Page 6 - Open access sep 2020

TECHNICAL PAPER

In extrusion-based printing, one of the major challenges is the Structural
performance
formation of the bond between the layers. Poor bonds among
layers can significantly decrease the strength carrying capacity
of the structure. The presence of such weak interfaces has been
Reinforcement
reported to cause anisotropy in the mechanical behaviour of
Pumpabiliy
printed concrete. The lack of models for mechanical parameters & Buildability
in 3D printed structures and the absence of structural design Mechanical property
codes require more research and attention.
Rheology Delivery & Placement
1.2 3D printing for concrete construction Tribology

In 3D concrete printing, both industry and academia are
VEA
progressing simultaneously. Concrete printing may provide SCM
advantages like rapid repetitive construction in mass housing Superplasticizer
projects, in-situ construction of specific complex building Figure 5: Multi-level material design (MMD) adapted from Lu et al. (2019)
components for either architectural or functional requirements
with less manual labour etc. These advantages are attracting rheology of the material (deformation and flow of materials)
the attention of the construction industry, which has not seen which consecutively affect the extrudability and buildability of
any significant technological change for the last few decades. materials, which will be discussed in the coming subsections.
The potential of 3D printing in the market (Figure 4) is studied The apexes in the middle pyramid can be rheology, and factors
comprehensively by BCG group in the article “Digital in such as granular contacts (tribology), delivery and placement,
Engineering and Construction: The Transformative Power of which decide the pumpability and buildability. Pumpability and
Building Information Modelling” (Castagnino et al., 2016). Figure buildability are affected by the material as well as equipment
4 describes the application of different digital technologies at parameters. The top-most pyramid in this model represents
different positions of the life-cycle of a structure. From designing structural performance, which has buildability as one factor
to operations, digital technology is changing the engineering and reinforcement and mechanical property as other factors.
and construction value chain. Additive manufacturing is a part This kind of approach can help to form a holistic overview of
of these digital technologies and has the potential to alter the 3D concrete printing and shows possible research areas. The
construction process.
upcoming sections discuss the different studies on the basic
As per the estimate of BCG, within 2026, a cost saving of 0.7 parameters controlling concrete 3D printing. The mix designs
trillion to 1.2 trillion dollars can be realized in the construction of 3D printable concrete, extrusion, buildability and early age
phase by full-scale digitalization of non-residential construction. mechanical behaviour, layer bonding, and hardened state
Even in the operation phase, around 0.3 trillion to 0.5 trillion mechanical strength are discussed in the following sections.
dollars can be saved. The BCG report also mentioned about
the enhanced quality of processes and improvement in safety, 2. MIX DESIGN OF 3D PRINTABLE CONCRETE
working conditions, and sustainability. So, in other words, we can
say that the future of quality assured safe construction lies on The mix is to be designed considering the three major
the shoulder of digital construction. characteristics of 3D concrete printing, namely pumpability,
extrudability, and buildability. Wangler et al. (2019) sorted a
Lu et al. (2019) proposed a design named multi-level material few important research areas considering mix components
design (MMD) which is made of three consecutive stages and parameters. The rheological parameters like yield stress,
(represented as pyramids), namely mixture design, printing plastic viscosity, and thixotropy and the difference in rheological
process, and composite structure. In each pyramid, the factors requirements for digital printing and conventional casting
of the lower three apexes affect the fourth upper apex, which are highlighted. They also emphasized on a relation between
affects the higher-level pyramids, as shown in Figure 5. processing and performance of the material, the effect of
admixtures and their interaction on rheological and mechanical
All pyramids are linked in a way that the upper apex of the properties, and developing computational modelling to
lower pyramid becomes a lower apex of the base of the upper understand the early age hydration effect at the micro-scale.
pyramid. In this way, all factors of lower pyramids impact Different researchers developed different mixes for 3D printing.
the upper pyramids. The lowest pyramid contains factors The requirement of the mix depends on the applicability as well
like raw ingredients (including SCMs), superplasticizer, and as printing constraints. A few of the designed mixes and the
viscosity-enhancing/modifying agents. These factors affect the properties considered for designing are summarized in Table 1.

10 THE INDIAN CONCRETE JOURNAL | SEPTEMBER 2020