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.     

Shear performance of reinforced concrete beams with corroded stirrups in chloride environment

Experiments were conducted for the investigation of the influence of reinforcing steel corrosion on the shear capacity of reinforced concrete beams. The shear performance of RC beams with different corrosion levels in both longitudinal reinforcing steel bars and stirrups was examined. Relationships of corrosion-induced crack widths in concrete cover with the corrosion level of the reinforcing steel bars were obtained. Engineering approaches were developed to predict the residual shear strength of the corroded beams.     

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.     

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.     

3D Numerical modelling of steel corrosion in concrete structures

The paper deals with a 3D numerical model for transient analysis of processes after depassivation of reinforcement in concrete, which are relevant for calculation of corrosion rate. The aim of the study is to investigate the influence of the concrete quality, cracking and water saturation in concrete on the current density. The results show that the corrosion rate is higher in poor quality concrete than in good quality concrete. The model predicts that cracks do not influence corrosion rate for the case where the only influence of the crack is on the rate at which oxygen can reach the steel.     

Stress distribution in thick-walled cylinder due to non-uniform radial pressure on the example of reinforcement corrosion in concrete

This paper presents an analytical solution to the non-uniform pressure on thick-walled cylinder. The formulation is based on the linear elasticity theory (plain strain) and stress function method. As an example, the proposed solution is used to model the stress distribution due to non-uniform steel reinforcement corrosion in concrete. The model is formulated considering different scenarios of corrosion pressure distribution. It is validated against the finite element model for different cases of non-uniform pressure distributions. The results show that the corrosion-induced cracks are likely to start just beyond the anodic zone. This is confirmed by the experimental tests on concrete cylinder exposed to non-uniform accelerated corrosion of steel reinforcement. The model can be effectively used to calculate the distribution of corrosion-induced stresses in concrete.     

Ursachen und Mechanismen der Spannungsrißkorrosion

Causes and mechanisms of stress corrosion cracking The paper provides a synopsis of the causes and mechanisms of stress corrosion cracking discussed at the present time. The discussion covers: selective metal dissolution (formation of cover layers, influence of lattice imperfections on the anodic metal dissolution, crack spreading rate, anodic hydrogen embrittlement); adsorption and brittle fracture (energy balance, atomistic and electro-chemical aspects); metallurgy (plastic deformation, alloying influence, sliding condition); fracture mechanics (crack formation and growth). Particularly important is the formation of cover layers where stress corrosion cracking depends on a certain ratio of the rate at which the cover layer is reformed and the rate at which surfaces free from cover layer are formed at the bottom of the crack. At high crack expansion rates, the adsorption-brittle fracture mechanism may play a part, as it tends to lower the cohesion at the bottom of the crack. As the crack represents an ideally sharp notch, it is possible to apply the linear elastic fracture mechanics to the crack expansion phenomena.     

Karbonatisierungszellen – Ein Beitrag zur Korrosion von Stahl in karbonatisiertem Beton

Carbonation cells – On the corrosion of steel in carbonated concrete Corrosion of steel in carbonated concrete involves the activity of active-passive galvanic couples (carbonation cells). Carbonation cells consisting of mortar electrodes in wet condition and subjected to wet-dry cycles have been investigated by means of current and potential measurements. The corrosion current can be explained in terms of oxygen diffusion through the mortar cover. The stationary current gives realistic values for the diffusion coefficient of oxygen in concrete. Changes from wet to dry and vice versa always increase the corrosion rate. Electrolytically and gravimetrically determined weight loss are in good agreement only for constant moisture conditions. In case of wetdry cycles oxidation and reduction reactions within the corrosion products are assumed to contribute to the total corrosion loss.     

Crack shape and rust distribution in corrosion-induced cracking concrete

This study investigated a reinforced concrete specimen that had deteriorated in an artificial environment for 2 years. The crack width and the rust distribution were observed by digital microscopy. The variation of the total circumferential crack width along the radial direction is presented using a linear function. Observation reveals that rust does not penetrate into the corrosion-induced cracks before concrete surface cracking. After concrete surface cracking, rust fills the cracks, lining the edges of the cracks due to the circulation of the outer solution. A schematic diagram is proposed to describe crack propagation and rust development.     

Einfluß einer Feuerverzinkung und PVC-Beschichtung von Bewehrungsstählen und von Inhibitoren auf die Korrosion von Stahl in gerissenem Beton

Influence of galvanizing and PVC-coating of reinforcing steels and of inhibitors on steel corrosion in cracked concrete Cracked concrete beams of concrete quality B15 and B35 with carbonized cracks were exposed in artificial seawater, under frost and deicing salt conditions and in industrial climate. The reinforcement was composed of black steel, galvanized and PVC-coated steel. For the purpose of additional protection the concrete was partly mixed with an inhibitor Ca(NO₂)₂. The concrete cover was 1,5 till 5,0 cm. It was found that the inhibitor protects satisfactorily only in the case of the higher concrete quality, cover 1,5 cm and not to high crack width. A protective effect of galvanizing is given by not to high chloride contents (< 1,5% relative to cement) and crack widths. The chosen PVC-coating failed because of chemical instability in the alcaline medium concrete.     

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.     

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.     

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。     

Effect of hydrogen peroxide on crack growth rate in X70 pipeline steel in weak acid solution

An investigation of the effect of hydrogen peroxide (H₂O₂) on corrosion crack growth can be considered as a necessary stage in a study of the SCC of pipeline steels in the presence of dissolved oxygen and other oxidants. It was found that the presence of hydrogen peroxide at a low concentration (5?mM) results in a deceleration of the crack growth. With an increase in the concentration of H₂O₂, the crack growth rate increases. The change in the steel corrosion rate at various H₂O₂ concentrations agrees with the dependence of the crack growth rate on the oxidant concentration. The conclusion has been made that the crack growth in a weakly acidic electrolyte (pH 5.5) is determined by the metal dissolution process. Hydrogen charging of the metal indirectly affects the crack growth by increasing the surface coverage with hydrogen, which decreases the steel dissolution rate.     

Local detailed inspection methods regarding reinforcement corrosion of concrete structures

Full surface surveys which are likely to include e.g. a deterioration and potential mapping as well as a cover depth survey, should be the base to identify critical areas which are suspect to be suffering from reinforcement corrosion 1 . Localised assessment in these areas should give further information about the type and extent of deterioration. It should include the determination of the chloride profile beyond the cover depth, the depth of carbonation and a direct visual assessment of the steel reinforcement by breaking away the cover concrete at selected locations. Further valuable information may be collected by determining the water content of the concrete or performing corrosion rate measurements. Within this publication the essential methods for local detailed inspections are discussed regarding the application and interpretation of the results. An overview of the survey procedure is given in ref. 2 .     

Modelling of reinforcement corrosion – Investigations on the influence of shrinkage and creep on the development of concrete cracking in the early propagation stage of reinforcement corrosion

Since the initiation stage of the damage process due to reinforcement corrosion had been successfully investigated in the recent past, the damage progress in the propagation stage is currently in the focus of research. This work deals with the mechanisms of cracking and spalling due to corrosion of reinforcement and aims for the development of an analytic prediction model of the damage process. On this occasion the influence of shrinkage and creep on the stress condition within the concrete cover is of major importance to the subsequent analysis of the crack formation due to reinforcement corrosion in the early propagation stage.     

黄铜应力腐蚀敏感性及其与脱Zn层拉应力的对应性

测量了不同极化电位下,Ｈ６２黄铜在氨水溶液腐蚀过程中表面脱Ｚｎ蔬松层引起的拉应力,并用单边缺口试样测量的不同极化电位下的应力腐蚀敏感性,结果表明：黄铜在氨水中自然腐蚀时,在脱Ｚｎ层界面会产生 大的拉应力,整个试样的平均应力为σ＝１８．１ＭＰａ;阳极极化使表面拉应力略有下降,阴极极化则使疏松层引起的拉应力急剧下降乃至为零,阴极极化在表层产生镀Ｃｕ层后出现尖力;应力腐蚀敏感性随外加电位的变化规律和疏松     

Improved concrete bridge deck evaluation using GPR by accounting for signal depth–amplitude effects

Ground-penetrating radar (GPR) surveys have tended to provide unpredictable accuracy in estimating areas of corrosion-induced damage in reinforced concrete bridge decks. Six in-service reinforced concrete bridge decks with exposed (unpaved) concrete wearing surfaces were surveyed using GPR to evaluate internal corrosion-induced damage for comparison against the chain drag and half-cell potential survey methods. A normalization procedure for removing GPR signal losses resulting from depth-dependent two-way travel time variations in the data was developed and thresholds for predicting chain-drag and half-cell potential-based estimates of deterioration quantity and location were established. Thresholds established using the amplitude–time correction provided significant improvements in the spatial and quantitative predictive capabilities of GPR for delineating corrosion-induced damage.     

Investigations of an ethanolamine‐based corrosion inhibitor system for surface treatment of reinforced concrete

Laboratory investigations were performed to assess the efficacy of a proprietary ethanolamine‐based corrosion inhibitor system when applied to the surface of reinforced concrete specimens that were chloride‐contaminated to varying extents in the presence or absence of carbonation. The corrosion responses of embedded steel bars at various depths of cover were monitored electrochemically during a controlled programme of cyclic wetting and drying undertaken for several months prior to the inhibitor treatment and for approximately eighteen months thereafter. Gravimetric measurements of the quantities and distribution of corrosion on the steel were also made on completion of the exposure tests. Analysis of aqueous extracts from treated concrete revealed that the ethanolamine component of the inhibitor system penetrated to depths of more than 15 mm within the concrete. It was found that, for inhibitor‐treated specimens, there was some reduction in the corrosion rate of pre‐corroding steel at low cover depths in non‐carbonated concrete with modest levels of chloride contamination. At higher levels of chloride contamination and in carbonated specimens, however, the ethanolamine‐based inhibitor was apparently ineffective under the conditions investigated.