Page 11 - Open access sep 2020

TECHNICAL PAPER

Figure 9: Static yield stress growth with time (left); compressive load vs deflection after different rest durations (right) adapted from Panda et al. (2019)

3.3 Assessment of layer bonding and of the mechanical behaviour of layered concrete. The three-
point bending test on prism specimens printed along different
mechanical properties of printed concrete
directional axes showed the variation due to layer orientation
In the hardened state, one of the critical parameters is (Shakor et al., 2019).
interlayer bonding, and a decrease in strength of the structure
is caused by improper interlayer bonding. Wolfs et al. (2019a) The mechanical properties for 3D printed foam concrete
experimentally studied the relationship between the process or fire performance of fibre reinforced concrete have also
parameters such as interlayer interval time, nozzle height, been studied. Weng et al. (2019a) evaluated the mechanical
and surface dehydration, and interlayer bond strength. The properties, and fire performance on a 3D printable fibre
mechanical properties like compressive strength and flexural reinforced cementitious composite. The flexural and
strength were studied along with all the three perpendicular compressive strength decreased at elevated temperatures.
directions. The influence of drying of the printed layer and The decrease in flexural strength is four times, and that of
interlayer interval time on the decrease of bond strength was compressive strength is two times when the temperature is
0
discussed. Le et al. (2012b) studied the hardened properties, increased from 25 to 800 C. Markin et al. (2019) created a 3D
and fresh properties for high-performance concrete with printable foam concrete as already discussed in the previous
strength tested parallel and perpendicular to layer orientation, section. The compressive strength at 28 days for printable foam
and bond strength for the different time gap between printing. concrete is reported to be 10.4 MPa for a dry density of 980
It was reported in this study that anisotropy in the layered kg/m3, which is similar to normally casted foam concrete.
structure led to a reduction in compressive strength. The voids The layer adhesion profoundly impacts the mechanical strength
created between layers due to the printing process reduced in the hardened state. The layer interfaces are the weakest
adhesion between layers leading to a reduction in strength. points in the structure. The crushing of bottom layers can
occur due to a dead load of subsequent printed layers. The
Marchment et al. (2019) explored a method to enhance the bond compressive strength of the material should be high enough to
strength by effective bond area amplification. The interlayer sustain the load, and light-weight concrete has a greater chance
bond can be strengthened by minimizing the voids at the for crushing of layers due to low compressive strength. The
interface using cement paste. The quantitative evaluation of the proper placing of layers also plays an essential part in hardened
effective bond area using image processing and experimental strength as eccentricities can develop, leading to failure of
and analytical methods is used to show an increase in bond structures. The discussions in this section indicate the significant
strength in proportion to the bond area. Rheological parameters potential for research in this area.
are found to control the weak bond formation in the case of
printed layers, but in the case of smooth interfaces, there could 3.4 Applications and demonstration projects
be a reduction in strength because of the evaporation of water with 3D printed concrete
from the free surface due to large printing time interval (Keita
et al., 2019). This water loss is localized, and cement hydration The projects executed by industry, primarily for demonstration
is incomplete at these dry local areas, which also show high purposes, are summarized in Table 2. The major companies
local porosity. Anisotropy is another important characteristic working in 3D concrete printing are mentioned along with their

THE INDIAN CONCRETE JOURNAL | SEPTEMBER 2020 15