Critical chloride content for reinforced concrete and its relationship to concrete resistivity

The critical chloride content for initiation of reinforcement corrosion is an essential element in service life design and modelling of concrete structures. The critical content is laden with questions regarding its definition, experimental assessment and practical aspects. It should be addressed by a statistical approach. The paper discusses such issues, presents experimental evidence and considers its relationship to the electrical resistivity of concrete. A low concrete resistivity can be theoretically argued to relate to a low critical chloride content. However, only part of the experimental data supports this. The resolution of available data does not allow distinction with regard to the critical content between cement types.     

Probabilistic considerations on the effect of specimen size on the critical chloride content in reinforced concrete

A probabilistic model was used to predict the critical chloride content (C_crit) for reinforcement corrosion as a function of specimen size. The specimen size is likely to be a reason for the large scatter of C_crit values as well as for the high threshold levels often observed in laboratory setups. Regarding test methods, it is discussed that the common combination of small specimens and low numbers of parallel samples results in poor reproducibility. If experimental data is to be used for service life modelling, the geometrical dimensions of the specimens on which C_crit was measured have to be taken into account.     

Influence of chloride and moisture content on steel rebar corrosion in concrete

Reinforced mortar samples were exposed in humidity chambers with different relative humidity or exposed in cyclic moisture conditions. The rebars were in an “as received” condition meaning that the preexisting oxide scale were intact. The lowest chloride concentration that initiated corrosion was 1% Cl^? by mass of cement, corrosion was then observed for samples exposed at 97% relative humidity. It is suggested that the corrosion rate decreases when samples are exposed to a relative humidity lower than 97%. The results indicate that threshold levels should be evaluated at rather humid conditions (97%) despite the fact that the maximum corrosion rate at higher chloride levels is observed in the interval 91–94%. For samples exposed to cyclic moisture conditions, a lower chloride concentration was needed to initiate corrosion compared to samples exposed in static moisture conditions.     

海工钢筋混凝土的腐蚀与防护

海水具有很强的腐蚀性。如果不采取防护措施，海工钢筋混凝土结构会受到严重腐蚀。本文结合当前现有研究成果，对海洋环境下钢筋混凝土结构的腐蚀特征和防护技术进行了论述。     

Covercrete with hybrid functions – A novel approach to durable reinforced concrete structures

Due to the corrosion of steel in reinforced concrete structures, the concrete with low water–cement ratio (w/c), high cement content, and large cover thickness is conventionally used for prolonging the passivation period of steel. Obviously, this conventional approach to durable concrete structures is at the sacrifice of more CO₂ emission and natural resources through consuming higher amount of cement and more constituent materials, which is against sustainability. By placing an economically affordable conductive mesh made of carbon fiber or conductive polymer fiber in the near surface zone of concrete acting as anode we can build up a cathodic prevention system with intermittent low current density supplied by, e.g., the solar cells. In such a way, the aggressive negative ions such as Cl^?, , and can be stopped near the cathodic (steel) zone. Thus the reinforcement steel is prevented from corrosion even in the concrete with relatively high w/c and small cover thickness. This conductive mesh functions not only as electrode, but also as surface reinforcement to prevent concrete surface from cracking. Therefore, this new type of covercrete has hybrid functions. This paper presents the theoretical analysis of feasibility of this approach and discusses the potential durability problems and possible solutions to the potential problems.     

Organic substances as inhibitors for chloride‐induced corrosion in reinforced concrete

Corrosion inhibitors are used to prevent chloride‐induced corrosion in reinforced concrete structures. Since performance of commercial organic inhibitors is only partially satisfactory, a 7‐year long research has been carried out in order to set‐up a new organic inhibitive mixture, able to prevent chlorides‐induced corrosion. A first screening, by means of potentiodynamic polarisation test in alkaline synthetic pore solution, was performed on 80 organic compounds, mainly primary and tertiary amines, aminoalcohols, carboxylates compounds and aminoacids, in order to select the best inhibiting substances. The nine best inhibitive organic substances were selected for long‐term tests: 2 amines (dimethylethanolamine and triethylentetramine), 4 aminoacids (aspartate, asparagine, glutamate and glutamine) and 3 carboxylates compounds (tartrate, benzoate and EDTA). Potentiostatic polarisation and free corrosion tests in synthetic pore solution were performed, as well as tests in concrete exposed to accelerated chlorides penetration. Five years of tests allow estimating the efficiency of the substances in preventing chlorides‐induced corrosion, in term of influence on chlorides penetration and on critical chlorides threshold.     

Stainless steels: Passive film composition,pitting potentials,and critical chloride content in concrete

Stainless steel reinforcing bars show excellent corrosion resistance in concrete structures exposed to harsh environments. Only a little information on the surface chemistry of these materials in alkaline media is available. This study reports the X-ray photoelectron spectroscopy surface analytical results (thickness, composition of the passive film and of the interface beneath the film) obtained on black steel, FeCr alloys, DIN 1.4301, DIN 1.4462 and the nickel-free DIN 1.4456 after exposure to alkaline solutions simulating concrete. The pitting potentials of the steels could be related to the Cr(III)oxy-hydroxide and Mo(VI) content in the passive film. C_crit, the critical chloride content for corrosion initiation in concrete, necessary for life-time predictions, can be determined only with time-consuming tests, especially for high-alloyed stainless steel. This study reports a correlation between C_crit in concrete (made with CEM II-A/LL and CEM I) and the pitting potential for carbon steel, Fe12%Cr alloy, DIN 1.4301, and DIN 1.4571 stainless steels. This could allow, for the first time, a quantitative estimation of C_crit for stainless steel in concrete based on short-term solution tests.     

Detecting critical chloride content in concrete using embedded ion selective electrodes – effect of liquid junction and membrane potentials

When studying critical chloride content in reinforced concrete, the amount of chloride dissolved in the pore solution is an important parameter. Ion selective electrodes offer the possibility of measuring the chloride ion concentration (activity) in the concrete pore solution non‐destructively. However, the potentiometric measurement might be disturbed by, amongst others, diffusion potentials (liquid junction potentials and membrane potentials). Laboratory samples designed to find critical chloride contents often involve chloride ingress by capillary suction, diffusion or migration, and thus chloride and pH profiles are usually present. As a result of these concentration gradients, membrane potentials are established, which markedly affect the determination of the chloride concentration. In addition, liquid junction potentials at the interface of the concrete sample and the reference electrode contribute to the measured potential. Experimental observations in the present work illustrate the effect of liquid junction potentials on the application of ion selective electrodes in concrete. Moreover, the influence of internal membrane potentials has been estimated by a theoretical model.     

Durability properties of concrete with blended cements

The study of chloride‐induced corrosion started more than three decades ago, after extensive cases of damage were observed on reinforced concrete structures in coastal regions and on infrastructural objects exposed to salt action. Nowadays, the basis for sustainable concrete industry lies in these three aspects: reducing CO₂ emissions by using by‐products of other industries (slag, fly ash, silica fume) for cement production, conservation of natural resources by replacing part of the aggregate with recycled construction waste, as well as use of recycled water in concrete production, and finally, construction of durable concrete structures. Durability design procedures for reinforced concrete structures in aggressive environments are still to a large extent empirical, especially in the case of using blended cements. The type of cement has a considerable effect on the properties of concrete, especially concrete resistance to the penetration of chloride ions. The paper presents results of testing durability and deformational properties of concrete produced with quaternary‐blended cement. A clearer insight into these properties is a necessary starting point for the performance‐based design of concrete prepared with blended cements, leading to more durable and corrosion‐free concrete structures in aggressive environments.     

Effect of the reinforcement bar arrangement on the efficiency of electrochemical chloride removal technique applied to reinforced concrete structures

This paper reports on the research done to find out the effect that different bar arrangements may have on the efficiency of the electrochemical chloride removal (ECR) technique when applied to a reinforced concrete structural member. Five different types of bar arrangements were considered, corresponding to typical structural members such as columns (with single and double bar reinforcing), slabs, beams and footings. ECR was applied in several steps. We observe that the extraction efficiency depends on the reinforcing bar arrangement. A uniform layer set-up favours chloride extraction. Electrochemical techniques were also used to estimate the reinforcing bar corrosion states, as well as measure the corrosion potential, and instant corrosion rate based on the polarization resistance technique. After ECR treatment, a reduction in the corrosion levels is observed falling short of the depassivation threshold.     

Critical chloride content and its influence on service life predictions

In service life predictions of concrete structures in marine environment, the critical chloride content is a crucial parameter. However, there is a lack of reliable data for this parameter. This paper presents a statistical model for the critical chloride content derived from measurements based on corrosion sensors installed in a 37 years old jetty. For each corrosion sensor location, the chloride content at different sensor rings is found from the chloride profile obtained from detailed profile grinding of the concrete core drilled out before installation of each sensor. The critical chloride content for each sensor location is then obtained by comparing the chloride profile with the sensor reading at the depth of depassivation. Further, the influence of the critical chloride content on the calculated service life is studied. From the probabilistic service life calculations, it is found that the service life increases from 50 to 100 years if the characteristic value of the critical chloride content increases from 0.34 to 0.67% (by weight of cement).     

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.     

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.     

A study of electrical chloride extraction of iron artifacts

The chloride extraction rule of iron artifacts was studied by electrical methods. The effect of the current and potential value on the desalination result of simulated iron artifacts was studied through the galvanostatic and potentiostatic experiments; the ingredients of the rust before and after treatments were also analyzed by the X‐ray diffraction (XRD). It has been found that the optimal current density was between ?0.50 and ?0.75 mA/cm² and the optimal potential was between ?1.175 and ?1.200 V. The phase of the samples rusts transformed after treatment, as well as the anti‐corrosion performance improved.     

Cost effectiveness and application of online monitoring in reinforced concrete structures

The annual costs for the maintenance of the Swiss national road networks amount to SFr. 560–640 millions 1 . Due to the limited means and the large number of engineering structures at the critical age between 30 and 45 years cost‐effective solutions for the monitoring and the maintenance of these structures have to be found 2 . In this paper the life cycle costs of an engineering structure using a sensor network (online‐monitoring system) were investigated. These costs were compared to the costs of classical inspection techniques.     

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.     

Comparative study on induced macrocell corrosion phenomenon in repaired ordinary reinforced and self-compacting concrete structures

Abstract

This paper aimed at an experimental investigation of the comparative corrosion phenomenon in the reinforcing steel of repaired chloride contaminated self-compacting concrete (SCRC) and ordinary concrete (OC) patches. This research is a continuation of a previous study in which macrocell specimens were prepared with OC having different chloride contaminations simulating the actual patch repair in the construction field. Here, in this research, the same specimens were prepared with SCRC to investigate more deeply the macrocell phenomenon in SCRC, which received limited attention in the past. Four prismatic specimens with two sets of 5 and 3% chloride contaminated SCRC at the two ends were prepared respectively, and the middle portion of these specimens was cast after 24 h with uncontaminated SCRC simulating the actual patch repair in the field. After 1 year of experimental observations of corrosion potentials and corrosion rates, interesting and novel results were obtained in comparison with OC and SCRC specimens.