Mechanical and microstructural properties of 3D printable concrete in the context of the twin-pipe pumping strategy

Abstract

To tackle the conflicts between the pumping and deposition process in 3D concrete printing, a twin-pipe pumping (TPP) system with a helical static mixer can be used. In this way, a cement-based mixture (without accelerator) and a limestone powder-based mixture (with a high dosage of accelerator) are transported separately and mixed within the helical static mixer just before extrusion. However, the helical static mixer will introduce striation patterns in the 3D printed concrete due to flow division and the effect of such a heterogeneous phenomenon on the mechanical behavior has not been studied yet. In this paper, the mechanical (compressive, flexural, and tensile bond strength) and the microstructural behavior of 3D printed specimens made using the TPP system was explored. During the mechanical tests, the cracks initiated within the limestone powder striation. In addition, the difference in the mechanical strength for the specimens under difference loading directions was observed. Microstructural investigations based on mercury intrusion porosimetry revealed that the pore size distribution of 3D printed specimens (critical pore size of 210.0 nm and 197.5 nm for the bulk and interface region, respectively) was distinct from that of the mould cast specimens (critical pore size of 62.7 nm). Although the mould cast specimens had higher porosity (19.75%), the volume of macropores was found to be higher in the 3D printed specimens, which was more detrimental to the compressive strength. Further, porosity analysis of micrographs showed that the macropores in the 3D printed specimens were primarily presented in the limestone powder-based striations. In addition, a small quantity of ettringite was also observed in the bulk of the limestone powder-based striation, which may be caused by ion migration between the two regions after the inline mixing process by the static mixer.