Materials for Fish Nets-Their Properties, Selection and Preservation

The properties of synthetic fibres vary with thc inherent physical characteristics of the basic raw materials used mode of preparation of yarns and method of construction of twines. Since the synthetic fibres as maufactured from polymers which are synthesized from simple chemical units, the qualities of man-made fibres can he influenced by the process of manufacture and certain modifications can even be introduced at the processing stage to meet any specific requirement to a certain extent. Hence, an elaborate study of the properties of fish not twines produced has been taken up with a view to determining their suitability for various types of fishing gear with particular reference to conditions prevailing in India.

“Fabrication and characterization of polymer optical fibers for photonic device applications

Although the main application of optical fibers are in the field of telecommunication, optical fiber based sensors of various designs are becoming valuable devices for wide industrial applications. The advantages of optical fiber-based sensors include high sensitivity, insensitivity to electromagnetic radiation; spark free, light weight and minimal intrusiveness due to their relatively small size and deployment in harsh and hostile environments. It has been proved that POI-7 based sensors can be employed to detect a great variety of parameters including temperature, humidity, pressure, refractive index etc. The proposed thesis presented in six chapters deals with the work carried on dye doped and undoped POF for photonic device applications such as amplifier, laser and sensor

“Studies on short polyester fiber - polyurethane elastomer composite with different interfacial bonding agents”

The work presented in this thesis is regarding the development and evaluation of new bonding agents for short polyester fiber - polyurethane elastomer composites. The conventional bonding system based on hexamethylenetetramine, resorcinol and hydrated silica was not effective as a bonding agent for the composite, as the water eliminated during the formation of the RF resin hydrolysed the urethane linkages. Four bonding agents based on MDI/'I‘DI and polypropyleneglycol, propyleneglycol and glycerol were prepared and the composite recipe was optimised with respect to the cure characteristics and mechanical properties. The flow properties, stress relaxation pattern and the thermal degradation characteristics of the composites containing different bonding agents were then studied in detail to evaluate the new bonding systems. The optimum loading of resin was 5 phr and the ratio of the -01 to isocyanate was 1:1. The cure characteristics showed that the optimum combination of cure rate and processability was given by the composite with the resin based on polypropyleneglycol/ glycerol/ 4,4’diphenylmethanediisocynate (PPG/GL/MDI). From the rheological studies of the composites with and without bonding agents it was observed that all the composites showed pseudoplastic nature and the activation energy of flow of the composite was not altered by the presence of bonding agents. Mechanical properties such as tensile strength, modulus, tear resistance and abrasion resistance were improved in the presence of bonding agents and the effect was more pronounced in the case of abrasion resistance. The composites based on MDI/GL showed better initial properties while composites with resins based on MDI/PPG showed better aging resistance. Stress relaxation showed a multistage relaxation behaviour for the composite. Within the-strain levels studied, the initial rate of relaxation was higher and the cross over time was lesser for the composite containing bonding agents. The bonding agent based on MDI/PPG/GL was found to be a better choice for improving stress relaxation characteristics with better interfacial bonding. Thennogravimetirc analysis showed that the presence of fiber and bonding agents improved the thennal stability of the polyurethane elastomer marginally and it was maximum in the case of MDI / GL based bonding agents. The kinetics of degradation was not altered by the presence of bonding agents

A Study On The Utilisation Of Plastic Wastes In Stabilised Masonry Blocks

At this era of energy crisis and resource depletion, availability of conventional materials throughout the year in quantity and quality, pose a hectic problem for the builders. Adding fuel to the fire, the demand of these materials increases day by day, since the housing and habitat requirements exponentially increase time to time. There is an international concern over this crisis and researchers are reorienting themselves, so as to evolve appropriate masonry units, using locally available cheap materials and technology. The concept of green material and construction has been well conceived in the research so that marginal materials and unskilled labour can be employed for the mass production of building blocks. In this context, considering earth as a sustainable material, there is a growing interest in the use of it, as a modern construction material. Solid waste management is one of the current major environmental concerns in our country. Our country is left with millions of cubic metre of waste plastics. One of the methods to satisfactorily address this solid waste management and the environmental issues is to suitably accommodate the waste in some form (as fibres). Their employability in block making in the form of fibres (plastic fibre- mud blocks) can be investigated through a fundamental research. Also, the review of the existing literature shows that most studies on natural fibres are focussed on cellulose based/ vegetable fibres obtained from renewable plant resources except in very few cases, where animal fibre, plastic fibre and polystyrene fabric were used. At this context, for the plastic fibre-mud blocks to be more widely applicable, a systematic quantification of the relevant physical and mechanical properties of the fibre masonry units is crucial, to enable an objective evaluation of the composite material’s response to actual field condition. This research highlights the salient observations from the detailed investigation of a systematic study on the effect of embedded fibres, made of plastic wastes on the performance of stabilised mud blocks.

Influence Of Additives On The Drain Down Characteristics Of Stone Matrix Asphalt Mixtures

In Kerala highways, where traditional dense graded mixtures are used for the surface courses, major distress is due to moisture induced damages. Development of stabilized Stone Matrix Asphalt (SMA) mixtures for improved pavement performance has been the focus of research all over the world for the past few decades. Many successful attempts are made to stabilize SMA mixtures with synthetic fibres and polymers. India, being an agricultural economy produces fairly huge quantity of natural fibres such as coconut, sisal, banana, sugar cane, jute etc.. Now- a -days the disposal of waste plastics is a major concern for an eco- friendly sustainable environment. This paper focuses on the influence of additives like coir, sisal, banana fibres (natural fibres), waste plastics (waste material) and polypropylene (polymer) on the drain down characteristics of SMA mixtures. A preliminary investigation is conducted to characterize the materials used in this study. Drain down sensitivity tests are conducted to study the bleeding phenomena and drain down of SMA mixtures. Based on the drain down characteristics of the various stabilized mixtures it is inferred that the optimum fibre content is 0.3% by weight of mixture for all fibre mixtures irrespective of the type of fibre. For waste plastics and polypropylene stabilized SMA mixtures, the optimum additive contents are respectively 7% and 5% by weight of mixture. Due to the absorptive nature of fibres, fibre stabilizers are found to be more effective in reducing the drain down of the SMA mixture. The drain values for the waste plastics mix is within the required specification range. The coir fibre additive is the best among the fibres investigated. Sisal and banana fibre mixtures showed almost the same characteristics on stabilization.

Sustainable Building Materials in Kerala – An Overview

Kerala, God’s own country is blessed with immense natural resources. It’s high time that the state’s natural resources being utilized effectively. While sustainable development is the need of the hour, we have to take lead in initiating activities that would minimize the exploitation of our natural resources resulting in their effective utilization. This paper narrates an overview of innovative building materials especially using natural fibres available in Kerala and discusses the feasibility of utilising such fibres in the context of sustainable building materials in Kerala. The paper also discusses how these materials can be effectively utilized to reduce the huge investment in the construction industry

Study of chemical properties and evaluation of collagen in mantle, epidermal connective tissue and tentacle of Indian Squid, Loligo duvauceli Orbigny

The chemical composition and evaluation of Indian squid (Loligo duvauceli) mantle, epidermal connective tissue and tentacle is investigated in this current study. It is observed that squid mantle contains 22.2% total protein; 63.5% of the total protein is myofibrillar protein. The unique property of squid myofibrillar protein is its water solubility. Squid mantle contains 12.0% total collagen. Epidermal connective tissue has highest amounts of total collagen (17.8%). SDS-PAGE of total collagen identified high molecular weight α-, β- and γ- sub-chains. Amino acid profile analysis indicates that mantle and tentacle contain essential amino acids. Arginine forms a major portion of mantle collagen (272.5 g/100 g N). Isoleucine, glutamic acid and lysine are other amino acids that are found in significantly high amounts in the mantle. Sulphur containing cystine is deficit in mantle collagen. Papain digest of mantle and epidermal connective tissue is rich in uronic acid, while papain digest, collagenase digest and urea digest of epidermal connective tissue has significant amounts of sialic acid (25.2, 33.2 and 99.8 μmol /100 g, respectively). PAS staining of papain digest, collagenase digest and urea digest also identify the association of hexoses with low molecular weight collagen fragments. Histochemical sectioning also emphasized the localized distribution of collagen in epidermal and dermal region and very sparse fibres traverse the myotome bundles

Utilization Of Weeds And Agriwaste By Popularizing Handpapermaking In Kerala, India

Kerala in south India grows several cash crops such as banana and pineapple, the crop residues of which are sources of natural fibres that can be used in hand papermaking. Kerala, however, does not have a tradition in hand papermaking. The following is an account of an attempt to popularize the art and craft of hand papermaking among self-help groups as a means of self-employment and waste utilization, using fibres extracted from agriwaste and local plants

Biopulping of Paddy Straw by Pleurotus eous

Biopulping being less energy intensive, inexpensive and causing lesser pollution, can be a viable alternative to chemical and mechanical pulping in paper and pulp industry. In view of shrinking forest reserves, agricultural residues are considered as an alternative raw material for making paper and board. By suitable treatment agriwaste can be converted into substrate for mushroom cultivation. Mushrooms of Pleurotus sp. can preferentially remove lignin from agriwaste with limited degradation to cellulose. The present study examines utilization of Pleurotus eous for biopulping of paddy straw by solid substrate fermentation. SMS, the mushroom growing medium that results from cultivation process, is a good source of fibre and can be pulped easily. Ligninases present in SMS were able to reduce lignin content to nearly half the initial amount by 21st day of cultivation. Highest cellulose content (% dry weight) was observed on 21st day, while cellulase production commenced from 28th day of cultivation. SEM images revealed that SMS fibres are still associated with non-cellulosic materials when compared to chemically (20% w/v NaOH) extracted fibres.

Design and study of self-assembled functional organic and hybrid systems for biological applications

The focus of self-assembly as a strategy for the synthesis has been confined largely to molecules, because of the importance of manipulating the structure of matter at the molecular scale. We have investigated the influence of temperature and pH, in addition to the concentration of the capping agent used for the formation of the nano-bio conjugates. For example, the formation of the narrower size distribution of the nanoparticles was observed with the increase in the concentration of the protein, which supports the fact that γ-globulin acts both as a controller of nucleation as well as stabiliser. As analyzed through various photophysical, biophysical and microscopic techniques such as TEM, AFM, C-AFM, SEM, DLS, OPM, CD and FTIR, we observed that the initial photoactivation of γ-globulin at pH 12 for 3 h resulted in small protein fibres of ca. Further irradiation for 24 h, led to the formation of selfassembled long fibres of the protein of ca. 5-6 nm and observation of surface plasmon resonance band at around 520 nm with the concomitant quenching of luminescence intensity at 680 nm. The observation of light triggered self-assembly of the protein and its effect on controlling the fate of the anchored nanoparticles can be compared with the naturally occurring process such as photomorphogenesis.Furthermore,our approach offers a way to understand the role played by the self-assembly of the protein in ordering and knock out of the metal nanoparticles and also in the design of nano-biohybrid materials for medicinal and optoelectronic applications. Investigation of the potential applications of NIR absorbing and water soluble squaraine dyes 1-3 for protein labeling and anti-amyloid agents forms the subject matter of the third chapter of the thesis. The study of their interactions with various proteins revealed that 1-3 showed unique interactions towards serum albumins as well as lysozyme. 69%, 71% and 49% in the absorption spectra as well as significant quenching in the fluorescence intensity of the dyes 1-3, respectively. Half-reciprocal analysis of the absorption data and isothermal titration calorimetric (ITC) analysis of the titration experiments gave a 1:1 stoichiometry for the complexes formed between the lysozyme and squaraine dyes with association constants (Kass) in the range 104-105 M-1. We have determined the changes in the free energy (ΔG) for the complex formation and the values are found to be -30.78, -32.31 and -28.58 kJmol-1, respectively for the dyes 1, 2 and 3. Furthermore, we have observed a strong induced CD (ICD) signal corresponding to the squaraine chromophore in the case of the halogenated squaraine dyes 2 and 3 at 636 and 637 nm confirming the complex formation in these cases. To understand the nature of interaction of the squaraine dyes 1-3 with lysozyme, we have investigated the interaction of dyes 1-3 with different amino acids. These results indicated that the dyes 1-3 showed significant interactions with cysteine and glutamic acid which are present in the side chains of lysozyme. In addition the temperature dependent studies have revealed that the interaction of the dye and the lysozyme are irreversible. Furthermore, we have investigated the interactions of these NIR dyes 1-3 with β- amyloid fibres derived from lysozyme to evaluate their potential as inhibitors of this biologically important protein aggregation. These β-amyloid fibrils were insoluble protein aggregates that have been associated with a range of neurodegenerative diseases, including Huntington, Alzheimer’s, Parkinson’s, and Creutzfeldt-Jakob diseases. We have synthesized amyloid fibres from lysozyme through its incubation in acidic solution below pH 4 and by allowing to form amyloid fibres at elevated temperature. To quantify the binding affinities of the squaraine dyes 1-3 with β-amyloids, we have carried out the isothermal titration calorimetric (ITC) measurements. The association constants were determined and are found to be 1.2 × 105, 3.6× 105 and 3.2 × 105 M-1 for the dyes, 1-3, respectively. To gain more insights into the amyloid inhibiting nature of the squaraine dyes under investigations, we have carried out thioflavin assay, CD, isothermal titration calorimetry and microscopic analysis. The addition of the dyes 1-3 (5μM) led to the complete quenching in the apparent thioflavin fluorescence, thereby indicating the destabilization of β-amyloid fibres in the presence of the squaraine dyes. Further, the inhibition of the amyloid fibres by the squaraine dyes 1-3, has been evidenced though the DLS, TEM AFM and SAED, wherein we observed the complete destabilization of the amyloid fibre and transformation of the fibre into spherical particles of ca. These results demonstrate the fact that the squaraine dyes 1-3 can act as protein labeling agents as well as the inhibitors of the protein amyloidogenesis. The last chapter of the thesis describes the synthesis and investigation of selfassembly as well as bio-imaging aspects of a few novel tetraphenylethene conjugates 4-6.Expectedly, these conjugates showed significant solvatochromism and exhibited a hypsochromic shift (negative solvatochromism) as the solvent polarity increased, and these observations were justified though theoretical studies employing the B3LYP/6-31g method. We have investigated the self-assembly properties of these D-A conjugates though variation in the percentage of water in acetonitrile solution due to the formation of nanoaggregates. Further the contour map of the observed fluorescence intensity as a function of the fluorescence excitation and emission wavelength confirmed the formation of J-type aggregates in these cases. To have a better understanding of the type of self-assemblies formed from the TPE conjugates 4-6, we have carried out the morphological analysis through various microscopic techniques such as DLS, SEM and TEM. 70%, we observed rod shape architectures having ~ 780 nm in diameter and ~ 12 μM in length as evidenced through TEM and SEM analysis. We have made similar observations with the dodecyl conjugate 5 at ca. 70% and 50% water/acetonitrile mixtures, the aggregates formed from 4 and 5 were found to be highly crystalline and such structures were transformed to amorphous nature as the water fraction was increased to 99%. To evaluate the potential of the conjugate as bio-imaging agents, we have carried out their in vitro cytotoxicity and cellular uptake studies though MTT assay, flow cytometric and confocal laser scanning microscopic techniques. Thus nanoparticle of these conjugates which exhibited efficient emission, large stoke shift, good stability, biocompatibility and excellent cellular imaging properties can have potential applications for tracking cells as well as in cell-based therapies. In summary we have synthesized novel functional organic chromophores and have studied systematic investigation of self-assembly of these synthetic and biological building blocks under a variety of conditions. The investigation of interaction of water soluble NIR squaraine dyes with lysozyme indicates that these dyes can act as the protein labeling agents and the efficiency of inhibition of β-amyloid indicate, thereby their potential as anti-amyloid agents.

Modification of Polypropylene/High Density Polyethylene Blend Using Nanokaolinite Clay and E-Glass Fibre: Preparation, Characterization and Micromechanical Modelling

Upgrading two widely used standard plastics, polypropylene (PP) and high density polyethylene (HDPE), and generating a variety of useful engineering materials based on these blends have been the main objective of this study. Upgradation was effected by using nanomodifiers and/or fibrous modifiers. PP and HDPE were selected for modification due to their attractive inherent properties and wide spectrum of use. Blending is the engineered method of producing new materials with tailor made properties. It has the advantages of both the materials. PP has high tensile and flexural strength and the HDPE acts as an impact modifier in the resultant blend. Hence an optimized blend of PP and HDPE was selected as the matrix material for upgradation. Nanokaolinite clay and E-glass fibre were chosen for modifying PP/HDPE blend. As the first stage of the work, the mechanical, thermal, morphological, rheological, dynamic mechanical and crystallization characteristics of the polymer nanocomposites prepared with PP/HDPE blend and different surface modified nanokaolinite clay were analyzed. As the second stage of the work, the effect of simultaneous inclusion of nanokaolinite clay (both N100A and N100) and short glass fibres are investigated. The presence of nanofiller has increased the properties of hybrid composites to a greater extent than micro composites. As the last stage, micromechanical modeling of both nano and hybrid A composite is carried out to analyze the behavior of the composite under load bearing conditions. These theoretical analyses indicate that the polymer-nanoclay interfacial characteristics partially converge to a state of perfect interfacial bonding (Takayanagi model) with an iso-stress (Reuss IROM) response. In the case of hybrid composites the experimental data follows the trend of Halpin-Tsai model. This implies that matrix and filler experience varying amount of strain and interfacial adhesion between filler and matrix and also between the two fillers which play a vital role in determining the modulus of the hybrid composites.A significant observation from this study is that the requirement of higher fibre loading for efficient reinforcement of polymers can be substantially reduced by the presence of nanofiller together with much lower fibre content in the composite. Hybrid composites with both nanokaolinite clay and micron sized E-glass fibre as reinforcements in PP/HDPE matrix will generate a novel class of high performance, cost effective engineering material.

VUV-Laserablation von Spinnenseide

Spinnenseide gehört zu den stabilsten bekannten Polymerverbindungen. Spinnfäden können bis auf das Dreifache ihrer ursprünglichen Länge gedehnt werden, bevor sie reißen, und dabei mit rund 160 MJ/m³ mehr als dreimal soviel Energie absorbieren wie die stärkste synthetisch hergestellte Faser Kevlar (50 MJ/m³). Dabei weisen Spinnfäden mit 2 bis 5 Mikrometer nur ein Zehntel des Durchmessers eines menschlichen Haares auf. Das präzise, berührungslose Bearbeiten von Spinnenseide ist für verschiedene technische Anwendungen interessant, insbesondere wenn dabei ihre außergewöhnlichen Eigenschaften erhalten bleiben. Könnten die von Natur aus dünnen Seidenfäden gezielt in ihrem Durchmesser verringert werden, so wären sie unter anderem in der Mikroelektronik einzusetzen. Hier könnten sie als Trägermaterial für eine dünne, elektrisch leitfähige Schicht fungieren. Man erhielte Nanodrähte, die auch in mechanisch besonders belasteten Mikroelektronikbauteilen (MEMS) Verwendung finden könnten. In dieser Arbeit wird die Verwendung der laserinduzierten Ablation zur gezielten Bearbeitung von Haltefäden der Schwarzen Witwe (Latrodectus hesperus) beschrieben. Eingesetzt wurde ein VUV-Excimerlaser vom Typ LPF 205 (Lambda-Physik, Göttingen) mit einer Wellenlänge von 157 nm und einer Pulsdauer von 18 ns. Eine berührungslose Laserbearbeitung bei 157 nm erlaubt einen effizienten und präzisen Abtrag von Material durch Ablation aufgrund der geringen optischen Eindringtiefe von unter 100 nm oberhalb einer Schwellenfluenz (Energie/Fläche) von Φth=29 mJ/cm², ohne dabei das umgebende Material thermisch zu beeinträchtigen. Parallel zur Ablation setzt allerdings eine wellenförmige Oberflächenstrukturierung auf der Faseroberfläche ein, wodurch die mechanische Belastbarkeit der Faser entscheidend geschwächt wird. Die Ursache hierfür liegt im Abbau materialbedingter Spannungsfelder („stress release“) innerhalb einer durch das Laserlicht induzierten dünnen Schmelzschicht. Im Rahmen dieser Arbeit ist es nun gelungen, diese Strukturen durch einen anschließenden Glättungsprozeß zu entfernen. Dabei wird auf der bestrahlten Oberfläche mittels Laserlichts eine glatte Ablation erzielt. Mit feinerer Abstufung dieser Prozeßschritte konnte der Durchmesser des verwendeten Spinnenseidefadens zum Teil um 70 Prozent bis auf ca. 750 nm verringert werden. Durch Zugfestigkeitsexperimente wurde belegt, daß die mechanischen Eigenschaften der so bearbeiteten Spinnenseide weitgehend erhalten bleiben. Die im Rahmen dieser Arbeit angewandte Methode erlaubt somit eine präzise Laserablation von Spinnenseide und ähnlichen hochabsorbierenden Materialien, ohne deren Kernsubstanz in ihrer Beschaffenheit zu verändern.

Ein Beitrag zur realitätsnahen Modellierung und Analyse von stahlfaserverstärkten Stahlbeton- und Stahlbetonflächentragwerken

Das Werkstoffverhalten von stahlfaserfreiem bzw. stahlfaserverstärktem Stahlbeton unter biaxialle Druck- Zugbeanspruchung wurde experimentell und theoretisch untersucht. Die Basis der experimentellen Untersuchungen waren zahlreiche Versuche, die in der Vergangenheit an faserfreiem Stahlbetonscheiben zur Bestimmung des Werkstoffverhaltens von gerissenem Stahlbeton im ebenen Spannungszustand durchgeführt wurden. Bei diesen Untersuchungen wurde festgestellt, dass infolge einer Querzugbeanspruchung eine Abminderung der biaxialen Druckfestigkeit entsteht. Unter Berücksichtigung dieser Erkenntnisse sind zur Verbesserung der Werkstoffeigenschaften des Betons, Stahlbetonscheiben aus stahlfaserverstärktem Beton hergestellt worden. Die aus der Literatur bekannten Werkstoffmodelle für Beton sowie Stahlbeton, im ungerissenen und gerissenen Zustand wurden hinsichtlich der in der Vergangenheit ermittelten Materialeigenschaften des Betons bzw. Stahlbetons unter proportionalen sowie nichtproportionalen äußeren Belastungen erklärt und kritisch untersucht. In den frischen Beton wurden Stahlfasern hinzugegeben. Dadurch konnte die Festigkeits- und die Materialsteifigkeitsabminderung infolge Rissbildung, die zur Schädigung des Verbundwerkstoffs Beton führt, reduziert werden. Man konnte sehen, dass der Druckfestigkeitsabminderungsfaktor und insbesondere die zur maximal aufnehmbaren Zylinderdruckfestigkeit gehörende Stauchung, durch Zugabe von Stahlfasern besser begrenzt wird. Die experimentelle Untersuchungen wurden an sechs faserfreien und sieben stahlfaserverstärkten Stahlbetonscheiben unter Druck-Zugbelastung zur Bestimmung des Verhaltens des gerissenen faserfreien und stahlfaserverstärkten Stahlbetons durchgeführt. Die aus eigenen Versuchen ermittelten Materialeigenschaften des Betons, des stahlfaserverstärkten Betons und Stahlbetons im gerissenen Zustand wurden dargelegt und diskutiert. Bei der Rissbildung des quasi- spröden Werkstoffs Beton und dem stahlfaserverstärkten Beton wurde neben dem plastischen Fließen, auch die Abnahme des Elastizitätsmoduls festgestellt. Die Abminderung der aufnehmbaren Festigkeit und der zugehörigen Verzerrung lässt sich nicht mit der klassischen Fließtheorie der Plastizität ohne Modifizierung des Verfestigungsgesetzes erfassen. Es wurden auf elasto-plastischen Werkstoffmodellen basierende konstitutive Beziehungen für den faserfreien sowie den stahlfaserverstärkten Beton vorgeschlagen. Darüber hinaus wurde in der vorliegenden Arbeit eine auf dem elasto-plastischen Werkstoffmodell basierende konstitutive Beziehung für Beton und den stahlfaser-verstärkten Beton im gerissenen Zustand formuliert. Die formulierten Werkstoffmodelle wurden mittels dem in einer modularen Form aufgebauten nichtlinearen Finite Elemente Programm DIANA zu numerischen Untersuchungen an ausgewählten experimentell untersuchten Flächentragwerken, wie scheibenartigen-, plattenartigen- und Schalentragwerken aus faserfreiem sowie stahlfaserverstärktem Beton verwendet. Das entwickelte elasto-plastische Modell ermöglichte durch eine modifizierte effektive Spannungs-Verzerrungs-Beziehung für das Verfestigungsmodell, nicht nur die Erfassung des plastischen Fließens sondern auch die Berücksichtigung der Schädigung der Elastizitätsmodule infolge Mikrorissen sowie Makrorissen im Hauptzugspannungs-Hauptdruckspannungs-Bereich. Es wurde bei den numerischen Untersuchungen zur Ermittlung des Last-Verformungsverhaltens von scheibenartigen, plattenartigen- und Schalentragwerken aus faserfreiem und stahlfaserverstärktem Stahlbeton, im Vergleich mit den aus Versuchen ermittelten Ergebnissen, eine gute Übereinstimmung festgestellt.

Rissbildung und Zugtragverhalten von mit Stabstahl und Fasern bewehrtem Ultrahochfesten Beton (UHPC)

Ultrahochfester Beton (UHPC) ist ein sehr gefügedichter zementgebundener Werkstoff, der sich nicht nur durch eine hohe Druckfestigkeit, sondern auch durch einen hohen Widerstand gegen jede Form physikalischen oder chemischen Angriffs auszeichnet. Duktiles Nachbruchverhalten bei Druckversagen wird meist durch die Zugabe dünner kurzer Fasern erreicht. In Kombination mit konventioneller Betonstahl- oder Spannbewehrung ermöglicht UHPC die Ausführung sehr schlanker, weitgespannter Konstruktionen und eröffnet zugleich neue Anwendungsgebiete, wie zum Beispiel die flächenhafte Beschichtung von Brückendecks. Durch das Zusammenwirken kontinuierlicher Bewehrungselemente und diskontinuierlich verteilter kurzer Fasern ergeben sich unter Zugbeanspruchung Unterschiede gegenüber dem bekannten Stahl- und Spannbeton. In der vorliegenden Arbeit wird hierzu ein Modell entwickelt und durch eine umfangreiche Versuchsreihe abgesichert. Ausgangspunkt sind experimentelle und theoretische Untersuchungen zum Verbundverhalten von Stabstählen in einer UHPC-Matrix und zum Einfluss einer Faserzugabe auf das Reiß- und Zugtragverhalten von UHPC. Die Modellbildung für UHPC-Zugelemente mit gemischter Bewehrung aus Stabstahl und Fasern erfolgt auf der Grundlage der Vorgänge am diskreten Riss, die daher sehr ausführlich behandelt werden. Für den elastischen Verformungsbereich der Stabbewehrung (Gebrauchslastbereich) kann damit das Last-Verformungs-Verhalten für kombiniert bewehrte Bauteile mechanisch konsistent unter Berücksichtigung des bei UHPC bedeutsamen hohen Schwindmaßes abgebildet werden. Für die praktische Anwendung wird durch Vereinfachungen ein Näherungsverfahren abgeleitet. Sowohl die theoretischen als auch die experimentellen Untersuchungen bestätigen, dass der faserbewehrte UHPC bei Kombination mit kontinuierlichen Bewehrungselementen selbst kein verfestigendes Verhalten aufweisen muss, um insgesamt verfestigendes Verhalten und damit eine verteilte Rissbildung mit sehr keinen Rissbreiten und Rissabständen zu erzielen. Diese Beobachtungen können mit Hilfe der bisher zur Verfügung stehenden Modelle, die im Wesentlichen eine Superposition isoliert ermittelter Spannungs-Dehnungs-Beziehungen des Faserbetons und des reinen Stahls vorsehen, nicht nachvollzogen werden. Wie die eigenen Untersuchungen zeigen, kann durch ausreichend dimensionierte Stabstahlbewehrung zielgerichtet und ohne unwirtschaftlich hohe Fasergehalte ein gutmütiges Verhalten von UHPC auf Zug erreicht werden. Die sichere Begrenzung der Rissbreiten auf deutlich unter 0,1 mm gewährleistet zugleich die Dauerhaftigkeit auch bei ungünstigen Umgebungsbedingungen. Durch die Minimierung des Material- und Energieeinsatzes und die zu erwartende lange Nutzungsdauer lassen sich so im Sinne der Nachhaltigkeit optimierte Bauteile realisieren.

Zur Druck-Querzug-Festigkeit von Stahlbeton und stahlfaserverstärktem Stahlbeton in scheibenförmigen Bauteilen

Am Fachgebiet Massivbau (Institut für Konstruktiven Ingenieurbau – IKI) des Fachbereichs Bauingenieurwesen der Universität Kassel wurden Bauteilversuche an zweiaxial auf Druck-Zug belasteten, faserfreien und faserverstärkten Stahlbetonscheiben durchgeführt. Dabei wurden die Auswirkungen der Querzugbeanspruchung und der Rissbildung auf die Druckfestigkeit, auf die Stauchung bei Erreichen der Höchstlast sowie auf die Drucksteifigkeit des stabstahl- und faserbewehrten Betons an insgesamt 56 faserfreien und faserverstärkten Beton- und Stahlbetonscheiben untersucht. Auf der Grundlage der experimentell erhaltenen Ergebnisse wird ein Vorschlag zur Abminderung der Druckfestigkeit des gerissenen faserfreien und faserbewehrten Stahlbetons in Abhängigkeit der aufgebrachten Zugdehnung formuliert. Die Ergebnisse werden den in DIN 1045-1 [D4], Eurocode 2 [E3, E4], CEB-FIP Model Code 1990 [C1] und ACI Standard 318-05 [A1] angegebenen Bemessungsregeln für die Druckstrebenfestigkeit des gerissenen Stahlbetons gegenübergestellt und mit den Untersuchungen anderer Wissenschaftler verglichen. Die bekannten Widersprüche zwischen den Versuchsergebnissen, den vorgeschlagenen Modellen und den Regelwerken aus U.S.A., Kanada und Europa können dabei weitgehend aufgeklärt werden. Für nichtlineare Verfahren der Schnittgrößenermittlung und für Verformungsberechnungen wird ein Materialmodell des gerissenen faserfreien und faserbewehrten Stahlbetons abgeleitet. Hierzu wird die für einaxiale Beanspruchungszustände gültige Spannungs-Dehnungs-Linie nach Bild 22 der DIN 1045-1 auf den Fall der zweiaxialen Druck-Zug-Beanspruchung erweitert.