the corrosion depth and width of the main crack that grew across the concrete cover. Furthermore, the crack pattern after the corrosion process was analyzed by using slices of the specimens impregnated with fluorescent resin. Experiments were conducted for several current densities. The results show that decreasing the corrosion current density results in an increase in the corrosion depth necessary for crack initiation, a smaller effective volumetric expansion and a more irregular crack pattern. When combined with finite-element simulations carried out by using a model that reproduces the expansive behavior of the oxide and the cohesive fracture of concrete, the results show that the effective expansion factor of the oxide becomes smaller as the density of current is reduced. This would suggest that the corrosion rate affects both the pressure build-up in the growing oxide layer and, closely linked to this, the diffusion of oxide within the pores and cracks in the surrounding concrete.     

Investigations on the influence of oxygen on corrosion of steel in concrete—Part 2

In Part I of this article, potentiostatic tests on reinforced concrete specimens under different environmental conditions have been described. The results of these tests have shown that it is possible to define four cases related to the humidity, in which different dominant parameters influence the corrosion rate for the reinforcement, discussed in detail in Part I. The main result is that the diffusion of oxygen is only a significant limiting factor for the corrosion rate when the concrete around the reinforcement is water-saturated and most of the oxygen within the concrete near the reinforcement has been consumed by the cathodic reaction of the corrosion process. As a consequence, the corrosion rate is only influenced by oxygen diffusion through the concrete cover in structures which are submerged or exposed to long term or cyclic water application, thereby causing water saturation of the concrete around the reinforcement for periods of several weeks. In Part II of this article, the determination of cathodic current-density/potential curves is described. Using the results of these tests and a simplified equivalent electric circuit model for the corrosion process, it is possible to calculate the influence of oxygen diffusion on the corrosion rate of the reinforcement quantitatively for defined conditions. One interesting consequence of this work is that the thickness of the concrete cover does not influence the corrosion rate by reducing the oxygen diffusion rate significantly when the concrete is not water-saturated. However, a sufficiently thick concrete cover is essential due to several other reasons (ingress of chlorides, carbonation, etc.).     

Insight into atmospheric corrosion evolution of mild steel in a simulated coastal atmosphere

The corrosion behavior of mild steel in a simulated coastal atmosphere environment has been investigated by the indoor accelerated wet/dry cyclic corrosion acceleration test(CCT),scanning electron microscopy(SEM),Raman spectroscopy and electrochemical measurements.During the CCT test of 60 cycles,the evolution of logarithmic(corrosion rate)vs.logarithmic(CCT cycles)presents a turning point at the 5th cycle,presenting a tendency to increase first and then decrease to gradually stabilize as the CCT cycle prolonged.Before the 5th cycle,γ-FeOOH and β-FeOOH and Fe3O4 were detected,respectively.And then,α-FeOOH as a new chemical composition was detected in the subsequent corrosion cycles.It is found that,after long term corrosion,the rust separated into a relatively dense inner layer rich with α-FeOOH and a loose outer layer rich with γ-FeOOH,both of which have poor electrical conductivity.The rapid increase of corrosion rate in the early stage since reducible corrosion products are involved in the reduction process of the cathode which promotes the dissolution of the anodic metal substrate.Afterward,as the rust layer thickens,the resistance of the rust increases,and the aggressive ions diffusion is blocked,gradually suppressing the electrochemical corrosion process.At last,when the composition and distribution of the rust layer remain stable,the corrosion presents a fluctuating speed around a certain value during the cracking and self-repairing process of the rust layer.     

纳米SiO2对钢纤维/混凝土高温后力学性能及微观结构的影响

研究了掺纳米SiO₂的钢纤维混凝土(NSFC)、 钢纤维混凝土(SFRC)和普通混凝土(NC)三种材料在不同加热温度后的抗压、 劈裂和抗折强度等力学性能, 对不同温度热处理后的微观结构进行了SEM分析, 对钢纤维与过渡区界面的相结构进行了XRD分析。结果表明: 在测试温度范围内, NSFC的抗压、 劈裂和抗折强度均高于SFRC和NC的强度, 且在400 ℃时达到最大值。在常温下, NSFC的抗压、 劈裂和抗折强度较NC分别提高27.01%、 63.28%和54.12%, 400 ℃高温热处理后比NC分别高35.09%、 84.62%和87.23%; SEM分析表明, 在钢纤维与过渡区的界面处, 致密度提高, 显微硬度提高。由于固相反应, 使界面区结构发生变化, 在钢纤维表层形成扩散渗透层(白亮层), 即化合物层, 呈锯齿状, XRD分析证明, 白亮层主要由FeSi₂和复杂的水化硅酸钙组成, 从而增强了钢纤维与基体的粘结力, 提高了混凝土的高温力学性能。     

Modeling of chloride-induced corrosion in reinforced concrete structures

A numerical procedure is presented in this paper for the prediction of chloride induced steel corrosion in reinforced concrete structures. Finite element analysis is introduced for the mechanical analysis of crack initiation and propagation due to the accumulation of corrosion products around the reinforcement, while the alternating direction implicit method is used to solve the transport equations of temperature, humidity, chloride ions and oxygen in concrete. Based on the assumption of a uniform distribution of corrosion products, a self-adaptation process for the variation of boundary conditions is proposed through a series of diffusion analyses together with crack propagation in concrete. Therefore, the interaction between the corrosion rate and the propagation of cracks in concrete is taken into account. Furthermore, a numerical program is developed and a case study involving bridge deck exposed to a marine environment in Hong Kong is investigated. The results show that interactive behavior has a significant effect on the corrosion rate of the reinforcement, and the non-cracking model significantly overestimates the service life of structures.     

Experimental verification and application of a practical corrosion model for uniformly depassivated steel in concrete

A practical corrosion model for predicting the corrosion rate of uniformly depassivated steel in concrete was previously developed. The model is built on Stern’s earlier work that an optimum anode-to-cathode ratio exists on the steel surface for which the corrosion current reaches a maximum value. Based on this principle, analytical and numerical techniques were used to obtain a relationship among the steel corrosion rate, concrete resistivity, the oxygen concentration on the surface of the member, oxygen diffusion coefficient, and cover thickness. The main objective of this paper is to present an extensive experimental verification and application of the developed model. For this purpose, three experimental programs carried out by other research groups were selected. The first set of data was used in the verification of the model when different types of cement are used in concrete. The second study was selected to verify the model with the measured corrosion rates obtained by different instruments to eliminate any concerns that may originate from the corrosion measurement technique. Finally, in the last verification study, the model’s interpretation of the effects of concrete resistivity and the oxygen concentration on the corrosion rate is verified. In all cases, it has been demonstrated that the developed model predicts the experimental observations with reasonable accuracy.     

Effect of chloride ion on corrosion behavior of low carbon steel in 0.1 M NaHCO3 solution with different dissolved oxygen concentrations

The corrosion behavior of low carbon steel in 0.1 M NaHCO_3+ 0.1 M NaCl solution with different dissolved oxygen concentrations was investigated with gravimetric tests and electrochemical measurements.Results show that the corrosion mass loss of steel is remarkably increased with the addition of chloride ion. In the initial stage, the carbon steel tends to active dissolution due to the dissolution effect of chloride ion on the oxide film; as a result, the corrosion potential maintains as low values. With the immersion time going by, the corrosion potential of the steel is promoted as a result of the accumulation of the corrosion products. However, the rust layer is loose and porous due to the deteriorating effect of chloride ion, which decreases the reduction resistance of oxygen. Meanwhile, the porous rust layer could be repaired by the depolarization of oxygen. Under the synergistic effect of chloride ion and oxygen, the corrosion of the steel is accelerated during the repeated process of dissolution and reparation of the oxide film.     

Modeling damage in concrete caused by corrosion of reinforcement: coupled 3D FE model

Reinforced concrete structures, which are exposed to aggressive environmental conditions, such as structures close to the sea or highway bridges and garages exposed to de-icing salts, often exhibit damage due to corrosion. Damage is usually manifested in the form of cracking and spalling of concrete cover caused by expansion of corrosion products around reinforcement. The reparation of corroded structure is related with relatively high direct and indirect costs. Therefore, it is of great importance to have a model, which is able to realistically predict influence of corrosion on the safety and durability of reinforced concrete structures. In the present contribution a 3D chemo-hygro-thermo-mechanical model for concrete is presented. In the model the interaction between non-mechanical influences (distribution of temperature, humidity, oxygen, chloride and rust) and mechanical properties of concrete (damage), is accounted for. The mechanical part of the model is based on the microplane model. It has recently been shown that the model is able to realistically describe the processes before and after depassivation of reinforcement and that it correctly accounts for the interaction between mechanical (damage) and non-mechanical processes in concrete. In the present paper application of the model is illustrated on two numerical examples. The first demonstrates the influence of expansion of corrosion products on damage of the beam specimen in cases with and without accounting for the transport of rust through cracks. It is shown that the transport of corrosion products through cracks can significantly influence the corrosion induced damage. In the second example the numerically predicted crack patterns due to corrosion of reinforcement in a beam are compared with experimental results. The influence of the anode?Ccathode regions on the corrosion induced damage is investigated. The comparison between numerical results and experimental evidence shows that the model is able to realistically predict experimentally observed crack pattern and that the position of anode and cathode strongly influences the crack pattern and corrosion rate.     

321不锈钢焊接波纹管失效分析

应用光学显微镜、扫描电镜和能谱仪等手段对321不锈钢焊接波纹管表面腐蚀物的化学成分、焊缝区域的显微组织、腐蚀裂纹的形态特征等方面进行检验和分析。结果表明，波纹管局部电镀端内壁腐蚀导致漏气失效，即加工应力、焊接残余应力与含Cl^-或含Cl^-和溶解氧的腐蚀性介质的联合作用使波纹管局部电镀端产生了点蚀和应力腐蚀开裂；退火工艺设计的不合理和钎焊工艺控制不恰当使焊缝热影响区敏化而出现了晶间腐蚀。据此提出了降低和避免321不锈钢焊接波纹管因腐蚀而漏气的措施。     

Relationship between concrete resistivity and corrosion rate – A literature review

The process of reinforcement corrosion in concrete is partially controlled by the transport of ions through the concrete microstructure. Ions are charged and the ability of a material to withstand transfer of charge is dependent upon the electrical resistivity. Thus, a connection could be expected between the corrosion process of steel embedded in concrete and the electrical resistivity of concrete. This paper reviews research concerning the relationship between corrosion rate and concrete resistivity. Overall, there exists an inverse proportional correlation between the parameters. However, the dependency varies between studies and one single relationship cannot be established between corrosion rate and resistivity. To address the variation, the article reviews and evaluates the influence of factors including the experimental setup, the concrete mix design and the cause of corrosion.     

Concrete cover cracking due to uniform reinforcement corrosion

Service life design (SLD) is an important tool for civil engineers to ensure that the structural integrity and functionality of the structure is not compromised within a given time frame, i.e. the service life. In SLD of reinforced concrete structures, reinforcement corrosion is of major concern and reinforcement de-passivation is a frequently used limit state. The present paper investigates an alternative limit state: corrosion-induced cover cracking. Results from numerical simulations of concrete cover cracking due to reinforcement corrosion are presented. The potential additional service life is calculated using literature data on corrosion rate and Faraday’s law. The parameters varied comprise reinforcement diameter, concrete cover thickness and concrete material properties, viz. concrete tensile strength and ductility (plain concrete and fibre reinforced concrete). Results obtained from the numerical simulations reveal that, depending on the serviceability limit state applied, the service life of a reinforced concrete structure can be increased significantly by allowing minor damage of the cover.     

Modelling steel corrosion in concrete structures

A comprehensive finite element model for predicting the rate of steel corrosion in concrete structures is developed. The model consists of initiation and propagation stages which are cast in the same time and space domains; i.e., processes which commence in the initiation stage, such as temperature, moisture, chloride ion, and oxygen transport within concrete, continue in the propagation stage while active corrosion occurs contemporaneously. This allows the model to include the effects of changes in exposure conditions during the propagation stage on corrosion and the effects of the corrosion reactions on the properties of concrete. The corrosion rates on steel surface are calculated by solving the Laplace's equation for electrochemical potential with appropriate boundary conditions. These boundary conditions include the relationship between overpotential and current density for the anodic and cathodic regions. Due to the non-linear nature of these boundary conditions, a non-linear solution algorithm is used. The developed model will enable designers to carry out comprehensive sensitivity analyses and to gauge the significance of variations in the values of certain parameters on the rate of corrosion in concrete structures.     

预应力钢绞线锈胀及混凝土开裂预测

针对钢绞线锈蚀导致混凝土开裂现象,开展了不同应力状态下混凝土的锈胀开裂试验,基于红外和热重分析研究了预应力钢绞线锈蚀产物的膨胀率,分析了预应力对保护层临界开裂时间和裂缝宽度的影响,综合考虑预应力、铁锈膨胀率和混凝土开裂损伤等因素,建立了开裂初始和发展全过程的锈胀裂缝预测模型,并通过试验结果进行了验证。结果表明:预应力会加速混凝土的锈胀开裂,在钢绞线抗拉强度75%的预应力水平下,保护层初始开裂时间降低了22%,裂缝扩展速率增加了9%;建立的模型具有较好的精度,可以合理地预测预应力混凝土的锈胀开裂。