A Study of Early Age Shear Properties of 3D Printable Cementitious Mixes with Fiber Reinforcements

The use of Fiber Reinforcement in digitally manufactured concrete is not only one of the viable techniques in the otherwise difficult reinforcement strategy but also a segue to the use of highly durable Ultra High Performance Concretes in 3D printing. For this, the characterization of early age mechanical properties such as tensile, shear and compressive strength of printable Fiber Reinforced Concrete is very important in the design process. The printability of fresh concrete which is defined as combination of pumpability, extrudability and buildability can be characterized using the tensile strength of filament, the shear strength and the compressive strength of the freshly printed element. This study aims on defining the shear strength of 3D printable cementitious mortars at early ages focusing on the “printability window” highlighting the phase transitions in printed concrete. Highly cohesive mortar with high Sulfo-Aluminate cement content is cast with Basalt fibers in specially designed molds to understand these said properties at various times from casting namely, 30, 45, 60 and 90 min. Repeatability of the methodology has been established using 3 iterations of each experiment and crack - failure patterns have been studied closely. Further study on developing the Kupfer’s failure envelopes is underway by including the compressive strength study obtained through penetration tests and tensile strength study

Experimental Investigation on the Early Age Tensile Strength of Fiber Reinforced Mortar Used in 3D Concrete Printing

Digital fabrication with cement-based materials requires specific attention to be paid to the rheological and mechanical material properties both in the fresh and hardened state. For the layered extrusion process, the cement-based material needs to satisfy the “printability” requirement. Generally, printable mortars exhibit brittle mechanical behaviour due to the absence of reinforcement. In order to overcome this issue, many different strategies can be implemented. Among them, the addition of short fibers in the mortar represents a first step towards the development of robust materials for 3D printing in construction. In this context, the paper focuses on the early stage tensile properties of fiber-reinforced cement-based material to be used in the layered extrusion process. The embedment of discrete fibers in a printable mix is expected to improve the mechanical behaviour but, at the same time, it implicates a loss of workability in the mix, which could lead to problems during the printing process (in terms of extrudability and pumpability of the mix). In this paper, the mechanical response under direct tensile is investigated as a function of the type/concentration of fibers as well as the mortar resting time. Furthermore, the effect of varying the amount of the superplasticizer to guarantee the printability requirement of the printable mortar is also investigated. In a quality control framework, the development of tensile fracture properties, in the considered production time frame, is fundamental to determine the printability of the mix, with reference not only to the quality of the finishing but also to the speed of the printing process