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.     

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。     

Damage analysis and cracking model of reinforced concrete structures with rebar corrosion

The damage of concrete cover in reinforced concrete structures induced by reinforcing steel corrosion is investigated in this study. The damage process of the concrete cover can be divided into two distinct stages: the non-cracking stage and the partial cracking stage. An analytical model based on damage mechanics and elastic mechanics is developed to predict the concrete cracking due to steel corrosion. Based on this model, the expansive pressure and the radial loss of steel bar are discussed. Parametric studies are carried out to examine the effects of the correlative factors on the expansive pressure and the steel loss.     

Corrosion of Carbon Steel and Corrosion-Resistant Rebars in Concrete Structures Under Chloride Ion Attack

Corrosion of reinforced concrete is the most challenging durability problem that threatens reinforced concrete structures, especially structures that are subject to severe environmental conditions (i.e., highway bridges, marine structures, etc.). Corrosion of reinforcing steel leads to cracking and spalling of the concrete cover and billions of dollars are spent every year on repairing such damaged structures. New types of reinforcements have been developed to avoid these high-cost repairs. Thus, it is important to study the corrosion behavior of these new types of reinforcements and compare them to the traditional carbon steel reinforcements. This study aimed at characterizing the corrosion behavior of three competing reinforcing steels; conventional carbon steel, micro-composite steel (MMFX-2) and 316LN stainless steel, through experiments in carbonated and non-carbonated concrete exposed to chloride-laden environments. Synthetic pore water solutions have been used to simulate both cases of sound and carbonated concrete under chloride ions attack. A three-electrode corrosion cell is used for determining the corrosion characteristics and rates. Multiple electrochemical techniques were applied using a Gamry PC4? potentiostat manufactured by Gamry Instruments (Warminster, PA). DC corrosion measurements were applied on samples subjected to fixed chloride concentration in the solution.     

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.     

提高海洋环境钢筋混凝土构筑物耐久性措施综述

针对海洋环境钢筋混凝土构筑物过早腐蚀破坏,从混凝土防护、钢筋保护、包覆隔离保护、施工技术四个方面综述目前国内外对海洋环境下保护钢筋减少腐蚀、延长构筑物服役寿命所采取的技术措施,内容涉及:高性能混凝土、抗海水腐蚀水泥、阻锈剂、混凝土表面涂层、提高保护层厚度、钢筋涂层、耐腐筋、电化学保护、纤维玻璃钢或钢护筒包覆隔离、预防早期开裂、透水模板。重点讨论各措施的基本原理、特点、主要缺点和发展方向及工程应用。其中海工高性能混凝土是最经济、最基本和不可替代的,其他作为高性能混凝土基本措施的有效补充。     

Gerissener Stahlbeton: Wie korrosionsgefährdet ist die Bewehrung?

Cracked reinforced concrete: What about corrosion risk? The formation of cracks within the tension zone of the concrete is part of the design principle of reinforced concrete structures. The width of these cracks can be limited by means of crack limiting reinforcement. Already by the end of the fifties first examinations have been carried out to determine the influence of cracks and their width on the corrosion risk. Compared to uncracked concrete CO₂ and chlorides as corrosion promoting substances are able to move faster towards the reinforcement within a crack, leading to an earlier depassivation of the reinforcement within the cracks. Before Schießl published his results, from experiments and literature research, it had been assumed that the corrosion rate of reinforcement within cracks increases with increasing crack width. Hence a differentiated maximum crack width linked to exposure classes had to be complied according to the DIN 1045 until the version of 1988. For the first time the DIN 1045 version 1988 did not enclose this differentiation, for durability reasons a maximum crack width of 0.25 mm has been fixed for reinforced concrete. In the version of 2001 the calculated crack width has been raised from 0.25 mm to 0.30 mm. Therewith the findings of Schießl (published in book no. 370 by the Deutscher Ausschuss für Stahlbeton) have been implemented by now. The main contributing factors influencing the corrosion rate of cracked concrete are the concrete quality and the concrete cover and not the crack width, whereas horizontal structural parts exposed to chlorides have to protected generally by additional measures.     

A field study of critical chloride content in reinforced concrete with blended binder

This paper presents the results from the investigation of chloride‐induced reinforcement corrosion in concrete slabs after over 13 years exposure in the marine environment. In the beginning of 1990s over 40 reinforced concrete slabs with different types of binder and water/binder ratios were exposed in a marine environment at Swedish west coast. In this study a new rapid technique was used for non‐destructive measurement of corrosion. Based on the results from the non‐destructive measurement, the actual corrosion of steel bars in five concrete slabs was visually examined and the chloride profiles in the penetrating direction as well as at the cover level were measured. The results show that the visible corrosion normally occurred about 10–20 cm under the seawater level, where the oxygen may be sufficiently available for initiating the corrosion. It is also found that chloride may easily penetrate through a poor interface between concrete and mortar spacer and initiate an early corrosion. As a conclusion, although the chloride level 1% by mass of binder may not be the same as the conventionally defined threshold value, it can be taken as the critical level for significant on‐going corrosion that is visible by destructive visual examination, despite types of binder.     

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.     

Design for durability including initiation and propagation period based on the fuzzy set theory

Corrosion of reinforced concrete structures occurs in two stages characterized by different intensities of the corrosion development: the initiation period of the corrosion process and the propagation period of the corrosion process. Consequences of the reinforcement corrosion in concrete are the reduction of the cross‐sectional area of the reinforcement bars, and cracking of the concrete cover. The reduction of the reinforcement structural capacity due to the corrosion, over a sufficiently long period of time, may result in structural failure of the designed structural element. It is impossible to quantitatively express all corrosion factors influencing loss of the reinforcement structural capacity. Their influences on the structural design could be considered through a factor of safety applicable to the corrosion limit state. Factor of safety is calculated considering fuzzy sets theory and qualitative inputs of various influencing factors.     

Reinforcement corrosion-induced cover cracking and its time prediction for reinforced concrete structures

A relation between the radial pressure induced by expansion corrosion products and weight loss percentage of corroded steel was developed and simplified by investigating the influence of the mechanical properties of corrosion layer on cover cracking. By taking account of the penetration of the rusts into open cracks, a mathematical model that predicts the cover cracking time was proposed based on Faraday’s law. Some experimental observations were compared with the predictions made by the proposed model and two existing models, which indicates that the presented model is more suitable for predicting cracking time for both accelerated and long-term corrosion conditions.     

Steel corrosion and corrosion-induced cracking in recycled aggregate concrete

This study investigated four reinforced concrete mixtures with different replacement percentages of recycled coarse aggregate (RA). The corrosion rate was measured using an electrochemical workstation, and the corrosion-induced cracks on the concrete surface were observed using digital microscopy. The results show that the use of RA introduces more interfaces in concrete, which accelerates the steel corrosion process and corrosion-induced crack propagation in concrete cover. However, steel corrosion and the corrosion-induced cracking process in concrete are not significantly influenced by replacing a small amount (33% in the study) of coarse aggregate with RA.     

聚丙烯纤维与海工钢筋混凝土的耐久性

在海工钢筋混凝土中加入少量网状聚丙烯纤维能阻止混凝土的早期开裂，从而提高混凝土的抗渗性能，抑制液态介质腐蚀和钢筋锈蚀，提高混凝土结构的耐久性。     

Influence of chloride ions and hot weather on isolated rusting steel bar in concrete based on NDT and electro-chemical model evaluation

The most prominent durability concern for reinforced concrete (RC) structures is the corrosion of steel reinforcement in concrete. RC buildings exposed to chloride and high temperature environments like sea and deserts suffer from accelerated corrosion of rebars. The chloride attack and increase in the electro-chemical reaction rate of corrosion due to high temperature is a thermodynamic phenomenon influenced by several parameters and some of them are being neglected in the past research works. The purpose of this present paper is therefore, to model and verify by NDT the coupled effects of chloride and temperature on corrosion of reinforcement throughout the life of concrete buildings by incorporating realistic thermodynamic model evaluations and actual field condition NDT. The model evaluation has been accomplished by the use of concrete durability model as a computational platform on which the corrosion based reinforced concrete building performance and quality at early age and throughout the life of concrete structure is examined in both space and time domains under environmental actions of chloride and temperature. On this line, the thermodynamic modeling evaluation of concrete forms the fundamental core of the theoretical approach to achieve both the scientific knowledge and engineering simulations of altering materials. The NDT results for the effect of chloride and temperature on corrosion have been compared with the DuCOM electro-chemical thermodynamic corrosion model evaluation and are found to be in close agreement with each other.     

钢筋阻锈剂的阻锈机理及性能评价的研究进展

钢筋腐蚀是引起钢筋混凝土构筑物提前失效的主要因素,应用钢筋阻锈剂是主要的防腐蚀措施之一。重点介绍了海洋环境中,氯盐的侵蚀机理和阻锈剂的作用机理;讨论了阻锈剂的性能评价方法和应用现状;指出高效且环境友好复合型阻锈剂的合成是今后研究的主要方向,这对于提高我国钢筋混凝土结构耐久性具有重要意义。     

Concrete cover cracking owing to reinforcement corrosion – theoretical considerations and practical experience

Reinforcement corrosion might lead to cracking and spalling of the concrete cover owing to the volume expansion associated with the deposition of some of the possible corrosion products. This is not only aesthetically unpleasing, it might also accelerate deterioration processes or become a safety issue for passing traffic. The present paper discusses first the mechanisms of carbonation‐ and chloride‐induced reinforcement corrosion and considers the chemistry of aqueous iron in order to identify the type of corrosion products as well as their location of formation. Furthermore, practical examples are summarised in order to compare the documented behaviour of a number of real structures with the theoretical considerations made. It is shown that for the case of purely chloride‐induced (pitting) corrosion, precipitation of corrosion products is strongly delayed or may even not occur. Implications are discussed with respect to time‐to‐corrosion prediction models and visual inspection of reinforced concrete structures. Both the theoretical considerations and the practical experience illustrate that relying on outwardly visible signs to detect internally on‐going corrosion must be done with caution if localised reinforcement corrosion cannot be excluded.     

Electrochemical realkalisation for rehabilitation of reinforced concrete structures

In steel reinforced and prestressed concrete structures depassivation of the reinforcing steel can take place due to carbonation of the concrete cover. Depending on humidity and oxygen availability subsequent corrosion reactions will be initiated. Such conditions require measures to exclude corrosion induced damages during the designed lifetime of the structure. In the last few years an electrochemical realkalisation treatment has been proposed as adequate rehabilitation technique for carbonated concrete. This temporary treatment should increase the pH-value of the concrete pore water solution due to penetration of alkaline electrolyte from the concrete surface as well as repassivate the reinforcement due to electro chemical reactions at the steel surface. In order to clarify the different mechanisms taking place during electrochemical realkalisation laboratory tests have been carried out using carbonated reinforced mortar specimens. The investigations were aimed at checking the influence of various parameters, e.g. treatment time and current density, as well as the efficiency and long-term durability of this rehabilitation method.     

DFG‐research‐group: Modelling reinforcement corrosion – overview of the project

A strong interest for the durability of reinforced concrete structures currently exists in industry and research [1]. Against the background of immense costs for maintaining reinforced concrete structures and repairing damage caused by corroding reinforcement steel, this interest lead to a German joint research project. The aim of this network‐based (www.bam.de/dfg537.htm) research group is, to deliver the basic knowledge of the corrosion propagation and to make a probabilistic tool available for engineers so that a complete design for durability, concerning reinforcement corrosion, will be possible.     

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.     

Modified hydrotalcites as a new emerging class of smart additive of reinforced concrete for anticorrosion applications: A literature review

Modified hydrotalcites (MTHs) represent a group of technologically promising materials for addition to concrete to improve its durability in aggressive environment, owing to their low cost, relative simplicity of preparation, and plenty of unique composition variables that may be adopted. Up to date, a lot of academic work and commercial interest on MHTs have been invested, but relatively few studies focus on cementitious materials, particularly in exploiting their potential applications in corrosion protection of reinforced concrete structures. In this article, the mechanism of corrosion in reinforced concrete and concrete properties that affect corrosion of reinforcement are briefly introduced. In addition, the existing knowledge with regard to synthesis and characterisation methods of MHTs, ion exchange within the MHT structure as well as the application of MHTs in the cementitious materials were reviewed accordingly. As a new emerging class of smart additive of reinforced concrete, MHTs are expected to contribute to the effort of searching for effective measures to improve the durability of reinforced concrete.