On the origins of anisotropy of extrusion-based 3D printed concrete: The roles of filament skin and agglomeration

One of the most significant challenges facing extrusion-based 3D concrete printing (3DCP) is the anisotropy present in the printed material: under load, the observed performance is typically lower than a cast equivalent and significantly so in certain directions. In addition, the performance is also more variable than cast material. These observations are, in part, due to surface moisture evaporation and air entrapment. Here, we investigate the hypothesis that the printed concrete comprises of agglomerated filament core and skin having distinct properties as a necessary consequence of the printing process. Through novel X-ray computed tomography measurements, we show that printed concrete comprises the core and Filament Interfacial Zone Network (FIZN) and that, in contrast to the cores, the FIZN is found to be free from pores except at boundaries where there is incomplete bonding. Through morphological, chemical and mechanical analysis, the FIZN is also found to contain 20% less sand and 60% more anhydrous cement than the filament cores, while the FIZ material was inferred to have 11% higher compressive strength, 28% lower flexural strength and 22% lower elastic modulus than the core. The findings from this work suggest that anisotropy will always exist and that care should be devoted to the material rheology, printing system and the filaments arrangement in order to produce consistent and predictable hardened material properties.