Page 9 - Open access sep 2020

TECHNICAL PAPER

to prevent the collapse during printing. The model considers 3.2 Buildability and early age mechanical
fresh concrete as a Herschel-Bulkley fluid. The deformation is behaviour of 3D printable concrete
assumed not to occur until the maximum shear stress of layered
concrete reaches yield stress. The CFD analysis was done, Buildability of 3D printable concrete is defined as the ability of
and the estimations were seen to be accurate. The model can the extruded layers to retain the printed shape and to carry the
estimate the occurrence and location of collapse and show the weight of subsequent layers without failure. Buildability can be
increase in yield stress of the fresh concrete. The model can be understood at different scales – microscopic, mesoscopic, or
used to reduce the use of numerical analysis for the computation macroscopic. The rheological behaviour of concrete governs a
of rheological parameters like hardening coefficient and initial large part of its buildability property.
yield stress (Jeong et al., 2019).
Cement-based materials are mostly considered as elasto-
Aggregate blocking during the extrusion of mortar is a very viscoplastic fluids, generally considered as Bingham fluids, as
critical problem. It was studied by El Cheikh et al. (2017) by they do not flow under self-weight when kept unsupported
experimental and numerical approaches, using a hard-core (Banfill, 1991). For the initiation of flow in the Bingham fluid,
(sand particles) soft-shell (cement paste) model for DEM. the initial yield stress is to be exceeded. The material exhibits
The governing parameter is the ratio of the bottom opening an infinite absolute viscosity until high stress is applied and the
diameter of the nozzle to the bead diameter of glass (to flow initiates (Chidiac and Habibbeigi, 2005). At rest, the static
compare with numerical results) as per numerical results. The yield stress keeps continuously increasing, but the application
model considers cement paste to be a fluid layer, which is of shear erases this growth and brings it back to a previous
dragged by the aggregates. The separation of cement pastes state, which is known as thixotropy (Roussel et al., 2012). There
from coarse aggregates during blockages in not considered in is an increase in the structural build-up (structuration) inside
this model. The model cannot predict properly for lower values the concrete, due to a physical phenomenon (flocculation)
and chemical phenomenon (hydration) (Roussel, 2012). If we
of paste volume proportion (value of 0.54 for this study). The consider the mesoscopic scale, concrete can be modelled as a
study suggests that the paste volume proportion is more critical suspension of aggregates in a cement paste (Roussel and Gram,
that maximum paste thickness for blocking. So, blocking of 2006).
extrusion needs to be further studied with different grain sizes
and other possible variations. In-line quantification using unit Concrete is a suspension of many different sized particles
extrusion energy (UEE) principle, which is the consumed energy in a continuous fluid flow. The mutual interactions can be
per unit volume of extruded material, is proposed by Nerella categorized mainly as Brownian forces, surface forces (colloidal
et al. (2019b). It is used as a measure of extrudability. UEE is interaction), hydrodynamic forces, and contact forces between
inversely related to extrusion, i.e., lower the value of UEE higher particles. At the microscopic scale, two phenomena control the
the extrudability. structuration - formation of a network of colloidal interactions
(Figure 6) between cement grains, and formation of initial
The importance of understanding the rheology of materials for cement hydrates (at the time of mixing) leading to the formation
extrusion is established in this section. The extrusion controls of CSH bridges between cement grains (Figure 7). The yield
the uniformity of the printed layers which can directly affect the stress increases by the rigidification of the bridges (Roussel et
geometry of the section and hence buildability. The next section al., 2012). The yield stress and elastic modulus development
describes the importance of buildability in 3D concrete printing. with time are assumed to be linear by the Roussel model and

(a) (b) (c) (d)
t = 0s t = a couple s t = several 10s t > 100s
Figure 6: Network of interacting cement particles in the dormant period, adapted from Roussel et al. (2012)

THE INDIAN CONCRETE JOURNAL | SEPTEMBER 2020 13