Twin-Pipe Pumping Method for Upscaled 3D Concrete Printing with Chemical Stiffening Control

Описание: Presented By: Geert De Schutter, Ghent University

Description: It is challenging to unify the contradictory rheology requirements for cementitious materials for 3D printing. In the first phase, the material needs to be sufficiently flowable to facilitate pumping towards the nozzle. In the second phase, the material has to be extrudable. In the third phase, once the layers are formed, the cementitious material needs to show a high buildability to support the weight of the next layers. Consequently, within a short time, the material needs to show a very quick change from more fluid-like to more solid-like. Different approaches could be followed to reach this goal, among which the addition of an accelerator in the nozzle in combination with the use of a dynamic mixer to homogenize. A new and alternative approach is now presented with the Twin-Pipe Pumping (TPP) method, basically consisting of two pumps combined with a motionless static mixer. During the printing process, two distinct mixtures are premixed. The first stream consists of a Portland cement-based mixture without accelerator, while the second stream is based on limestone powder and contains a relatively high dosage of accelerator. Both mixtures are easily pumpable, with a long open time. Two separate pipes convey the different streams towards an in-line static mixer, located immediately before the extrusion nozzle. By means of the chemical action induced by the accelerator, the stiffening rate of the combined mixture is then drastically increased. TPP enables a high construction rate with good shape stability. As an example, a 3 m high column could be printed within a record-low time window of only 9 minutes. As a further advantage, TPP can also be extended to potentially more sustainable alkali-activated materials, following the same principle of two separate streams and chemical activation within the static mixer. Some examples of remarkable 3D printed elements will be shown, including large overhang and bridging window and door openings.