Optimisation of Interlayer Bond Strength in 3D-Printed Concrete Using Response Surface Methodology and Artificial Neural Networks

Advancements in 3D printing of cementitious materials: A review of mineral additives, properties, and systematic developments

Interlayer bond strength is one of the key aspects of 3D concrete printing. It is a well-established fact that, similar to other 3D printing process material designs, process parameters and printing environment can significantly affect the bond strength between layers of 3D printed concrete. The first section of this review paper highlights the importance of bond strength, which can affect the mechanical and durability properties of 3D printed structures. The next section summarizes all the testing and bond strength measurement methods adopted in the literature, including mechanical and microstructure characterization. Finally, the last two sections focus on the influence of critical parameters on bond strength and different strategies employed in the literature for improving the strength via strengthening mechanical interlocking in the layers and tailoring surface as well as interface reactions. This concise review work will provide a holistic perspective on the current state of the art of interlayer bond strength in 3D concrete printing process.

In structures manufactured using 3D concrete printing, cracks can easily propagate along the interface between printed layers. Therefore, it was necessary to determine the interlayer bond strength. In this study, direct shear and tensile tests were performed to determine the interlayer bond stability of the 3DCP members. To confirm the appropriateness of the mix proportion used to fabricate the specimens, the open time available for printing was identified via a mixing test, and the extrudability and buildability were verified via a printing test. In addition, direct shear and tensile tests were performed using the specimen manufacturing method (i.e., mold casting and 3D printing) and printing time gap (PTG) between the laminated layers as key test variables. The interlayer bond strengths of the specimens, according to the variables obtained from the test results, were compared and analyzed based on the interfacial shear strength standards presented in the current structural codes. In the direct shear test, failure occurred at the interlayers of all the specimens, and the interlayer bond strength tended to decrease with increasing PTG. In addition, the interlayer bond strength of the direct shear specimens exceeded the interfacial shear strength suggested by current structural codes. In contrast, in the direct tensile test, interlayer surface failure occurred only in some specimens, and there was no distinct change in the interlayer bond strength owing to PTG.

Recent experimental research by Reiter et al. (Cem. Concr. Res., 132:106047, 2020) indicates that the buildability of fresh concrete used in extrusion-based 3D printing processes can be significantly enhanced by chemically accelerating the curing process. In the present contribution the effect of accelerated curing on failure by plastic collapse and elastic buckling during 3D concrete printing is explored by incorporating a power-law curing function in the parametric 3D printing model developed by Suiker (Int. J. Mech Sci, 137:145–170, 2018). A structural yield criterion is derived for the case of accelerated curing, and the main advantages on the resistance against plastic collapse are demonstrated through a comparison of the predicted failure characteristics to those for linear curing and exponentially-decaying curing. Subsequently, the elastic buckling behaviour under accelerated curing is derived for a free wall configuration, and the competition between elastic buckling and plastic collapse of the free wall structure is assessed via the construction of failure mechanism maps. In addition, a modelling recipe is proposed for consistently accounting for the vertical deformations of layers in the prediction of structural failure during 3D concrete printing. The modelling of this effect may further increase the accuracy of the prediction of the number of layers at structural failure. For failure under plastic collapse, results are computed for linear curing, exponentially-decaying curing and accelerated curing. The model outcome for linear curing is used for a comparison with results from 3D concrete printing experiments recently presented in the literature, showing an excellent agreement. It is further demonstrated that the effect of vertical wall deformations on the prediction of failure by elastic buckling typically is minor, so that for this failure mechanism this contribution may be left out of consideration. All design graphs presented in this communication are generic, in a sense that they are not restricted to concrete, but can be applied for other printing materials as well.

This 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.

A review of “3D concrete printing”: Materials and process characterization, economic considerations and environmental sustainability

In this chapter, the theoretical concepts of magneto-rheology control in extrusion-based three-dimensional (3D) concrete printing are first illustrated. Based on the fact that some residual magnetic clusters exist in cementitious suspension after removing external magnetic field, a conceptual examination of the application of magneto-rheology control to 3D printing is presented by rheological experiments on cementitious paste with nano-Fe3O4. It is revealed that a faster structural build-up is observed compared to the situation without magnetic field. This finding offers an innovative methodology to actively improve the buildability of 3D printed concrete by introducing a short-pulsed magnetic field during extrusion. Further research regarding the print head design for magneto-rheology control of 3D concrete printing is prospected.

Automatic construction technologies have become the primary focus of the global construction sector. 3D printing is one of the disruptive technologies emerging from Industrial Revolution 4.0. 3D printing has grown increasingly popular in concrete construction due to its architectural freedom, speed, formwork-free printing, lesser waste creation, eco-friendliness, affordability, and safety. There were issues with the printing process when manufacturing 3D-Printed Concrete (3DPC) mixes, such as poor extrusion and buildability issues. This study investigates the use of Viscosity Modifying Agents (VMAs) in 3DPC to improve printability as well as structural integrity. VMAs, known for their capacity to change the rheological properties of concrete mixtures, are used selectively to optimise the material’s flow behaviour throughout the 3D printing process. The study compares the effect of VMA concentrations on the workability and buildability properties of 3DPC mixtures. Comparative examinations of VMA-enhanced and traditional 3DPC specimens indicate that an optimised VMA dose improves structural performance. The findings of this study hold significant relevance for the advancement of 3D printing technology in construction, offering a more nuanced understanding of the role of VMAs in optimising concrete mixtures for additive manufacturing.

Factors affecting 3D printing and post-processing capacity of cookie dough

Robust prediction of workability properties for 3D printing with steel slag aggregate using bayesian regularization and evolution algorithm

3D printing has been getting increasing attention from both industry and academy in recent years. Unlike traditional construction processes, 3D cementitious material printing requires more accurate control on the pumping flow rate, which is known to be affected by the material rheology. In this paper, the effect of fresh rheology on the flow rate of cementitious material was studied experimentally in 3D cementitious material printing process. The material viscosity was measured via a large gap vane viscometer at different time points after mixing, and printing tests were conducted at the same time period to measure the flow rate based on volume conservation principle. Experiments showed that the flow rate was significantly affected by rheology change with respect to time. An open loop control method was then implemented to harmonize the flow volume per unit length during printing processes to improve printing quality.

Crack propagation and failure mechanism of 3D printing engineered cementitious composites (3DP-ECC) under bending loads

The objective of this study was to derive a cementitious material for three-dimensional (3D) concrete printing that fulfills key performance functions, extrudability, buildability and bondability for 3D concrete printing. For this purpose, the rheological properties shown by different compositions of cement paste, the most fundamental component of concrete, were assessed, and the correlation between the rheological properties and key performance functions was analyzed. The results of the experiments indicated that the overall properties of a binder have a greater influence on the yield stress than the plastic viscosity. When the performance of a cementitious material for 3D printing was considered in relation with the properties of a binder, a mixture with FA or SF was thought to be more appropriate; however, a mixture containing GGBS was found to be inappropriate as it failed to meet the required function especially, buildability and extrudability. For a simple quantitative evaluation, the correlation between the rheological parameters of cementitious materials and simplified flow performance test results-time taken to reach T-150 and the number of hits required to reach T-150—in consideration of the flow of cementitious materials was compared. The result of the analysis showed a high reliability for the correlation between the rheological parameters and the time taken to reach T-150, but a low reliability for the number of hits needed for the fluid to reach T-150. In conclusion, among several performance functions, extrudability and buildability were mainly assessed based on the results obtained from various formulations from a rheological perspective, and the suitable formulations of composite materials for 3D printing was derived.

Emerging horizons in 3D printed cement-based materials with nanomaterial integration: A review

Failure analysis of 3D concrete printing bolted laminates mimicking geological strata