Effect of surface moisture on inter-layer strength of 3D printed concrete

•Factors influencing the inter-layer strength in concrete 3D printing are studied.•The moisture level at the surface is identified as an important factor.•Low moisture level is linked to low inter-layer strength in 3D concrete printing.•The surface moisture level is affected by bleeding and evaporation rates.•Strengths of 3D printed concrete were different in x, y and z directions. The extrusion-based 3D concrete printing is a new technology under development for construction of buildings and structures of complex geometries without the use of expensive formwork. The weak inter-layer strength between printed layers is one of the limitations of this technology when compared with cast-in-the-mold concrete. This study investigates effects of print-time interval on the inter-layer strength, along with compressive and flexural strengths of extrusion-based 3D printed concrete in different directions. Specimens were printed with 10, 20 and 30 min delay times (print-time intervals). Compressive, flexural and inter-layer strengths of the 3D printed concretes were measured. The inter-layer strengths of the specimens printed with 10 and 30 min delay times were comparable but higher than that of the specimens printed with 20 min delay time. A correlation was found between the results of inter-layer strength and the surface moisture content at the interface of the layers. The surface moisture content, in turn, depends on the bleed rate of the concrete and the rate of drying of moisture from the surface among other factors. The results also indicated that the compressive and flexural strengths of 3D printed concrete depended on the testing direction. The orthotropic phenomenon was more pronounced in compressive than flexural strength.