Capillary extrusion rheometry for characterising wall slip behaviour in 3D printed concrete

The 3D concrete printing (3DCP) processes involve the flow of fresh concrete through a pipe, which is dominated by a lubrication layer at the interface between the bulk concrete and the boundary wall. However, the physical and rheological properties of the lubrication layer in 3DCP concrete have not been fully understood, and therefore further research is required. This paper addresses this challenge by conducting a series of comprehensive experimental studies to characterize the rheological properties and wall slip behaviour of printable concrete mortar. The experiments employ multiple rheometric tools, including a rotational vane viscometer, tribometer, capillary extruder, and micro-Computed Tomography (micro-CT). The classic analytical model of wall slip correction is applied to quantify the physical properties of the lubrication layer for fresh concrete with varying aggregate content. The results reveal a linear relationship between the slip velocity and the wall shear stress for all mixtures, indicating stable slip coefficients at different wall shear stresses. Furthermore, it is observed that the thickness of the lubrication layer decreases as aggregate content increases. Assuming the layer consists solely of pure paste, rheological analysis estimated the thickness to lie between 10 and 70 μm for the tested concrete mixtures. However, micro-CT indicated a looser aggregate packing near the wall and revealed a thicker lubrication layer than that predicted by the rheological model. A thicker, paste-rich lubrication layer is shown to facilitate the pumping and extrusion process, which may also influence the interlayer bond strength between the printed filaments.