Page 19 - Open-Access-Sep-2019

TECHNICAL PAPER

In general, curing in tap water achieves low compressive however, the opposite is the case for conventional cast
strength test results, predominantly due to the slight reaction specimens. This phenomenon could be due to better flowability
that occurs among particles. The low compressive strength from the feeder tank and better compaction on the build-
results occurred when the printed specimens were placed chamber for the bigger specimens. The feeder tank contains
directly in water, resulting in most of the unreacted particles a rotor motor which is in the middle of the printer. Therefore,
becoming detached from the printed part and weak bonds the powders for the bigger specimens deposit from the middle
breaking, as found in the study by . The small concentration of the feeder tank with greater homogeneity and fewer voids
[9]
of OH- ions in tap water works as a reactive chemical agent in between particles. This results in a better deposition of powder
the cementitious process since it leads to ineffective dissolution and more powder flowing from the feeder. When the powders
and formation of hydroxyl substances [27] [28] . Consequently, low have more particles in the feeder chamber, the result is that the
compressive strength will occur due to the densify reaction not roller spreads more powder on the build-chamber. The more
being established appropriately. In addition, it must be noted particle powders on the build-chamber allow them to be more
that water curing at very high temperatures adversely affects effectively compacted.
compressive strength, due to heat acceleration, which leads
to the leaching of Al 2O 3 and SiO 2 from the existing gel in the The previous work of Feng, Meng, Chen and Ye [16] confirmed
specimens. that the gypsum 3D printed parts are orthotropically
characterised. This is consistent with cement mortar 3D printed
Cube specimens for the compressive test were printed and parts . Figure 8 presents the optimum compressive stress-
[9]
tested in all three planes. This study found that compressive strain relationship for three selected specimens in each plane
strength is predictably influenced by the printed plane of the and the selected manual conventional mix of mortar. The tests
specimen. The YZ printed plane exhibited noticeably lower were performed by attaching the strain gauge to the printed
compressive strength results while, for the XY and XZ planes, the specimens, then recording the strain of the printed specimens
results were generally similar for the 20 mm cubes (Figure 7a) until they reached the failure point. During the compression test,
and strength in the XY plane was overwhelmingly better in the it was observed that the stress-strain of the specimens’ response
50 mm cubes (Figure 7b) [11] . faced a collision at the starting stage when the load was applied
on the specimens. This response of the stress-strain is consistent
These results are positive for the construction industry and with the studies of [29] and [30] .
precast construction applications. This study utilised optimal
saturation levels that were presented in the authors’ earlier Figure 8 shows that the conventional mix has the lowest
studies [9] [10] [11] . They identify the strongest plane and direction in compressive stress of 14.34 MPa. It apparently shows that the
addition to the optimised elevated temperature, which are most strain in the conventional mix has a larger value (0.00322 mm/
suitable for medium curing to gain the highest compressive mm) compared to the printed specimens. This could be due
strength.

a) 20×20×20 mm cubes (b) 50×50×50 mm cubes

Figure 7: Compressive strength of mortar specimens using different
curing media for cubes

The results presented in Figure 7 demonstrate that size Figure 8: Stress-strain relationship of mortar specimens of 50×50×50 mm
influences the printed specimens. For printed specimens, an at temperature (80°C) in the three planes (XY, XZ, YZ) and for a manually
increase in their size also increases their mechanical strength; prepared mortar mix.

70 The IndIan ConCreTe Journal | SepTember 2019