Closed-loop testing systems provide the ability to directly control the deformation of the loaded... more Closed-loop testing systems provide the ability to directly control the deformation of the loaded specimen. This considerably enhances the precision, stability, and scope of the experiments. Closed-loop machines can be used to determine the stable response of a test specimen or structure by monitoring and controlling the physical quantities that are sensitive to its behavior. The importance of the various components of the closed-loop controlled system and the test configuration is reviewed in the paper. The most critical aspect of designing the test is the choice of the controlled variable. With appropriate controlled variables and good system performance, several interesting and intricate testing techniques can be developed, as seen in the examples presented here.
The characterization of the behaviour in the fresh state is critical for self compacting concrete... more The characterization of the behaviour in the fresh state is critical for self compacting concrete (SCC), and it is usually performed through tests such as the slump flow or the V-funnel. A better description of SCC behaviour can be performed by using viscometers. It has been recognized that in SCC an adequate combination of the rheological parameters (yield stress and plastic viscosity) is required to obtain a mix with enough mobility but avoiding risks of segregation. This paper analyses the relationship between the engineering tests results and the rheological parameters. The experimental data corresponds to an extensive research program, where the effects of the mixing energy, time, concrete temperature and environmental exposure conditions on the rheological behaviour of SCC were studied. Simultaneous measurements with a BML viscometer, the slump flow and the V-funnel tests were carried out on different types of SCC. Relationships between the slump flow and the yield stress, and between the plastic viscosity and the flow times have been found for the same component materials. In addition, typical ranges of yield stress and plastic viscosity for each class of SCC have been identified.
The Marsh cone test is a simple approach for obtaining a practical measure of the fluidity of cem... more The Marsh cone test is a simple approach for obtaining a practical measure of the fluidity of cement pastes containing superplasticizer and silica fume. Using this method, a study of the effect of water/cement ratio, type and dosage of mineral admixture, and superplasticizer type on cement pastes with different superplasticizer dosages is presented here. Results are presented for cement pastes with silica fume and micronized diatomites as mineral admixtures. In all cases, there is a superplasticizer saturation dosage beyond which there is no significant increase in fluidity and, therefore, can be used for the selection of superplasticizer type and dosage. The data obtained indicate that the relative fluidity generally decreases with an increase in the silica fume content. The Marsh cone approach is also used to compare the effects of the mixing sequence used in the preparation of the pastes. A study of the loss of fluidity over time as a function of the superplasticizer type shows that the trends can vary considerably from one product to another. L'expérience réalisée avec le cône de Marsh constitue un instrument simple pour obtenir une mesure pratique de la fluidité des pâtes de ciment avec ajouts de minéraux et de superplastifiants. Cet article expose une étude sur l'influence de différents paramètres sur la fluidité de la pâte lorsqu'on utilise le cône de Marsh. Les paramètres analysés sont: le rapport eau/ciment, le type et le dosage d'ajout minéral (fumées de silice et diatomite), le type et le dosage de superplastifiant, l'ordre d'introduction des constituants dans le mélange et la perte de fluidité en fonction du temps. Les résultats obtenus révèlent l'existence d'un pourcentage optimum de superplastifiant, appelé point de saturation, au-delà duquel aucune augmentation significative sur la fluidité ne se manifeste. Les résultats mettent d'autre part en évidence l'influence significative du type de superplastifiant sur la perte de fluidité de la pâte.
Los aditivos reductores de retracción (SRAs) se plantean, hoy en día, como una alternativa viable... more Los aditivos reductores de retracción (SRAs) se plantean, hoy en día, como una alternativa viable para reducir la fisuración por retracción plástica. El objetivo del presente trabajo es conocer mejor y predecir el comportamiento a primeras edades de la retracción plástica en elementos estructurales coaccionados, a los que se les ha añadido diversos aditivos reductores de retracción (tres tipos diferentes). Esta influencia se analiza a través las siguientes propiedades: presión capilar, evaporación, evolución de temperaturas, evolución de fisuración, y deformaciones verticales de asentamiento. Los ensayos para estudiar la fisuración y las deformaciones se han realizado sobre diferentes configuraciones (prisma restringido con estrangulamiento y panel restringido), en un túnel de viento, con temperaturas y velocidades de viento controladas. Las conclusiones obtenidas señalan la viabilidad del empleo de este tipo de aditivos y la bondad de los métodos experimentales utilizados.
The water-absorption behavior of cement pastes ͑w / c = 0.30͒ containing varying concentrations ͑... more The water-absorption behavior of cement pastes ͑w / c = 0.30͒ containing varying concentrations ͑i.e., 0, 0.2, and 5%͒ of a shrinkage-reducing admixture ͑SRA͒ was measured. Moisture ingress was monitored using X-ray absorption. A decrease in both the depth of water penetration and the rate of water absorption was observed with increasing specimen maturity and admixture concentration. This agrees with theoretical considerations that suggest water sorption is a function of the surface tension and the viscosity of the fluid ingressing into the pores. The Boltzmann-Matano method was successfully employed to determine the moisture content dependent moisture diffusivity of the material, which exhibited a dependence on both the pore structure ͑specimen maturity͒ and the SRA concentration.
The modulus of elasticity of dam concrete is difficult to determine directly from tests due to th... more The modulus of elasticity of dam concrete is difficult to determine directly from tests due to the necessity for large specimens and testing machines. In order to study the applicability of simple elastic models for predicting the modulus from standard size specimens, tests were conducted on prisms of 45×45×90 cm fabricated with dam concrete (maximum aggregate of 120 mm). The tests on standard 15×30 cm cylinders were made with the mortar and wet-screened components of this concrete. It is seen that the use of the data from these components together with estimated values of the modulus of the aggregates gives reasonable predictions of the moduli of the dam concrete. This has been verified for a range of ages, from 7 to 180 days.
Melamine and naphthalene-based superplasticizers have been used, over the past few decades, in or... more Melamine and naphthalene-based superplasticizers have been used, over the past few decades, in order to improve the workability of concrete. Recently, more efficient copolymer formulations have been introduced for the same purpose. However, the influence of these chemical admixtures on the microstructure of the hardened concrete and, consequently, on its properties still needs to be extensively evaluated. Accordingly, the present work analyzes the hydration characteristics of cement pastes with naphthalene, melamine and copolymerbased superplasticizers, using the techniques of X-ray diffraction (XRD) and nuclear magnetic resonance (NMR), up to the age of 28 days.
The electrical resistivity and conductivity of cement pastes with different dosages of a superpla... more The electrical resistivity and conductivity of cement pastes with different dosages of a superplasticizer have been determined using impedance spectroscopy. The evolution of these parameters during the first 24 h after mixing indicates that the incorporation of the superplasticizer leads to a retardation of the setting process, the significance of which increases with the dosage. It appears that the incorporation of the superplasticizer extends the dormant stage of hydration and delays the onset of the accelerating stage, without affecting its rate. Comparisons are made with penetration measurements with the Vicat apparatus.
Closed-loop testing systems provide the ability to directly control the deformation of the loaded... more Closed-loop testing systems provide the ability to directly control the deformation of the loaded specimen. This considerably enhances the precision, stability, and scope of the experiments. Closed-loop machines can be used to determine the stable response of a test specimen or structure by monitoring and controlling the physical quantities that are sensitive to its behavior. The importance of the various components of the closed-loop controlled system and the test configuration is reviewed in the paper. The most critical aspect of designing the test is the choice of the controlled variable. With appropriate controlled variables and good system performance, several interesting and intricate testing techniques can be developed, as seen in the examples presented here.
The characterization of the behaviour in the fresh state is critical for self compacting concrete... more The characterization of the behaviour in the fresh state is critical for self compacting concrete (SCC), and it is usually performed through tests such as the slump flow or the V-funnel. A better description of SCC behaviour can be performed by using viscometers. It has been recognized that in SCC an adequate combination of the rheological parameters (yield stress and plastic viscosity) is required to obtain a mix with enough mobility but avoiding risks of segregation. This paper analyses the relationship between the engineering tests results and the rheological parameters. The experimental data corresponds to an extensive research program, where the effects of the mixing energy, time, concrete temperature and environmental exposure conditions on the rheological behaviour of SCC were studied. Simultaneous measurements with a BML viscometer, the slump flow and the V-funnel tests were carried out on different types of SCC. Relationships between the slump flow and the yield stress, and between the plastic viscosity and the flow times have been found for the same component materials. In addition, typical ranges of yield stress and plastic viscosity for each class of SCC have been identified.
The Marsh cone test is a simple approach for obtaining a practical measure of the fluidity of cem... more The Marsh cone test is a simple approach for obtaining a practical measure of the fluidity of cement pastes containing superplasticizer and silica fume. Using this method, a study of the effect of water/cement ratio, type and dosage of mineral admixture, and superplasticizer type on cement pastes with different superplasticizer dosages is presented here. Results are presented for cement pastes with silica fume and micronized diatomites as mineral admixtures. In all cases, there is a superplasticizer saturation dosage beyond which there is no significant increase in fluidity and, therefore, can be used for the selection of superplasticizer type and dosage. The data obtained indicate that the relative fluidity generally decreases with an increase in the silica fume content. The Marsh cone approach is also used to compare the effects of the mixing sequence used in the preparation of the pastes. A study of the loss of fluidity over time as a function of the superplasticizer type shows that the trends can vary considerably from one product to another. L'expérience réalisée avec le cône de Marsh constitue un instrument simple pour obtenir une mesure pratique de la fluidité des pâtes de ciment avec ajouts de minéraux et de superplastifiants. Cet article expose une étude sur l'influence de différents paramètres sur la fluidité de la pâte lorsqu'on utilise le cône de Marsh. Les paramètres analysés sont: le rapport eau/ciment, le type et le dosage d'ajout minéral (fumées de silice et diatomite), le type et le dosage de superplastifiant, l'ordre d'introduction des constituants dans le mélange et la perte de fluidité en fonction du temps. Les résultats obtenus révèlent l'existence d'un pourcentage optimum de superplastifiant, appelé point de saturation, au-delà duquel aucune augmentation significative sur la fluidité ne se manifeste. Les résultats mettent d'autre part en évidence l'influence significative du type de superplastifiant sur la perte de fluidité de la pâte.
Los aditivos reductores de retracción (SRAs) se plantean, hoy en día, como una alternativa viable... more Los aditivos reductores de retracción (SRAs) se plantean, hoy en día, como una alternativa viable para reducir la fisuración por retracción plástica. El objetivo del presente trabajo es conocer mejor y predecir el comportamiento a primeras edades de la retracción plástica en elementos estructurales coaccionados, a los que se les ha añadido diversos aditivos reductores de retracción (tres tipos diferentes). Esta influencia se analiza a través las siguientes propiedades: presión capilar, evaporación, evolución de temperaturas, evolución de fisuración, y deformaciones verticales de asentamiento. Los ensayos para estudiar la fisuración y las deformaciones se han realizado sobre diferentes configuraciones (prisma restringido con estrangulamiento y panel restringido), en un túnel de viento, con temperaturas y velocidades de viento controladas. Las conclusiones obtenidas señalan la viabilidad del empleo de este tipo de aditivos y la bondad de los métodos experimentales utilizados.
The water-absorption behavior of cement pastes ͑w / c = 0.30͒ containing varying concentrations ͑... more The water-absorption behavior of cement pastes ͑w / c = 0.30͒ containing varying concentrations ͑i.e., 0, 0.2, and 5%͒ of a shrinkage-reducing admixture ͑SRA͒ was measured. Moisture ingress was monitored using X-ray absorption. A decrease in both the depth of water penetration and the rate of water absorption was observed with increasing specimen maturity and admixture concentration. This agrees with theoretical considerations that suggest water sorption is a function of the surface tension and the viscosity of the fluid ingressing into the pores. The Boltzmann-Matano method was successfully employed to determine the moisture content dependent moisture diffusivity of the material, which exhibited a dependence on both the pore structure ͑specimen maturity͒ and the SRA concentration.
The modulus of elasticity of dam concrete is difficult to determine directly from tests due to th... more The modulus of elasticity of dam concrete is difficult to determine directly from tests due to the necessity for large specimens and testing machines. In order to study the applicability of simple elastic models for predicting the modulus from standard size specimens, tests were conducted on prisms of 45×45×90 cm fabricated with dam concrete (maximum aggregate of 120 mm). The tests on standard 15×30 cm cylinders were made with the mortar and wet-screened components of this concrete. It is seen that the use of the data from these components together with estimated values of the modulus of the aggregates gives reasonable predictions of the moduli of the dam concrete. This has been verified for a range of ages, from 7 to 180 days.
Melamine and naphthalene-based superplasticizers have been used, over the past few decades, in or... more Melamine and naphthalene-based superplasticizers have been used, over the past few decades, in order to improve the workability of concrete. Recently, more efficient copolymer formulations have been introduced for the same purpose. However, the influence of these chemical admixtures on the microstructure of the hardened concrete and, consequently, on its properties still needs to be extensively evaluated. Accordingly, the present work analyzes the hydration characteristics of cement pastes with naphthalene, melamine and copolymerbased superplasticizers, using the techniques of X-ray diffraction (XRD) and nuclear magnetic resonance (NMR), up to the age of 28 days.
The electrical resistivity and conductivity of cement pastes with different dosages of a superpla... more The electrical resistivity and conductivity of cement pastes with different dosages of a superplasticizer have been determined using impedance spectroscopy. The evolution of these parameters during the first 24 h after mixing indicates that the incorporation of the superplasticizer leads to a retardation of the setting process, the significance of which increases with the dosage. It appears that the incorporation of the superplasticizer extends the dormant stage of hydration and delays the onset of the accelerating stage, without affecting its rate. Comparisons are made with penetration measurements with the Vicat apparatus.
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Papers by Ravindra Gettu