1,720,981 research outputs found
On the Potential of Lattice Type Model for Predicting Shear Capacity of Reinforced Concrete and SHCC Structures
No full textDifferent analytical models exist to predict the shear strength of reinforced concrete members. Generally, each of these shear strength models consists of a formulation based on certain underlying theory and fitted model coefficients. The model fitting parameters are usually established from the comparison with test data. Hence, the predictive value of a shear strength model depends, to some extent, on the quality and representativeness of the used test data.This work investigates the predictive capability of several shear strength models for reinforced concrete beams without shear reinforcement. Particular attention is given to the application domain of relatively low reinforced and high depth concrete beams where limited shear test data is available. The predictive capability of the models for this area of interest is analyzed with Bayesian Inference. This probabilistic technique calculates the posterior distributions of uncertain parameters, given a set of measured test data and some prior knowledge.The predictive capability of each shear strength model is quantified by means of a calculated model uncertainty. Furthermore, the influence of the uncertainty in model parameter values on the calculated model uncertainties is evaluated. Bayesian Inference is also used to estimate the model evidences conditionally on the used data.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Concrete StructuresSteel & Composite StructuresMaterials and Environmen
Chloride Ingress of Carbonated Blast Furnace Slag Cement Mortars
No full textIn the Netherlands civil engineering structures, such as overpasses, bridges and tunnels are generally built using blast furnace slag cement (BFSC, CEM III/B) concrete, because of its high resistance against chloride penetration. Although the Dutch experience regarding durability performance of BFSC concrete has been remarkably good, its resistance to carbonation is known to be sensitive, especially when the used slag percentage is high. In a field investigation on a highway overpass damage was found in sheltered elements such as abutments and intermediate supports, which was attributed to chloride induced corrosion enhanced by carbonation that occurred prior to the chloride exposure.Many structures built using BFSC could be prone to this mechanism, i.e. carbonation enhanced chloride induced corrosion, negatively affecting their durability. Focus of the research was given on the influence of carbonation on the chloride penetration resistance of BFSC mortars with varying slag content. In light of the characteristics from the overpass case, it was assumed that first there is a period of carbonation during sheltered exposure, and subsequently joint leakage causes exposure to chlorides. In order to identify the influence of slag content on carbonation, chloride penetration resistance and their coupled effect, mortars with twelve cement blends in a range of 0–70% slag were evaluated based on chloride migration coefficient, accelerated carbonation and electrical resistivity.This study shows that carbonation of BFSC mortars increases the porosity, consequently decreasing the chloride penetration resistance. Binders with 50% or more slag were found to have a significantly lower resistance after carbonation. Consequently, the chloride penetration resistance of a given concrete cover strongly depends on the duration of carbonation and the resulting carbonation depth, hence influencing its lifespan. The service life was estimated using a simplified model for the chloride penetration time of a combined carbonated and uncarbonated layer. It was found that mortar with a slag content between 35 and 50% that was carbonated before chloride exposure show the lowest influence of carbonation on the chloride penetration resistance.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Materials and Environmen
Deliberate Deformation of Concrete in the Fresh State: Crack Risk and Efficient Production of Curved Precast Elements
No full textThe production of double-curved precast concrete elements for cladding or shell structures requires expensive CNC (computer numerical control)-milled formwork. As an alternative method, the innovative flexible mould for economically efficient and sustainable production of such elements is discussed in this paper. This method comprises the use of a flexible, CNC-controlled formwork, which is filled with self-compacting concrete. After a short period of thixotropic stabilization in the fresh state, the flexible mould is then deformed into its desired geometry, typically having a strong curvature radius of only a few metres in one or two direction(s). After hardening and de-moulding, the flexible mould can be reused for elements with the same or different curved geometry. The present paper describes the outcomes of a study focussing on two aspects relevant for the abovementioned production method: effect of change of rheological properties in the first 90 min after casting and assessment of the risk of cracking and development of cracks during the deformation process. In an experimental study the following parameters were modified: radius of deformation, moment of deformation in time, panel thickness and water-cement ratio. The presence of cracks after deformation was investigated quantitatively, using a petrographic technology. The results show that for the application of the flexible mould method the plastic stage of concrete is important to be considered.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Steel & Composite StructuresMaterials and Environmen
INNOVATION IN SOLAR VEHICLES: FROM THE IDEA TO THE PROTOTYPE IN LESS THAN 24 MONTHS
Full text linkThe article aims to describe the integrated path used for the conceptual, functional and constructive design of an exclusive solar vehicle. The project was based on the massive implementation of concurrent engineering and quality tools, rarely used in such an integrated way. New and attractive design, 3D CAD modelling, details design, structural and fluid dynamic validations, in-scale rapid prototyping, functional tests, multi-objective optimization, parts manufacturing and assembly. Thanks to this approach, the solar prototype presents high technological contents, especially in terms of materials, structures and processes, together with their optimizations. Furthermore, large CNC-machined multi-material molds, hybrid manufacturing solutions: everything was used to speed up phases permitting to move from the initial idea to the final prototype in 24 months. Since June 2018, the solar vehicle is on the road, transporting 4 people, weighing less than 300kg, reaching speeds of 120km/h and able to run hundreds of km without fuel
Influence of interface and strain hardening cementitious composite (SHCC) properties on the performance of concrete repairs
No full textIn the construction industry the demand for repair and maintenance of concrete structures constantly increases. Still, the performance of current concrete repairs is not satisfactory and there is an urgent need for improvement. Understanding the damage development in a repair system, and how to predict, model and prevent its failure is critical to improving performance of concrete repairs. Therefore, the aim of this thesis is to study damage development and failure modes in repair systems. For this purpose, interaction between the repair material and the existing substrate was investigated. First the dynamics of moisture exchange in repair systems is studied, because this interaction is critical for the development of properties of repair material and interface. Furthermore, the interaction between the repair material and the substrate under different exposure and loading conditions was investigated. The most common causes of repair failure were studied here: flexural effects, drying shrinkage and ongoing corrosion of reinforcing bars in the repair system. The main repair material used in this study is SHCC (strain-hardening cementitious composite). MOISTURE MOVEMENT IN THE REPAIR SYSTEM In Chapter 3 moisture movement in repair systems was investigated using X-ray absorption technique. Influence of w/c of the repair material, the duration of sealed curing, primer application and the initial moisture content of the substrate on moisture exchange and formed microstructure in the repair system were investigated. Experimental results show that the absorption rate by the dry substrate in the first 5 hours after casting of the repair material (regular OPC paste without any chemical admixtures, as used in this research) is independent of the w/c of the repair material. Furthermore, water from the repair material is absorbed by the substrate at the same rate as pure water. The substrate absorption significantly reduced the water content of the repair material and was found to be critical for development of the repair material properties. Repair material cast on the initially dry substrate achieved lower degree of hydration compared to the repair material cast on the initially saturated substrate. Wet substrate ensures more uniform water distribution and microstructure development in the repair material compared to the repair material cast on a dry substrate. At later age, water migrates back from the substrate to the repair material. The lower the w/c of the repair material, the more water goes back from the substrate to the repair material. Duration of sealed curing has an effect on development of properties of the repair material and interface but its effect is dependent on the initial saturation state of the substrate. It was found that, when the substrate was initially saturated, increasing duration of (sealed) curing both from1 to 3 days and from3 to 5 days had a beneficial influence on the hydration of the repair material. However, when the substrate was initially dry, curing samples for 3 instead of 1 day was beneficial, but curing samples for 5, instead of 3 days did not result in significant improvements. Moisture movement not only affects the moisture distribution but also the microstructure around the interface in a repair material. It was found that with higher substrate absorption significantly more voids in the repair material, close to the interface, formed. Dry substrate absorbs water from the repair material and releases air. This air stays entrapped at the interface resulting in high void content. This may affect the strength of the material in this region. MICROMECHANICAL INTERFACE PROPERTIES IN THE REPAIR SYSTEM In Chapter 4 the properties of the interface, repair material, and substrate were investigated at the microscale using nanoindentation. Measured moduli of elasticity and hardness were used indirectly as input in micromechanical modelling. Uniaxial tension test is simulated with the lattice model and interface, repair material, and substrate fracture properties were obtained. It was shown that the ratio between interface tensile strength and repair material tensile strength at the age of 28 days, is lower than 0.9 for pure Portland Cement paste. This ratio decreases further as the addition of BFS increases. FAILURE MODES IN THE REPAIR SYSTEM In Chapter 5 experimental and numerical studies were performed to investigate the influence of the interface and SHCC material properties on the fracture performance of repair systems due to mechanical loading. Three point bending test, DIC (digital image correlation) and epoxy impregnation were used for investigating the fracture behaviour of the repair system in experiments. The lattice fracture model was used as a numerical tool for studying damage development. The influence of substrate surface roughness was explained. Surface roughness of the substrate does not affect the load-bearing capacity in flexural tests, but there is a substantial difference in crack pattern and debonding tendency in the repair system. When the substrate surface is rough, cracks from the repair material are interlocked by grooves and directed to the substrate. In flexural and reflective cracking tests low interface toughness (low interface strength and smooth surface of the substrate) are beneficial. With less restraint at the interface, there is more local debonding around the cracks, resulting in more microcracking in the repair material (SHCC). As a result, higher ductility of the repair system is achieved. This is different from what we find in standard recommendations for surface preparation, which advises roughening of the substrate surface. However, very low bond strength and smooth surface might also lead to uncontrolled debonding and shift in failure mode (complete delamination). In Chapter 6 experimental and numerical studies were performed to investigate the influence of interface and SHCC material properties on the fracture performance of repair systems due to drying shrinkage of repair material. Free drying shrinkage of repair materials was measured and the effects of drying shrinkage (cracking and delamination)on repair system beams were investigated. Lattice moisture model and lattice fracture model were used for studying damage development due to restrained shrinkage. Influence of substrate surface roughness, repair material thickness, interface strength and type of repair material were investigated. It was shown that interface strength and surface roughness are more important for performance of thinner overlays compared to the thicker ones due to the higher moisture gradient. With weak bond and thinner repair material, the system is susceptible to large debonding and a big crack. If there is no continuous delamination, with the same bond and drying conditions, thin overlays result in more cracks with smaller spacing and smaller crack widths. As already shown in Chapter 5, damage development and final failure mode are sensitive to the interface strength and surface roughness. In contrast to recommendations from mechanical tests (i.e. Chapter 5), high interface strength and high surface roughness are necessary for optimal performance of the repair system with SHCC as a repair material under restrained shrinkage conditions. In Chapter 7 experimental and numerical studies were performed to investigate the influence of interface and SHCC material properties on the fracture performance of repair systems subjected to ongoing corrosion of rebars in the repair material. Experimentally, rebars in the repair material were exposed to accelerated corrosion. For the numerical study, the lattice model, used in Chapters 5 and 6, is applied. Influence of type of repair material, interface strength, substrate strength and substrate surface roughness on damage development is studied. It was shown that in case of continuing rebar corrosion, surface roughness had a similar influence as in Chapters 5 and 6. Grooves enable cracks to continue to propagate to the substrate. A high interface strength and a rough surface are beneficial to exploit the ductility of the repair material. On the contrary, a smooth substrate surface and a low interface strength cause uncontrolled failure through delamination. However, some delamination around the crack is also beneficial, especially when the substrate is stronger than the repair material, as the stresses will be partially relieved resulting in smaller cracks in the repair material (SHCC). A CASE STUDY In Chapter 8 SHCC is applied as a repair material in a trial patch repair in a deteriorated concrete tunnel (the Maastunnel). For surface preparation of the substrate, recommendations and conclusions from the previous chapters were used. Five different types of repair materials were applied in small patches in the tunnel. Laboratory tests were combined with on-site investigations in order to study the performance of different repair materials. It was shown that when concrete used as a repair material (as opposed to repair mortar), the repair system exhibited the best performance (no cracking nor delamination). However, in case that corrosion continues in the bars, use of SHCC is beneficial as it will result in smallest crack widths and more ductility of the repair system. CONCLUSIONS AND RECOMMENDATIONS In Chapter 9, conclusions of this study and practical recommendations for the repair application are given.Structural EngineeringCivil Engineering and Geoscience
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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