Filtern
Erscheinungsjahr
Dokumenttyp
- Beitrag zu einem Tagungsband (90) (entfernen)
Schlagworte
- Rheology (25)
- Cement (22)
- Concrete (16)
- Superplasticizer (8)
- Admixtures (7)
- Africa (7)
- Self-compacting concrete (7)
- Cement hydration (6)
- Polycarboxylate ether (6)
- Rheologie (6)
Organisationseinheit der BAM
- 7 Bauwerkssicherheit (22)
- 7.4 Baustofftechnologie (19)
- 6 Materialchemie (5)
- 6.6 Physik und chemische Analytik der Polymere (5)
- 7.0 Abteilungsleitung und andere (3)
- 1 Analytische Chemie; Referenzmaterialien (2)
- 1.2 Biophotonik (2)
- 7.2 Ingenieurbau (2)
- 8 Zerstörungsfreie Prüfung (2)
- 8.0 Abteilungsleitung und andere (2)
Selbstverdichtender Beton (SVB) findet kaum Anwendung als Transportbeton. Ein wesentlicher Grund hierfür ist, dass die Außentemperaturen auf der Baustelle nicht gesteuert werden können. Während des Betonierens können allerdings Schwankungen in der Umgebungstemperatur die rheologischen Eigenschaften aber auch das Erstarrungsverhalten signifikant beeinflussen, wodurch in der Folge auch Veränderungen im Festbeton auftreten können. Durch geeignete Wahl der Komponenten kann die Robustheit von SVB gegenüber Einflüssen aus den Umgebungsbedingungen deutlich verbessert werden.
In der vorgestellten Arbeit werden anhand von Untersuchungen am Betonrheometer, am Setzfließmaß und am Erstarren nach Vicat die Einflüsse von veränderlichen Umgebungstemperaturen auf die frühen Eigenschaften von SVB dargestellt. Der Fokus liegt auf Effekten, die durch die Modifikation von Polycarboxylatether-Fließmitteln und durch Stabilisierer eingetragen werden. Darüber hinaus wird der Einfluss des Mischungsentwurfes gezeigt. Die Ergebnisse werden mit Relevanz für praktische Betonanwendungen analysiert und es werden die wichtigen Einflussgrößen zur Verbesserung der Robustheit von SVB gegenüber Temperaturschwankungen herausgearbeitet.
Flowable concretes can differ significantly from traditional vibrated concrete. Concrete types like self-compacting concrete (SCC), ultra high performance concrete (UHPC) and high performance fibre reinforced cementitious composites (HPFRCCs) require novel mix design approaches. This has consequences for the production and the performance in the hardened state. Mix designs for flowable concretes can incorporate a wide variety of innovative admixtures or components: e.g. superplasticisers increase the flowability and allow for significant reduction of the water content, shrinkage compensating admixtures or superabsorbent polymers support sound and damage free curing processes, viscosity modifying admixtures enhance the robustness, and new fibre types allow for sophisticated and tailored structural performance.
The new Model Code has limitations regarding the application of flowable concrete, e.g. thresholds for the minimum aggregate size and the maximum strength. Provisions are added to include fibres for structural design. fib Task Group 4.3 aims at facilitating the use of innovative flowable materials for designing concrete structures and considers three aspects of flowable concrete: material properties, production effects and structural boundary conditions and performance. This paper reports about the progress of fib TG 4.3 related to the mix design of flowable concrete and discusses the present state-of-the-art concerning admixtures and robustness.
During the last three decades concrete has emerged from a rather simple mass construction material based on only the three components cement, water, and aggregates towards a high performance material, which can be adjusted for high performance applications and according to ultimate user specifications. The reason for the rapid evolvement was the increasing awareness about how the rheology of concrete can be improved without negatively affecting the mechanical properties of concrete. Hence, mineral additions and in particular Chemical admixtures have been the most influential factors for the technological boost since approximately the 1980s.
The incorporation of superplasticizers into concrete mixture compositions eventually facilitated concrete engineers to improve the workability properties without need to increase the water-cement-ratio (w/c) and furthermore to significantly reduce the w/c without loss of workability. This finally resulted in concrete with higher performance and specified properties. Fillers are basically used to improve the particle packing of cementitious Systems, but they can also be used beneficially to support the rheology, since they modify the water demand and may interact with superplasticizers as well.
Finally, supplementary admixtures like polysaccharides have become more populär in the field of mortar, plaster, and grout technology. They can have a variety of rheological effects on cementitious Systems, which can be used to individually adjust their performance.
The possibility to control the rheology and the significantly widened ränge of consistencies that can be adjusted opened up the gates for modern concrete and any type of high performance concrete. Therefore, without doubt, it can be concluded that the capability to control the rheology of concrete Systems can be considered as the catalyst for the invention of the many recent mortar and concrete innovations such as polymer modified cementitious composites (PCC) self-compacting concrete (SCC), high-performance concrete (HPC), ultra-high performance concrete (UHPC) or engineered cementitious composites (ECC).
Therefore, understanding the rheology of cementitious Systems and how to control the workability by the use of Chemical admixtures is the key to innovations in concrete technology.
However, the other side of the coin of versatility is that sophisticated cementitious Systems have become more sensitive. Hence, concrete mixture composition with admixtures demands for a high level of expertise and often there is lack of awareness about the mode of Operation of rheology modifying admixtures among concrete technoiogists. The paper gives a comprehensive overview about rheology modifying constituents such as superplasticizers, stabilising agents, and mineral fillers, and how they can be used depending upon the application in the most favourable way.
Hochduktiler Beton eröffnet neue Möglichkeiten bei der Instandsetzung von Bauwerken, der Herstellung dünnwandiger Bauelemente oder dem Einsatz von Dämpfungselementen in stoßartig beanspruchten Bauwerken (z.B. in Erdbebengebieten).
Die Steigerung der Duktilität kann durch die Zugabe von Kurzfasern erreicht werden. Im Falle einer Rissbildung in der Zementsteinmatrix überbrücken die Fasern den Riss, nehmen die Spannung vollständig auf und stoppen lokal das Risswachstum. Bei weiterer Steigerung der Last reißt die Matrix an anderer Stelle. Auf diese Weise wird die Rissbildung fein verteilt und es werden große Dehnungen erreicht, bevor das Bauteil versagt. Zur Optimierung der Festbetoneigenschaften müssen die Matrixfestigkeit und die Faserart sowie deren Gehalt aufeinander abgestimmt werden. Dabei sind auch die Verarbeitungseigenschaften des Frischbetons zu berücksichtigen.
Anhand der Schallemissionen kann die Rissbildung in der Matrix und die Interaktion mit den Fasern analysiert werden. So konnte in ersten Zugversuchen gezeigt werden, dass sich Schallereignisse dem Versagen der Zementsteinmatrix bzw. dem Auszug der Fasern aus der Matrix zuordnen lassen. Damit steht zur Beurteilung des mechanischen Verhaltens von hochduktilem Beton neben der Spannungs-Dehnungs-Linie ein weiteres Verfahren zur Verfügung.
Concrete of today has only little in common with the traditional concrete used a few decades ago. It has become a high performance material, which can be adjusted for high performance applications and according to ultimate user specifications. The reason for the rapid evolvement was the increasing awareness about how the rheology of concrete can be improved without negatively affecting the mechanical properties of concrete by chemical admixtures.
Polysaccharides are incorporated into cement based systems in order to modify the rheological properties. Typically cellulose ethers, sphingan gums or starch ethers are applied. Depending upon their chemistry, molecular architecture, and adsorption tendency, polysaccharides interact differently with the entire cementitious system. Some stabilising agents like diutan gum mainly affect the cementitious paste; other stabilising agents like starch tend to interact with the sand fraction and even with the coarse aggregates. Cellulose shows more divers performances. Often stabilising admixtures like polysaccharides are used, when sophisticated rheological properties are adjusted. Therefore, polysaccharides are often used in combination with superplasticisers. This can cause interactions, particularly when the stabilising agent shows a strong tendency to adsorb on particle surfaces. Adsorptive stabilising agents may reduce the amount of adsorbed superplasticisers, thus affecting both viscosity and yield stress, while non-adsorptive stabilising agents mainly affect the plastic viscosity independently of the superplasticiser. Due to the strong influence of superplasticisers on the yield stress, influences of the stabilising agent on the yield stress retreat into the background, so that their major effect is an increase of the viscosity. The paper provides a comprehensive overview of how different polysaccharide superplasticisers affect cementitious flowable systems and points out the challenges of the combined use of polysaccharides and superplasticisers. Based on rheometric experiments and Observations of the hydration process, time dependent effects on the workability as well as of the hydration of cement are presented.