Quality Assessment of Printable Strain Hardening Cementitious Composites Manufactured in Two Different Printing Facilities

Over the past few years, several studies have shown the potential of three-dimensional concrete printing (3DCP) for applications in building and civil engineering. However, only a few studies have compared the properties of the fresh printing material and the quality of the printed elements from different printing facilities. Variations in the manufacturing conditions caused by the mixing procedures, the pumping device and the nozzle shape and/or dimensions may influence the quality of the printed elements. This study investigates the differences in the fresh and hardened properties of a printing material tested in two different printing facilities. The pump pressure and temperature experienced by the printing material during the printing session are monitored real-time. Hardened properties are measured for the printed elements, such as the bending capacity, the apparent density, and the air void content. The research shows that two different printing facilities may result in printed elements with relative differences in flexural strength and volumetric density of 49% and 7%, respectively

Potentials and challenges in 3D concrete printing

\u3cp\u3eReinforced concrete structures have constantly become more safe and durable over the past century and the materials properties have improved tremendously, but the design and construction method have not changed much over time. Concrete structures face a series of challenges like a new degree of freedom for architecture, sustainability, health, increased productivity and a better integration to BIM models. 3D Concrete printing has the potential to meet these demands, although it is not clear yet which type of structures will benefit most from this additive type of manufacturing. However, in order to explore the benefits, the first task of research is to make the print process robust. This is needed since the load bearing capacity of a structure depends on the design and the printing strategy. Besides, the issue of a lack of reinforcing steel must be solved to ensure safe structures, for example by developing new and ductile types of concrete.\u3c/p\u3

Structural failure during extrusion-based 3D printing processes

\u3cp\u3eThis contribution studies failure by elastic buckling and plastic collapse of wall structures during extrusion-based 3D printing processes. Results obtained from the parametric 3D printing model recently developed by Suiker (Int J Mech Sci, 137: 145–170, 2018), among which closed-form expressions useful for engineering practice, are validated against results of dedicated FEM simulations and 3D concrete printing experiments. In the comparison with the FEM simulations, various types of wall structures are considered, which are subjected to linear and exponentially decaying curing processes at different curing rates. For almost all cases considered, the critical wall buckling length computed by the parametric model turns out to be in excellent agreement with the result from the FEM simulations. Some differences may occur for the particular case of a straight wall clamped along its vertical edges and subjected to a relatively high curing rate, which can be ascribed to the approximate form of the horizontal buckling shape used in the parametric model. The buckling responses computed by the two models for a wall structure with imperfections of different wavelengths under increasing deflection correctly approaches the corresponding bifurcation buckling length. Further, under a specific change of the material properties, the parametric model and the FEM model predict a similar transition in failure mechanism, from elastic buckling to plastic collapse. The experimental validation of the parametric model is directed towards walls manufactured by 3D concrete printing, whereby the effect of the material curing rate on the failure behaviour of the wall is explored by studying walls of various widths. At a relatively low curing rate, the experimental buckling load is well described when the parametric model uses a linear curing function. However, the experimental results suggest the extension of the linear curing function with a quadratic term if the curing process under a relatively long printing time is accelerated by thermal heating of the 3D printing facility. In conclusion, the present validation study confirms that the parametric model provides a useful research and design tool for the prediction of structural failure during extrusion-based 3D printing. The model can be applied to quickly and systematically explore the influence of the individual printing process parameters on the failure response of 3D-printed walls, which can be translated to directives regarding the optimisation of material usage and printing time.\u3c/p\u3

Correlation between destructive compression tests and non-destructive ultrasonic measurements on early age 3D printed concrete

\u3cp\u3e3D printing of concrete and related digital fabrication techniques are enjoying rapid growth. For these technologies to be broadly accepted in structural applications and to be economically competitive, quality control methods of the process will be required. Additive concrete manufacturing processes are sensitive to process settings and conditions, which calls not only for preprint structural modelling to establish printability, but also for in-print monitoring to ensure expected properties are indeed achieved. Non-destructive test methods are highly suitable for this aspect of quality control, as they usually allow efficient, high frequent digital measurements that require relatively little effort. However, as they generally do not directly measure the appropriate parameter(s), correlations between non-destructive and destructive testing have to be established. The preprint structural modelling is based on a number of time-dependent mechanical properties, including the compressive strength and the Young's modulus. If concrete is still in the dormant state, as it often is in 3D concrete printing, these properties require difficult, time consuming destructive tests to establish. In the present work, the correlation between these two mechanical properties on the one hand, and the pulse velocity on the other, was studied. A (destructive) unconfined uniaxial compression test was applied to determine the former, while a (non-destructive) ultrasonic wave transmission test was used for the latter. As expected from previous research on a similar mortar, both the compressive strength and the Young's modulus were found to increase linearly in a time frame of 5–90 min after extrusion. This is attributed to thixotropic build-up. Within that time frame, the pulse velocity also grew in a linear fashion. Thus, a simple linear correlation between the destructive and non-destructive test results could be established. For now, this allows continuous quality control on simply obtainable control batches. Furthermore, it stimulates the development of ultrasonic online monitoring methods for the objects during printing.\u3c/p\u3

Triaxial compression testing on early age concrete for numerical analysis of 3D concrete printing

In 3D concrete printing processes, two competing modes of failure are distinguished: material failure by plastic yielding, and elastic buckling failure through local or global instability. Structural analysis may be performed to assess if, and how, an object may fail during printing. This requires input in the form of transient material properties obtained from experimental testing on early age concrete. In this study, a custom triaxial compression test setup was developed, to characterize all essential parameters to assess failure by elastic buckling, and material yielding according to the Mohr-Coulomb criterion. The results of the triaxial tests were compared to simultaneously run unconfined uniaxial compression tests and ultrasonic wave transmission tests. The correlation between these experimental methods was reviewed. It was concluded that the triaxial compression test is an appropriate method to determine all relevant transient properties from one series of experiments. Subsequently, the experimental results were used for structural analyses of straight printed walls of different lengths with a Finite Element Modelling approach. These walls have been printed up to failure during print trials and the results were compared to the numerical predictions. The failure mode is predicted accurately by the numerical model, as is the critical height at which failure occurs for relatively small objects. For larger objects and/or longer printing processes, the quantitative agreement of the critical height with the print experiments could be improved. Two possible causes for this deviation are discussed.\u3cbr/\u3e\u3cbr/\u3

Hardened properties of 3D printed concrete:the influence of process parameters on interlayer adhesion

\u3cp\u3eThe technology of 3D Concrete Printing (3DCP) has progressed rapidly over the last years. With the aim to realize both buildings and civil works, the need for reliable mechanical properties of printed concrete grows. As a consequence of the additive manufacturing technique, 3D printed structures may consist of several layers that should exhibit bond to guarantee a safe structural design. This paper presents the results of an experimental study on the relation between the 3DCP process parameters and the bond strength of 3D printed concrete. The effect of 3 process parameters (interlayer interval time, nozzle height, and surface dehydration) on two mechanical properties (compressive strength and tensile strength, determined through flexural and splitting tests), has been established, in three perpendicular directions. A very limited influence of layer orientation was found for the given process-material combination, given a sufficiently short interlayer interval time. However, the bond strength between the layers reduced for increasing interlayer interval times. This was also reflected by the failure mode of the samples. The reduction in strength became more pronounced for the samples that were left uncovered during the interval time, exposed to drying. No clear relation was found between the height of the nozzle, and the bond strength between layers. The results of this study, in comparison to various other works on 3DCP, emphasize the need for standardization of test methods and characterization of 3D printed concrete, as individual process parameters clearly must be considered in relation to the applied material and other process parameters.\u3c/p\u3

Bond of Reinforcement Cable in 3D Printed Concrete

The use of high strength steel cables directly entrained into printed concrete during the printing process, has previously been introduced as a method to provide reinforcement to objects being manufactured through a layer-extrusion based 3D concrete printing process. The bond between the cable and the cementitious mortar is a crucial parameter for the structural performance of such reinforcement, and was hence subject of a detailed study presented in this paper. The bond performance was studied in direct and flexural pull-out tests on cast and printed specimens and further analyzed by microscopic analysis of the bond surface. Two effects were identified that significantly decrease the bond strength. Firstly, chemical reactions create a spongy interface of poor strength. Secondly, the flow of mortar around the cable tends to create a cavity underneath the cable which reduces the effective bond surface. Mortar viscosity, nozzle design and filament pressure, were thus identified as important parameters for the bond quality. The average bond quality seems to reduce with embedment length. As a consequence, cable breakage was not achieved, in spite of considerable embedment lengths that were tested. Likely, this was caused by the cumulative probability of critical defects along the increasing embedment length, in combination with a non-constant shear distribution. All test series showed significant scatter. It was concluded that, although this reinforcement method is promising as it can potentially provide sufficient post-cracking strength, the bond quality must be improved considerably both in terms of average strength and reduction of scatter

Magnetic orientation of steel fibres in self-compacting concrete beams : effect on failure behaviour

The magnetic orientation of steel fibres in transparent silicone oil and in fresh, self-compacting concrete (SCC) beams is studied experimentally. The effect of the generated fibre locations and orientations on the failure response of the SCC beams is determined by means of three-point bend tests. A relatively small coil was designed for the magnetic orientation of single and multiple fibres in the transparent silicone oil. The time required for orienting a single fibre was measured for a range of magnetic fluxes, which showed to strongly decrease with increasing magnetic field strength. The presence of gravel on the fibre orientation behaviour was considered in order to mimic the influence by a concrete aggregate, indicating that the gravel does not prevent rotations and chain formations of fibres. A larger coil was developed for the magnetic orientation of fibres in freshly casted SCC beams. The energy absorption capacity of SCC beams subjected to three-point bending scales approximately proportionally with the number of “well-oriented fibres” bridging the catastrophic failure crack, which emphasizes the importance of adequately orienting steel fibres with the magnetic orientation technique

Magnetic orientation of steel fibres in self-compacting concrete beams : effect on failure behaviour

The magnetic orientation of steel fibres in transparent silicone oil and in fresh, self-compacting concrete (SCC) beams is studied experimentally. The effect of the generated fibre locations and orientations on the failure response of the SCC beams is determined by means of three-point bend tests. A relatively small coil was designed for the magnetic orientation of single and multiple fibres in the transparent silicone oil. The time required for orienting a single fibre was measured for a range of magnetic fluxes, which showed to strongly decrease with increasing magnetic field strength. The presence of gravel on the fibre orientation behaviour was considered in order to mimic the influence by a concrete aggregate, indicating that the gravel does not prevent rotations and chain formations of fibres. A larger coil was developed for the magnetic orientation of fibres in freshly casted SCC beams. The energy absorption capacity of SCC beams subjected to three-point bending scales approximately proportionally with the number of “well-oriented fibres” bridging the catastrophic failure crack, which emphasizes the importance of adequately orienting steel fibres with the magnetic orientation technique