Printable Concrete Research Articles

Yield stress criteria to assess the buildability of 3D concrete printing

3D concrete printing (3DCP) using the extrusion method is a commonly used additive manufacturing technique to construct the structures layer by layer without using formwork. Therefore, the green (wet or fresh) strength of the printable concrete determines the possible printable height of the structure and the rate of printing to avoid collapse during the printing process. This paper aims to identify the buildability criteria based on the green strength of concrete and the effect of early age material properties on the stability of printed structures. The commonly available strength-based failure models based on the material yield stress and the vertical stresses induced were developed considering higher aspect ratios (height to width ratio). This may be not suitable for lower aspect ratios used in 3DCP layer geometries. Also, the frictional behaviour due to the applied vertical stresses should be considered in the material used for 3DCP applications. In this work, the Mohr-Coulomb based buildability criterion was developed and validated with laboratory experiments and numerical simulations. The laboratory experiments were carried out to establish the time-dependent material and rheological properties of 3D printable concrete mixes. Further, 3D printing of hollow circular sections was carried out to study the failure modes and to obtain the build height at the time of failure. The nonlinear finite difference model of the 3D printed sections was developed with the user-defined time-dependent material models to assess the accuracy of the proposed buildability criterion. Additionally, the proposed buildability criterion was further validated with the 3DCP experimental and numerical data presented in literature and all confirm that the proposed criterion shows higher accuracy over existing methods in assessing the buildability.

Increasing structuration rate of 3D printable concretes: the effect of viscosity enhancing admixtures

Abstract In the last decades, 3D printing has emerged as a promising new paradigm for manufacturing. Even in the civil construction industry, it has gained attention from companies and researchers around the world. Despite of that, the properties of materials applied in the additive manufacturing process are still understudied. One of the challenges is the need to conciliate both high bearing capacity, sparing the need of any confining measures, and the ability of keeping fluidity for enough time, in order to avoid cold joints between the layers. In that scenario, viscosity enhancing admixtures (VEAs) can be a solution, because they are able of promoting flocculation, viscosity gain and yield strength increase, reducing the deposition time in between the layers, which may decrease the formation of cold joints. This research evaluated rheological parameters of four different VEAs and found out that they show potential for increasing the cohesion and buildability of concretes for 3D-printing. The results showed that this effect varies with the type and amount of the admixture adopted and bentonite clay, as a mineral powder material, performed best in comparison to other polymeric VEAs, presenting structuration rates of up to 62% higher than the reference mixture.

Design of 3D printable concrete based on the relationship between flowability of cement paste and optimum aggregate content

This paper deals with the mixture design of 3D printable concrete (3DPC) based on the theoretical analysis of the yield stress of mortar. It has been found that there is a linear relationship between the flowability of paste and optimum aggregate content in 3DPC mixes which comply well with printability requirements. Additionally, the linear relationship obtained from the 3DPC with coarser natural sand can be also applied to the 3DPC with finer sand in accordance with the excess slurry theory. It is revealed that if the excess slurry thickness and the mix proportion of paste are the same, the mortar has similar rheological behavior. The verification experiment confirms that combined with the linear relationship between the flowability of cement paste and optimum aggregate content, the flowability of paste is a useful parameter for the mixture proportion design of 3DPC.

Mechanical characterization of 3D printable concrete

The current study deals with test methods for characterizing the mechanical behaviour of 3D printed wall elements. At first, the effect of interfaces present in a printed concrete element was examined through an assessment of porosity and bond shear test. The porosity was evaluated using specimens extracted from the bulk as well as from the interfaces between horizontal and vertical layers of printed concrete. It was found that the porosity in the bulk was lower by 6–8% as compared to the mould cast concrete. On the other hand, the porosity at the interface between horizontal and vertical layers was found to be higher by 11–14% and 10–16% respectively. Therefore, although the bulk in 3D printed elements is more densified, the interfaces are weak with higher porosity. The interfaces were further evaluated by a direct bond shear test. Compared to the shear strength of the mould cast concrete, the shear strength at interface between the horizontal and vertical layers was found to be lower by 24–25% and 22–30% respectively. The compressive strength of the printed concrete was similar when tested in different loading directions but lower by 12–22% as compared to the mould cast concrete. This reduction in compressive strength may be due to the higher porosity at the interfaces present in printed concrete elements. The flexural strength was found to depend on the region subjected to the maximum bending moment. When tested along the direction where the maximum bending moment and therefore the maximum bending stress occurs at the weaker interfaces between layers, the flexure strength was lower by about 32–40%. On the other hand, the flexural strength was found to increase by 13–20% when tested in directions where maximum bending stress is induced in the bulk concrete, having a lower porosity as compared to the mould cast concrete. Finally, some perspectives are provided on formulating a structural design procedure for a 3D printed wall element.

Drying and shrinkage of cement paste for 3D printable concrete

The article presents the results of the study of drying and shrinkage of three compositions of cement paste different in the content of microsilica and superplasticiser within the range of variation typical for 3D printable concrete compositions. Plate samples kept in the environment with RH = 20, 30, 50, 70 % were used to simulate different drying conditions of material in thin layers of 3D printing structures. Rates of mass loss and shrinkage of hardened cement paste are compared with criterial parameters of their structure. It is shown that increased dosage of microsilica and superplasticiser ensures the reduction of total volume of pores, increase of the content of nanopores, while simultaneously the area and energy activity of solid phase surface increases. These factors secure the ability of cement paste to better retain the water in the structure under all drying conditions. As a result, reduction of absolute values of mass loss by 3 - 4 times and shrinkage by 2 times in comparison with reference system is ensured for cement paste with maximally dense structure. At the same time, there is a twofold increase of relative deformability on each percent of mass loss, which may lead to significant tension in 3D printing structures due to moisture gradient.

Direct shear test for the assessment of rheological parameters of concrete for 3D printing applications

Rheology of concrete plays a major role in concrete 3D printing applications, where the concrete is pumped at high pressure and extruded through a nozzle at low speed to build the structural component. The 3D printable concrete should be stiff and is different to normal or self-compacting concrete. Hence, the common testing methods used to estimate the rheological parameters are not suitable for 3D printable concrete. In this study, the direct shear test is trialled as a potential method to measure the rheological parameters of different mixes of concrete. The effects of water–cement ratio and shear rates on rheological parameters were examined. The tests were carried out with varying shear rates, ranging from 0.5 to 15 min−1, and normal stresses, ranging from 2 to 15 kPa, for mixes with water–cement ratios of 0.3, 0.4 and 0.6. Further testing was carried out on mixes with varying aggregate to cement and fine to total aggregates ratios to study the effect of binder and aggregate proportions on the rheology of mortar. It was found that the shear rates, 0.5 to 15 min−1, have little effect on the cohesion values and friction angles. Further, the behaviour of the mixes was found to be following the Mohr–Coulomb model.

3D printable concrete: Mixture design and test methods

The current study deals with a yield stress based mixture design approach for 3D printable concretes. The mixtures were evaluated based on buildability, extrudability, robustness and tests for structural build-up. For the print parameters (such as pump type, nozzle size and extrusion velocity) used in the study, it was found that both extrudability and buildability could be achieved only when the material yield stress was within a range of 1.5–2.5 kPa. Below this range, the material lacked enough strength to achieve shape stability, while above this range, the extrudabilty of the material was difficult. The robustness of the mixtures was quantified in terms of a variability factor defined in terms of the variation in yield stress with small changes in the superplasticizer dosage. Inclusion of 10% of silica fume, 0.1% of viscosity modifying agent (VMA) and 0.1–0.3% addition of nanoclay resulted in decreasing the variability factor, hence improving the robustness. The structural changes due to thixotropy and cement hydration increased the yield stress with time. This structural build-up was assessed by measuring the yield stress with increasing rest duration. The mixture with silica fume showed the maximum structural build-up while the mixture with VMA showed the least. Heat curves from semi-adiabatic calorimetry and penetration curves were also used to assess the structural build-up. They showed a similar trend to that of the yield stress vs time plots.