Successful implementation of extrusion-based three-dimensional (3-D) printing requires the development of print materials with adapted rheology. In this study, filtration characteristics coupled with rheological properties of mortar mixtures are investigated to characterize the extrudability of print materials and establish a “printability window” (that is, the acceptable range of material properties for successful extrusion and shape stability). The extrudability was measured as the maximum force needed for the ram extrusion of the material. The fluid filtration rate was assessed in terms of desorptivity of the fresh mixture under pressure. The yield stress, plastic viscosity, and desorptivity were varied by changing the water-cement ratio (w/c), high-range water-reducing admixture (HRWRA) dosage, and welan gum (WG) content. Regression analysis indicated that during extrusion-based printing, the yield stress and desorptivity values can exhibit a more significant effect on extrudability than plastic viscosity.

As one of the key factors influencing the hydration process, as well as the microstructure formation and evolution of ultra-highperformance concrete (UHPC), the action mechanism of different curing regimes have been studied to some extent. However, the current knowledge of the underlying mechanisms that control the different effects of different curing regimes is limited. In this study, the composition of hydration products, micromorphology, and migration and evolution of aluminum-phase hydration products of UHPC under three combined curing regimes (standard curing, steam curing + standard curing, and autoclave curing + standard curing) were investigated in depth. Micromorphology observation shows that heat treatment promoted the formation of higher-stiffness hydration products (tobermorite and xonotlite) in UHPC, and the higher the polymerization degree, the higher the Si/Ca ratio of the hydration product. Meanwhile, 29Si and 27Al nuclear magnetic resonance (NMR) spectroscopy shows that specimens with higher strength had higher Al[4]/Si and a lower amount of ettringite and AFm at the early curing stage. The elevated curing temperature reduced the formation of ettringite and AFm and allowed more Al3+ to replace Si4+ into the structure and interlayer of the calcium- (alumino)silicate-hydrate (C-(A)-S-H) gel, which increased the mean chain length (MCL) and polymerization degree of the C-(A)-S-H gel. However, the polymerization effect of Al ions is limited, so the provision of the silicon source to improve the Si/Ca ratio of the system is important.

This experimental study evaluated the correlation between measured concrete expansion from a modified version of the miniature concrete prism test (MCPT) with the concentration of chemical markers leached from the prisms into an alkaline soak solution. Fifteen concrete mixture designs were tested for expansion and soak solution concentrations over time. The changes in expansion and soak solution concentrations were found to correlate well even with variations in alkali loading and substitution of cement with Class F fly ash. A model was developed to estimate the expansion potential of concrete based on an expansion reactivity index (ERI) that incorporated the concentrations of silicon, sulfate, calcium, and aluminum. The relationship between ERI and expansion was then used to identify potentially expansive concrete mixtures using the ERI of cores taken from a structure exhibiting potential alkalisilica reaction (ASR) expansion and concrete cylinders matching the mixture designs of the MCPT specimens.

The need to constantly improve the quality and properties of manufactured products leads to the development of hybrid materials that combine different elements, complementing one another. Fiber-reinforced mortar is one of those products, as the fibers are used to improve cementitious materials’ flexural weakness. Experimental data on different metallic fibers dispersed in mortar demonstrate the correlation between early-age rheological properties and long-term mechanical strength. Both quantities depend on the ratio of the solid volume fraction of the fiber to a critical solid volume fraction characteristic of the form factors of the fiber. It is demonstrated that both effects arise from the packing stress of the fibers in the mortar when their concentrations are close to their maximum packing fraction. Geometrical arguments are used to explain how this critical volume fraction is related to the fiber form factor. Then, it enables the building of master curves using geometrical arguments.

The early-age material parameters of three-dimensional (3-D)-printable concrete defined under the umbrella of printability, namely, pumpability, extrudability, buildability, and the “printability window/open time,” are subjective measures. The need to correlate and successively substitute these subjective measures with objective and accepted material properties, such as tensile strength, shear strength, and compressive strength, is paramount. This study validates new testing methodologies to quantify the tensile and shear strengths of printable fiber-reinforced concretes still in their fresh state. A tailored mixture with high sulfoaluminate cement and nonstructural basalt fibers has been assumed as a reference. The relation between the previously mentioned parameters and rheological parameters, such as yield strength obtained through International Center for Aggregates Research (ICAR) rheometer tests, is also explored. Furthermore, in an attempt to pave the way and contribute toward a better understanding of the mechanical properties of 3-D-printed concrete, to be further transferred into design procedures, a comparative study analyzing the work of fracture per unit crack width in three-point bending has been performed on printed and companion nominally identical monolithically cast specimens, investigating the effects of printing directions, position in the printed circuit, and specimen slenderness (length to depth) ratio.