A reação álcali-agregado no concreto é um fenômeno que tem como causa uma reação química que ocorre entre os hidróxidos alcalinos provenientes do cimento e alguns minerais reativos presentes no agregado. Esta reação pode causar a deterioração do concreto, pois os seus subprodutos podem tornar-se expansivos na presença de umidade, originando fissuração, diminuição da resistência, aumento da permeabilidade e, eventualmente, a ruptura da estrutura. O uso de adições minerais em concretos suscetíveis à reação álcali-agregado tem sido apontado como uma alternativa eficiente na prevenção da reação expansiva, juntamente com o uso de agregados não reativos e a limitação dos teores de álcalis no cimento ou concreto. Neste contexto, o presente trabalho teve como objetivo principal a investigação do processo de deterioração do concreto devido à reação álcali-sílica, principalmente no que se refere ao tipo ou mineralogia do agregado e à utilização de cinza de casca de arroz, como substituição parcial do cimento Portland. Desta forma, o programa de pesquisa compreendeu, a realização de ensaios de expansão acelerada em barras de argamassa (ASTM C1260/94) moldadas com cimento Portland tipo CP-I S 32, com teores de 12,5; 25 e 50% de dois tipos de cinza de casca de arroz, em substituição parcial ao cimento, e quatro diferentes tipos de agregados (basalto B, basalto BGO, granito e riodacito). A microestrutura dos materiais utilizados e das barras submetidas ao ensaio acelerado foi avaliada através de técnicas analíticas e experimentais, tais como, petrografia, difração de raios x, porosimetria por intrusão de mercúrio, microscopia eletrônica de varredura e de transmissão (MEV e MET), com microanálise por detecção de energia dispersiva (EDS). Os resultados obtidos no ensaio acelerado comprovaram a potencialidade reativa das rochas analisadas e identificaram uma correlação entre o tipo de rocha e o teor de cinza de casca de arroz. A análise da microestrutura indicou que existe uma reação química da CCA com o meio alcalino utilizado no ensaio que acaba interferindo na formação e na relação C/S dos produtos expansivos resultantes da reação álcali-sílica. / The alkali-aggregate reaction in concretes is a phenomenon caused by a chemical reaction that occurs between alkaline hydroxides from Portland cement and some reactive minerals from aggregates. Such reaction can cause severe concrete deterioration, as its by-products can become expansive in the presence of water, originating fissuration, strength reduction, permeability increase, and eventually, the failure of concrete structures. The use of mineral admixtures in concretes susceptible to the alkali-aggregate reaction has been pointed out as an efficient alternative to prevent concrete expansion, along with the use of non-reactive aggregates and the limitation of the alkali amount in cement or concrete composition. In this context, the main purpose of the present work was the investigation of concrete deterioration due to the alkali-silica reaction, focusing the aspects related to the type or mineralogy of the aggregate and the utilization of rice husk ash as partial substitution of Portland cement. The research program comprised initially accelerated expansion tests carried out in mortar bars (ASTM C 1260/94), which were molded using CP-I S 32 Portland cement, 12.5, 25, and 50% contents of two types of rice husk ash, as partial replacement to the cement, and four different types of rock aggregates (basalt B, Basalt BGO, granite, and rhyodacite). Also, the microstructure of the concrete mixtures investigated, after being submitted to expansion in the accelerated tests, were evaluated through experimental and analytical techniques such as petrography, mercury intrusion porosimetry, x-ray diffraction, scanning and transmission electron microscopy (SEM and TEM), and energy dispersive detection (EDS). The results obtained have proved the reactivity potential of the investigated rock aggregates and identified a correlation between type of aggregate and rice husk ash content. The microstructure analysis indicated that the occurrence of a chemical reaction involving the rice husk ash in the alkaline environment established in the tests, had a significant effect on the amount of expansive by-products as C/S relation resulting from alkali-silica reactions.

The objectives of this research were to investigate the sensitivity of two-prong and fourprong resistivity measurements to certain operator-controlled variables and to conduct a direct comparison of the sensitivity of two-prong and four-prong resistivity measurements to certain concrete material variables. Four full-factorial experiments were designed for this research. In the experimentation on operator-controlled variables with two-prong resistivity testing, main effects that are both statistically significant and practically important include hole depth and surface water. In the experimentation on operator-controlled variables with four-prong resistivity testing, probe position, surface water, and prong spacing are all neither statistically significant nor practically important. This high degree of unexplained variation may be of concern to practitioners. In the experimentation on concrete material variables with two-prong and four-prong resistivity testing, main effects that are both statistically significant and practically important include chloride concentration and temperature, both of which exhibit inverse relationships with resistivity. These research findings support several important recommendations for resistivity testing. Operators of the two-prong resistivity device should use an accurately positioned drill stop to ensure that the prepared holes are consistently the correct depth, and they should expect to obtain different values depending on the presence of surface water on the deck surface. Operators considering use of the four-prong resistivity device should not expect the measurements to be sensitive to probe position with respect to rebar, presence of surface water, or prong spacing for conditions similar to those investigated in this research. Operators interested in monitoring resistivity values over time to ascertain material changes in a bridge deck should develop protocols for measuring concrete temperature in the field and subsequently normalizing resistivity measurements to a standard temperature.

Un béton exposé à des cycles de gel-dégel peut subir des dégradations généralement identifiées sous deux formes. Le gel interne, d’une part, qui affecte le coeur du matériau et peut aboutir à une microfissuration généralisée de la pâte cimentaire. La sensibilité d’un béton courant à ce mode de détérioration peut être diminuée par la présence d’un réseau de bulles d’air au sein de la pâte cimentaire. L’écaillage, d’autre part, qui est une détérioration de surface. Cette dernière est le plus souvent quantifiée par la perte de masse sur la surface exposée. Cette forme de dégradation est fortement influencée par la présence d’une solution aqueuse proche de la surface de béton exposée et de sa concentration en sels fondants (généralement du chlorure de sodium). Lorsque la formulation d’un béton courant n’est pas adaptée pour résister à ce type d’environnement, les dégradations liées au gel interne et à l’écaillage peuvent pénaliser l’exploitation de l’ouvrage concerné dans des conditions de services attendues. Des essais laboratoire, exposant des corps d’épreuve en béton à des cycles de gel-dégel accélérés, permettent d’évaluer la résistance d’un béton face à ces deux formes de détériorations. La communauté scientifique s’accorde sur les essais permettant de caractériser la détérioration par le gel interne et l’aspect protecteur d’un réseau de bulles d’air. Par contre, l’écaillage est encore abordé par diverses procédures expérimentales qui ne font pas consensus et ont peu évolué depuis le milieu des années 1990 (...) / A concrete submitted to freeze-thaw cycles may undergo damages generally identified under two forms. On one hand, the internal frost affects the whole volume of the concrete. It can resul in generalized microcracks into the ciment paste. The sensibility of a common concrete to this deterioration can be decreased by the presence of a network of air bubbles within the ciment paste. On the other hand, the scaling, is a surface deterioration generaly quantified by the loss of mass from the exposed surface. The scaling is strongly influenced by the presence of a solution and its concentration in salts (generally some sodium chloride), over the exposed concrete surface. When a common concrete is not correctly formulated to resist this environment, its degradation can penalize the exploitation of the concerned structure in expected conditions of services. Laboratory tests exposing bodies of concrete to accelerated freeze-thaws cycles, allow to estimate the concrete resistance when exposed to these two kinds of deteriorations. The scientific community agrees on the internal frost laboratory test and on the protective aspect of a network of air bubbles. But the scaling is still approached by experimental procedures which do not make consensus and have little evolved since the middle of the 1990s (...)

This report describes a 3 year study carried out to determine the effects of modern coal power generation technologies on the properties of fly ash and how these may affect the use of the material in concrete. A total of 18 fly ashes, from 11 different sources, produced under a range of conditions and technologies were investigated. These primarily included co-combustion, low NOx, supercritical and oxy-fuel technologies, although other available materials (run-of-station, air-classified, processed and stockpiled fly ashes) were included for comparison. The initial experimental work involved physical and chemical characterization of the fly ash samples. Thereafter, tests covering fresh properties, strength development and durability were carried out on selected concretes. A fly ash level of 30% was used with w/c ratios covering the practical range considered (0.35 to 0.65). Equal strength comparisons were also made where appropriate. Finally, granular (unbound fly ash) and monolithic (fly ash concrete) leaching tests were carried out to assess the environmental implications of using the fly ashes. The results from the physical and chemical characterization tests suggest that modern technologies used for coal fired power generation can have an influence on the properties of fly ash produced. The co-combustion, oxy-fuel and in-combustion low NOx fly ashes had reduced fineness and greater LOI, which had a negative effect on foam index and water requirement of the materials. However reactivity was largely unaffected. The post-combustion low NOx and supercritical fly ashes appeared to be unaffected by their production methods compared to that produced by conventional/establish means. Tests on fresh concrete properties showed that fly ashes with high LOI and low fineness required higher SP doses than the reference PC concrete. However, fly ashes with high fineness and low surface area were found to require a lower SP dose than the PC concrete. The concrete compressive strength tests indicate that, in general, finer fly ash concretes tended to have higher strengths than those containing coarser material. However, there did not appear to be any significant difference in performance between fly ash concretes, which suggests that, although modern technologies can have an impact on fly ash properties, if account is taken of these they should not have any significant influence on strength development. Comparison with an earlier study from the 1990s considering BS EN 450-1 fly ashes showed general agreement between the data. The durability study showed that finer, low LOI fly ashes had higher chloride resistance and at equal strength fly ash concretes performed better than those with PC. Equal strength fly ash concretes covering the modern technologies were found to have similar levels of durability for sulfate attack, abrasion and carbonation. High alkali concrete (following the BS 812-123 method) gave similar expansion levels and good resistance with respect to AAR. With air-entrainment, it was found that the fly ash concretes required high doses of AEA (relative to the PC concrete), with high LOI/BET fly ashes requiring greatest quantities. At equal strength, the fly ash concretes had poorer freeze-thaw scaling resistance than PC concrete. However, the majority of the fly ashes did manage to achieve acceptable scaling resistance according to the Swedish criteria. In general, the findings of the durability study are in agreement with the earlier study from the 1990s. Overall, no effect of production technology on the durability of concrete was observed. The leaching studies showed that, in general, in both granular and concrete form, modern fly ashes met the non-hazardous waste requirements in the WAC for all components tested except chromium. For the granular test, there were instances where elevated chromium levels were observed. Similarly, the fly ash concretes failed to meet the non-hazardous limit for chromium. However, chromium from the cement may have contributed to this, since the PC reference also failed to meet this requirement. Based on the results, there is no effect of production technology on the leaching characteristics of fly ash or concrete and the materials do not appear to pose a significant environmental risk. The practical implications of the study have been considered and overall, it has been shown that modern fly ashes behave in much the same way as traditional materials, and therefore, if these materials meet the requirements of BS EN 450-1, and their properties are taken into account in the proportioning of concrete, they should give satisfactory performance.

Alkali-silica reaction (ASR) is a common deterioration mechanism responsible for numerous concrete durability issues. Since ASR was first discovered in the 1940's, a significant number of investigations have been carried out in order to understand its mechanisms. However, due to the complexity of the reaction and to the numerous factors that affect its development, many aspects still remain unexplained. The research described in this document was funded by the Texas Department of Transportation (TxDOT), and it focused on a specific type of reactive aggregates, known as 'pessimum'; they present an unexpected behavior with respect to the relation between the amount of material present in the mixture and the extent of ASR related damage. The main objective of this investigation was to determine a method for identifying aggregates that exhibit the 'pessimum' behavior by means of a short-term testing regime. Modified versions of the Accelerated Mortar Bar Test (AMBT) and the Concrete Microbar Test (CMBT) were considered for this purpose. In addition, the behavior of a selected group of 'pessimum' aggregates in the Concrete Prism Test (CPT) and the Chemical Method was evaluated. The petrographic characteristics for a reduced number of the aggregates studied were linked to their performance in the ASR tests. The results obtained from the experimental program conducted were combined with results from previous investigations performed at UT Austin to draw conclusions about the overall behavior of ‘pessimum’ aggregates. ‘Pessimum’ aggregates were successfully identified with a modification proposed to the AMBT. As for their behavior, it was found that depending on the amount of reactive constituents present in each test, these aggregates are classified as reactive (for low chert contents) or as non-reactive (for chert contents above the 'pessimum' proportion). Whether these aggregates will generate durability problems depends on the amount of reactive silica in the concrete mixture. / text

Made available in DSpace on 2014-07-29T15:03:38Z (GMT). No. of bitstreams: 1 dissertacao ricardo.pdf: 2894239 bytes, checksum: c7ade6470d36ad62bc264e4ee243965b (MD5) Previous issue date: 2007-10-17 / The increasing demand for aggregate for uses in engineering stimulated the search for alternative materials. In this way, the civil construction potentially started to be a great consumer of residues from other industries, becoming a great recycle plant. The use of by-product, brings great environmental benefits in relation to the degradation of the environment, as for example, less settlement of residues in earth fill, the reduction of extraction of natural resources, and the exploration of aggregate ores. Steel plants from around the world have a commom problem, that is what to do with all the steel slags generated as a by-product from the steel refining in electric steel plants or oxygen steel plants. There are two points of these process where slags can be generated: the first comes from the electric or oxygen furnace itself (called oxidizing refining) and the second is the laddle metallurgy slag (reduction refining slag), which is the last stage of the steel production. This aims to demonstrate related studies to the use of steel slags (EAF slags) in electric steel plants as coarse aggregate. Expansibility of the steel slag tests had been carried through, compressive strength, tensile strength for diametrical compression, related modulus of deformation and durability tests, such as: intensity of corrosion, potential of corrosion, ohmic resistivity and depth of carbonation. After this, it was observed the similarity of the results between the concrete made with steel slag and the concrete made with conventional aggregates, natural aggregate known as mica schist. It was observed improvements in the properties of the concrete with steel slag, such as: increase in its strength (compressive / tensile) and increase of the modulus of deformation. Concerning the durability, it was noted, in a general way, that the substitution of the aggregates, conventional for siderurgical, has not influenced in the performance of the concrete. / A crescente demanda por agregados para usos em engenharia estimulou a procura por materiais alternativos. Desta forma, a construção civil passou a ser potencialmente uma grande consumidora de resíduos provenientes de outras indústrias, tornando-se uma grande usina recicladora. A utilização de subprodutos traz benefícios ambientais bastante expressivos em relação à degradação do meio ambiente, como por exemplo, a menor deposição de resíduos em aterros, a diminuição da extração predatória de recursos naturais e a exploração de jazidas de agregados. As siderúrgicas, em nível mundial, vêm enfrentando um problema comum, que consiste no que fazer para que a totalidade da escória gerada no refino do aço em aciarias elétricas ou a oxigênio tenha uma solução de aproveitamento melhor do que vem sendo feito atualmente. Na fabricação do aço as escórias são geradas em duas etapas: a primeira provém do chamado refino oxidante (forno elétrico a arco ou convertedor à oxigênio) e a segunda do refino redutor em processos de metalurgia na panela (forno-panela). Este trabalho tem como objetivo específico demonstrar, por meio de estudos, a potencialidade do uso das escórias de aciaria elétrica (escórias FEA) como agregado graúdo. Foram realizados ensaios de expansibilidade, resistência à compressão, à tração por compressão diametral, módulo de deformação e ensaios relacionados à durabilidade, tais como: intensidade de corrente de corrosão, potencial de corrosão, resistência ôhmica e profundidade de carbonatação. Após o estudo realizado observou-se a semelhança dos resultados dos concretos confeccionados com escória de aciaria elétrica com os concretos confeccionados com agregados convencionais, no caso o micaxisto. Podem-se constatar melhorias nas propriedades dos concretos produzidos com escória, tais como: aumento em suas resistências (compressão / tração) e aumento do módulo de deformação. No que diz respeito à durabilidade verificou-se, de uma maneira geral, que a substituição dos agregados, convencional por siderúrgico, não influenciaram no desempenho dos concretos.

The deliberate use of fly ash in the production of concrete has been used for decades, but recent trends tend to maximise its utility properties. These are different types of activations from chemical through thermal to mechanical. An alkaline agent is added to chemical activation which, by etching the glass structure of the ash grain, promotes its reactivity. Mechanical activation is mainly focused on granulometry and particle size distribution curve. In this thesis three mechanical activation methods are compared: separation the ash into fractions by grain size, mixing these fractions to achieve the ideal particle size distribution curve and grinding. Four types of ash were collected from the Tušimice power plant – ash from individual electrostatic separators (I, II and III) and SESYP ash which represents the total volume of production. Ash from individual separators differ not only in the particle size but also in the chemical composition and reactivity. Better properties generally have smaller ash particles. This is used by mechanical activation by sorting. When mixing the individual ash fractions, the aim is to achieve an optimal grading curve of either the ash itself or the fine particle set in the concrete. In this work, ash is only used to calculate the mixing ratio and the grading curve according to Funk was selected as optimal. The last compared option of activation was grinding ash. Grinding was carried out in an industrial mill for 15, 30, 45, 60 and 75 minutes. The efficiency of the modified ash was monitored for cement paste in terms of rheology and for concrete in terms of consistency, strength and durability. Based on the results we can say that mechanical activation by sorting is suitable for special purposes, but it’s necessary to realise that coarser ash fractions remain unused. When mixing the fractions to ensure matrix density, the efficiency was shown mainly on the durability characteristics. Grinding cause positive effect.

Thin-walled, high-strength concrete elements exhibiting low system weight and great slenderness can be created with a large degree of lightweight structure using the textile-reinforced, load-bearing concrete (TRC) slab and a shell with a very high level of sound absorption. This was developed with the objective of lowering system weight, and then implemented operationally in construction. Arising from the specifications placed on the load-bearing concrete slab, the following took place: an adapted fine-grain concrete matrix was assembled, a carbon warp-knit fabric was modified and integrated into the fine concrete matrix, a formwork system at prototype scale was designed enabling noise barriers to be produced with an application-oriented approach and examined in practically investigations within the context of the project. This meant that a substantial lowering of the load-bearing concrete slab’s system weight was possible, which led to a decrease in transport and assembly costs.

The intrusion of chlorides and carbon dioxide into a reinforced concrete (RC) structure can initiate corrosion of the reinforcing steel, which, due to its expansive nature, can damage the structure and adversely affects its serviceability and safety. Corrosion will initiate if at the steel surface the concrete free chloride concentration exceeds a defined limit, or its pH falls below a critical level. Hence, determination of the time to reaching these critical limits is key to the assessment of RC structures durability and service life. Due to the ionic nature of the chlorides and the bicarbonate anion (HCO3-) formed by the CO2 in the multi-ionic pore solution, the transport of both species is driven by Fickian diffusion combined with electromigration and ionic activity, which can be mathematically expressed by the Nernst-Planck-Poisson (NPP) equations. For a complete representation of the phenomenon, however, the NPP equations must be supplemented by the relevant chemical equilibrium equations to ensure chemical balance among the various species within the concrete pore solution. The combination of NPP with the chemical equilibrium equations is often termed the NPP reactive transport model. In this study, such a model is developed, coded into the MATLAB platform, validated by available experimental data, and applied to analyze the time-dependent concrete carbonation and the movement of chlorides in carbonated and non-carbonated concrete. The results of these analyses can be used to predict the time to corrosion initiation. The transient one-dimensional governing equations of NPP are numerically solved using the Galerkin’s finite element formulation in space and the backward (implicit) Euler scheme in the time domain. The associated system of chemical equilibrium equations accounts for the key homogeneous and heterogeneous chemical reactions that take place in the concrete during carbonation and chlorides transport. At each stage of the analysis, the effects of these reactions on the changes in the pore solution chemical composition, pH, cement chloride binding capacity, concrete porosity, and the hydrated cement solids volumetric ratio are determined. The study demonstrates that given accurate input data, the presently developed NPP reactive transport model can accurately simulate the complex transport processes of chlorides and CO2 in concrete as a reactive porous medium, and the ensuing physical and chemical changes that occur due to the reaction of these species with the pore solution and the other cement hydration products. This conclusion is supported by the good agreement between results of the current analyses with the corresponding available experimental data from physical tests involving carbonation, and chloride diffusion in non-carbonated and carbonated concrete. / Thesis / Doctor of Philosophy (PhD)

Rebar corrosion in reinforced concrete structures is a big issue to technical studies, because it has, besides the maintenance costs, indirect costs which affect bridges and building users. Corrosion inhibitors are chemical substances that added to concrete reduce or retard rebar corrosion. In this thesis we described the behavior of sodium molybdate and thiourea used as corrosion inhibitors (CI) for concrete mixes submitted to chloride medium. Calcium hydroxide solution, cement paste, mortar and concrete were used to understand corrosion inhibitors effects on their properties. Dosage study was performed using solutions, with and without the CI, applying cyclic voltammetry and steady-state polarization. Effects on cement paste and concrete were studied using four different dosages of the proposed CI and two commercial products, two different cements and silica fume as supplementary cementitious material. Concretes mixes were tested by workability, compression strength and resistance to chloride penetration, using ASTM C 1202 and ASTM C 1543. ASTM G 109 was used as standard to determine CI efficiency on corrosion inhibition and two techniques were used to determine concrete electrical resistivity: electrochemical impedance spectroscopy and conductivimetry. The proposed inhibitors showed, in 2% sodium molybdate and 0,67% thiourea, slight effects on concrete properties and good effects on improving the resistance to chloride penetration and to corrosion due chloride ions. / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / A corrosão de armaduras de concreto armado em pontes, viadutos e edifícios tem preocupado o meio técnico, pois além do custo de manutenção, há o custo indireto devido à quedas de estruturas, interdições e incômodos causados aos usuários. Entre os métodos para minimizar a corrosão, podem ser citados os inibidores de corrosão, os quais são adicionados ao concreto e atuam na interface concreto/aço, diminuindo a taxa de corrosão. Neste trabalho, são relatados os estudos do comportamento de inibidores de corrosão à base de molibdato de sódio e tiouréia, aplicados em concretos submetidos à ação de íons cloreto e comparados a dois inibidores comerciais. Foram realizados estudos em solução e em pastas, argamassas e concretos, com dois cimentos diferentes, sem e com adição de 5 e 10% de sílica ativa. A determinação das dosagens de inibidores foi feita em solução saturada de hidróxido de cálcio, utilizando técnicas eletroquímicas como polarização em estado quase-estacionário e voltametria cíclica. Foram estudados os efeitos dos inibidores propostos, em várias dosagens, e de dois inibidores comerciais, nas propriedades de pastas e concretos sem e com sílica ativa. Além da trabalhabilidade e da resistência à compressão, foi testada a resistência à penetração de cloretos pelos métodos ASTM C 1202 e ASTM C 1543. A avaliação da eficiência frente à corrosão provocada por íons cloreto seguiu a norma ASTM G 109. A resistividade elétrica foi determinada utilizando espectroscopia de impedância eletroquímica e condutivimetria. Os inibidores propostos, nas concentrações de 2% de molibdato de sódio e 0,67% de tiouréia, mostraram resultados bastante promissores, tanto na avaliação quanto à influência sobre as propriedades físicas e mecânicas do concreto, quanto à diminuição da penetração de cloretos e eficiência na inibição da corrosão.