257 research outputs found
Corrosion inhibitors for steel in concrete – an update
Full text link16 anni fa un primo stato dell’ arte “Inibitori di corrosione per il calcestruzzo armato” è stato pubblicato come EFC numero 35. Gli inibitori di corrosione vengono utilizzati maggiormente come tecnica preventive, dunque aggiunti durante l’impasto del calcestruzzo. In questo caso i risultati di prove in laboratorio ed il comportamento in pratica indicano che il tempo di innesco di corrosione viene prolungato da un fattore di circa due. La riduzione della velocità di corrosione invece non è significativo. Inibitori applicati sul calcestruzzo indurito come sistema di ripristino hanno mostrato risultati positivi solamente a tenori di cloruro bassi e quando la corrosione è in fase iniziale.16 years ago a first state of the art report “Corrosion Inhibitors for Steel in Concrete” has been published as EFC publication No. 35. Industry continued to develop and improve admixtures that can be added to the fresh concrete (preventative) or systems that can be used as surface applied inhibitors (curative). This paper presents an update on the state of the art on corrosion inhibitors, with particular emphasis on long-term and especially field studies. There is a general agreement from both laboratory and field studies that mixed-in corrosion inhibitors can delay the onset of chloride-induced corrosion in good quality concrete by a factor of about 2. The reduction of the corrosion rate once corrosion has initiated appears to be much less significant. Surface applied inhibitors can show positive results only at low chloride concentrations and with beginning corrosion
PH-monitoring in mortar with thermally-oxidized iridium electrodes
Full text linkThe pH of the concrete pore solution plays a vital role in protecting the reinforcing steel from corrosion. Here, we present results from embeddable pH sensors that permit the continuous, in-situ monitoring of the pH in the concrete pore solution. These are potentiometric sensors, based on thermally-oxidized iridium/iridium oxide (IrOx) electrodes. We propose an iterative calculation algorithm taking into account diffusion potentials arising from pH changes, thus permitting the reliable, non-destructive determination of the pore solution pH over time. This calculation algorithm forms an essential part of the method using IrOx electrodes. Mortar samples were exposed to accelerated carbonation and the pH was monitored at different depths over time. Comparative tests were also performed using thymolphthalein pH-indicator. The results from the pH sensors give insight in the carbonation process, and can, in contrast to thermodynamic modelling and titration experiments, give insight in kinetic processes such as transport and phases transformations. Additionally, it was found that the front at which the pH is decreased from initially 13-14 down to 12.5 can be significantly ahead of the common carbonation front corresponding to pH 9-10. This has major implications for laboratory testing and engineering practice
A new setup for rapid durabiity screening of new blended cements
Full text linkIn order to reduce CO2 emission and energy consumption of cement production substantially, the clinker content in the cements must be reduced. Blended cements, with high clinker substitution by a large variety of supplementary cementitious materials (SCMs), are appearing on the market. Today’s performance tests are time consuming, focus on transport properties for service-life modeling and neglect factors directly playing a role in corrosion (pH of the pore solution, alkalinity reserve). Thus a new way of testing blended cements that is both rapid and relevant for assessing corrosion performance is needed.
We propose a new test setup that consists of small (8 x 8 cm2) and thin (6 mm) mortar samples instrumented with reference electrode, 5 steel wire electrodes and a stainless steel grid counter electrode, allowing performing both traditional and sophisticated electrical and electro¬chemical measurements. This innovative setup can be used to test any cement blend, w/c ratio and admixture content. Durability testing including the transport properties of the mortar and corrosion behavior of the steel can be performed both for chloride or carbonation induced corrosion. Specific applications that are envisaged are:
- Fast durability screening of new blended cements in standard conditions;
- Durability testing of already established blended cements in a variety of environmental conditions or testing of mixes for specific structures or environments;
- Research on the mechanisms and controlling factors of steel corrosion in concrete.
The users are cement producers, test laboratories, research institutions, standardization bodies, owners of new structures etc. The potential for application world¬wide is huge, especially because cement producers and users are still local and many different new blends have to be tested. The setup contributes to tackle the main future challenge of the building industry: to guarantee long-term durability of reinforced concrete infrastructure with minimum amount of clinker in the cement
The mechanism controlling corrosion of steel in carbonated cementitious materials in wetting and drying exposure
No full textExperiments were performed to study the mechanism of corrosion of steel in carbonated concrete in cyclic wetting/drying exposure. It was found that the corrosion rate increased during wetting, followed by a decrease during drying. The increase and decrease of the corrosion rate can be explained by changes in electrochemically active steel area as a function of the moisture state of the mortar and its porosity. The corrosion rate achieved in the wet phase stabilizes during the first few wetting and drying cycles. The mechanism of the electrochemical process is discussed on the basis of theoretical considerations. It is concluded that the corrosion kinetics are under activation control. The availability of oxygen does not seem to be a limiting parameter for the corrosion process in cyclic wetting/drying exposure
Merging Electrochemistry and Water Capillary Condensation to Understand the Corrosion Mechanism of Steel in Carbonated Concrete
No full textReinforced concrete is one of the most used man-made materials worldwide, one of its main causes of degradation worldwide is carbonation induced corrosion, accounting every year for billions of dollars for maintenance, repair and rebuilding. The alkali produced by the Portland cement hydration (mainly Ca(OH)2) allows steel passivation. CO2 from the atmosphere can enter the concrete pores and react with the Ca(OH)2, leading to a decrease of pH that can propagate to the reinforcement level. In presence of oxygen the steel starts to corrode, the corrosion products precipitate in the surrounding of the reinforcement, generating stresses that can lead to cracking and spalling of the concrete cover. The topic of carbonation induced corrosion is becoming of key importance in the reduction of environmental footprint of Portland cement production, responsible for about 8% of the man-made CO2 and thus for global warming and climatic change. The replacement of Portland cement by using supplementary cementitious materials (SCM) is one of the most viable, short-term solutions. This, however, might impair the durability of reinforced concrete; reducing the Portland cement content leads to a reduced production of calcium hydroxide and therefore to a much faster carbonation process. When the carbonation front reaches the reinforcement before the end of service life, the corrosion propagation becomes an important part of the life of the structure.
To be able to collect data of corrosion propagation rates in a reasonable time a new experimental set up has been designed, which consists of small (8x8 cm) and thin (0.6 cm) mortar samples instrumented with a reference electrode (Ag/AgCl), 5 steel wire electrodes and a stainless steel grid counter electrode. The thin sample allows rapid carbonation and equilibration of environmental humidity. Electrical resistivity, oxygen cathodic current, open circuit potential and corrosion rate of the steel can be measured. The results have shown that the corrosion rate increases markedly with increasingly higher relative humidity (up to 99% RH), maximum values around 2 μA/cm2 have been found in wet/dry cycles. Increasing corrosion rate was associated with a decrease of the corrosion potential, the ohmic resistance of the electrochemical cell was found to correlate with the corrosion rate. The oxygen reduction limiting current was always about five to ten times higher than the corrosion rate. The influence of cement type and water/binder ratio was of minor importance.
The mechanism of corrosion of steel in carbonated concrete is under debate since 1980 - and after almost 40 years there is still no agreement. The traditionally proposed controlling mechanisms such as resistive, cathodic or anodic control have been found not suitable for corrosion of steel in carbonated mortar, which has always been considered as a uniform process. Instead, being the cement paste a porous system, the capillary water condensation had to be taken into account. With the Kelvin equation and the porosity curves, the volumetric water content could be calculated for every different sample and relative humidity condition. A correlation between the water content and the corrosion rate was found.
Merging electrochemistry and water capillary condensation theory allowed explaining the mechanism of corrosion of carbon steel embedded in a porous system. The porosity and degree of pore saturation define the electrochemically active area with respect to the total steel surface. This active area varies by orders of magnitude depending on the exposure condition: from immersed conditions to 99% RH the active area goes down to ca. 40 %, at 80% RH only 1 %. The volumetric water content also determines the available diffusion volume of produced Fe2+ ions, which modifies the anodic reaction reversible potential. A larger diffusion volume leads to a lower Fe2+ concentration and therefore lower reversible potential and higher corrosion current according to the Evans diagram. The two contributions can be recognized and the huge variation of the corrosion rate with the exposure condition can thus be explained. In conclusion the corrosion mechanism of carbon steel in carbonated concrete could be determined and the corrosion rate can be uniformly expressed, for every different humidity state, water to binder ratio and binder type, as a function of pore solution pH, water content and porosity
Influence of Calcium Nitrate and Sodium Hydroxide on Carbonation Induced Steel Corrosion in Concrete
Full text linkRecent developments in concrete technology, together with sustainability concerns and requirements related to digital concrete fabrication technologies, lead to increased usage of chemical admixtures in order to achieve the needed concrete performance. However, the consequences that these compounds may have for the long-term durability are unknown, particularly concerning steel reinforcement corrosion. Here, we study the effect of NaOH and Ca(NO3)2 as common examples of a hydration activator and accelerator, respectively. It is found that both admixtures considerably increase the steel corrosion rate in carbonated concrete (by up to a factor of 20). Corrosion tests in mortar as well as in aqueous solutions, together with porosity measurements of the mortars, provided evidence that the impact of the admixtures can be mainly found in modifying the electrochemistry, that is, by introducing an additional reduction reaction or by a catalyzing effect. An estimation revealed that at usual dosages, these adverse effects will prevail for a substantial portion of the design service life of a structure, as these species will not be consumed during decades
Corrosion rates in carbonated low clinker cements: are the new binders really sustainable?
Full text linkLowering the clinker content of concrete using SCMs can contribute significantly to reduce the en-ergy consumption and the CO2 emissions of building materials. Substitution of clinker up to 65% is now possible according to European and Swiss standards. Uncertainty about durability, especially carbonation induced corrosion, is the main factor limiting the practical use of these blended ce-ments: containing less CaO they have less capacity to neutralize CO2 and thus higher carbonation rate, which may lead to premature corrosion of steel reinforcement. Results in literature on the corrosion rate in carbonated concrete are rare and refer to ordinary Port-land cement only. For service life prediction of concrete structures with new, blended cements, cor-rosion rate data are urgently needed because the so-called “corrosion propagation stage” might be a significant part of the total service life of the structure. To be able to collect data of corrosion propagation rates in a reasonable time a new experimental set up has been designed. Parameters that can be measured are electrical resistivity of the sample, cor-rosion potential and corrosion rate of the steel, oxygen diffusion and consumption rate; their evalua-tion should allow to investigate the protective nature of the low clinker material for steel in concrete and the mechanism, particularly the kinetics, of carbonation induced corrosion. The first results show that in certain environments blended cements could be more susceptible to corrosion
Innovative sample design for corrosion rate measurements in carbonated blended concrete
No full textLowering the clinker content of concrete using SCMs can contribute significantly to reduce the energy consumption for cement production and the CO2 emissions of building materials. Substitution of clinker up to 65% is now possible according to European and Swiss standards. Uncertainty about durability, especially carbonation induced corrosion, is the main factor limiting the practical use of these blended cements. For service life prediction of concrete structures with new, blended cements corrosion rate data are urgently needed because the so called “corrosion propagation stage” might be a significant part of the total service life. Such data so far can be obtained only with very time consuming tests due to the sample size of standard concrete samples and the slow carbonation process. In this paper an innovative experimental setup is presented that overcomes these limitations. The setup consists in miniaturised thin samples with a size of 80 x 80 x 6 mm, thus with an effective carbonation depth of only 3 mm. With this new sample design the corrosion rate of steel in carbonated samples can be studied at any relative humidity. Our preliminary results show that these samples allow any kind of electrochemical measurements can be performed – thus the study of corrosion and related influencing parameters, in homogenous carbonated conditions, is possible within a short time scale
Kinetics of electrochemical dissolution of metals in porous media
Full text linkMetals embedded in porous media interact electrochemically with the liquid phase contained in the pores. A widespread form of
this, adversely affecting the integrity of engineered structures, is corrosion of steel in porous media or in natural environments.
While it is well documented that the rate of this electrochemical dissolution process can vary over several orders of magni-
tude, understanding the underlying mechanisms remains a critical challenge hampering the development of reliable predictive
models. Here we study the electrochemical dissolution kinetics of steel in meso-to-macro-porous media, using cement-based
materials, wood and artificial soil as model systems. Our results reveal the dual role of the pore structure (that is, the influence
on the electrochemical behaviour through transport limitations and an area effect, which is ultimately due to microscopic inho-
mogeneity of the metal/porous material interface). We rationalize the observations with the theory of capillary condensation
and propose a material-independent model to predict the corrosion rate
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