An Experimental Study of Consistency and Strength Variation of 3D Printed Concrete Mixes (2025-01)¶

Concrete three-dimensional (3D) printing is an innovative technology that allows for building three-dimensional objects layer by layer. Although the process is traditionally called concrete 3D printing, most of the mixes used have a maximum particle size of 2–3 mm, and, thus, calling the process mortar 3D printing would be more technically correct. In this chapter, the terms “3D-printed mortar” and “3D-printed concrete” are used interchangeably. Typical concrete 3D printing involves extrusion of a mortar filament through a nozzle with a diameter ranging from 6 to 50 mm [1]. The printing head is mounted on a gantry or robotic arm that positions the material to form the printed structure layer by layer. The advantages of concrete 3D printing include the absence of the need for formwork and shape flexibility, whereas the challenges include difficulties in applying reinforcement to 3D-printed structures and the lack of standards for the structural use of 3D-printed parts. A substantial portion of the research on 3D-printed concrete is dedicated to the properties of the fresh concrete and mix design, ensuring the pumpability, extrudability and buildability of the mix. Interested readers are referred to reviews by Zhang et al. [2] and Chen et al. [3] for more details on mix design and characterization methods. When it comes to the strength and durability of hardened 3D-printed concrete, researchers are mainly concerned with the anisotropy and influence of the layerinterface properties on the overall performance of the structure as reported by e.g. Nerella et al. [4] and Rahul et al. [5]. When a mortar mix is tested in a laboratory, the water-to-binder-ratio (w/b) is usually known and fixed, whereas in the case of an industrial printing process, the concrete printer operator specifies not w/b but the amount of water in litres per hour and routinely adjusts this amount based on a feel of good consistency. Thus, the behaviour of the same mix in the laboratory and at the printing site might vary due to a fluctuating w/b. In this chapter, we present an approach that allows quantifying this variation, which has not yet received enough attention in the literature.