The Effect of Accelerator Dosage on Fresh Concrete Properties and on Interlayer Strength in Shotcrete 3D Printing

Recently, the progress in 3D concrete printing has developed enormously. However, for the techniques available, there is still a severe lack of knowledge of the functional interaction of processing technology, concrete rheology and admixture usage. For shotcrete 3D printing technology, we present the eect of accelerator dosages (0%, 2%, 4% and 6%) on fresh concrete properties and on interlayer strength. Therefore, early yield stress development up to 90 min is measured with penetration resistance measurements. Deformation of layers under loading is investigated with digital image correlation and a mechanical testing machine. One point in time (10 min after deposition) is examined to quantify vertical buildability of elements depending on the accelerator dosage. Four dierent interlayer times (0, 2, 5 and 30 min), which occur for the production of small and large elements as well as due to delay during production, are investigated mechanically as well as quantitatively with computed tomography regarding the formation of cold joints. With increased accelerator dosage, an instantaneous increase in early age yield stress and yield stress evolution was observed. An increase in interlayer time leads to a reduced strength. This is mainly attributed to the observed reduced mechanical interlocking eect of the strands. Finally, a model to describe interlayer quality is presented. In the end, advantages as well as limitations of the findings are discussed

Schalentragwerke mit funktionaler Gradierung

Betone für schlanke Schalentragwerke weisen zur Sicherstellung ausreichender Zugfestigkeiten oft einen hohen Stahlfasergehalt auf. Dies ist mit hohen ökologischen und monetären Kosten verbunden. Das Ziel war es daher, die Voraussetzungen für die Herstellung effizienter Schalentragwerke aus funktional fasergradierten Betonfertigteilen zu schaffen.Concrete for slender load-bearing shell structures often has a high steel fibre content to ensure sufficient tensile strength. This is associated with high ecological and financial costs. Thus, the aim of this project was to create the prerequisites for the production of efficient shell structures made of functional fibre-graded precast concrete elements

Experimental investigations on the compaction energy for a robotic rammed earth process

Rammed earth is a construction material with a long history of traditional manufacturing. Due to its low environmental impact, positive impact on indoor climate and completely recyclable nature, its demand is also increasing in modern construction industry. However, as a consequence of the predominantly manual manufacturing processes, the production of rammed earth components is both inefficient and costly. Through the implementation of automated and robot-aided fabrication processes in the field of rammed earth construction, the opportunity to advance the digitalization of the field can raise to a new level. In this paper, general studies on the interrelation of process and material parameters and their influence on the compaction results were conducted as a basis for the development of a prototypic robotic manufacturing process. The results show that reducing the layer height can significantly decrease the impact energy. Additionally, it was shown that there is a minimum number of strokes and a minimum ramming frequency required for sufficient compaction. Furthermore, a possible workflow for a specific control of the required dry density through variation of the compaction energy with regard to the present moisture content was identified

Effect of Pre-Shear on Agglomeration and Rheological Parameters of Cement Paste

Cementitious pastes are multiphase suspensions that are rheologically characterized by viscosity and yield stress. They tend to flocculate during rest due to attractive interparticle forces, and desagglomerate when shear is induced. The shear history, e.g., mixing energy and time, determines the apparent state of flocculation and accordingly the particle size distribution of the cement in the suspension, which itself affects suspension's plastic viscosity and yield stress. Thus, it is crucial to understand the effect of the mixing procedure of cementitious suspensions before starting rheological measurements. However, the measurement of the in-situ particle agglomeration status is difficult, due to rapidly changing particle network structuration. The focused beam reflectance measurement (FBRM) technique offers an opportunity for the in-situ investigation of the chord length distribution. This enables to detect the state of flocculation of the particles during shear. Cementitious pastes differing in their solid fraction and superplasticizer content were analyzed after various pre-shear histories, i.e., mixing times. Yield stress and viscosity were measured in a parallel-plate-rheometer and related to in-situ measurements of the chord length distribution with the FBRM-probe to characterize the agglomeration status. With increasing mixing time agglomerates were increasingly broken up in dependence of pre-shear: After 300 s of pre-shear the agglomerate sizes decreased by 10 µm to 15 µm compared to a 30 s pre-shear. At the same time dynamic yield stress and viscosity decreased up to 30% until a state of equilibrium was almost reached. The investigations show a correlation between mean chord length and the corresponding rheological parameters affected by the duration of pre-shear

Effect of Different Shear Rates on Particle Microstructure of Cementitious Materials in a Wide Gap Vane-in-cup Rheometer

Rheological properties of cementitious suspensions are affected not only by their mixture composition but also by process-related factors such as shear history. To enable a model-based description, investigations were carried out on the effect of shear history (shear rate variation over time) on the cement paste agglomeration state. Therefore, a Focused Beam Reflectance Measurement (FBRM) system and a wide gap rheometer were coupled to study the relation between shear history and in-situ chord length distribution simultaneously, indicating particle agglomeration. Hence, the effect of average shear rates (resulting from the applied shear profile), as well as shear rate distribution within the gap (local shear rates) on the particle agglomeration state have been investigated. The rheological properties of cement paste were evaluated with the Reiner-Riwlin approach. Furthermore, the agglomeration state of the particles was compared for different average shear rates and local shear rates at various positions of the FBRM probe. The results show that the median chord length increases in all positions when the average shear rate is decreased, indicating increasing particle agglomeration. Moreover, due to variable local shear rates at different FBRM probe positions, different agglomeration states are observed, resulting from two factors, shear rate dependent particle agglomeration and shear-induced particle migration

Bayesian inference of mesoscale mechanical properties of mortar using experimental data from a double shear test

In this work, we propose Bayesian parameter estimation of a nonlinear mechanics based model describing the behaviour of mortar subjected to double shear test with externally bonded carbon fibre reinforced polymer (CFRP) plates. With the Bayesian approach, it is possible to identify mechanical material parameters of different phases of the mortar mesostructure, i.e. hardened cement paste, aggregates and interface transition zone (ITZ). Due to nonlinearity of the concerned problem, we use a novel sequential approach for the parameter inference, which does not require coupling between the finite element solver and software for the stochastic analysis. The model geometry and material mesostructure are learned based on micro-computed tomography (μCT) scans of the real specimen, whereas the unknown boundary conditions are assumed to be uncertain and are also identified from experimental data. Mortar is modelled through a discrete lattice model consisting of spatial Timoshenko beams with embedded discontinuities. The latter allows the description of the distinct stages in material degradation, from the appearance of microscopic material damage to its evolution into macroscopic cracks leading to localised failure.</p

Additive Fertigung frei geformter Betonbauteile durch selektives Binden mit calciumsilikatbasierten Zementen

Die additive Fertigung erlaubt ein bisher nicht gekanntes Maß an geometrischer Freiheit bei der Gestaltung von Bauteilen. In der Medizin- und Dentaltechnik wird die additive Fertigung beispielsweise eingesetzt, um an die individuelle Anatomie des Patienten angepasste Prothesen und Implantate herzustellen. Im Maschinenwesen ermöglicht sie den Bau von multifunktionalen und formoptimierten Bauteilen und damit eine Senkung des Materialeinsatzes bei gleichzeitiger Steigerung der Leistungsfähigkeit. In der Luft- und Raumfahrttechnik wird sie genutzt, um mittels Topologieoptimierung oder durch das Zusammenfassen von ganzen Baugruppen zu einem einzelnen Bauteil Gewicht zu sparen [1]. (Aus: Motivation]Additive manufacturing allows a previously unknown degree of geometric freedom in the design of components. In medical and dental technology for example, additive manufacturing is used to produce prostheses and implants adapted to the individual anatomy of the patient. In mechanical engineering, it enables the construction of multifunctional and shape-optimized components thus reducing the amount of material used while simultaneously increasing performance. In aerospace engineering, it is used to save weight by means of topology optimization or by combining entire assemblies into a single component [1]. [Off: Motivation

Effiziente Schalentragwerke aus funktional gradierten Betonfertigteilen: Funktionale Gradierung, Schalungs- und Herstelltechnologie unter Berücksichtigung der Segmenttopologie

Ziel des Forschungsprojektes ist es, die Voraussetzungen für die Herstellung effizienter Schalentragwerke aus funktional gradierten Betonfertigteilen zu schaffen. Hierfür wurde zunächst die Frage nach dem optimalen Entwurfsraum geklärt. Weiterhin galt es, die Effizienz des Tragwerks durch Erhöhung der Zugtragfähigkeit sowie durch Gradierung des Stahlfasergehaltes zu steigern. Die Herstellung von faserbewehrten Bauteilen und insbesondere die rheologische Optimierung stellt einen weiteren Schwerpunkt im Vorhaben dar. Daher wurde eine Methode entwickelt, mit der es möglich ist, das Formfüllungsvermögen von feinkörnigen Betonen in dünnwandigen Schalungen mit numerischer Strömungsmechanik vorab zu simulieren

Injection 3D Concrete Printing (I3DCP): Basic Principles and Case Studies

Today, the majority of research in 3D concrete printing focuses on one of the three methods: firstly, material extrusion; secondly, particle-bed binding; and thirdly, material jetting. Common to all these technologies is that the material is applied in horizontal layers. In this paper, a novel 3D concrete printing technology is presented which challenges this principle: the so-called Injection 3D Concrete Printing (I3DCP) technology is based on the concept that a fluid material (M1) is robotically injected into a material (M2) with specific rheological properties, causing material M1 to maintain a stable position within material M2. Different to the layered deposition of horizontal strands, intricate concrete structures can be created through printing spatially free trajectories, that are unconstrained by gravitational forces during printing. In this paper, three versions of this method were investigated, described, and evaluated for their potential in construction: A) injecting a fine grain concrete into a non-hardening suspension; B) injecting a non-hardening suspension into a fine grain concrete; and C) injecting a fine grain concrete with specific properties into a fine grain concrete with different properties. In an interdisciplinary research approach, various material combinations were developed and validated through physical experiments. For each of the three versions, first architectural applications were developed and functional prototypes were fabricated. These initial results confirmed both the technological and economic feasibility of the I3DCP process, and demonstrate the potential to further expand the scope of this novel technology

Modelling the influence of material and process parameters on Shotcrete 3D Printed strands - cross-section adjustment for automatic robotic manufacturing

Due to its high interlayer strength and application flexibility, Shotcrete 3D Printing (SC3DP) is a promising method for the additive manufacturing of structural concrete components. The printing process is based on a layer-wise material application, conducted along a pre-designed printing path. However, material batch inhomogeneities and environmental alteration lead to varying concrete properties over the production processes. These material irregularities stochastically affect the layer geometry and thus limit the achievable reproducibility and accuracy. To enhance the process stability and improve the dimensional component quality in case of environmental changes, a reliable mapping between the strand geometry and the process and material parameters is fundamental for systematic cross-section adjustment. In this paper, we present an experimental-based approach for attaining a flexible regression model of the cross-section of Shotcrete 3D Printed concrete strands. The width and height of the layer are chosen for the strand representation, which we considered as the main factors for the printing-path planning. Regarding the modelling parameters, we focus on the volume flow parameters of concrete and air, and on the accelerator dosage. These inertia afflicted parameters can provide a consistent strand geometry, while factors of lower latency such as printing speed or spray distance are conserved for online adaptation. Based on the presented proceeding, an adjustable layer height and width model has been successfully used to predict the strand properties. The production of a medium sized sample wall further proves the applicability to the production process. In addition, we demonstrated that the chosen parameters not only affect the geometry but also the mechanical performance of SC3DP-specimens. This is evaluated based on flexural strength measurements. Given the geometrical and mechanical properties, the study defines applicable limits for the investigated parameters