- Original article
- Open Access
Method for the Enhancement of Buildability and Bending Resistance of 3D Printable Tailing Mortar
© The Author(s) 2018
- Received: 24 August 2017
- Accepted: 29 March 2018
- Published: 29 May 2018
The innovative 3D printing has been successfully applied to layeredly build-up construction-scale structures through the extrusion of various cementitious materials. Favourable buildability of fresh cement mixture and the hardened properties of the printed structures are essential requirement for the application of 3D concrete printing. This paper firstly proposed a 3D printable cement mixture containing 40% mining tailings. The influence of paste age on the buildability of forty-layer structure was evaluated, as well as the bending resistance of prism specimen sawed from the printed structure. The bonding between layers is a critical factor that influences the structural capacity. In particular, the weak bonding interface formed in the layered extrusion process was identified through high-resolution X-ray CT scanning. It is necessary and desirable for the cement paste to perform both well buildability and mechanical performances. Thereafter, a proper amount of viscosity modify agent (VMA) was used to improve the structural integrity by increasing the contact behaviour between the adjacent extruded layers. Meanwhile, the impact of curing method on the hardened properties of 3D printed structures was accessed. Results indicated that the prepared tailing mortar achieved sufficient buildability to be used in an extrusion-typed 3D printer at the paste age of 45 min. The mould-cast specimens process flexural strength of 7.87 MPa. In contrast, the flexural strength of printed specimens values 5.22 MPa and 12.93 MPa, respectively, after the addition of 1.5% VMA and 90 °C steam curing.
- 3D concrete printing
- bending behaviour
- weak bonding interface
- layered structure
In recent few years, various 3D printing techniques have been developed and pave a promising way for the optimal design of complex solid models and the guidance to real engineering practices (Ju et al. 2017a, b). Lots of attempts have been conducted to explore the potential of 3D printing in the building and construction industries, such as D-shape, contour crafting and concrete printing (Buswell et al. 2007; Gibbons et al. 2010; Le et al. 2012; Cesaretti et al. 2014; Kazemian et al. 2017). Such techniques are well suited to the production of one-off, complex structures that would often be difficult to produce using traditional manufacturing methods. Cementitious materials that are compatible with 3D printing promote rapid application of this innovative technique in the construction field with advantages of cost effective, high efficiency, design flexibility and environmental friendly (Khoshnevis 2004; Zhang and Khoshnevis 2013; Labonnote et al. 2016; Attaran 2017). It is critical to ensure a complementary connection between the designs of the printable mix and printing machine. Up to date, various 3D printable mixtures have been continuously developed, such as high-performance composite (Le et al. 2012; Gosselin et al. 2016), fibre reinforced mixtures (Hambach and Volkmer 2017), etc. A number of specific implementation practices have been presented. For example, a five-story apartment 3D printed by WinSun (2015), the BigDelta project (WASP 2016), a castle printed in situ (Rudenko 2015), architectural elements (Gosselin et al. 2016) etc., which have all demonstrated the great potential and feasibility of 3D printing in constructing large-scale building components.
The printable property of fresh cementitious materials and the mechanical behavior of the hardened structures are of great concerns for current 3D printing technologies (Le et al. 2012; Feng et al. 2015; Perrot et al. 2016; Ma and Wang 2017). One of the important printable characteristics is the buildability, which refers to the ability of cement paste to retain its extruded shape under self-weight and the resistance to the pressure from upper layers (Le et al. 2012; Lim et al. 2012; Perrot et al. 2016). Buildability can be considered as the early stage stiffness. Good buildability is a basic requirement for 3D printable mixtures. A feasible approach for improving the buildability is to properly extent the paste age of the mixture (Reinhardt and Grosse 2004; Voigt et al. 2006). A longer time allows the further cement hydration and therefore contributing the mix to acquire certain stiffness.
However, this approach would scarify the fluidity and adhesive behavior of the fresh paste, which may result in poor hardened structural capability and integrity. In the printing process, the concrete components are created by bonding the extruded filaments together to form each layer without using extra formworks (Pegna 1997; Khoshnevis and Dutton 1998; Feng and Liang 2014). The rheology and flowability of the fresh material must allow its fluent extrusion to form small filaments. Concrete paste of low fluidity and adhesion is likely to form voids between filaments and weak bonding interface between adjacent filament and layers, which are negative for the overall mechanical performances (Le et al. 2012). Therefore, it is of great significance for the cement paste to optimize and coordinate the well buildability and mechanical properties.
Proper treatment can be applied to improve the bonding between the layers and to decrease the voids formed by the filaments. Viscosity modifying agents (VMA) are water soluble polymers that control the flow characteristics and rheological performance of concrete mortars (Lachemi et al. 2004; Leemann and Winnefeld 2007; Benaicha et al. 2015). VMA is also a good material to ensure extrudability as it reduces the cement paste permeability, and therefore reduces the less risk of water drainage (Perrot et al. 2006, 2012, 2015). Research investigations have indicated that adding VMA in cement pastes decreases the flowability at a constant water content, however, it can increase corresponding yield stress and plastic viscosity (Leemann and Winnefeld 2007; Benaicha et al. 2015). Proper addition of VMA can improve the water retention and reduce the bleeding, contributing to the contact and bonding between the adjacently deposited filaments. The overall mechanical properties are therefore enhanced (Kovler and Jensen 2005; Lin and Huang 2010). It is therefore feasible and promising to improve the bonding effect of extruded layers by incorporating proper dosage of VMA. Besides, the 3D printing process of construction differs from the traditionally fabrication process. This innovative technique is appropriate for pre-fabricated structural components. Proper curing method can be employed to the hardened structures to improve the degree of cement hydration and the compactness of microstructure of the matrix. However, there is little investigation available regarding to the influence of VMA and post-curing method on the viscous and bonding property of cement motors used for 3D printing.
The objective of study is to optimize the structural integrity and mechanical performance of the components printed at a favourable buildability situation. To this end, this paper firstly proposed a 3D printable cementitious material that is suitable for the extrusion-based printing process. The influence of paste age on the buildability of fresh motors was evaluated to reach a desirable buildability. Meanwhile, the bending resistance of printed prism specimens fabricated at different paste ages was tested. Thereafter, a proper amount of viscosity modify agent (VMA) and different curing method were applied to improve the structural capacity by increasing the bonding force between the adjacent extruded layers. Particularly, X-ray CT scanning was implemented to characterize the weak bonding interfaces formed in the layered extrusion processes.
2.1 Material Preparation
Mixture proportions of the raw materials used for 3D printable cementitious material.
PP fiber (kg/m3)
Chemical Composition of tailings by mass ratio.
Particle size distribution parameters for copper tailings.
In the preparation process, PP fibers and the dry powders, i.e., cement, fly ash, silica fume, natural sand and tailings are firstly mixed and blended for three minutes to obtain a uniform mixture. Then, one half of the total amount of water along with the superplasticizer is added in and stirred for 2 min. Subsequently, the second half of the total amount of water together with superplasticizer is poured in and stirred for an additional two minutes.
2.2 Prism Specimen Manufacture
Thereafter, prism specimens with size of 30 mm × 30 mm × 120 mm are sawed from the 3D printed structure. As the picture shown in Fig. 1(b), the prism samples are of corrugate surfaces and the layers are perpendicular to printing direction. Then the printed specimens are smoothed to eliminate the influence of corrugate surface on the fracture behaviours, since that cracks are prone to initiate from the transition zones between two layers. Meanwhile, specimens manufactured in mould-cast state are taken as the reference specimens.
2.3 Testing Procedure
Testing procedures designed for the evaluation of bending resistance.
Viscosity modify agent (%)
90 Steam curing
Paste age refers to the duration from the ending of the blending of raw materials to the starting of the printing. The liquid and viscous property of fresh pastes are crucial to the bonding performance between layers, which greatly depends on the paste age. It is expected that the shorter the paste age, the higher bonding strength between layers. Three paste ages range from 15 to 45 min and three VMA contents range in 0.5–1.5% are taking into account. A longer paste age was not considered due to the fresh mortar became stiff and then could not be printed. Once the specimens are demoulded after 24-h curing at room temperature, they are cured by three different methods to monitor the strength development with time. The water curing method is designed to directly immerse the specimens into water at room temperature (approximately 20 °C) for 7 days. The steam curing method is to cure the samples with steam at 90 °C for 72 h. While the standard curing method is to place the samples in the moist cabinet for proper curing with an ambient temperature of 20 ± 1 °C and the relative humidity of 95 ± 5% for 7 days.
2.4 X-ray CT Characterization
Non-destructive X-ray computed tomography (CT) has been widely adopted to provide an accurate identification of the meso/microscopic structure of engineering materials (Lu et al. 2006; Gallucci et al. 2007; Zhang et al. 2012). In this section, we intend to employ the advanced CT technology to detect the voids and weak bonding interfaces formed by the filaments as well as the relative position between the fracture path and interfaces. For scanning, a high-resolution CT with a maximum spatial resolution of 10 μm was adopted, which satisfied the needs of reconstructing meso-level structures. Due to the specimen was beam-shaped, the scanning was focus on the zones in neighbourhood of the fracture surfaces rather than the whole specimen, aiming to improve the clearness of the detecting images. Moreover, the separated two parts of prism sample after bending test are assembled together to facilitate the CT characterization.
3.1 Effect of Paste Age on the Buildability
At the paste age of 45 min, the average thickness of layers under pressure of self-weight measures 7.5 mm, accounting for 93.8% of the optimal designed value 8.0 mm. The optimal value is the thickness of the right printed filament with no slump. In most cases, it equals to the size of the nozzle. Measured results indicate that the material at the paste age of 45 min can perform favourable loading capacity. Good buildability is featured by a sufficient stiffness and unobvious deformation. It is feasible to improve the buildability by increasing the paste age. However, prolong the paste age will reduce the surface chemical activity to a large extent, produce more voids between two adjacent filaments and form relative weak interfaces between layers, therefore producing negative influences to the mechanical integrity of the printed structures (Panda et al. 2017). In most cases, proper deformation of the filament is expected to fill the voids to improve mechanical capacity of the printed structures through enhancing the contact and bonding of adjacent filaments. Therefore, there is a balancing relationship between the buildability (stiffness) and the void filling (mechanical capacity).
3.2 Effect of Paste Age on the Bending Performance
3.3 Effect of VMA on the Flowability
3.4 Effect of VMA on the Bending Behaviour
3.5 Effect of Curing Method on the Bending Behaviour
From the test results, steam curing significantly contributes to the improvement of the microstructures of the cement matrix as well as elimination of the interlayer delamination, therefore resulting in the structural capacity enhancement of the printed structures. While this is sufficient for some construction applications, further improvements to strength will be necessary for many construction applications.
As a result, the buildability of the proposed tailing material can be controlled by adjusting the paste age. The longer the paste age, the better the buildability. At the paste age of 45 min, the average layer thickness measures for 75 mm, accounting for 93.8% of the optimal designed value. The Low-slump characteristic represents a well buildability. It is feasible to improve the buildability by adjusting the paste age.
The flexural strength fflx of specimens printed at the paste age of 45 min accounts for 46.1% of the mould-casted samples. From the CT identification, the weak bonding interface are characterized by discontinuously distributed small voids along the boundary of extruded filaments. The weak interface becomes more noticeable as the paste age increases. The inherent nature of layer delamination negatively influences the structural integrity and capacity of printed models.
Incorporating a content of 1.5% viscous modify agent can increase the flexural strength and fracture energy by 25 and 54.5%, respectively. The addition of VMA eliminate the influence of the interlayer delamination on the fracture path to a large extent. The flexural strength of material with 1.5% VMA measures 67% of the mould-casted ones. The flowability of fresh paste shall be taken into account to meet the basic requirement of a desirable printability when a certain amount of VMA is adopted.
Steam curing method introduced in this study increased the strengths approximately four times from the original strength. Flexural strengths about 12.93 MPa can be achieved with this post-processing method. The inherent nature of the layered structure becomes less distinct as components are cured. Heat curing at 90 °C may not be an applicable means for the rapid manufacturing processing, however, it is indeed a promising post-treatment method for enhancing the mechanical performances.
The paper has investigated the structural integrity and bending resistance of the components printed at a favourable buildability situation. It is applicable and favourable to enhance the 3D printed structures by the proposed methods. However, there still exist certain mechanical mismatch between the printed with mould-cast specimens. Next step researches shall be continuously explored to reduce the weakening impact of layer delamination on the structural performance of components. Meanwhile, the mechanical anisotropy of the printed laminar structure is necessary to be investigated in the ongoing works. Since the current printed objects are either unreinforced, or reinforcement is applied manually, fibre reinforced cement mixtures or fibre-reinforced polymers (FRPs) that is of great potential to increases the ductility of the printed mortar should be developed. Further research will also be devoted to explore the frontiers of 3D printing and promote its effective application in the real-life construction sectors.
The authors are grateful to the financial support by the National Major Research Instrument Development Project of the National Natural Science Foundation of China (Grant No. 51627812), and the opening project of State Key Laboratory of Explosion Science and Technology (Beijing Institute of Technology, Grant No. KFJJ13-11 M).
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