Evaluating the printability of concretes containing lightweight coarse aggregates

This paper examines the printability of concrete formulations made using lightweight expanded clay aggregates (LECA) of maximum size 10 mm. The effect of coarse aggregate addition on the two essential aspects of printing, i.e., extrudability and buildability are examined using a piston-pump-based 3D printer system. The extrudability is assessed by measuring the desorptivity, a fundamental material parameter for assessing the phase separation tendency during the extrusion process. It is found that increasing the coarse aggregate content increases the desorptivity, thereby decreasing the water retention capacity. Mixes with up to 30% volume substitution with coarse aggregate were extruded successfully. However, for mixes with volume substitution higher than 30%, a significant amount of phase separation is found to occur during the extrusion process, resulting in blockages. To assess the effect of coarse aggregate on buildability, uniaxial compression tests were carried out on concretes after different ages of casting. For both the mixes with and without coarse aggregates, the material behaviour changes from an elasto-plastic type response at early ages to a strain softening type response (typical of hardened concrete) at later ages. At any given age, it is found that the addition of LECA, increases both the strength and elastic modulus of the concrete. This may be attributed to the increase in the internal friction angle as the size of the aggregate increases, as well to the higher amount of dewatering that occurs during the extrusion of mixes containing coarse aggregates resulting in a relatively stiffer mix after extrusion. Finally, discussions are provided on the effect of using low-density aggregates like LECA on improving the resistance to both plastic and buckling type collapse that may occur during printing.