Obtaining carbonation depth of concrete used two different procedures standard EHE-08

In the Standard EHE 08, for the first time, durability acquires the status of Limit State. Article 8 provides that the term Durability limit state, produced by physical and chemical actions, different loads and actions of structural analysis, which can degrade the concrete and reinforcement to unacceptable limits. The verification of this limit state can be done through a procedure set out in the provisions of the Standard. This procedure is based on the use of tables that, depending on the aggressiveness of the environment in which the structure is the concrete strength and the life of the project, setting the quality of the concrete cover (minimum thickness and maximum water cement ratio of concrete used) and the maximum crack width. This procedure, simple in its application, provides highly secure solutions. In addition, on Annex 9, the Standard EHE 08 offers models for testing the durability limit state in cases of corrosion of reinforcement due to carbonation of concrete or entry of chloride ions. The results obtained with these models are tighter than those obtained with the procedure of the articles. In this paper we use both methods in the study of reinforced concrete structures with potential problems of corrosion of reinforcement due to carbonation of concrete. Later checking the results obtained by both procedures. Results demonstrate that the use of the models listed in Annex 9 of Standard EHE 08 offer cheaper solutions than those obtained using the procedure of the articles

Development of corrosion of steel bars embedded in mortar made with slag from secondary metallurgy

The aim of this work is to study the evolution of the corrosion rate of reinforcements embedded in mortar specimens that have been partly or fully replaced by the sand ladle furnace white slag. Prisms are manufactured mortar 6cm x 8cm x 2cm in which are embedded reinforcing steel bars of 6mm diameter B500SD. At the time of mixing were added varying amounts of chloride ion content by weight of cement (0%, 0.4%, 0.8%, 1.2%, 2%). The specimens were made totally or partially replacing the white slag, getting four different mixes depending on the degree of substitution. After curing the specimens for 28 days in moist chambers proceeded to dry up naturally. Here are gradually dampened by its conservation in a moist chamber, periodically measuring the corrosion rate of the bars using the technique of polarization curve. The results, in terms of corrosion current and corrosion potential, were compared with those obtained on standard samples, without replacement by slag aggregate. The analysis of results allows us to know, depending on the type of mortar used, the chloride threshold with the depassivation produced steel and the corrosion rates achieved in steels in the active state in terms of mortar moisture, obtained from qualitatively using gravimetric techniques. The results achieved to date support the conclusion that no significant differences in the behavior against corrosion induced by chloride ions, between the steel bars embedded in standard samples and the steel bars embedded in samples including with aggregates from slag. Both the chloride threshold resulting in the depassivation steel as the corrosion rate reached through the bars in an active state are very similar in both types of mortars when they have the same moisture content.

Evaluación del comportamiento mecánico, estructural y frente a la corrosión, de una nueva armadura de acero inoxidable dúplex bajo en níquel

La durabilidad de las estructuras de hormigón armado no es ilimitada, en especial en determinados ambientes. El ingreso de agentes agresivos en el hormigón, fundamentalmente dióxido de carbono e iones cloruros, rebasando el espesor del recubrimiento y alcanzando las armaduras, reducen el alto pH del hormigón hasta alcanzar un umbral crítico, por debajo del cual, el acero queda despasivado. Posteriormente, si existe el suficiente aporte de humedad y oxígeno, el acero se corroe, lo que supone drásticas reducciones de la vida de servicio de estas estructuras y su inevitable reparación. La utilización de armaduras de acero inoxidable es una alternativa que está recibiendo cada vez más consideración. Su resistencia a la corrosión en los ambientes más agresivos, incluso con ataque de cloruros, lo convierte en el material idóneo para prolongar de forma muy considerable la vida útil de la estructura. En este trabajo se ha evaluado el comportamiento mecánico y estructural, y de resistencia a la corrosión, de un nuevo acero inoxidable dúplex de bajo contenido en níquel, el EN 1.4482 (AISI 2001), y se ha comparado con el inoxidable austenítico más utilizado, el EN 1.4301 (AISI 304), con el dúplex EN 1.4362 (AISI 2304) y con el tradicional acero al carbono B-500-SD. El estudio mecánico y estructural se ha realizado en tres niveles diferentes: a nivel de barra, estudiando las propiedades mecánicas y de ductilidad de los cuatro aceros citados; a nivel de sección, estudiando su comportamiento a flexión con diferentes cuantías de armado por medio de los diagramas momento-curvatura; y a nivel de pieza, ensayando una serie de vigas armadas con diferentes aceros y cuantías, y comprobando su comportamiento a desplazamiento y resistencia por medio de los diagramas carga-desplazamiento. El estudio de resistencia a la corrosión se ha realizado embebiendo barras corrugadas, de los tres aceros inoxidables mencionados, en probetas de mortero contaminadas con diferentes cantidades de cloruros, y realizando mediciones electroquímicas durante un periodo de al menos un año. Se han preparado probetas de mortero para dos comparativas diferentes. La primera, manteniendo las probetas en un desecador con el 95 % de humedad relativa durante todo el periodo de mediciones. La segunda, sumergiendo parcialmente las probetas en una solución tampón para carbonatar el mortero. Los resultados de los ensayos mecánicos han demostrado dos aspectos diferentes. Uno, que las armaduras de acero inoxidable tienen un comportamiento muy similar a las de acero al carbono en lo referente a las resistencias alcanzadas, en el límite elástico y en rotura, pero distinto en cuanto al módulo de deformación longitudinal, cuyo valor es claramente inferior al del acero al carbono, por lo que su utilización en las estructuras de hormigón necesita tener en cuenta ese dato en los análisis lineales de cálculo. El segundo aspecto es que las armaduras de acero inoxidable laminadas en caliente presentan una ductilidad muy superior a las de acero al carbono, por lo que ofrecen una mayor seguridad frente a su rotura o al colapso de la estructura, lo que se debe tener en cuenta en el análisis de cálculo plástico. En cambio, las armaduras de acero inoxidable laminadas en frío sólo cumplen con los límites mínimos de ductilidad establecidos en la instrucción EHE-08 para los aceros soldables, y no para los aceros con características especiales de ductilidad. El estudio a nivel de sección refleja la paradoja de obtener secciones menos dúctiles con las armaduras de acero inoxidable laminadas en caliente que con las armaduras de acero al carbono. Para subsanarlo, se definen los conceptos de curvatura última de rotura y ductilidad de la sección en rotura, que tienen en cuenta las altas deformaciones alcanzadas por las armaduras de acero inoxidable. Los resultados a nivel de pieza permiten identificar el comportamiento estructural del hormigón armado con barras corrugadas de acero inoxidable y compararlo con el de las estructuras de hormigón armado convencionales, verificando los resultados experimentales con los teóricos obtenidos con la formulación recogida en la instrucción EHE- 08. Los ensayos de resistencia a la corrosión por cloruros demuestran, durante el primer año y medio de vida de las probetas, un comportamiento muy similar entre el nuevo acero inoxidable dúplex bajo en níquel y el austenítico y el dúplex utilizados para la comparación, incluso para las probetas carbonatadas. Por último, se añade una comparativa económica, realizada sobre dos edificaciones tipo, para cuantificar el sobrecoste que supone la utilización de armaduras de acero inoxidable respecto a las de acero al carbono. El alto coste inicial de las armaduras de acero inoxidable se ve compensado en el coste final de la estructura de muy diferentes formas, principalmente dependiendo del grado de acero elegido y de si se emplean en el total de la estructura o solamente en los elementos más expuestos. The durability of the concrete structures is limited, especially in certain environments. The attack of aggressive agents in the concrete, mainly carbon dioxide and chloride ions, penetrating the thickness of concrete cover and reaching the reinforcements, reduce the high pH of concrete to the point of reaching a critical threshold, under which, the steel despasivates. Therefore, if there is enough humidity and oxygen, the steel corroes, causing drastic reductions in the service life of these structures and its inevitable repair. Despite the high initial cost compared to carbon steel, the usage of stainless steel reinforcements is an alternative with a major consideration nowadays. Its resistance to corrosion in the most aggressive atmospheres, including chlorides attack, makes the stainless steel a suitable material to extend considerably its lifetime. In this study, it’s been evaluated the mechanical and structural behaviour, and the corrosion resistance, of a new low-nickel duplex stainless steel EN 1.4482 (AISI 2001), and it has been compared with the most widely used austenitic type EN 1.4301 (AISI 304), with duplex steel EN 1.4362 (AISI 2304) and with the traditional carbon steel B-500-SD. The mechanical and structural study has been carried out in three different levels: bar level, studying mechanical properties and ductility of the four steels; section level, studying its behaviour when blending with different amounts of reinforcement through the moment-curvature diagrams; and structural element level, testing a series of reinforced beams with different steels and amounts, and checking its sag and resistance through the load-deflection diagrams. The corrosion resistance study was performed by embedding ribbed bars, using the three stainless steel listed, on mortar specimens contaminated with different amounts of chlorides, and taking electrochemical measurements over a period of at least one year. Mortar specimens have been prepared for two different comparisons. The first, keeping the specimens at 95% of relative humidity during the measurement period. The second, immersing the specimens partially in a carbonate buffer solution. The results of those tests have proved two different aspects. Firstly, that stainless steel reinforcements show a very similar behaviour to carbon steel, according to the reached levels of mechanical resistance, yield stress and steel strength, but a different behaviour in Young’s modulus, which value is clearly lower than the carbon steel. Therefore, when using in concrete structures it is need to consider on that point the existing calculus of linear analysis. The second aspect is that stainless steel reinforcement manufactured by hot-rolling process show a very higher ductility than carbon steel, offering a better security on cracks or structure collapse, which it has to be taken into account on plastic calculus analysis. However, the stainless steel reinfor9 cement cold-rolled bars only meet the minimum thresholds of ductility established by EHE-08 for welded steel, and not for steels with special ductility. The results at the section level reflect the paradox of getting less ductile sections with hot rolled stainless steel reinforcement than with carbon steel reinforcements. To overcome that, the concepts of last break curvature and break ductility section have been defined, which take into account the high deformation value achieved by stainless steel reinforcements. The results at the structural element level allow to identify the structural behaviour of reinforced concrete with stainless steel reinforcements and compared with that of conventional steel reinforcement, contrasting the experimental with the theoretical results obtained from the formulation contained in the instruction EHE-08. Tests on resistance of chloride corrosion show during the first year and a half of specimens life, a similar behaviour between the new low nickel duplex stainless steel and austenitic and duplex used for comparison, even for carbonated specimens. Finally, it has been included an economic comparison on two differents building types, to quantify the additional cost involved on the use of stainless steel reinforcement compared to that of carbon steel. The high initial cost of stainless steel reinforcements is offset in the final cost of the structure in many different ways, mainly depending on the chosen steel grade and whether the reinforcement is used in the total structure or only in risky structural elements.

Coupled model of initiation and propagation of corrosion in reinforced concrete

Corrosion of reinforcing steel in concrete due to chloride ingress is one of the main causes of the deterioration of reinforced concrete structures. Structures most affected by such a corrosion are marine zone buildings and structures exposed to de-icing salts like highways and bridges. Such process is accompanied by an increase in volume of the corrosión products on the rebarsconcrete interface. Depending on the level of oxidation, iron can expand as much as six times its original volume. This increase in volume exerts tensile stresses in the surrounding concrete which result in cracking and spalling of the concrete cover if the concrete tensile strength is exceeded. The mechanism by which steel embedded in concrete corrodes in presence of chloride is the local breakdown of the passive layer formed in the highly alkaline condition of the concrete. It is assumed that corrosion initiates when a critical chloride content reaches the rebar surface. The mathematical formulation idealized the corrosion sequence as a two-stage process: an initiation stage, during which chloride ions penetrate to the reinforcing steel surface and depassivate it, and a propagation stage, in which active corrosion takes place until cracking of the concrete cover has occurred. The aim of this research is to develop computer tools to evaluate the duration of the service life of reinforced concrete structures, considering both the initiation and propagation periods. Such tools must offer a friendly interface to facilitate its use by the researchers even though their background is not in numerical simulation. For the evaluation of the initiation period different tools have been developed: Program TavProbabilidade: provides means to carry out a probability analysis of a chloride ingress model. Such a tool is necessary due to the lack of data and general uncertainties associated with the phenomenon of the chloride diffusion. It differs from the deterministic approach because it computes not just a chloride profile at a certain age, but a range of chloride profiles for each probability or occurrence. Program TavProbabilidade_Fiabilidade: carries out reliability analyses of the initiation period. It takes into account the critical value of the chloride concentration on the steel that causes breakdown of the passive layer and the beginning of the propagation stage. It differs from the deterministic analysis in that it does not predict if the corrosion is going to begin or not, but to quantifies the probability of corrosion initiation. Program TavDif_1D: was created to do a one dimension deterministic analysis of the chloride diffusion process by the finite element method (FEM) which numerically solves Fick’second Law. Despite of the different FEM solver already developed in one dimension, the decision to create a new code (TavDif_1D) was taken because of the need to have a solver with friendly interface for pre- and post-process according to the need of IETCC. An innovative tool was also developed with a systematic method devised to compare the ability of the different 1D models to predict the actual evolution of chloride ingress based on experimental measurements, and also to quantify the degree of agreement of the models with each others. For the evaluation of the entire service life of the structure: a computer program has been developed using finite elements method to do the coupling of both service life periods: initiation and propagation. The program for 2D (TavDif_2D) allows the complementary use of two external programs in a unique friendly interface: • GMSH - an finite element mesh generator and post-processing viewer • OOFEM – a finite element solver. This program (TavDif_2D) is responsible to decide in each time step when and where to start applying the boundary conditions of fracture mechanics module in function of the amount of chloride concentration and corrosion parameters (Icorr, etc). This program is also responsible to verify the presence and the degree of fracture in each element to send the Information of diffusion coefficient variation with the crack width. • GMSH - an finite element mesh generator and post-processing viewer • OOFEM – a finite element solver. The advantages of the FEM with the interface provided by the tool are: • the flexibility to input the data such as material property and boundary conditions as time dependent function. • the flexibility to predict the chloride concentration profile for different geometries. • the possibility to couple chloride diffusion (initiation stage) with chemical and mechanical behavior (propagation stage). The OOFEM code had to be modified to accept temperature, humidity and the time dependent values for the material properties, which is necessary to adequately describe the environmental variations. A 3-D simulation has been performed to simulate the behavior of the beam on both, action of the external load and the internal load caused by the corrosion products, using elements of imbedded fracture in order to plot the curve of the deflection of the central region of the beam versus the external load to compare with the experimental data.

Evaluación de parámetros que influyen en el transporte de cloruros en hormigón parcialmente saturado

El deterioro del hormigón debido a la presencia del ion cloruro es causa frecuente de problemas en estructuras localizadas en ambiente marino y alta montaña. Su principal efecto consiste en la despasivación del acero de refuerzo embebido en el hormigón y su consecuente inicio de la corrosión del mismo. El ingreso del ion cloruro al interior del hormigón, está condicionado por una serie de parámetros de origen medioambiental e intrínsecos del hormigón. En función de estos parámetros el ingreso de cloruros en el hormigón puede deberse principalmente a los siguientes mecanismos: difusión y succión capilar. El estudio y evaluación de la resistencia del hormigón frente a cloruros, se ha desarrollado principalmente en condiciones saturadas del hormigón. Lo que ha significado que parámetros de importancia no sean considerados. Debido a esto, distintos procesos que suceden en estructuras reales no han sido identificados y estudiados correctamente. En este trabajo, se diseñó un programa de investigación para evaluar los parámetros que influyen en el transporte cloruros en hormigones no saturados. Para esto se diseñaron tres dosificaciones diferentes de hormigón. En la primera se empleó únicamente cemento portland, para el resto se utilizaron adiciones minerales (humo de sílice y escoria de alto horno). El empleo de adiciones se debió a que tienen un papel importante en la durabilidad de hormigones frente a cloruros. Los hormigones fueron dosificados con una relación agua/material cementício de 0,40 para el hormigón elaborado únicamente con cemento portland y 0,45 para las mezclas con adiciones. Para evaluar las propiedades de los hormigones en estado fresco y endurecido se realizaron ensayos vigentes en las normativas. Con los resultados obtenidos se determinaron parámetros de resistencia mecánica, microestructurales, resistencia al transporte de cloruros e higroscópicos. Una vez caracterizados los hormigones, se diseñó una propuesta experimental para estudiar los principales parámetros presentes en estructuras reales con presencia de cloruros. Tanto la concentración de cloruro como las condiciones ambientales se han variado teniendo como referencia las situaciones reales que podrían producirse en ambientes de alta montaña en la zona centro de España. La propuesta experimental consistió en tratar de evaluar la capacidad de los hormigones al transporte de iones en ambientes de alta montaña con presencia de sales fundentes. Para esto se establecieron 5 fases experimentales donde los principales parámetros ambientales y la presencia de iones agresivos sufrieron variaciones. Al término de cada fase se obtuvieron perfiles de penetración de cloruros en los hormigones y se evaluó la influencia de los parámetros presentes en cada fase. Los resultados experimentales se implementaron en un modelo numérico basado en la teoría de elementos finitos, desarrollado por el grupo de investigación del Departamento de Materiales de Construcción. Para esto fue necesario realizar la calibración y validación del modelo numérico para cada hormigón. El calibrado del modelo precisa de datos químicos y microestructurales de cada hormigón, tales como: capacidad de combinación de cloruros y propiedades difusivas e higroscópicas. Para la validación del modelo numérico se realizaron simulaciones de la propuesta experimental. Los resultados obtenidos se compararon con los valores experimentales. Con el objeto de poder estudiar en mayor profundidad la influencia del grado de saturación del hormigón durante la difusión de cloruros, se llevó a cabo una campaña experimental que consideró distintos grados de saturación en los hormigones. Para esto se establecieron en los hormigones cuatro grados de saturación distintos (50%, 60%, 80% y 100%, aproximadamente), posteriormente se expusieron a cloruro de sodio finamente molido. Una vez transcurrido el tiempo necesario se obtuvieron experimentalmente los perfiles de penetración de cloruros para cada grado de saturación y se calcularon los coeficientes de difusión. Los datos obtenidos durante la campaña experimental han demostrado la influencia positiva que ejercen las adiciones en las mezclas de hormigón. Sus principales ventajas son el refinamiento de la red porosa y el aumento en la capacidad de combinación de cloruros, además de mejorar sus propiedades mecánicas. La porosidad total en las mezclas no presentó grandes cambios, sin embargo, el cambio en la distribución del tamaño de poros es importante en las muestras con adiciones. En especial las fabricadas con humo de sílice. Los coeficientes de difusión y migración de cloruros para las mezclas con adiciones disminuyeron significativamente, igual que los valores de resistividad eléctrica. En los ensayos de penetración del agua bajo presión, fueron las muestras con adiciones las que mostraron las menores penetraciones. Los resultados obtenidos al final de la propuesta experimental permitieron estudiar los distintos parámetros involucrados. Se observó claramente que el proceso de difusión provoca el mayor transporte de cloruros hacia el interior del hormigón. Así mismo se comprobó que el lavado superficial y el secado de las probetas, trasladan cloruros hacia las zonas externas del hormigón. El primero debido a una baja concentración de cloruros externa, mientras que el secado provoca el movimiento de la solución de poro hacia las zonas de secado depositando cloruros en ellas. Las medidas higroscópicas permitieron determinar la existencia de dos zonas distintas en el interior del hormigón. La primera se localizó en el rango de 0-10mm, aproximadamente, en ésta se puso de manifiesto una mayor sensibilidad a los cambios experimentados en el exterior de las probetas. La segunda zona se localizó a una profundidad mayor de 10mm, aproximadamente. Se observó claramente una baja influencia de los cambios externos, siendo la difusión de cloruros el principal mecanismo de transporte presente en ella. En cuanto al estudio de la influencia del grado de saturación en la difusión de cloruros, se observó claramente una marcada diferencia entre los coeficientes de difusión de cloruro obtenidos. Para grados de saturación mayores del 80% el mecanismo de penetración de cloruros por difusión existe de forma significativa. Mientras que para valores inferiores los resultados revelaron que las vías de acceso disminuyen (poros conectado con agua) considerablemente limitando en un alto grado la penetración del agresivo. Para grados de saturación inferiores del 50% los valores del coeficiente de difusión son despreciables. The deterioration of concrete due to chloride ions is a frequent problem identified in structures located in marine and high-mountain environments. After entering the outer layer of the concrete, the chlorides tend to penetrate until they reach and then depassivate the steel bars. Subsequently, this induces the deterioration process of the reinforced concrete. This chloride penetration depends on the environmental conditions and intrinsic parameters of the concrete. Several transport mechanisms, such as diffusion, capillary suction and permeability can be present into the concrete. While recent research into the study and evaluation of concretes with chloride presence has been carried out in saturated concrete, it has not considered certain parameters that can modify this condition. Consequently, at the time of writing several processes that take place in real structures have not been identified and studied. In this work a research programme is designed to evaluate the parameters that influence chloride transport into non-saturated concrete. For this, three concrete mixes were designed by using high-early-strength Portland cement and mineral admixtures (silica fume and blast-slag furnace). The water-cement ratio was 0.40 for the concrete made solely with Portland cement and 0.45 for the concretes that used mineral admixtures as a cement replacement. A set of experimental tests were performed to evaluate the concrete properties both in fresh and hardened state. In addition, an experimental simulation was carried out under laboratory conditions in which the main objective was to assess resistance of concrete to chloride penetration under high-mountain conditions with the presence of de-icing salts. The environmental conditions and surface chloride concentration of the concrete used during the experimental simulation were chosen by considering conditions found in the high-mountain environment in central Spain. For the experimental simulation five phases were designed by varying the environmental parameters and concrete surface concentration. At the end of each phase a chloride profile was obtained with the aim of assessing the influence of the parameters on chloride transport. The experimental results were then used to calibrate and validate a numerical model based on finite element theory developed by the research team from the Construction Materials Department in a previous work. In order to carry out model calibration chemical and microstructural data for the concretes was required, such as binding capacity and the diffusive and hygroscopic properties. The experimental results were compared with the numerical simulations and provided a good fit. With the objective of studying the influence of the degree of concrete saturation on chloride diffusion, an experimental programme was designed. This entailed four saturation degrees (50%, 60%, 80% and 100%) being established in several concrete samples. The samples were then exposed to ground sodium chloride. Once the time required was achieved, the chloride profiles and diffusion coefficients were obtained for each saturation degree. The results obtained from the experimental program revealed a positive influence of the mineral admixtures on the concretes. Their effects were reflected in the pore-network refinement and the increase of chloride binding capacity, together with the improvement of the mechanical properties of the concretes. Total porosity did not reveal any notable change, though the pore-size distribution showed a significant degree of change in the concretes with mineral admixtures, specifically the samples prepared through use of silica fume. The chloride diffusion and migration coefficient, as well as the electrical resistivity values, decreased significantly in the concretes with admixtures. In the water penetration under pressure test, the concretes with admixtures presented the lowest penetration depth. The results obtained in the experimental simulation allowed study of the main parameters involved during the chloride penetration processes in non-saturated concretes in the presence of chlorides. According to the results, the diffusion process was the transport mechanism that transferred the greatest amount of chlorides into the concrete samples. In addition, a chloride movement toward external zones of the concrete, caused by the washing of concrete surface and the drying processes, was identified. The washing occurred when the concrete surface came into contact with a low-chloride concentration solution which initiated the outward movement of chloride diffusion. The drying processes corresponded to a movement of pore solution launched by water evaporation from the outer layer. Furthermore, hygroscopic measurements made in the concrete allowed two areas with distinct behavioural patterns to be identified. The first one, located in the range of approximately 0-10mm, showed a greater degree of influence regarding the changes of the external conditions. The second, situated at depths greater than 10mm, displayed a low influence of external conditions. The main process in this area was diffusion. Study of the influence of the degree of concrete saturation on chloride diffusion showed a clear difference among the chloride diffusion coefficients obtained. For degrees of concrete saturation higher than 80%, chloride penetration by diffusion tends to be significant. However, in the case of extent of saturation of lower than 80%, the results revealed that the access zone through which chlorides can penetrate decreased considerably. For degrees of concrete saturation lower than 50%, the chloride diffusion coefficients were negligible.

Retinal glial cell glutamate transporter is coupled to an anionic conductance.

Application of L-glutamate to retinal glial (Müller) cells results in an inwardly rectifying current due to the net influx of one positive charge per molecule of glutamate transported into the cell. However, at positive potentials an outward current can be elicited by glutamate. This outward current is eliminated by removal of external chloride ions. Substitution of external chloride with the anions thiocyanate, perchlorate, nitrate, and iodide, which are known to be more permeant at other chloride channels, results in a considerably larger glutamate-elicited outward current at positive potentials. The large outward current in external nitrate has the same ionic dependence, apparent affinity for L-glutamate, and pharmacology as the glutamate transporter previously reported to exist in these cells. Varying the concentration of external nitrate shifts the reversal potential in a manner consistent with a conductance permeable to nitrate. Together, these results suggest that the glutamate transporter in retinal glial cells is associated with an anionic conductance. This anionic conductance may be important for preventing a reduction in the rate of transport due the depolarization that would otherwise occur as a result of electrogenic glutamate uptake.

Estudos mecanísticos da origem da inibição da reação foto-Fenton por íons cloreto

O objetivo principal deste estudo foi determinar a origem da inibição do processo foto-Fenton [Fe(II)/Fe(III), H2O2, luz UV] pelo íon cloreto. Um estudo das reações primárias da etapa fotocatalítica do processo foto-Fenton por fotólise por pulso de laser na presença de NaCl mostrou que a inibição reflete: i) fotólise competitiva dos complexos Fe(Cl)2+ e Fe(Cl)2+; ii) captura do radical hidroxila (dependente do pH) pelo íon cloreto. Esses dois processos formam o ânion radical menos reativo Cl2•- em lugar do radical HO•-, provocando uma progressiva inibição da reação de degradação com a diminuição do pH. Modelagem cinética destes resultados previa que a manutenção do pH em 3,0 durante a fotodegradação evitaria a formação do Cl2•-, o que foi confirmada através de experimentos de fotodegradação do fenol e da gasolina em meio aquoso na presença de NaCl. Por outro lado, na degradação do fenol pela reação térmica de Fenton [Fe(II)/Fe(III), H2O2], o radical hidroxila não parece ter um papel muito importante. A degradação térmica não foi inibida pela presença de íon cloreto e a cinética de mineralização do fenol pela reação térmica de Fenton é indistinguível da degradação do fenol pelo processo foto-Fenton inibido por NaCl. Isso sugere que a reação proposta por Hamilton, isto é, a redução de Fe(III) a Fe(II) por catecol (o principal intermediário inicial da oxidação do fenol) na presença de H2O2, é o mecanismo principal de catálise da reação térmica de Fenton no nosso sistema.

Polímeros condutores aplicados a sistemas-modelo de liberação controlada eletroquimicamente de drogas

Este trabalho descreve a síntese, caracterização e aplicação de sistemas poliméricos baseados em polímeros condutores em sistemas de liberação controlada de drogas. Esta tese pode ser dividida em duas partes: na primeira se apresentam os resultados da aplicação de filmes de polianilina e polipirrol na liberação de drogasmodelo como a dopamina protonada e o ácido salicílico. Na liberação de salicilato utilizou-se um filme polianilina eletrosintetizado e dopado com íons cloreto. Já para a liberação de dopamina protonada (um cátion) a liberação foi conduzida a partir de um sistema bicamadas, com um filme de polianilina recoberta com uma camada de Náfion. É mostrada a liberação controlada nos dois casos, porém também se discutem limitaçãoes deste tipo de sistema que levaram ao estudo de uma forma alternativa de controle eletroquímico utilizando polímeros condutores. A segunda parte do trabalho mostra então esta nova metodologia que se baseia em compósitos de poianilina eletropolimerizada no interior de hidrogéis de poliacrilamida. É mostrado que este novo material é eletroativo e mantém as características de intumescimento dos hidrogéis, tanto necessárias ao desenvolvimento destes sistemas de liberação controlada. Mecanismos para o crescimento e distribuição da polianilina na matriz isolante e para a atuação do compósito no controle eletroquímico da liberação são propostos com base nos dados de microscopia de força atômica, Raman e eletrônica de varredura, além de testes de liberação controlada com moléculas de diferentes cargas.

Complexos de dirutênio com os fármacos ibuprofeno, ácido acetilsalicílico, naproxeno, indometacina, e com ácido y-linolêncio: Síntese, caracterização, avaliação da ação antiproliferativa sobre células tumorais e estudo da interação da unidade dimetálica com adenina e adenosina

Este trabalho tem como principal objetivo contribuir para o desenvolvimento de novos potenciais metalofármacos de rutênio. Nele são descritas a síntese, a caracterização e a avaliação da ação antiproliferativa de alguns complexos de dirutênio (II,III) com os fármacos antiinflamatórios não-esteróides (AINEs): ibuprofeno (ibp), ácido acetilsalicílico (aas), naproxeno (npx) e indometacina (ind) e também com o ácido γ-linolênico (lin), sobre células cancerígenas. Os compostos obtidos foram caracterizados por análise elementar, espectroscopia de absorção eletrônica, medidas de susceptibilidade magnética, espectroscopia vibracional FTIR e Raman, difratometria de raios X de pó, medidas de condutividade molar e análise térmica (TG, OTAe OSC). Todos os complexos sintetizados apresentam estrutura em gaiola, com os carboxilatos derivados dos fármacos AINEs coordenados à unidade dimetálica Ru2( (II,III), em ponte equatorial, estabilizando assim a ligação direta rutênio-rutênio. As posições axiais são ocupadas por íons cloreto, no caso dos complexos [Ru2(O2(CR)4(Cl] (O2(CR = ibp, aas, npx ou ind), ou por moléculas de água, nas espécies do tipo [Ru2(O2(CR)4(H2O)2]PF6(O2CR =npx e ind). Ensaios biológicos demonstraram que os compostos [Ru2(ibp) 4Cl]•½H2O e [Ru2(npx)4(H2O)2]PF6 apresentam ação antiproliferativa sobre células de glioma de rato C6 in vitro, dependendo do tempo de exposição do meio celular ao complexo. O complexo [Ru2 (lin)4Cl] também apresenta efeito sobre a proliferação de células C6; entretanto, nesse caso, efeitos significativos são observaçlos já nas primeiras 24 h de exposição. Estudos mostraram que as bases adenina e adenosina reagem com o complexo [Ru2(OAc)4(H2O)2]PF6 sem que ocorra quebra da estrutura em gaiola. As bases nitrogenadas substituem axialmente as moléculas de água, formando pontes axiais entre duas unidades de dirutênio (II,III) no estado sólido.

Correlação entre microestrutura e comportamento de corrosão em duas ligas do sistema Al-Fe-Si produzidas pelos processos industriais de lingotamento contínuo e semi-contínuo.

As chapas de ligas de alumínio trabalháveis são produzidas atualmente por dois processos, o método de vazamento contínuo conhecido TRC (Twin Roll Continous Casting) ou pelo método tradicional de vazamento de placas DC (Direct Chill). A fabricação de ligas de alumínio pelos dois processos confere características microestruturais diferentes quando comparadas entre si, o que se reflete em suas propriedades. Além disto, ocorrem variações microestruturais ao longo da espessura, especialmente nas chapas produzidas pelo processo TRC. Neste sentido, é importante estudar a evolução microestrutural que ocorre durante o seu processamento e sua influência com relação à resistência à corrosão. Dessa forma foi realizado neste trabalho um estudo comparativo do comportamento de corrosão, bem como das microestruturas do alumínio de alta pureza AA1199 (99,995% Al) e das ligas de alumínio AA1050 (Fe+Si0,5%) e AA4006 (Fe+Si1,8%) produzidas pelos processos industriais de lingotamento contínuo e semi-contínuo. Os resultados obtidos evidenciaram que as microestruturas das ligas AA4006 DC e AA4006 TRC são distintas, sendo observada maior fração volumétrica dos precipitados na liga fabricada pelo processo TRC comparativamente ao DC. Para caracterizar o comportamento de corrosão foram realizados ensaios de Espectroscopia de Impedância Eletroquímica e Polarização Potenciodinâmica, que mostraram a maior resistência à corrosão localizada para a liga fabricada pelo processo TRC em comparação ao processo DC. Além disso, foi verificada, em ordem decrescente, uma maior resistência à corrosão do alumínio AA1050, seguida pela superfície da liga AA4006 e por fim, pelo centro da chapa desta última. Os resultados obtidos por espectroscopia de impedância eletroquímica para as ligas AA4006 fabricadas pelo processo TRC apresentaram melhor desempenho que o processo DC, principalmente em intervalos de 2 a 12 horas de imersão na solução de sulfato de sódio contaminada com íons cloreto. Para tempos de imersão acima de 4 horas foi observado comportamento indutivo em baixas frequências para os dois tipos de processamento investigados, o que foi associado à adsorção de espécies químicas, principalmente íons sulfato e oxigênio, na interface metal/óxido. As curvas de polarização anódica mostraram maior resistência à corrosão localizada para a liga fabricada pelo processo viii TRC em comparação ao processo DC. Este comportamento foi associado às diferentes características microestruturais, observadas para liga AA4006 obtida pelos dois processos.

A study on transition of iron from active into passive state

An investigation was carried out on the transition of an iron electrode from active to passive state in a sulphuric acid solution. It was found that the active-passive transition was an auto-catalytic process in which a pre-passive film grew on the electrode surface. The growing pre-passive film had a fractal edge whose dimension was affected by the applied passivating potential and the presence of chlorides in the solution. Applying a more positive passivating potential led to a faster active-passive transition and resulted in a more irregular pre-passive film. If chlorides were introduced into the sulphuric acid solution, the active-passive transition became more rapid and the pre-passive film more irregular. Apart from the influence on the growth of the pre-passive film, the presence of chlorides in the passivating solution was found to deteriorate the stability of the final passive film. All these phenomena can be understood if the passivating iron electrode is regarded as a dissipative system. To explain these results, a fractal pre-film model is proposed in this paper. (C) 2004 Elsevier Ltd. All rights reserved.

Cholinergic and adrenergic innervation of lingual salt glands of the estuarine crocodile, Crocodylus porosus

Many marine reptiles and birds possess extrarenal salt glands that facilitate the excretion of excess sodium and chloride ions accumulated as a consequence of living in saline environments. Control of the secretory activity of avian salt glands is under neural control, but little information is available on the control of reptilian salt glands. Innervation of the lingual salt glands of the salt water crocodile, Crocodylus porosus, was examined in salt water-acclimated animals using histological methods. Extensive networks of both cholinergic and adrenergic nerve fibres were identified close to salt-secreting lobules and vasculature. The identification of both catecholamine-containing and cholinergic neurons in the salt gland epithelium and close to major blood vessels in the tissue suggests the action of the neurotransmitters on the salt-secreting epithelium itself and the rich vascular network of the lingual salt glands.

Effect of pulsed current on the electrodeposition of chromium and copper

This research was concerned with the effects of pulsed current on the electrodeposition of chromium and copper. In the case of the latter metal, a novel application has been studied and a theory proposed for the ability to improve throwing power by the joint use of organic additives and pulsed reverse current. During the course of the research, several improvements were made to the pulse plating unit.Chromium. A study was made of the effect of square wave pulsed current on various physical properties of deposits from three hard chromium plating electrolytes. The effect of varying frequency at a duty cycle of 50% on the mean bulk internal stress, visual appearance, hardness, crack characteristics and surface topography of the electrodeposits was determined. X-ray diffraction techniques were used to study the phases present in the deposits. The effect of varying frequency on the cathodic efficiencies of the electrolytes was also determined. It was found that pulsed current reduced the internal stress of deposits from the sulphate catalysed electrolyte. It also reduced or eliminated cracking of deposits and reduced deposit brightness. Under certain conditions, pulsed current was found to induce the co-deposition of hydrides of chromium. Deposit hardness was found to be reduced by the use of pulsed current. Cathodic efficiencies of the high efficiency electrolytes were reduced by use of pulsed current although this effect was minimised at high frequencies. The sulphate catalysed electrolyte showed an increase in efficiency over the frequency range where hydrides were co-deposited.Copper. The polarisation behaviour of acid copper solutions containing polyethers, sulphopropyl sulphides and chloride ions was studied using both direct and pulse reverse current. The effect of these additives on the rest potentials of copper deposits immersed in the electrolyte was also studied. Hole Throwing Power on printed circuit boards was determined using a specially designed test cell. The effect of pulsed reverse current on the hole throwing power of commercially produced printed circuit boards was also studied. Polyethers were found to have an inhibiting effect on the deposition of copper whereas the sulphopropyl sulphides produced a stimulating (i.e. depolarising) effect. Studies of rest potentials made when both additives were present indicated that the sulphopropyl sulphide was preferentially adsorbed. The use of pulsed reverse current in solutions containing both polyether and sulphopropyl sulphide was found to cause desorption of the sulphopropyl sulphide at the cathode surface. Thus, at higher current densities, the inhibiting effect of the polyether produced an increase in the cathodic polarisation potential. At lower current densities, the depolarisation effect of the sulphopropyl sulphide could still occur. On printed circuit boards, this effect was found to produce an increase in the `hole throwing power' due to depolarisation of the holes relative to the surface of the boards. Typically, using direct current, hole/surface thickness ratios of 40% were obtained when plating 0.6 mm holes in a 3.2 mm thick board at a current density of 3 A/dm2 whereas using pulsed reverse current, ratios of 80% could be obtained at an equivalent rate of deposition. This was observed both in laboratory tests and on commercially plated boards.

Microstructural analysis of surface and interface zones in concrete

Several of OPC paste and concrete specimens, with different mix proportions, were cast against CPF and impermeable formwork (IF) and the profiles of pore structure, microhardness and scratch hardness of the cover zone were established. The chloride ingress and the depth of carbonation of the surface zone of concrete cast against CPF and IF were investigated. The main mechanisms controlling the ECR processes and the factors affecting such treatment were critically reviewed. Subsequently, as a means of restoring passivation of steel embedded in carbonated concrete, such HCP specimens were subjected to ECR. The influence of ECR on the chemistry of the pore solution and the microstructure of the surface and the steel/cement past interface zones were also studied. The main findings of this investigation were as follows: (a) The thickness of the microstructure gradient of cover concrete is significantly decreased with increasing period of water curing but is relatively unaffected by curing temperature, w/e ratio and the use of cement replacement materials. (b) The scratch hardness technique was shown to be potentially useful for characterising the microstructure and microhardness gradients of the surface zone. (c) A relationship between the microstructure gradient and mass transport properties of the surface zone was established. (d) The use of CPF resulted in a significant reduction in porosity of both the cement paste matrix and the aggregate/cement paste transition zone, and a marked improvement in the resistance of the surface zone to carbonation and the ingress of chloride ions. (e) The ECR treatment resulted in a marked densification of the pore structure and in changes to the pore solution chemistry and the cement phases of near-surface and steel/cement paste transition zones. This effect was more pronounced with current density, period of treatment and particularly with the use of sodium phosphate as an electrolyte.

Metakaolin as a cement extender

The effect of 10% and 20% replacement metakaolin on a number of aspects of hydration chemistry and service performance of ordinary Portland cement pastes has been investigated. The analysis of expressed pore solutions has revealed that metakaolin-blended specimen pastes possess enhanced chloride binding capacities and reduced pore solution pH values when compared with their unblended counterparts. The implications of the observed changes in pore solution chemistry with respect to chloride induced reinforcement corrosion and the reduction in expansion associated with the alkali aggregate reaction are discussed. Differential thermal analysis, mercury intrusion porosimetry, and nuclear magnetic resonance spectroscopy have been employed in the analysis of the solid phase. It is suggested that hydrated gehlenite (a product of pozzolanic reaction) is operative in the removal and solid state binding of chloride ions from the pore solution of metakaolin-blended pastes. Diffusion coefficients obtained in a non-steady state chloride ion diffusion investigation have indicated that cement pastes containing 10% and 20% replacement metakaolin exhibit superior resistance to the penetration of chloride ions in comparison with those of plain OPC of the same water:cement ratio. The chloride induced corrosion behaviour of cement paste samples, of water:cement ratio 0.4, containing 0% , 10%, and 20% replacement metakaolin, has been monitored using the linear polarization technique. No significant corrosion of embedded mild steel was observed over a 200 day period.