Concrete three-dimensional (3D) printing technology has the potential to significantly reduce on-site labour activities, as 3D-printed concrete eliminates the need for formwork. However, challenges persist in ensuring the quality and strength of 3D-printed concrete. One of the factors likely to impact its strength is the fluctuation of the water-to-binder ratio (w/b) during the printing process. In contrast to traditional cast-in-place concrete, where the mix is prepared at the factory with a fixed water-to-binder ratio, 3D-printed concrete is continuously mixed from a dry mix during printing. The amount of water added to the dry mix is routinely adjusted by the printer operator to maintain optimal consistency. This adjustment affects the water-to-binder ratio, leading to variations in concrete strength. This chapter investigates the extent to which the consistency and strength of 3D-printed mortar may vary while remaining printable using an industrial-scale concrete printer. Using the same dry mix, we observed a difference of approximately 15% in the mean strength for samples cast at the 3D printing site on two different dates. Additionally, there was about a 20% difference in the strength between the thickest and thinnest printable consistencies mixed in the laboratory. This variation must be considered when determining the design strength of 3D-printed concrete.