Page 11 - ICJ Jan 2026

TECHNICAL PAPER

a two-component design of the material is conducted so that
an accelerator could be added in line at the nozzle, which is
equipped with a mixing arrangement. However, printers with a
two-component capability are quite complex, expensive and
not easily available. Therefore, in order to maximize the output
from conventional 3D printing systems, it is necessary to come
up with a suite of tests that can capture the complex rheological
behavior of 3D printable concrete.

The use of the vane shear test as a first level indicator of
the rheology in combination with the flow table test was
suggested by Rahul et al. [14] , and the methodology could
easily demonstrate the robustness of 3D printed mixtures
Figure 11: Results of the slump – yield stress comparison for 60 MPa incorporating different types of additives such as VMA,
[13]
mixtures with 0 – 35% replacement of cement by fly ash (reported in ) ; silica fume, and nano clay. Further, in order to capture the
the yield stress measured using the test is well correlated with the slump
propensity for phase separation caused by pumping pressures
the slump-yield stress (measured using rheometer) relationships in the delivery system, Rahul et al. [15] adapted a simple water
suggested in literature. The procedure was applied to several desorptivity test, which is based on a simple principle of water
HDC mixtures with and without fly ash and slag, designed for retention by the mixture under an applied pressure, similar
40-70 MPa compressive strength. The yield stress determined as to the water retentivity test conducted in masonry mortars. A
per the procedure correlated well with the slump measured on suitable index was suggested that could clearly indicate the
the fresh concrete, as shown in Figure 11 (for 60 MPa concrete 3D printable mixtures that could sustain the pressures during
with different levels of fly ash). Further, the measured yield stress printing without any adverse effects on the fresh properties.
was also able to capture the differences between a paste rich
mixture (such as the higher grade concrete) and a lower paste The use of vane shear to determine the static yield stress of 3D
mixture, which could not be differentiated by the slump test. printable mixtures using different types of binders, including
Tests on each mixture were performed multiple times, with the Portland cement systems with fly ash, slag, or limestone-
results for the measured yield stress being within 10 % of each calcined clay combination, as well as alkali activated systems,
other. This indicates that the test has good repeatability. Being was performed by Patel [16] . One of the results from the study,
a simple test, this can be easily conducted on worksites to showing the temporal variation of the static yield stress for 3D
assess the fresh state rheological characteristics instead of using printable mixtures with a few of the binder types is shown in
rheometers. Figure 12. Further, the gravity-driven slug test methodology
suggested by Ducolombier et al. [17] was adopted to determine
5. RHEOLOGICAL CONTROL AND STABILITY the yield stress during printing, which is indicative of the
dynamic yield stress of the mixtures. Using the measures of
OF 3DCP
the static and dynamic yield stress, key differences between
Concrete for 3D printing needs to be designed with a careful the behavior of concretes with different binders could be
control of the rheological characteristics. The difficulty in the highlighted. The work by Patel [16] clearly indicated that
design comes from the different rheological requirements irrespective of the binder type, the static and dynamic yield
exhibited by the concrete in the different stages of the printing stress right after mixing were almost in the same range.
process. When the concrete is being delivered to the nozzle for However, the growth of the yield stress with time was dictated
printing, it needs to exhibit a combination of low to moderate by the thixotropy and hydration kinetics of the different binder
yield stress and low plastic viscosity (for ease of material delivery systems.
and attainment of the shape of the nozzle while extruding),
but when the filament is printed, it needs to be able to rapidly Figure 13 presents the suite of tests recommended from the
develop a high resistance to flow so as not to get deformed work of IIT Madras [14-16, 18] to provide a complete assessment
when the next layer is printed on it. In other words, there is a of the printability of the material. The design process for 3D
rapid change in the material in the fresh state without the actual printable concrete starts with the choice of the w/b and the
start of the setting process. If the material has a rapid setting binder content. While the w/b is primarily dependent on
behavior, then the delivery through the pump system and the the strength required, the suggested values for the same
extrusion through the nozzle could get compromised. Often, are between 0.3 and 0.4. Higher w/b may lead to excessive

56 THE INDIAN CONCRETE JOURNAL | JANUARY 2026