Cover cracking of reinforced concrete due to rebar corrosion induced by chloride penetration

Cracking of concrete cover due to corrosion induced expansion of steel rebar is one of the major causes of the deterioration of reinforced concrete (RC) structures exposed to marine environments and de-icing salts.This paper presents two models that deal with the chloride-induced corrosion and subsequent cracking of concrete cover in RC structures. The former analyses the chloride diffusion within partially saturated concrete. A comprehensive model is developed through the governing equations of moisture, heat and chloride-ion flow. Nonlinearity of diffusion coefficients, chloride binding isotherms and convection phenomena are also highlighted. The latter describes the internal cracking around the bar due to expansive pressures as corrosion of the reinforcing bar progresses. Once a certain chloride concentration threshold is reached in the area surrounding the bar, oxidation of steel begins and oxide products are generated, which occupy much greater volume than the original steel consumed by corrosion. An embedded cohesive crack model is applied for cracking simulation.Both models are incorporated in the same finite element program. The models are chained, though not explicitly coupled, at first instance. Comparisons with experimental results are carried out, with reasonably good agreements being obtained. The work is a step forward for the integration of the two traditional phases (initiation and propagation) widely used in the literature and usually analysed separately. The estimation of the service life of the structure needs to evaluate the associated time for each one.     

Modeling bond strength of corroded reinforcement without stirrups

Deterioration of bond strength between concrete and reinforcement is of great importance in studying the strength of structural members with corroded reinforcements. A simple analytical model is proposed to demonstrate the effect of corrosion of reinforcing bar on reduction of bond strength. The corrosion pressure due to expansive action of corrosion products before and after corrosion cracking is firstly estimated. Then, reduction of bar confinement caused by cover cracking, change of friction coefficient between the steel and the concrete, and reduction of the friction force on the bearing face as well as deterioration of the ribs of the deformed bars due to steel corrosion are considered in calculating the mechanical interactions between reinforcing bar and concrete. As a result, the bond strength of corroded bars is calculated. The theoretical results are compared with the experimental results and agree with those results well.     

Modeling of concrete cracking due to corrosion process of reinforcement bars

The reinforcement corrosion in Reinforced Concrete (RC) is a major reason of degradation for structures and infrastructures throughout the world leading to their premature deterioration before design life was attained. The effects of corrosion of reinforcement are: (i) the reduction of the cross section of the bars, and (ii) the development of corrosion products leading to the appearance of cracks in the concrete cover and subsequent cover spalling. Due to their intrinsic complex nature, these issues require an interdisciplinary approach involving both material science and structural design knowledge also in terms on International and National codes that implemented the concept of durability and service life of structures.In this paper preliminary FEM analyses were performed in order to simulate pitting corrosion or general corrosion aimed to demonstrate the possibility to extend the results obtained for a cylindrical specimen, reinforced by a single bar, to more complex RC members in terms of geometry and reinforcement. Furthermore, a mechanical analytical model to evaluate the stresses in the concrete surrounding the reinforcement bars is proposed. In addition, a sophisticated model is presented to evaluate the non-linear development of stresses inside concrete and crack propagation when reinforcement bars start to corrode. The relationships between the cracking development (mechanical) and the reduction of the steel section (electrochemical) are provided. Finally, numerical findings reported in this paper were compared to experimental results available in the literature and satisfactory agreement was found.     

A strain-softening model for steel-concrete bond

A new analytical model is proposed to describe the bond strength between a ribbed reinforcement bar and concrete. The model is based on the partly cracked solution of a thick-walled cylinder exposed to radial internal pressure. Instead of considering the minimum crack number in previous solutions, smeared cracking and average stress-strain of concrete in tension are used in radial direction to describe the softening behavior of concrete. The results are compared with Tepfers' classic solutions and other previous solutions.     

Effects of non-uniform corrosion on the cracking and service life of reinforced concrete structures

The purpose of this study is to explore the effects of non-uniform corrosion on cracking behavior of concrete cover. The effects of non-uniform corrosion distribution, cover-to-rebar diameter ratio, and concrete compressive strength on the cracking pressure of concrete cover were studied. The present study indicates that the pressures to cause cracking of concrete cover under non-uniform corrosion conditions are much smaller than those under uniform corrosion case. The cracking pressure decreases up to about 60% depending upon the types of non-uniform corrosion distributions. It was also shown that cover-to-rebar diameter ratio and concrete compressive strength affect greatly the cracking pressure of concrete cover. Realistic equations on the cracking pressure of concrete cover were derived. The comparisons of analysis results with the test data on the cracking pressure of concrete cover show fairly good agreement. Finally, the effect of non-uniform corrosion on the service life of concrete structures was discussed.     

Corrosion products of carbonation induced corrosion in existing reinforced concrete facades

Active corrosion in reinforced concrete structures is controlled by environmental conditions and material properties. These factors determine the corrosion rate and type of corrosion products which govern the total achieved service life. The type and critical amount of corrosion products were studied by electron microscopy and X-ray diffractometry on concrete and reinforcement samples from existing concrete facades on visually damaged locations. The corrosion products in outdoor environment exposed concrete facades are mostly hydroxides (Feroxyhite, Goethite and Lepidocrocite) with a volume ratio to Fe of approximately 3. The results can be used to calibrate calculation of the critical corrosion penetration of concrete facade panels.     

Cover zone properties influencing acoustic emission due to corrosion

The deterioration of reinforced concrete is a serviceability problem worldwide. The cover zone plays an important role in the durability and serviceability of concrete and provides the initial barrier to aggressive species. The research presented investigates the potential of acoustic emission (AE) as a means of identifying corrosion at an early stage, before any significant cover damage has occurred. The purpose of part of the study was to identify the influential cover zone factors that affect the magnitude of the AE measurements per gram of steel loss. Prisms with various combinations of strength, cover thickness, aggregate and rebar diameters were studied to ascertain the important variables likely to be encountered on reinforced structures. The experimental results confirmed that early corrosion, verified by internal visual inspection and mass loss, can be detected by AE and before any external signs of cracking. They also show that the most influential parameter affecting the AE measurement is concrete strength, being exponentially related to the AE energy. Material properties, such as cover thickness, had a negligible effect on AE energy during the initial stages of reinforcement corrosion, whereas from this initial work, the rebar diameter indicated a promising relationship with AE energy per gram of steel loss.     

Predicting carbonation in early-aged cracked concrete

Carbonation in cracked concrete is considered as one of major deteriorations accelerating steel corrosion in reinforced concrete structures. For durable concrete structures, it is necessary to control crack in concrete through crack resistance evaluation for early-aged concrete structures, but often unavoidable cracks in early-aged concrete may occur. These cracks become a main path for CO₂ penetration inside concrete so that the carbonation is accelerated in cracked concrete.In this study, an analytical technique for carbonation prediction in early-aged cracked concrete was developed for considering both CO₂ diffusion of pore water in sound concrete and in cracked concrete. Then, characteristics of diffusivity on the carbonation in early-aged concrete are studied through finite element analysis implemented with the so-called multi-component hydration heat model and micro-pore structure formation model. The carbonation behaviour in sound concrete and cracked concrete are also simulated by using the derived diffusivity with consideration of reaction with dissolved CO₂. Finally, numerical results obtained for cracked concrete made with 3 different W / C ratios (45%, 55%, and 65%) with different crack widths were compared with experimental results.     

An experimental study on the bond strength between reinforcement bars and concrete as a function of concrete cover,strength and corrosion level

The effect of corrosion on the bond strength between reinforcement bars and concrete was studied in a series of experiments. An accelerated corrosion method was used to corrode the reinforcement bars embedded in concrete specimens. Pullout tests were performed to develop an empirical model for the ultimate bond strength by evaluating bond strengths in two different concrete mixes, three concrete cover depths and different mass losses of reinforcement bars after corrosion. Bond-slip relationships for the different corrosion levels were compared. It was found that the relationship between bond strength and concrete strength in uncorroded specimens differed from that of corroded specimens set in high-strength concrete because of brittleness in the corroded specimens, which caused a sudden loss of bond strength. The results revealed that specimens with higher concrete strength levels and corroded reinforcements showed a higher percentage of bond strength degradation due to concrete cracking during the pullout tests.     

掺氧化镁膨胀剂混凝土的抗硫酸盐侵蚀性能研究

由于环境侵蚀,混凝土结构易开裂破坏,掺入适宜的外加剂可减轻其损伤劣化。本文研究了不同氧化镁膨胀剂掺量对混凝土抗侵蚀性能的影响,测试了混凝土经MgSO₄浸泡后的质量损失、强度变化,结合微观结构变化,评价了掺氧化镁膨胀剂混凝土的抗硫酸盐侵蚀性能。结果表明:硫酸盐长期浸泡环境下,混凝土试件的质量和力学性能均先增加后降低,掺入氧化镁膨胀剂以及降低硫酸盐溶液浓度都能降低硫酸盐的侵蚀速率;氧化镁膨胀剂的掺入一方面能够细化混凝土内部孔结构,降低硫酸盐的侵蚀损伤速率,延缓试件损伤开裂形成微裂纹;另一方面氧化镁膨胀剂填充了开裂后的微裂纹,阻断或减小了硫酸盐继续传输通道,从而抑制裂纹继续扩展。     

Sensitivity analysis of simplified diffusion-based corrosion initiation model of concrete structures exposed to chlorides

This paper presents the results of a sensitivity analysis of the diffusion-based corrosion initiation model for reinforced concrete structures built in chloride-laden environments. Analytical differentiation techniques are used to determine the sensitivity of the time to corrosion initiation to the four governing parameters of the model, which include chloride diffusivity in concrete, chloride threshold level of steel reinforcement, concrete cover depth, and surface chloride concentration. For conventional carbon steel, the time to corrosion initiation is found to be most sensitive to concrete cover depth, followed by chloride diffusion coefficient, with normalized sensitivity coefficients of about 2 and − 1. For corrosion resistant steels, the time to corrosion initiation is most sensitive to the surface chloride concentration and chloride threshold level followed by the concrete cover depth and chloride diffusion coefficient. The results of this sensitivity analysis are discussed in detail, including the variations in predicted time to corrosion initiation induced by variations of the four model parameters and their implications for the design and maintenance of concrete structures built in corrosive environments.     

Reinforcement corrosion initiation and activation times in concrete structures exposed to severe marine environments

The corrosion of steel reinforcement bars in reinforced concrete structures exposed to severe marine environments usually is attributed to the aggressive nature of chloride ions. In some cases in practice corrosion has been observed to commence already within a few years of exposure even with considerable concrete cover to the reinforcement and apparently high quality concretes. However, there are a number of other cases in practice for which corrosion initiation took much longer, even in cases with quite modest concrete cover and modest concrete quality. Many of these structures show satisfactory long-term structural performance, despite having high levels of localized chloride concentrations at the reinforcement. This disparity was noted already more than 50 years ago, but appears still not fully explained. This paper presents a systematic overview of cases reported in the engineering and corrosion literature and considers possible reasons for these differences. Consistent with observations by others, the data show that concretes made from blast furnace cements have better corrosion durability properties. The data also strongly suggest that concretes made with limestone or non-reactive dolomite aggregates or sufficiently high levels of other forms of calcium carbonates have favourable reinforcement corrosion properties. Both corrosion initiation and the onset of significant damage are delayed. Some possible reasons for this are explored briefly.     

低热硅酸盐水泥水化及性能研究现状

低热硅酸盐水泥因水化热低而被大量应用于高等级大体积混凝土工程以降低温度应力给结构带来的开裂风险。此外,高温下强度增长稳定的特点决定其能在高热施工环境发挥作用,优良的体积稳定性有利于解决混凝土结构开裂问题,较高的后期强度和优良的抗侵蚀性能适合用于高性能混凝土的制备。本文从水化、性能等角度出发,分析了低热硅酸盐水泥在水化调控、水化产物及微观结构、性能优化等方面存在的部分问题,总结了低热硅酸盐水泥高温耐受、抗侵蚀、体积稳定等性能特点,提出了低热硅酸盐水泥在严酷环境、高热环境中的应用展望。     

Corrosion process and structural performance of a 17 year old reinforced concrete beam stored in chloride environment

The long-term corrosion process of reinforced concrete beams is studied in this paper. The reinforced concrete elements were stored in a chloride environment for 17years under service loading in order to be representative of real structural conditions. At different stages, cracking maps were drawn, total chloride contents were measured and mechanical tests were performed. Results show that the bending cracks and their width do not influence significantly the service life of the structure. The chloride threshold at the reinforcement depth, used by standards as a single parameter to predict the end of the initiation period, is a necessary but not a sufficient parameter to define service life. The steel-concrete interface condition is also a determinant parameter. The bleeding of concrete is an important cause of interface de-bonding which leads to an early corrosion propagation of the reinforcements. The structural performance under service load (i.e.: stiffness in flexure) is mostly affected by the corrosion of the tension reinforcement (steel cross-section and the steel-concrete bond reduction). Limit-state service life design based on structural performance reduction in terms of serviceability shows that the propagation period of the corrosion process is an important part of the reinforced concrete service life.     

Corrosion effects on pullout behavior of hooked steel fibers in self-compacting concrete

Fiber reinforced concrete structures are subjected to chloride and carbonation penetration that could initiate corrosion of steel fibers, with eventual pernicious consequences in terms of structural and durability performance. Cracks in concrete are known to hasten initiation of steel corrosion in reinforced concrete structures. The investigation of the impact of cracks on the corrosion initiation and the associated interfacial damage between concrete and steel fibers is important for understanding the mechanical behavior of steel fiber reinforced concrete.In the present work, with the aim of studying the corrosion action on the mechanical behavior of cracked Steel Fiber Reinforced Self-Compacting Concrete (SFRSCC), an experimental program was performed to characterize the corrosion of hooked-end steel fibers and to assess the fiber pullout behavior in cracked concrete, previously subject to the action of corrosion by exposure to aggressive chloride environment.     

The corrosion pattern of reinforcement and its influence on serviceability of reinforced concrete members in chloride environment

This paper deals with two corroded reinforcement concrete beams, which have been stored under sustained load in a chloride environment for 14 and 23 years respectively. The evolution of corrosion pattern of reinforcement and its influence on serviceability are studied. In chloride-induced corrosion process, corrosion cracking affects significantly the corrosion pattern. During the corrosion cracking initiation period, only local pitting corrosion occurs. At early stage of cracking propagation, localized pitting corrosion is still predominant as cracks widths are very small and cracks are not interconnected, but a general corrosion slowly develops as the cracks widen. At late cracking stage, interconnected cracking with wide width develops along large parts of the beam leading to a general corrosion pattern. Macrocells and microcells concepts are used for the interpretation of the results.Mechanical experiments and corrosion simulation tests are performed to clarify the influence of this corrosion pattern evolution on the serviceability of the beams (deflection increase). Experimental results show that, when the corrosion is localized (early cracking stage), the steel–concrete bond loss is the main factor affecting the beams serviceability. The local cross-section loss resulting from pitting attack does not significantly influence the deflection of the beam. When corrosion is generalized (late cracking stage), as the steel–concrete bond is already lost, the generalized steel cross-section reduction becomes the main factor affecting the beams serviceability. But, at this stage, the deflection increase is slower due to the low general corrosion rate.     

Capillary transport of water through textile-reinforced concrete applied in repairing and/or strengthening cracked RC structures

The use of textile-reinforced concrete (TRC) has great potential for innovative solutions in repairing, protecting, and strengthening concrete and RC structures. The article at hand reports on an investigation on composite concrete specimens made of cracked ordinary concrete as substrate and textile-reinforced concrete (TRC) as a cover layer for its strengthening and repair. The TRC cover layer was assessed with regard to its effectiveness as a protective layer against the ingress of water through capillary action. Since in real applications such TRC layers may be cracked or presumed to be so, thereby activating the load-carrying function of the textile reinforcement, the TRC layer was cracked for purposes of this study. The water transport in the cracked ordinary concrete specimens without the TRC layer was used as a reference. Gravimetric measurements and neutron radiography served as the testing techniques. In ordinary concrete quick and deep ingress of water through relatively wide macro-cracks of approximately 100 μm width, followed by transport through the capillary pore system, caused saturation of large areas in a rather short time. TRC applied to the RC surface reduced the ingress of water to a large extent. Its small crack widths of 15 to 20 μm changed suction behaviour fundamentally. In the cracked substrate of ordinary concrete, capillary suction was prevented, and transport through the pore system of the matrix became the prevailing transport mechanism of capillary action. Not only was the mechanism altered, but the transport of water deep into inner regions was markedly retarded as well.     

混凝土中钢筋腐蚀速率模型研究进展

预测混凝土中钢筋的腐蚀速率对腐蚀扩展阶段的发展非常重要,同时也能表征腐蚀导致的混凝土保护层开裂的时间。讨论了已有的钢筋腐蚀速率模型,包括经验模型、反应（氧气扩散与混凝土电阻率）控制模型和电化学模型。此外,根据以前的研究成果总结了钢筋腐蚀的时变性、局部腐蚀性与随机性等特征,并提出了准确预测腐蚀速率所需要解决的关键问题。     

复合盐侵蚀再生混凝土耐久性能退化试验研究

我国西北地区土壤及其地下水中含有大量侵蚀性离子,造成了再生混凝土结构耐久性能快速退化,严重制约着再生混凝土在主体结构中的应用。基于西北地区耐久性环境中侵蚀离子组成及区域气候环境特征,以7.5%(质量分数)MgSO₄-7.5%(质量分数)Na₂SO₄-5%(质量分数)NaCl为侵蚀介质,采用干湿交替方式,开展复合盐侵蚀再生混凝土耐久性能退化试验研究。以粉煤灰、矿渣、硅灰、偏高岭土等活性矿物掺合料取代水泥,研究矿物掺合料搭配方式对再生混凝土耐久性能影响。采用XRD、FTIR、TG-DSC及SEM、EDS等表征手段,分析侵蚀产物矿物组成、含量及微结构变化,研究复合盐侵蚀再生混凝土损伤过程。随着干湿交替次数增加,再生混凝土相对动弹性模量、质量变化率、相对抗压强度、相对劈裂抗拉强度及损伤层厚度的变化规律与矿物掺合料搭配方式有着显著的相关性。随着侵蚀离子的不断扩散,混凝土碱度降低,在表面依次形成了以水镁石、石膏及钙钒石为主要产物的致密侵蚀产物层,短暂阻碍了侵蚀离子进一步扩散。随着侵蚀产物不断堆积及无胶凝性水化硅酸镁的形成,在膨胀应力及盐结晶压力共同作用下,再生混凝土表面大量剥蚀并开裂,耐久性能快速退化。     

水泥基材料裂缝微生物修复技术的研究与进展

混凝土结构因为开裂而失效的事故屡屡发生,这不仅给国家、人民带来了巨大的经济损失,而且还严重威胁着人们的人身安全.目前混凝土裂缝的修复技术主要有灌浆修复(水泥砂浆或环氧树脂)、电化学修复、微生物修复等.也有相当一部分科研人员根据生物体损伤修复的原理,在混凝土传统组分中复合特殊组分使其具有自修复功能,即令混凝土材料在出现损伤或裂缝时能自动触发修复机制使裂缝愈合.介绍了水泥基材料裂缝的微生物修复技术的研究与进展,分析了这种技术的优势及其存在的问题,展望了其发展趋势和应用前景,最后对该技术进一步的研究方向提出了建议.