Hardened properties of high-performance printing concrete
Abstract
This paper presents the hardened properties of a high-performance fibre-reinforced fine-aggregate concrete extruded through a 9 mm diameter nozzle to build layer-by-layer structural components in a printing process. The printing process is a digitally controlled additive method capable of manufacturing architectural and structural components without formwork, unlike conventional concrete construction methods. The effects of the layering process on density, compressive strength, flexural strength, tensile bond strength and drying shrinkage are presented together with the implication for mix proportions. A control concrete (mould-cast specimens) had a density of approximately 2250 kg/m3, high strength (107 MPa in compression, 11 MPa in flexure) and 3 MPa in direct tension, together with a relatively low drying shrinkage of 175 μm (cured in water) and 855 μm (cured in a chamber at 20 °C and 60% relative humidity) at 184 days. In contrast well printed concrete had a density of 2350 kg/m3, compressive strength of 75–102 MPa, flexural strength of 6–17 MPa depending on testing direction, and tensile bond strength between layers varying from 2.3 to 0.7 MPa, reducing as the printing time gap between layers increased. The well printed concrete had significantly fewer voids greater than 0.2 mm diameter (1.0%) when compared with the mould-cast control (3.8%), whilst samples of poorly printed material had more voids (4.8%) mainly formed in the interstices between filaments. The additive extrusion process was thus shown to retain the intrinsic high performance of the material.
... This might be explained by the material being compacted in the printing direction due to the extrusion process' mechanical pressure [24,25]. Similar anisotropic behaviour for several types of printed concrete has been shown in earlier studies [26,27]. ...
... The results showed that the average hardened density of printed cubes in X, Y and Z directions are 2150, 2149, and 2145 kg/m 3 , respectively, which are slightly more than that of cast concrete with 2100 kg/m 3 . The higher density of printed cubes, which also seen in [20,26], is because it gets well compacted under the pressure during extrusion, as well as these cubes sawed from the middle part of the printed slab which compacted well compared to the outer layers. In contrast, the average hardened density of cast cubes with size 150 mm is 2071 kg/m 3 , which is higher than that of fully printed cubes in Path1, Path2, and Path3 with 2008, 1888, and 1930 kg/m 3 , respectively. ...
3D concrete printing, also known as 3D CP, offers an advantage in creating intricate and unique shapes using a printer equipped with a pump, hose pipe and nozzle. The speed at which the printing process occurs is crucial for construction. It depends on factors such as the size and complexity of the printed element, the pump rate and the quality of the concrete used. To achieve precision during printing, it is essential to use high-performance construction materials. Unlike cast methods, 3D CP does not require support formwork. Thus, certain factors like the fresh properties of the material being used, the orientation in which it is printed and how long it is printed can significantly affect the capacity of the printed objects. The layering involved in printing concrete can introduce weaknesses in joints, which affect all mechanical characteristics of 3D printed elements. This study examines how the printing direction and printing paths influence the compressive strength of 3D printed specimens. Additionally, conventional mould cast specimens were tested for comparison purposes. The findings indicate that both the printing directions and paths impact the strength of these printed specimens.
... El proceso de impresión 3D empleado en el sector de la construcción más popular es el que utiliza la extrusión, debido a su rendimiento, bajo costo y capacidad para producir piezas de una manera rápida. Para garantizar la eficacia de este proceso, es fundamental que los materiales utilizados posean ciertas propiedades específicas, como la "extrudabilidad", la "fluidez" y la "edificabilidad" [18], [19]. La extrudabilidad se refiere a la capacidad de los materiales para ser extruidos sin obstrucciones en los componentes de la maquinaria, como los tubos y las boquillas. ...
En los últimos años, se ha generado gran interés en la construcción sostenible, lo que ha llevado a un mayor interés en la impresión 3D o manufactura aditiva. Sin embargo, el uso de esta técnica con materiales convencionales no es suficiente para disminuir el gran impacto ambiental que genera el sector de la construcción. Aunque la mayoría de las investigaciones y avances están centralizadas en la impresión 3D de concreto Portland, esta revisión se ha trabajado orientada hacia la impresión 3D de materiales de construcción basados en suelos y arcillas, los con los cuales se puede proporcionar un enfoque asequible (ya que es un material localmente disponible en muchas regiones del planeta), sostenible ambientalmente, y con bajo costo, lo cual es altamente beneficioso para la construcción de viviendas. Este documento se ha orientado hacia la búsqueda de literatura científica y prototipos que se han elaborado utilizando materiales ancestrales, como son suelos-arcillas-arena-fibras como paja y agua, para elaboración de piezas constructivas tipo muros o adobes impresos en 3D. El objetivo de este documento es cerrar la brecha sobre la utilización de mezclas basadas en suelos, que, aunque parezcan totalmente estudiadas por varios siglos, a la fecha su aplicación en impresión 3D es reducida. Reajustes en propiedades de las mezclas de suelos como la fluidez para el bombeo o extrusión, edificabilidad y buen tiempo de trabajo, son variables que se reportan en este documento. Además, en esta revisión se describen las mezclas que han sido desarrolladas para impresión 3D a partir de suelos y arcillas, y las principales características que se han encontrado. Finalmente, se presentan los desafíos que aún persisten para que las mezclas puedan aplicarse a una escala industrial masiva. Palabras clave: manufactura aditiva; impresión 3D de suelos; materiales basados en tierra; adobe; cob.
... According to the test results, the cast concrete has a higher compressive strength than the printed concrete. Similar compressive strength results were reported [19,20,28,29]. An anisotropic behaviour was observed in the compressive strength of the printed cubes, depending on the loading direction. ...
Due to the molding-free property and dry shrinkage of extrusion-based three-dimensional printable concrete (3DPC), the precision issues of 3DPC have not been solved effectively. One of the viable solutions for 3DPC precision improvement is to print using ultra-thin filaments. The challenges of ultra-thin-filament printing are extrudability, flowability, and fast solidification. To overcome these challenges and enhance precision, a customized 3D concrete printer with an ultra-thin diameter nozzle (6 mm) and fully sealed extrusion system was developed, and the mix design of ultra-thin-filament 3DPC (UTF-3DPC) was studied, including ingredients such as fly ash (FA), silica fume (SF), ordinary Portland cement (OPC), sodium dodecyl sulfate and cellulose (SDSC), water reducer, water, and sand. The function of UTF-3DPCs flowability and fast solidification with the proportion of water and SDSC was explored to obtain the optimal mix design. The standard compressive and flexural strengths of UTF-3DPC specimens were compared with the mold-cast vibrated and the mold-cast non-vibrated concrete. Their meso-scale and micro-scale structures were analyzed to expose the strength mechanism, according to the scanning electron microscope (SEM) images. A suitable mix design of UTF-3DPC was obtained and UTF-3DPC strength reached 80% of standard mold-cast concrete. The findings reported here provide a pathway to improve the precision of 3DPC and extend the application of 3D printing technology in engineering.
This paper presents insights into the design and development of a geopolymer consisting of fly ash and ground granulated blast furnace slag (GGBFS) usable for extrusion-based 3D printing. It also reports on factors influencing the printability of the geopolymer, such as the amount of GGBFS, the solids content, mixing time, and activator ratio. Besides measuring the setting behaviour, the yield stress, and the ultrasonic velocity, we also conducted a modified flow spread test to investigate the geopolymer’s early physical material properties. We also used analytical methods (SEM, XRD, and TG) to explain its physical behaviour. We introduced an adjusted test for the flow spread (VFS test), which gives information about the thixotropic behaviour depending on the shear stress and the opentime for printing. By comparing the derivatives of the yield stress curve with printing tests on an XYZ gantry printer and analysis of the yield stress measurement’s transition point, we also present a unique method that allows researchers to assess the printability of the geopolymer solely on one small-scale laboratory tests without requiring printing tests on a physical 3D printer.
This article was published in the journal, Magazine of Concrete Research, and the definitive version is available from: http://www.thomastelford.com/journals/ This paper, which reports on part of a three year research project into wet-process sprayed concrete for repair, examines the influence of rheology on the pumping and spraying of mortars. The performance of seven commercially available pre-packaged repair mortars and six laboratory designed fine mortars was examined using the Tattersall two-point and Viskomat rotational viscometers, the pressure bleed test, the slump test, a build test and a vane shear strength test. These tests were used to form a rheological audit of each mortar. The two-point apparatus was successful with low-workability mortars and their flow resistance and torque viscosities were determined. These parameters were also obtained with the Viskomat, although problems were encountered due to their low workability. The pressure bleed test measured both the rate and the total volume of liquid emitted from the mixes whilst the vane shear strength test provided an instantaneous reading of the shear strength of the mortars and is compared with their slump. The mortars were pumped and sprayed through a worm pump to assess their suitability and to measure their adhesion to a substrate by build thickness. This value is a measure of sprayability and is converted into values of maximum shear and bending stress which are then compared with the workability parameters in order to determine their inter-relationship. Published (author's copy) peer reviewed
The in-place bond test is a valuable tool to determine directly the bond strength and the quality of the prepared surface. Two important advantages are that the test is performed in-situ and represents actual field conditions and it is a useful tool for quality control during construction repairs. Using pneumatic hammers in concrete causes a bruised surface layer and creates a zone of weakness below the interface. Hydrodemolition has demonstrated the ability to remove concrete without damage below the prepared surface. In view of the accurate and reliable nature of the in-place bond test and its representation of actual field conditions, it is recommended that ACi consider adoption of the procedure as a standard for general use.
Increased productivity and improved working environment have had high priority in the development of concrete construction over the last decade. Development of a material not needing vibration for compaction—i.e. self-compacting concrete (SCC)—has successfully met the challenge and is now increasingly being used in routine practice. The key to the improvement of fresh concrete performance has been nanoscale tailoring of molecules for surface active admixtures, as well as improved understanding of particle packing and of the role of mineral surfaces in cementitious matrixes. Fundamental studies of rheological behaviour of cementitious particle suspensions were soon expanded to extensive innovation programmes incorporating applied research, site experiments, instrumented full scale applications, supporting technology, standards and guides, information efforts as well as training programmes. The major impact of the introduction of SCC is connected to the production process. The choice and handling of constituents are modified as well as mix design, batching, mixing and transporting. The productivity is drastically improved through elimination of vibration compaction and process reorganisation. The working environment is significantly enhanced through avoidance of vibration induced damages, reduced noise and improved safety. Additionally, the technology is improving performance in terms of hardened material properties like surface quality, strength and durability.
This paper, which reports on part of a three-year research project into wet-process sprayed mortars and concretes for repair, investigates the hardened performance of wet-process sprayed mortars. Seven commercially available preblended repair mortars were pumped and sprayed through a worm pump, three through a piston pump and two through a dry-spray machine. A laboratory-designed mortar was also worm and piston pumped. The properties measured included compressive and flexural strength, tensile bond strength, hardened density, modulus of elasticity, air permeability, sorptivity, and drying and restrained shrinkage. In situ test specimens were extracted from 500 mm×500 mm×100 mm deep sprayed panels. Tests were also conducted on corresponding cast specimens and, where possible, on specimens that had been sprayed directly into a cube or beam mould. A new test to quantify the degree of reinforcement encasement has been developed and an initial investigation into the measurement of the restrained shrinkage of in situ repairs is presented. The compressive and flexural strengths of the laboratory mix were comparable to those of the best of the commercially available preblended mortars. The values for modulus of elasticity, when compared with the compressive strength, were lower than published formulae for this relationship would suggest, especially at lower strengths. The air permeability of most of the mortars was lower than that for normal wet-cured concrete and decreased with an increase in compressive strength. The sorptivity values showed no clear relationship to the compressive strength. The type of wet-process pump was found to have little effect on the in situ compressive and flexural strengths but did affect the bond strength, although this was mainly because of the stream velocity and w/c ratio rather than the pumping process. The pump type also affected the reinforcement encasement, with higher stream velocities producing better encasement. The mixes exhibited a wide range of drying shrinkage, but the data from the restrained specimens suggest an actual repair is influenced as much by ambient conditions as it is by the mix proportions.
The present paper is directed towards developing a better understanding on the isolated contribution of silica fume on the tensile strengths of high-performance concrete (HPC). Extensive experimentation was carried out over water–binder ratios ranging from 0.26 to 0.42 and silica fume–binder ratios from 0.0 to 0.3. For all the mixes, compressive, flexural and split tensile strengths were determined at 28 days. The compressive, as well as the tensile, strengths increased with silica fume incorporation, and the results indicate that the optimum replacement percentage is not a constant one but depends on the water–cementitious material (w/cm) ratio of the mix. Compared with split tensile strengths, flexural strengths have exhibited greater improvements. Based on the test results, relationships between the 28-day flexural and split tensile strengths with the compressive strength of silica fume concrete have been developed using statistical methods.
Drying shrinkage can be a major cause of the deterioration of concrete structures. The contraction of the material is normally hindered by either internal or external restraints and tensile stresses are induced. These stresses may exceed the tensile strength and cause concrete to crack. The evaluation of the stress distribution in the material requires the knowledge of the “real” free shrinkage deformation. This paper presents the results of a study performed to evaluate this deformation and obtain a better understanding of the behavior of concrete under drying conditions. Shrinkage tests were carried out on cement pastes, mortars, and concretes. The influences of different key parameters were evaluated: relative humidity, specimen size, water/cement ratio, and paste volume. The results indicate that between 48 and 100% relative humidity, the shrinkage of cement paste is approximately inversely proportional to relative humidity. Results also show that the ultimate shrinkage of pastes and mortars measured on 50 × 50 × 400-mm specimens does not differ much from the “real” shrinkage measured on 4 × 8 × 32-mm specimens. Thus, for the specimen dimensions investigated in this study, the existence of a humidity gradient did not affect to a large extent the ultimate shrinkage strain. The influence of the water/cement ratio, within the range investigated (0.35–0.50), was found to be relatively small. Conversely, paste volume was observed to have a very strong influence.
This paper discusses a research programme in which the 3D computer aided industrial design geometry for a consumer product was translated into appearance models using the contrasting techniques of workshop-based fabrication techniques and rapid prototyping using stereolithography. The research also examined the capacity to extend the use of the rapid prototype components for the production of a fully working prototype. The ability to combine an appearance model and a working prototype into a single “appearance prototype” was a significant advance in the application of RP within industrial design.
In this paper we investigate the influence of the shape and of the size of the specimens on the compressive strength of high-strength concrete. We use cylinders and cubes of different sizes for performing stable stress–strain tests. The tests were performed at a single axial strain rate, 10− 6 s− 1. This value was kept constant throughout the experimental program. Our results show that the post-peak behavior of the cubes is milder than that of the cylinders, which results in a strong energy consumption after the peak. This is consistent with the observation of the crack pattern: The extent of cracking throughout the specimen is denser in the cubes than in the cylinders. Indeed, a main inclined fracture surface is nucleated in cylinders, whereas in cubes we find that lateral sides get spalled leading to the so-called hour-glass failure mode. The remaining cube core gets fragmented due to crushing, in some cases exhibiting a dense columnar cracking in the bulk of the specimen. Finally, we investigate the relationship between the compressive strength given by both types of specimen for several specimen sizes.