Page 12 - ICJ Jan 2026

TECHNICAL PAPER

Figure 12: Static yield stress variation with time for different binder
systems (LC = Limestone – calcined clay combination as a replacement
[16]
of cement, SL = slag, FA = fly ash)
flowability, which can only be corrected by a suitable accelerator
Figure 13: Suite of tests for establishing printability – Vane shear test to
added at the nozzle, while lower w/b may make the mixtures establish the static yield stress, Gravity driven slug test for dynamic yield
difficult to print (however, this can be done with appropriate stress, Desorptivity test for assessing the ability of mix to withstand the
pressure, Compressive rheology test to evaluate the buildability and flow
choice of SP). The aggregate to binder ratio is generally chosen
table test to assess the open time
between 1.2 and 2.0, with the maximum size of aggregate
typically being restricted to 2.36 mm (however, several studies
6. SUMMARY
also attempt up to 5 mm size or more). The suite of tests can
then be applied to determine the printability of the mixture. This paper showed how the choice of a systematic approach
While the vane shear test, as indicated above, provides a good addressing the key aspects of particle packing and rheology
estimate of the rheology required for the initial extrusion, the was able to help in the mixture design of special concretes. The
flow table test is a simple indicator of the retention of the approach adopted in the projects undertaken by IIT Madras
workability of the 3D printable mixture. This is important from showed that simple test methods and analysis techniques
the view of establishing the ‘open time’ for the mixture, i.e. could be utilized for the material selection process, which is
the time during which the mixture can be workable enough often very complex as in the case of rheological assessment,
for printing. The desorptivity coefficient is an indicator of the or experiment-intensive as in the case of determination of
ability of the mixture to avoid phase separation under pressure, aggregate and binder combinations by trial and error. While the
while the gravity-driven slug test provides an estimate of the necessity for trial mixtures in each of the instances cannot be
yield stress in a dynamic state. Finally, in order to understand overcome, the adoption of the systematic approach leads to a
the buildability, i.e. the ability of the layers to retain their shape reduction in the trials required to attain the desired properties.
when subsequent layers are printed on top, the compressive
[18]
rheology test is critical. Using the compressive rheology test , The lessons learnt from the studies can be summarized as
Shantanu et al. [19, 20] were able to demonstrate the ability follows:
of limestone-calcined clay combinations to take on higher • The design of special concretes needs specific attention to
aggregate to binder ratios for 3D printing as compared to fly ash the aggregate gradation and paste rheology, in addition
based mixtures. Further, the complete suite of tests was applied to the conventional requirement of w/b and binder type/
alongside a computer vision technique by Haripan et al. to content.
[21]
assess the printability of mixtures incorporating recycled fine
aggregate. The methodology developed through the work at • Optimal packing can reduce the trials required to achieve
IIT Madras is robust enough to be applied to any type of 3D the required mechanical and durability properties of HPC,
printable mixture. while also reducing the consumption of cement.

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