Page 10 - Open access sep 2020

TECHNICAL PAPER

Cement grain d

D

h CHS

d
Cement grain

Figure 7: CSH bridge between cement grains adapted from Roussel Figure 8: Yield stress growth with time - Perrot model (exponential) and
et al. (2012) Roussel model (linear) adapted from Perrot et al. (2015)

exponential by the Perrot model (Perrot et al., 2015), as shown in al. (2018a) developed a model based on the Mohr-Coulomb
Figure 8. failure criterion with linear stress-strain behaviour to analyze
the mechanical behaviour from 0 to 90 minutes after extrusion.
Some attempts to understand the development of the structural
build-up of cementitious matrices are presented here. Structural The model parameters like Young’s modulus, Poisson’s ratio,
build-up due to change in rheological characteristics and cohesion, and friction angle were obtained by unconfined
hydration is critical for buildability. Further, this section also uniaxial compression tests, and direct shear tests.
explores research efforts to capture early age mechanical An analytical model was developed by Suiker (2018). It is
response, which affects the buildability directly.
reported to have a relative error of 10-13% for free wall and
Ultrasonic pulse velocity test with compressional (P) and shear rectangular wall segments from experimental observations.
(S) waves were conducted to gain information on microstructural The critical length ratio of elastic buckling and plastic collapse
changes at the time of setting as well as hydration evolution. failure depends on the stiffness ratios, which again depend
The S waves provide information on the evolution of interparticle on the curing rate. Wolfs et al. (2019a) used the model
bonding during the hydration process (Voigt et al., 2006). Nerella presented by Suiker (2018) and found that for lower curing
et al. (2019a) presented a strain-based approach by keeping a rate, both linear and quadratic curing function gave similar
constant strain rate for measuring structural build-up at different results for experiments, whereas, for higher curing rate, the
shear rates, and the performance at the paste level was studied. quadratic function provided a better result than a linear one.
Replacement with SCMs showed the highest structuration rate
due to an increase in flocculation. Practice-oriented buildability Further development was done by Wolfs et al. (2019b); a
criteria were presented considering time interval, printing width, numerical model was developed based on structural properties
and cost of printing for 3D concrete printable concrete by characterized by triaxial compression test as this test can provide
Nerella et al. (2018). all details to assess the failure mechanisms already discussed.

The rheology can further be altered using external agents. It Triaxial test of uncompacted specimens provides a good
is concluded in Reiter et al. (2018) that the structural build- representation of the development of yield stress during the
up of concrete can be controlled chemically by on-demand actual printing process. The yield stress growth is seen to be
set accelerators (set on demand), thickeners, and continuous exponential (Figure 9) (Panda et al., 2019), for the mix which had
preparation and pumping of concrete. The thickeners increase w/c of 0.3, 2% silica fume, 60% fly ash, 38% OPC and 3% Sodium
the ratio of instantaneous yield stress to apparent viscosity or Sulphate (to gain early strength). Recently, a thermodynamic-
in other words, increase the thixotropy. Stability gain was also based approach was used to develop a complete 3-dimensional
observed upon heat application on fresh concrete (Kazemian et constitutive model for cement-based material that can capture
al., 2017).
the thixotropic structural build-up (Rahul et al., 2020c). Such
For early age strength development of 3D printed concrete, models can also be promising in performing simulation studies
several numerical models have been developed. Wolfs et of failure during printing.

14 THE INDIAN CONCRETE JOURNAL | SEPTEMBER 2020