Production of the first concrete and reinforced concrete columns by means of 3D printing with concrete
To allow individual, fast and cost-effective construction of high quality in the future, for one thing, requires the entire digitalization of manufacturing processes and, for another thing, the manufacturing of structural members needs to be designed in an automated and more flexible manner. In this regard, the concrete 3D printing technology combined with an automated assembly line production is a promising approach.
1. Introduction
According to estimates of the Federal Statistical Office, Germany has a lack of about one million homes. Moreover, many building structures in Germany are old and dilapidated and need to be replaced as soon as possible. Even if, at present, substantial efforts are undertaken on the part of government and industry to react to these drawbacks, new construction is faced with essential problems due to an industry-specific, individual planning and production of building structures, the long construction periods depending on weather conditions, the manual, time-consuming and fault-prone on-site production as well as the high costs associated. The precast concrete industry, with its partially industrial and standardized production of precast parts and elements [1], enables savings in terms of time and money, but often does not allow considering the clients‘ individual requirements on architecture optimally.
3D printing is an additive manufacturing technique that builds three-dimensional components adding layer by layer without formwork. The calculation of the layer structure and the corresponding printing paths are computer controlled using previously defined target geometries. Even through additive manufacturing of structural members is still in the development stage, a distinction between the following concrete printing technologies is possible already now:
particle-bed method,
extrusion method,
shotcrete method.
In the particle-bed process, thin layers of sand are selectively saturated with a cement-water mixture layer by layer. As soon as the saturated layers are setting, the surplus sand is removed and the finished concrete component is revealed. In contrast to this process, the concrete is mixed in advance when using the extrusion method and the component is printed by depositing small strings of fresh concrete. In the shotcrete process, in turn, the concrete is applied layer by layer as shotcrete. Apart from the actual printing process, the integration of the reinforcement in the concrete printing process, among others, is an object of current research.
2. Digital Building Fabrication Laboratory (DBFL)
In 2016, the Digital Building Fabrication Laboratory (DBFL) at the ITE was put into operation for the digital manufacture of structural members. The DBFL is a DFG-funded major research instrumentation, which is unique in respect of its conception and performance, and is the focus of current research in the field of additive manufacturing at the ITE and iBMB (Fig. 2). The working space of the DBFL is 15 m long and approximately 7 m wide, with a height of up to 3 m. A 6-axis industrial robot, connected to a 3-axis portal machine, is the additive manufacturing unit of the DBFL. The robot has a load-carrying capacity of 150 kg, can move about freely in the entire working space, and can be provided with different end effectors (e.g., spray nozzle, welding apparatus, etc.) depending on the production process. The DBFL comprises another portal machine that is equipped with a 5-axis, CNC-controlled milling and sawing attachment. The CNC portal machine is the subtractive manufacturing unit of the DBFL which was also developed for the processing of hard and abrasive materials such as ultra-high performance concrete, for example. A wide range of applications is covered by combining the flexibility of the robot with the rigidity of the milling and sawing unit. Both units can be controlled separately as well as synchronously. The synchronous gear allows for setting up complex process chains consisting of additive and subtractive manufacturing steps.
3. 3D printing of columns
3.1 General
Columns are structural members with essential importance for the structural safety regarding their integrity and dimensional accuracy, while requiring a high expenditure in manufacturing simultaneously. The iBMB has gained extensive experiences concerning columns, among others, made from
normal and high-performance concrete with large bar diameters (e.g., [7], [8]),
ultra-high performance fiber-reinforced concrete with high-strength reinforcement (e.g., [9]) and
ultra-high performance spun concrete with high-strength reinforcement (e.g., [10]).
In a first step, the shotcrete used in the project sponsored by the Ministry of Science and Culture of Lower Saxony was investigated regarding the achievable concrete parameters. Afterwards, investigation was carried out on concrete printing of non-reinforced concrete columns and, based on this experience, on reinforced concrete columns (integration of reinforcement in the concrete printing process).
3.2 Concrete mixture and concrete strength values
The shotcrete used was „Emcefix-Spachtel G extra“, a polymer-modified, microfiber-reinforced fine-grained concrete made by MC Bauchemie, with a maximum grain size of 2.0 mm. The fine-grained shotcrete was pre-mixed in a compulsory mixer according to the instruction of the manufacturer and then conveyed by a screw pump through a hose up to the nozzle of the spraying device, where it was accelerated by means of compressed air. An accelerator was added to the concrete stream via the compressed air in order to control the setting behavior of the fine-grained concrete.
3.3 3D printed concrete columns
3.4 3D printed reinforced concrete columns
Because of the prefabricated reinforcement cages, it was not possible to lead the spraying nozzle at a right angle to the horizontal, as in case of the non-reinforced concrete columns. In addition, the concrete printing process had to be carried out in such a way that (hardly) any spray shadow occurred. Based on extensive preliminary investigations and process simulations regarding possible guidance of the spray nozzles, the reinforcement cage was finally fixed by means of two shuttering boards on an electrically driven rotary plate and the spraying nozzle that is inclined to the horizontal by 60° was led from the bottom to the top along the rotating reinforcement cage. During the printing process, the rotary plate was operated at a rotational speed of 0.1 m/s. The series of photos shown in Figure 6 clearly demonstrates that the production of reinforced concrete columns could be realized with this method quite well.
Figure 8 (center and right), however, also clearly reveals that the concrete exhibits fine grain fractions at the cross-sectional edge and inside the core and that the cross-sectional shape is still very rough (see also section 3.3). Further investigations are also planned on this process-related phenomenon.
4. Summary and outlook
At the ITE and iBMB, Division of Concrete Construction of the TU Braunschweig, segmented, non-reinforced concrete columns as well as continuously printed reinforced concrete columns were printed in order to investigate the realization of 3D printed precast reinforced-concrete components. The structural members were manufactured in the so-called Shotcrete 3D Printing (SC3DP) process, a robot-supported 3D printing method without formwork that is based on the shotcrete technology. The production of the columns took place in the Digital Building Fabrication Laboratory (DBFL) of the ITE, a research laboratory where new, digital manufacturing methods for structural members made of concrete (and other materials) can be investigated at large scale. Whereas in the case of concrete columns, apart from the production, the focus was segmenting them as well as the realization of the associated dry joints, the research concentrated, in case of the reinforced concrete columns, on the integration of the longitudinal and stirrup reinforcement into the robotic manufacturing process. The following structural investigation of the reinforced concrete columns at the iBMB revealed that the production of reinforced concrete columns could be realized with this method quite well. However, there is still need for research so as to minimize process-related structural defects such as the still too low dimensional accuracy, the high surface roughness, occasional spray shadows and the inhomogeneity of the shotcrete.
In addition, systematic investigations on the performance of the 3D printed columns have been planned to allow for comparison with conventionally manufactured reinforced concrete columns. These investigations intend to ensure a robust and repeatable production process which will be integrated in an automated assembly line production, thus leading to an economic and practical application in building and enabling fast and individual construction in the future.