Examining layer height effects on the flexural and fracture response of plain and fiber-reinforced 3D-printed beams

A significant amount of work has focused on the development of concrete mixtures for digital manufacturing (3D printing), and their rheological and mechanical properties. However, for extrusion-based layered manufacturing, it is also important to select the appropriate printing parameters that have the potential to impact the performance of 3D printed elements. Among the many such parameters, this paper places emphasis on layer height, which has a direct bearing on rheology requirements, print quality, overall printing time, and interlayer bonding. Specifically, this paper examines the effects of layer height (5, 10, and 15 mm layer heights corresponding to 25, 50, and 75% of the nozzle diameter, which is 20 mm) on the flexural strength and fracture properties of 3D printed beams. Flexural and fracture properties indicate that smaller layer heights are beneficial for unreinforced and fiber-reinforced 3D printed mortars, even though this results in greater number of interfaces and longer printing times. A small amount of steel fiber reinforcement is shown to be useful in eliminating the negative effects of weak interfaces on the measured bulk properties, with average flexural strengths higher by 30–40% and fracture toughness and crack tip opening displacement higher by almost 30% as compared to plain mixtures. Strain energy release rates, digital image correlation, and optical images/micrographs are used to explain crack propagation in layered 3D printed mortars under unnotched four-point, and notched three-point bending.