Métodos electroquímicos semicuantitativos de estudio de lo corrosión por picaduras del acero para armaduras de hormigón

El acero embebido en el hormigón se encuentra sometido a un medio de elevada alcalinidad (pH comprendido entre 12 y 13), debido a la cal libre, a los álcalis que contiene el cemento, y al Ca(0H)2 que se forma durante la hidratación de los silicatos. En esta situación, el acero se halla recubierto por una capa de óxidos e hidróxidos estables que lo mantienen pasivado, interponiendo una barrera entre él y los agentes agresivos

Determinación electroquímica cuantitativa de la velocidad de corrosión de un acero de construcción. Evaluación de la susceptibilidad al ataque por picaduras. Parte I: Ensayos en disolución saturada de hidróxido cálcico

En un trabajo anterior (1) se analizaron las distintas características de los métodos electroquímicos empleados hasta la actualidad para la detección del ataque localizado en el caso de un acero de construcción sumergido en disolución saturada de hidróxido cálcico, así como el efecto despasivante e inhibidor que sobre la corrosión del acero tienen los iones Cl- y NO2-, respectivamente, añadidos a dicho medio

The effect of two types of modified Mg-Al hydrotalcites on reinforcement corrosion in cement mortar

Two modified Mg-Al hydrotalcites (MHTs), (MHT-pAB and MHT-NO) were incorporated into mortar (with different w/c ratios) in two different ways: (1) as one of the mixing components in bulk mortar; (2) as part of cement paste coating of the reinforcing steel. Accelerated chloride migration, cyclic wetting-drying and diffusion tests were performed to investigate their effect on reinforcement corrosion. The results indicated that MHTs could be promising alternatives for preventing chloride-induced corrosion when an appropriate dosage is adopted and applied in a proper way, particularly, replacing 5% mass of cement by MHT-pAB in bulk mortar or as a coating of reinforcing steel (MHT-pAB/MHT-NO to replace 20% mass of cement). The effect of MHT-pAB on time-to-corrosion initiation (TTC) of reinforcing steel was estimated using the DuraCrete model. It was found that the incorporation of 5% MHT-pAB in bulk mortar led to a more than double TTC relative to reference mortar without MHTs.The research was carried out under project number M81.609337 in the framework of the Research Program of the Materials innovation institute (M2i) (www.m2i.nl). Dr. Hartmut Fischer at TNO Materials Performance is acknowledged for his contribution in previous stages of this project. Sasol Germany GmbH is acknowledged for providing the carbonate hydrotalcites PURAL® MG 63 HT that was used in this work

Effect of damage on permeability and hygro-thermal behaviour of HPCs at elevated temperatures: Part 1. Experimental results

This paper presents an analysis of some experimental results concerning micro-structural tests, permeability measurements and strain-stress tests of four types of High-Performance Concrete, exposed to elevated temperatures (up to 700∘C). These experimental results, obtained within the "HITECO" research programme are discussed and interpreted in the context of a recently developed mathematical model of hygro-thermal behaviour and degradation of concrete at high temperature, which is briefly presented in the Part 2 paper (Gawin, et al. 2005). Correlations between concrete permeability and porosity micro-structure, as well as between damage and cracks' volume, are found. An approximate decomposition of the thermally induced material damage into two parts, a chemical one related to cement dehydration process, and a thermal one due to micro-cracks' development caused by thermal strains at micro- and meso-scale, is performed. Constitutive relationships describing influence of temperature and material damage upon its intrinsic permeability at high temperature for 4 types of HPC are deduced. In the Part II of this paper (Gawin, et al. 2005) effect of two different damage-permeability coupling formulations on the results of computer simulations concerning hygro-thermo-mechanical performance of concrete wall during standard fire, is numerically analysed

Durability of concrete structures in marine atmosphere zones - The use of chloride deposition rate on the wet candle as an environmental indicator

Durability of concrete structures under marine environments has been studied for a long time. This work was focused on marine atmosphere zone and studied the deposition of chlorides on wet candle devices and its relation with chlorides accumulated into concrete. Concrete specimens with three different mixtures were exposed at places located at four different distances from the sea. Periodically, chloride profiles were obtained and analysed taking into account environmental data. Results of numerical extrapolations show that chloride deposition rate on the wet candle can be used as an environmental indicator, helping to preview the expectancy of service life of concrete structures or suggesting minimum concrete cover thicknesses for a required service life. Regarding the studied region, service life decreases between 30% and 60% were observed when changing chloride deposition from 120 mg/m2 day to 500 mg/m2 day, which shows that chloride deposition plays an important role as an environmental indicator on service-life analysis of concrete structures in marine atmosphere zone. © 2010 Elsevier Ltd. All rights reserved

Oxygen and chloride diffusion in cement pastes as a validation of chloride diffusion coefficients obtained by steady-state migration tests

When chloride ions diffuse through concrete, it has been found that they interact with the surface charge and electrical double layer developed at the cementitious matrix/pore solution interface. As a consequence of this interaction, the diffusion of chloride ions is retarded in comparison with that of dissolved oxygen molecules, although the two species have very similar diffusion coefficients in infinitely aqueous dilute solutions. This is taken to imply that electrostatic, rather than purely steric, factors limit the ability of chloride ions to diffuse through pores below a critical radius. If an electrical field is applied to accelerate chloride transport through concrete, it is uncertain what effect this may have on the size range of pores that are accessible to the migrating ions. At high applied voltages, it might even be that the normal electrostatic interactions that influence ionic diffusion are nullified so that chloride ions can traverse pores of similar radii to those that are accessible to dissolved oxygen. This would be expected to give rise to an apparent increase in the value of ‘diffusion coefficient’ obtained for chloride. To investigate whether, during a steady-state migration test, chloride ions diffuse through the essentially the same range of pores as during a natural steady-state diffusion test, experimental studies (oxygen diffusion, natural diffusion and migration tests) have been carried out with a limited range of cement pastes. The results indicate that, at the recommended applied voltage (12 V) and for cementitious materials of the type studied, steady-state migration tests can be used to determine ‘effective diffusion coefficients’ that are of similar magnitude to those obtainable from natural steady-state diffusion measurements.Peer reviewe

Chloride penetration into concrete structures in the marine atmosphere zone – Relationship between deposition of chlorides on the wet candle and chlorides accumulated into concrete

The relationship between chlorides from marine aerosol and chlorides accumulated into concrete is discussed in this paper. The experimental programme comprised an environmental characterisation, with climatic and chloride deposition data, and a study of chloride penetration into concrete based on natural exposure of specimens in a marine atmosphere zone. Results show that salt concentration in marine aerosol strongly decreases in the first meters from the sea. Chlorides present in the atmosphere can be studied using the wet candle method and correlated with chlorides accumulated into concrete. This relationship can be represented by the equation View the MathML source, where kd is a coefficient which depends on concrete and environmental characteristics, Ctot is the average total amount of chlorides accumulated into concrete, C0 is the chloride content in concrete before exposure and Dac is the accumulated dry deposition of chlorides.Peer reviewe

Modelling sea-salt transport and deposition in marine atmosphere zone - A tool for corrosion studies

Atmospheric corrosion of metals and corrosion of reinforcements in concrete structures are influenced by salt concentration of marine aerosol inland. This work presents a model that represents marine aerosol behaviour in marine atmosphere zone. The model has the distance from the sea and wind speed as the major variables and takes into account changes in marine aerosol due to settlement of salts when being transported inland. The proposed equations M = M0 e(v dep0 / α h) [e(- α x / v) - 1], D = D0 e(v dep0 / α h) [e(- α x / v) - 1] show good consistency with literature data and well represent the results obtained. This model can be a useful tool to indicate the salinity level expected at different distances from sea in marine atmosphere zone. © 2008 Elsevier Ltd. All rights reserved.Authors thank CAPES (Brazilian Government Agency for Graduated Professionals Improvement) for supporting the Sandwich Doctorate of Gibson R. Meira in Eduardo Torroja Construction Research Institute – IETcc (Spain), making a reciprocal collaboration between these institutions possible