Neste estudo foram analisados experimentalmente o comportamento de 24 pilares curtos de Concreto de Ultra Alta Resistência - CUAR, confinados por armaduras helicoidais, avaliando especificamente os acréscimos de resistência e ductilidade obtidos com diferentes níveis de pressão lateral de confinamento. Na etapa experimental foram realizados ensaios de pilares curtos de CUAR com as seguintes características: - seção circular de 7,2 cm de diâmetro e comprimento de 23 cm, e quatro níveis de resistência à compressão do concreto sendo eles, 165, 175, 200 e 229 MPa, dosados sem e com adição de fibras metálicas; - diferentes espaçamentos das armaduras helicoidais, de modo que fossem obtidas situações com baixo, médio e alto índice de confinamento e taxa de armadura longitudinal fixa. Os ensaios de compressão centrada foram realizados com controle de deslocamento, de modo que foram obtidas as curvas força x deslocamento completas. Constatou-se que a seção resistente dos pilares de CUAR é a formada pelo núcleo de concreto confinado, área delimitada pelo eixo da armadura transversal. Observou-se que o CUAR com fibras metálicas apresenta maior deformação do núcleo de concreto confinado em relação ao núcleo de concreto confinado de CUAR sem adição de fibras metálicas, indicando dessa forma, que os pilares de CUAR com fibras metálicas apresentam comportamento mais dúctil. Para as situações de alto confinamento foram gerados ao concreto do núcleo confinado significativos acréscimos de resistência e deformação axial, aumentando a resistência do concreto confinado em relação a resistência do concreto não confinado em: 82,26%, 75,34%, 90,46% e 70,51%, respectivamente, e as deformações axiais do concreto confinado em relação a deformação axial do concreto não confinado em: 433%, 474%, 647% e 550%. Finalmente, acredita-se que os resultados obtidos poderão trazer subsídios para aplicações futuras desta técnica de confinamento na construção de novos elementos estruturais e no reforço de pilares submetidos a elevados níveis de solicitação axial. / This study evaluated experimentally the behavior of 24 short columns of Ultra High Strength Concrete - UHSC confined by helical transverse reinforcement, specifically evaluating strength increases and ductility obtained with different levels of lateral pressure of confinement. In the experimental phase short UHSC columns with the following characteristics were tested: - circular cross section of 7.2 cm diameter and 23 cm length, four levels of concrete strength (165, 175, 200 and 229 MPa), with and without addition of metallic fibers; - different spacing of transverse reinforcement, so that situations were obtained with low, medium and high level of confinement, while the longitudinal reinforcement ratio was fixed. The centered compression tests were conducted with displacement control, so that complete force x displacement curves were obtained. It was found that the resistant section of UHSC columns is formed by the confined concrete core delimited by the axis of the transverse reinforcement. It was observed that the axial displacement reached in columns with steel fibers was higher than without fibers, indicating that columns with steel fibers exhibit more ductile behavior. For high confinement levels significant axial strength and displacement increases were observed. Increases of axial strength of confined concrete in comparison to unconfined concrete were 82.26%, 75.34%, 90, 46% and 70.51%. Axial displacements were increased by 433%, 474%, 647% and 550%. Finally, it is believed that the results could provide information for future applications of this technique in construction of a new type of columns or in strengthening of columns subjected to high levels of axial force.

Neste estudo foram analisados experimentalmente o comportamento de 24 pilares curtos de Concreto de Ultra Alta Resistência - CUAR, confinados por armaduras helicoidais, avaliando especificamente os acréscimos de resistência e ductilidade obtidos com diferentes níveis de pressão lateral de confinamento. Na etapa experimental foram realizados ensaios de pilares curtos de CUAR com as seguintes características: - seção circular de 7,2 cm de diâmetro e comprimento de 23 cm, e quatro níveis de resistência à compressão do concreto sendo eles, 165, 175, 200 e 229 MPa, dosados sem e com adição de fibras metálicas; - diferentes espaçamentos das armaduras helicoidais, de modo que fossem obtidas situações com baixo, médio e alto índice de confinamento e taxa de armadura longitudinal fixa. Os ensaios de compressão centrada foram realizados com controle de deslocamento, de modo que foram obtidas as curvas força x deslocamento completas. Constatou-se que a seção resistente dos pilares de CUAR é a formada pelo núcleo de concreto confinado, área delimitada pelo eixo da armadura transversal. Observou-se que o CUAR com fibras metálicas apresenta maior deformação do núcleo de concreto confinado em relação ao núcleo de concreto confinado de CUAR sem adição de fibras metálicas, indicando dessa forma, que os pilares de CUAR com fibras metálicas apresentam comportamento mais dúctil. Para as situações de alto confinamento foram gerados ao concreto do núcleo confinado significativos acréscimos de resistência e deformação axial, aumentando a resistência do concreto confinado em relação a resistência do concreto não confinado em: 82,26%, 75,34%, 90,46% e 70,51%, respectivamente, e as deformações axiais do concreto confinado em relação a deformação axial do concreto não confinado em: 433%, 474%, 647% e 550%. Finalmente, acredita-se que os resultados obtidos poderão trazer subsídios para aplicações futuras desta técnica de confinamento na construção de novos elementos estruturais e no reforço de pilares submetidos a elevados níveis de solicitação axial. / This study evaluated experimentally the behavior of 24 short columns of Ultra High Strength Concrete - UHSC confined by helical transverse reinforcement, specifically evaluating strength increases and ductility obtained with different levels of lateral pressure of confinement. In the experimental phase short UHSC columns with the following characteristics were tested: - circular cross section of 7.2 cm diameter and 23 cm length, four levels of concrete strength (165, 175, 200 and 229 MPa), with and without addition of metallic fibers; - different spacing of transverse reinforcement, so that situations were obtained with low, medium and high level of confinement, while the longitudinal reinforcement ratio was fixed. The centered compression tests were conducted with displacement control, so that complete force x displacement curves were obtained. It was found that the resistant section of UHSC columns is formed by the confined concrete core delimited by the axis of the transverse reinforcement. It was observed that the axial displacement reached in columns with steel fibers was higher than without fibers, indicating that columns with steel fibers exhibit more ductile behavior. For high confinement levels significant axial strength and displacement increases were observed. Increases of axial strength of confined concrete in comparison to unconfined concrete were 82.26%, 75.34%, 90, 46% and 70.51%. Axial displacements were increased by 433%, 474%, 647% and 550%. Finally, it is believed that the results could provide information for future applications of this technique in construction of a new type of columns or in strengthening of columns subjected to high levels of axial force.

Freileitungsmaste aus herkömmlichen Beton werden bereits heute in großer Zahl in Mittelspan-nungsnetzen eingesetzt. Im Bereich der Hochspannungsfreileitungen existieren bisher international nur wenige erste Freileitungen mit Masten aus herkömmlichen Beton. Um zukünftig Elektroenergie über große Entfernungen über Trassen mit geringen Flächenbedarf transportieren zu können, sind neue Hochspannungsfreileitungen in kompakter Bauweise notwendig. Um dieses Ziel zu erfüllen, sollen die Kompaktmaste aus ultra-hochfestem Beton (UHPC) hergestellt werden. Dafür ist eine neue Sorte von UHPC mit hoher Festigkeitsklasse zu entwickeln. Die mechanischen, elektrischen und thermischen Eigenschaften des neuen Betonmaterials waren zunächst unbekannt. Bisher gab es kaum Kenntnisse über die elektrischen und thermischen Belas-tungen, die auf die Betonmaste einer Freileitung in kompakter Bauweise einwirken. Ein Teilthema im interdisziplinären Forschungsprojekt „KoHöMaT“ (gefördert durch das Bundesmi-nisterium für Wirtschaft und Energie) war es, gemeinsam mit Forschungsinstituten (IMB, Fichtner, Lapp, Europoles, KIT, iBMB) die Materialparameter des neuen UHPC zu bestimmen. Den Einfluss der elektromagnetischen Belastungen auf die Lebensdauer und die Festigkeit des Ver-bundes aus Stahl und Beton habe ich untersucht. Aufgabe meiner Arbeit ist es auch, die elektrischen und thermischen Eigenschaften, wie die elektrische Leitfähigkeit, die elektrische Festigkeit, die Per-mittivität, den Verlustfaktor und die Wärmeleitfähigkeit experimentell zu bestimmen. Anhand der experimentellen Untersuchungen wurde der Versagensmechanismus des UHPC-Betons bei Span-nungsbelastung identifiziert. Die am Betonmast auftretenden elektrischen und thermischen Belas-tungen wurden mit Hilfe von verschiedenen FEM-Modellen berechnet und den gemessenen Fes-tigkeiten gegenübergestellt. Es wurde der Einfluss permanenter elektrischer Felder auf die mechanischen Eigenschaften des UHPC bestimmt. Hierfür wurde die Druckfestigkeit des Betons vor und nach Dauerversuchen bei verschiedenen Spannungsbelastung gemessen. Der Verbund zwischen Stahl und Beton wurde in Lastwechselversuchen thermisch hoch beansprucht und dessen mechanische Festigkeit vor und nach der thermischen Belastung bei Auszugsversuchen gemessen. Aufgrund der befürchtenden gesundheitlichen Risiken für Menschen und Tiere, sowie der gegen-seitigen Beeinflussung benachbarter elektronischer Systeme (EMV) dürfen die elektromagnetischen Felder von Freileitungen die jeweiligen maximal zulässigen Grenzwerte nicht überschreiten. Ich habe die Berechnungen der elektrischen und magnetischen Feldverteilung für die im Verbundvorhaben entwickelten Mastdesigns durchgeführt. Gemeinsam mit den Forschungsinstituten (Europoles, Fichtner, Lapp) wurden die Mastdesigns hinsichtlich der Feldverteilung optimiert. / The Overhead line towers made of conventional concrete are already used in large numbers in the medium voltage nowadays. So far, only a few towers of overhead transmission line made of con-ventional concrete which exists internationally in the area of high voltage. In order to be able to transmit electrical energy over long distances by routes of less floor space requirements, new high voltage overhead lines in compact construction are necessary. To achieve this goal, the compact towers have to be made of ultra-high-performance concrete (UHPC). Therefore, a new kind of UHPC with a high strength class has to be developed. For this kind of new concrete, the mechanical, electrical and thermal characteristics were unknown till now either, there was rare knowledge about the electric and thermal loads which have an effect on the concrete towers of an overhead line in compact construction method. The main purpose part of this interdisciplinary research project 'KoHöMaT “, which funded by the Federal Ministry for Economic Affairs and Energy), was to identify the material parameters of the new UHPC together with the following research institutes (IMB, Fichtner, Lapp, Europoles, KIT, iBMB). It was examined the influence of electromagnetic loads on the lifetime and its’ strength bond be-tween both of composite steel and concrete, also as my major involve was to determine the elec-trical and thermal properties experimentally, such as electrical conductivity, electrical strength, per-mittivity, dissipation factor and finally thermal conductivity. As a result, the failure mechanism of the UHPC under the electrical stresses has been identified then,the electrical and thermal loads on the concrete towers were calculated by using various FEM models accordingly, the measured values were used in the determination of electrical strength. All mentioned theoretical calculated parameters were compared with the real measured parameters. The influence of permanent electric fields on mechanical properties of the UHPC was determined as well. Mainly, the compressive strength of the concrete was measured before and after durability tests at different voltage loads. In addition, the composite (interface) between steel and concrete was thermal extremely loaded by alternating load tests. Its mechanical strength has been measured by pull-out tests before and after this thermal loads. Due to the fear of health risks for both humans and animals, as well as the mutual influence of neighboring electronic systems (EMV), the electromagnetic fields of open lines must not exceed the respective maximum permissible limit values. The calculations of the electrical and magnetic field distribution were carried out for the mast design developed in the composite project. Together with the other research institute (Europoles, Fichtner, Lapp). the tower designs were optimized with re-gard to the field distribution.

The autogenous shrinkage due to self-desiccation of high- and ultra-high performance concretes with very low water-cement ratio in case of restraint leads to considerable stresses starting from very early age. The resultant risk of cracking presently cannot be adequately investigated. Parameters that are particularly difficult to capture experimentally are the concrete temperature and the viscoelasticity. The primary objective of this work was to assess as precise as possible the autogenous shrinkage cracking propensity of representative concretes at strong restraint and constant room temperature. Test methods needed to be chosen and enhanced in a way that preferably allowed for the efficient and precise investigation of all relevant factors in the future. Ideally, a method suitable for a complete empirical modeling was provided. First the methodological requirements and the advantages and disadvantages of existing test methods were discussed. Based on this, optimized test methods were proposed. Their suitability was verified using the example of ultra-high strength concrete. The choice of concrete compositions considered the essential measures for reducing shrinkage (internal curing, shrinkage-reducing admixtures, reduction of the fraction of Portland cement in the binder). The autogenous shrinkage was measured with the shrinkage cone method. This new test method was validated by investigations of the repeatability and reproducibility and proved efficient and precise. It allows for measurements under non-isothermal conditions; no established test method exists for that purpose to date. The autogenous shrinkage of the ultra-high strength concretes at the age of 24 h, investigated under quasi-isothermal conditions (20 °C), was between 0,25 mm/m and 0,70 mm/m. It was particularly low when a shrinkage-reducing admixture was added and when superabsorbent polymers were used. The stresses due to restraint were determined with the restrained ring test. A large part of the stresses to be expected according to Hooke’s Law were eliminated by creep and relaxation. The relaxation capacity being very pronounced at very early age was the main reason that no visible cracking occurred, not even with the concretes with high autogenous shrinkage. The development of the autogenous shrinkage cracking propensity was described as ratio of restraint stress and splitting tensile strength. By means of modified ring tests, used to determine the maximum tensile stress, it could be shown that the ratio of stress to strength is an appropriate failure criterion. However, the cracking propensity can be calculated correctly only if the strongly age-dependent ratio of uniaxial to splitting tensile strength is accounted for. Besides, it needs to be considered that at very early age a plastic stress redistribution may occur in restrained ring tests. The reference concrete showed a high cracking propensity of up to 0.68. The fact that shrinkage-reducing measures led to significantly lower values reveals their relevance for the safe application of ultra-high strength concrete. However, the investigations carried out here at 20 °C do not allow for a final assessment of the cracking propensity under typical on-site conditions. To empirically model the autogenous shrinkage cracking propensity as a function of temperature and stress level in the future, an analytical stress solution for non-isothermal restrained ring tests and a new approach for investigating the residual stress and relaxation capacity by means of non-passive restrained ring tests was suggested. / Das durch Selbstaustrocknung verursachte autogene Schwinden von besonders leistungsfähigen Betonen mit sehr niedrigem Wasserzementwert führt bei Dehnungsbehinderung bereits in sehr frühem Alter zu erheblichen Zwangsspannungen. Die Gefahr der Rissbildung, die sich daraus ergibt, lässt sich bislang nur unzureichend untersuchen. Experimentell besonders schwer zu erfassende Faktoren sind die Betontemperatur und die Viskoelastizität. Das vorrangige Ziel der Arbeit war die möglichst genaue Ermittlung der autogenen Schwindrissneigung repräsentativer Betone bei starker Dehnungsbehinderung und konstanter Raumtemperatur. Dabei waren die Prüfverfahren möglichst so zu wählen und weiterzuentwickeln, dass sich zukünftig alle relevanten Faktoren effizient und genau untersuchen lassen. Im Idealfall sollte eine Methode entstehen, die eine vollständige empirische Modellierung erlaubt. Zunächst wurden die methodischen Anforderungen und die Vor- und Nachteile existierender Prüfverfahren diskutiert. Darauf aufbauend wurden optimierte Verfahren vorgeschlagen. Ihre Eignung wurde an ultrahochfestem Beton überprüft. Bei der Auswahl der Betone wurden die wesentlichen Maßnahmen zur Schwindreduzierung berücksichtigt (innere Nachbehandlung, schwindreduzierende Zusatzmittel, Verringerung des Portlandzementanteils am Bindemittel). Das autogene Schwinden wurde mit dem Schwindkegelverfahren gemessen. Das neue Verfahren wurde durch Untersuchungen zur Wiederhol- und Vergleichsgenauigkeit validiert und erwies sich als effizient und genau. Es ermöglicht Messungen unter nicht-isothermen Bedingungen; hierfür existiert bisher kein etabliertes Verfahren. Das autogene Schwinden der untersuchten ultrahochfesten Betone unter quasi-isothermen Bedingungen (20 °C) betrug im Alter von 24 h zwischen 0,25 mm/m und 0,70 mm/m. Besonders gering war es bei Zugabe eines schwindreduzierenden Zusatzmittels bzw. Verwendung superabsorbierender Polymere. Mit dem Ring-Test wurden die bei Dehnungsbehinderung entstehenden Spannungen ermittelt. Ein großer Teil der gemäß Hooke’schem Gesetz zu erwartenden Spannungen wurde durch Kriechen und Relaxation abgebaut. Die im sehr frühen Alter stark ausgeprägte Relaxationsfähigkeit war der wesentliche Grund dafür, dass es selbst bei Betonen mit hohem autogenen Schwinden zu keiner erkennbaren Rissbildung kam. Die Entwicklung der autogenen Schwindrissneigung wurde als Verhältnis von Zwangsspannung und Spaltzugfestigkeit beschrieben. Durch modifizierte Ring-Tests, mit deren Hilfe die maximale Zugspannung ermittelt wurde, konnte gezeigt werden, dass das Verhältnis von Spannung und Festigkeit als Versagenskriterium geeignet ist. Die Rissneigung lässt sich aber nur dann korrekt berechnen, wenn das stark altersabhängige Verhältnis von einaxialer Zugfestigkeit und Spaltzugfestigkeit berücksichtigt wird. Außerdem ist zu beachten, dass es im sehr frühen Alter zu einer plastischen Spannungsumlagerung in Ring-Tests kommen kann. Der Referenzbeton wies eine hohe Rissneigung von bis zu 0,68 auf. Dass die schwindreduzierenden Maßnahmen zu deutlich geringeren Werten führten, zeigt deren Bedeutung für den sicheren Einsatz von ultrahochfestem Beton. Die hier bei 20 °C durchgeführten Untersuchungen erlauben allerdings keine abschließende Bewertung der Rissneigung unter baustellentypischen Bedingungen. Um die autogene Schwindrissneigung zukünftig als Funktion der Temperatur und des Lastniveaus empirisch modellieren zu können, wurden eine analytische Spannungslösung für nicht-isotherme Ring-Tests und ein neuer Ansatz zur Untersuchung der Resttrag- und Relaxationsfähigkeit mit Hilfe nicht-passiver Ring-Tests vorgeschlagen.

The autogenous shrinkage due to self-desiccation of high- and ultra-high performance concretes with very low water-cement ratio in case of restraint leads to considerable stresses starting from very early age. The resultant risk of cracking presently cannot be adequately investigated. Parameters that are particularly difficult to capture experimentally are the concrete temperature and the viscoelasticity. The primary objective of this work was to assess as precise as possible the autogenous shrinkage cracking propensity of representative concretes at strong restraint and constant room temperature. Test methods needed to be chosen and enhanced in a way that preferably allowed for the efficient and precise investigation of all relevant factors in the future. Ideally, a method suitable for a complete empirical modeling was provided. First the methodological requirements and the advantages and disadvantages of existing test methods were discussed. Based on this, optimized test methods were proposed. Their suitability was verified using the example of ultra-high strength concrete. The choice of concrete compositions considered the essential measures for reducing shrinkage (internal curing, shrinkage-reducing admixtures, reduction of the fraction of Portland cement in the binder). The autogenous shrinkage was measured with the shrinkage cone method. This new test method was validated by investigations of the repeatability and reproducibility and proved efficient and precise. It allows for measurements under non-isothermal conditions; no established test method exists for that purpose to date. The autogenous shrinkage of the ultra-high strength concretes at the age of 24 h, investigated under quasi-isothermal conditions (20 °C), was between 0,25 mm/m and 0,70 mm/m. It was particularly low when a shrinkage-reducing admixture was added and when superabsorbent polymers were used. The stresses due to restraint were determined with the restrained ring test. A large part of the stresses to be expected according to Hooke’s Law were eliminated by creep and relaxation. The relaxation capacity being very pronounced at very early age was the main reason that no visible cracking occurred, not even with the concretes with high autogenous shrinkage. The development of the autogenous shrinkage cracking propensity was described as ratio of restraint stress and splitting tensile strength. By means of modified ring tests, used to determine the maximum tensile stress, it could be shown that the ratio of stress to strength is an appropriate failure criterion. However, the cracking propensity can be calculated correctly only if the strongly age-dependent ratio of uniaxial to splitting tensile strength is accounted for. Besides, it needs to be considered that at very early age a plastic stress redistribution may occur in restrained ring tests. The reference concrete showed a high cracking propensity of up to 0.68. The fact that shrinkage-reducing measures led to significantly lower values reveals their relevance for the safe application of ultra-high strength concrete. However, the investigations carried out here at 20 °C do not allow for a final assessment of the cracking propensity under typical on-site conditions. To empirically model the autogenous shrinkage cracking propensity as a function of temperature and stress level in the future, an analytical stress solution for non-isothermal restrained ring tests and a new approach for investigating the residual stress and relaxation capacity by means of non-passive restrained ring tests was suggested.:1 Introduction 2 Autogenous shrinkage 5 2.1 Shrinkage and hydration 5 2.2 Definitions and research approaches 10 2.3 Metrological issues 14 2.3.1 Multitude of test methods 14 2.3.2 Time-zero 16 2.3.3 Other metrological issues 18 2.4 Corrugated tube method 19 2.5 Influencing parameters 21 2.5.1 Concrete composition 21 2.5.2 Temperature 23 2.5.3 Specific countermeasures 25 2.6 Summary and conclusions with respect to the own work 25 3 Concretes used in the own investigations 27 3.1 Preliminary remarks 27 3.2 Concrete compositions 27 3.3 Constituents 28 3.3.1 Cement 28 3.3.2 Ground-granulated blast furnace slag 28 3.3.3 Silica fume 28 3.3.4 Admixtures 29 3.3.5 Aggregates 29 3.4 Mixing 29 3.5 Basic properties 30 3.5.1 Compressive strength 30 3.5.2 Splitting tensile strength 31 3.5.3 Modulus of elasticity 33 3.5.4 Analysis of mechanical properties 35 3.5.5 Coefficient of thermal expansion 38 3.5.6 Isothermal calorimetry 39 3.6 Summary 39 4 Shrinkage cone method for measuring autogenous shrinkage 41 4.1 Introduction 41 4.2 Setup and measurement procedure 41 4.3 Temperature control 44 4.4 Precision under quasi-isothermal conditions 47 4.4.1 Repeatability 47 4.4.2 Reproducibility 49 4.4.3 Shrinkage cone method vs. corrugated tube method 49 4.5 Autogenous shrinkage of the investigated concretes at 20 °C 54 4.6 Tests under non-isothermal conditions 55 4.7 Summary 56 5 Stress and cracks due to restrained autogenous shrinkage 58 5.1 Introduction 58 5.2 Degree of restraint 58 5.3 Formation of cracks 60 5.4 Very early age and importance of stress relaxation 63 5.5 Creep and cracking - further methodological aspects 65 5.6 Autogenous shrinkage cracking propensity 69 5.7 Role of temperature history 70 5.8 Further state of knowledge 72 5.8.1 Preliminary remarks on test methods 72 5.8.2 Quantitative investigations under restraint conditions 73 5.8.3 A full-scale model for assessing the cracking risk at very early age 77 5.9 Summary 78 6 Investigation of the autogenous shrinkage cracking propensity 80 6.1 Introduction 80 6.2 Suitability of temperature-stress testing machines 80 6.2.1 Development, setup and use 80 6.2.2 Results of round robin tests 83 6.3 Restrained ring test - methodological foundations 86 6.3.1 Setup and use 86 6.3.2 Evaluation of restrained ring tests 90 6.3.3 Use of temperature changes for the investigation of creep and relaxation 96 6.4 Own investigations with the restrained ring test 97 6.4.1 Setup 97 6.4.2 Compensation of disturbing temperature effects 99 6.4.3 Repeatability 100 6.4.4 Measured steel ring strains 101 6.4.5 Simple stress analysis 102 6.4.6 Autogenous shrinkage cracking propensity - further analysis 106 6.4.7 Thermal stress component 116 6.4.8 Period of maximum cracking propensity 118 6.4.9 Restraint stress versus autogenous shrinkage 119 6.4.10 Cracking propensity versus autogenous shrinkage 120 6.4.11 Further considerations on creep 121 6.5 Summary 126 7 Summary, conclusions and outlook 128 7.1 Summary and conclusions 128 7.2 Outlook 130 8 Literature 131 9 Annex 159 / Das durch Selbstaustrocknung verursachte autogene Schwinden von besonders leistungsfähigen Betonen mit sehr niedrigem Wasserzementwert führt bei Dehnungsbehinderung bereits in sehr frühem Alter zu erheblichen Zwangsspannungen. Die Gefahr der Rissbildung, die sich daraus ergibt, lässt sich bislang nur unzureichend untersuchen. Experimentell besonders schwer zu erfassende Faktoren sind die Betontemperatur und die Viskoelastizität. Das vorrangige Ziel der Arbeit war die möglichst genaue Ermittlung der autogenen Schwindrissneigung repräsentativer Betone bei starker Dehnungsbehinderung und konstanter Raumtemperatur. Dabei waren die Prüfverfahren möglichst so zu wählen und weiterzuentwickeln, dass sich zukünftig alle relevanten Faktoren effizient und genau untersuchen lassen. Im Idealfall sollte eine Methode entstehen, die eine vollständige empirische Modellierung erlaubt. Zunächst wurden die methodischen Anforderungen und die Vor- und Nachteile existierender Prüfverfahren diskutiert. Darauf aufbauend wurden optimierte Verfahren vorgeschlagen. Ihre Eignung wurde an ultrahochfestem Beton überprüft. Bei der Auswahl der Betone wurden die wesentlichen Maßnahmen zur Schwindreduzierung berücksichtigt (innere Nachbehandlung, schwindreduzierende Zusatzmittel, Verringerung des Portlandzementanteils am Bindemittel). Das autogene Schwinden wurde mit dem Schwindkegelverfahren gemessen. Das neue Verfahren wurde durch Untersuchungen zur Wiederhol- und Vergleichsgenauigkeit validiert und erwies sich als effizient und genau. Es ermöglicht Messungen unter nicht-isothermen Bedingungen; hierfür existiert bisher kein etabliertes Verfahren. Das autogene Schwinden der untersuchten ultrahochfesten Betone unter quasi-isothermen Bedingungen (20 °C) betrug im Alter von 24 h zwischen 0,25 mm/m und 0,70 mm/m. Besonders gering war es bei Zugabe eines schwindreduzierenden Zusatzmittels bzw. Verwendung superabsorbierender Polymere. Mit dem Ring-Test wurden die bei Dehnungsbehinderung entstehenden Spannungen ermittelt. Ein großer Teil der gemäß Hooke’schem Gesetz zu erwartenden Spannungen wurde durch Kriechen und Relaxation abgebaut. Die im sehr frühen Alter stark ausgeprägte Relaxationsfähigkeit war der wesentliche Grund dafür, dass es selbst bei Betonen mit hohem autogenen Schwinden zu keiner erkennbaren Rissbildung kam. Die Entwicklung der autogenen Schwindrissneigung wurde als Verhältnis von Zwangsspannung und Spaltzugfestigkeit beschrieben. Durch modifizierte Ring-Tests, mit deren Hilfe die maximale Zugspannung ermittelt wurde, konnte gezeigt werden, dass das Verhältnis von Spannung und Festigkeit als Versagenskriterium geeignet ist. Die Rissneigung lässt sich aber nur dann korrekt berechnen, wenn das stark altersabhängige Verhältnis von einaxialer Zugfestigkeit und Spaltzugfestigkeit berücksichtigt wird. Außerdem ist zu beachten, dass es im sehr frühen Alter zu einer plastischen Spannungsumlagerung in Ring-Tests kommen kann. Der Referenzbeton wies eine hohe Rissneigung von bis zu 0,68 auf. Dass die schwindreduzierenden Maßnahmen zu deutlich geringeren Werten führten, zeigt deren Bedeutung für den sicheren Einsatz von ultrahochfestem Beton. Die hier bei 20 °C durchgeführten Untersuchungen erlauben allerdings keine abschließende Bewertung der Rissneigung unter baustellentypischen Bedingungen. Um die autogene Schwindrissneigung zukünftig als Funktion der Temperatur und des Lastniveaus empirisch modellieren zu können, wurden eine analytische Spannungslösung für nicht-isotherme Ring-Tests und ein neuer Ansatz zur Untersuchung der Resttrag- und Relaxationsfähigkeit mit Hilfe nicht-passiver Ring-Tests vorgeschlagen.:1 Introduction 2 Autogenous shrinkage 5 2.1 Shrinkage and hydration 5 2.2 Definitions and research approaches 10 2.3 Metrological issues 14 2.3.1 Multitude of test methods 14 2.3.2 Time-zero 16 2.3.3 Other metrological issues 18 2.4 Corrugated tube method 19 2.5 Influencing parameters 21 2.5.1 Concrete composition 21 2.5.2 Temperature 23 2.5.3 Specific countermeasures 25 2.6 Summary and conclusions with respect to the own work 25 3 Concretes used in the own investigations 27 3.1 Preliminary remarks 27 3.2 Concrete compositions 27 3.3 Constituents 28 3.3.1 Cement 28 3.3.2 Ground-granulated blast furnace slag 28 3.3.3 Silica fume 28 3.3.4 Admixtures 29 3.3.5 Aggregates 29 3.4 Mixing 29 3.5 Basic properties 30 3.5.1 Compressive strength 30 3.5.2 Splitting tensile strength 31 3.5.3 Modulus of elasticity 33 3.5.4 Analysis of mechanical properties 35 3.5.5 Coefficient of thermal expansion 38 3.5.6 Isothermal calorimetry 39 3.6 Summary 39 4 Shrinkage cone method for measuring autogenous shrinkage 41 4.1 Introduction 41 4.2 Setup and measurement procedure 41 4.3 Temperature control 44 4.4 Precision under quasi-isothermal conditions 47 4.4.1 Repeatability 47 4.4.2 Reproducibility 49 4.4.3 Shrinkage cone method vs. corrugated tube method 49 4.5 Autogenous shrinkage of the investigated concretes at 20 °C 54 4.6 Tests under non-isothermal conditions 55 4.7 Summary 56 5 Stress and cracks due to restrained autogenous shrinkage 58 5.1 Introduction 58 5.2 Degree of restraint 58 5.3 Formation of cracks 60 5.4 Very early age and importance of stress relaxation 63 5.5 Creep and cracking - further methodological aspects 65 5.6 Autogenous shrinkage cracking propensity 69 5.7 Role of temperature history 70 5.8 Further state of knowledge 72 5.8.1 Preliminary remarks on test methods 72 5.8.2 Quantitative investigations under restraint conditions 73 5.8.3 A full-scale model for assessing the cracking risk at very early age 77 5.9 Summary 78 6 Investigation of the autogenous shrinkage cracking propensity 80 6.1 Introduction 80 6.2 Suitability of temperature-stress testing machines 80 6.2.1 Development, setup and use 80 6.2.2 Results of round robin tests 83 6.3 Restrained ring test - methodological foundations 86 6.3.1 Setup and use 86 6.3.2 Evaluation of restrained ring tests 90 6.3.3 Use of temperature changes for the investigation of creep and relaxation 96 6.4 Own investigations with the restrained ring test 97 6.4.1 Setup 97 6.4.2 Compensation of disturbing temperature effects 99 6.4.3 Repeatability 100 6.4.4 Measured steel ring strains 101 6.4.5 Simple stress analysis 102 6.4.6 Autogenous shrinkage cracking propensity - further analysis 106 6.4.7 Thermal stress component 116 6.4.8 Period of maximum cracking propensity 118 6.4.9 Restraint stress versus autogenous shrinkage 119 6.4.10 Cracking propensity versus autogenous shrinkage 120 6.4.11 Further considerations on creep 121 6.5 Summary 126 7 Summary, conclusions and outlook 128 7.1 Summary and conclusions 128 7.2 Outlook 130 8 Literature 131 9 Annex 159