@phdthesis{01GTE5YZBYMDQ21DM3E6JCGFH8,
  abstract     = {{Concrete is intrinsically one of the greenest construction materials and is widely
used in a myriad of applications. Also, concrete is one of the most economical,
robust, easy-to-use and widely available materials around the world. It is estimated
that the annual consumption of concrete is almost 25 gigatons. Even though the
concrete itself is greener compared to other alternative construction materials, 8 %
of the total global CO2 emission is caused by the cement and concrete industry.
This is because of the tremendous use of concrete around the world. Therefore,
one of the ways to reduce CO2 emissions is to minimize concrete consumption. In
this regard, extrusion-based concrete 3D printing is a promising technology and
has the potential to be a sustainable construction solution by utilizing structural
optimization and reducing material usage. However, concrete mixtures used for
3D printing processes require a higher amount of binder and chemical admixtures
to meet the stringent rheological requirements. Therefore, one of the main
objectives of this thesis is to develop 3D printable concrete mixtures with lower
environmental impact. This thesis outlines different strategies to improve the
sustainability of 3D printable concrete mixtures.
Sustainability can be improved by lowering the binder content and
increasing the aggregate content, as the binder phase is the highest CO2-intensive
phase. Also, the use of natural and recycled coarse aggregate in the granular
skeleton can reduce the environmental impact. A set of 3D printable mixtures were
developed with increasing aggregate content and changing the granular skeleton
by incorporating natural and recycled coarse aggregates. However, such changes
in the granular skeleton can influence the fresh and hardened state performance of
the 3D printable mixtures. The increasing aggregate content resulted in increased
plastic viscosity, yield stress, pumping pressure and also improved buildability.
On the other hand, the incorporation of coarse aggregates and changing the
granular skeleton reduced these parameters due to the lower specific surface area
and better packing as the paste film thickness increased.
Another strategy explored through the thesis is the use of a low CO2
alternative binder system for Portland cement. In this regard, 3D printable
mixtures with calcium sulfoaluminate (CSA) cement were designed. Two major
challenges for the mixtures with CSA cement were the extremely fast hydration
and significantly high plastic viscosity compared to Portland cement. Using a
suitable retarder, the rapid hydration was controlled, and the plastic viscosity was
lowered by replacing part of the CSA cement with limestone powder. Though the
use of a retarder was beneficial in providing the sufficient open time needed for
the pumping phase, buildability was adversely affected due to the suppression of
hydration. Using a two-stage mixing strategy, i.e., the hydration of a retarded CSA
cement mixture with borax was re-initiated at the nozzle/print head with a static
mixer, and the conflicting requirements for pumping and stiffening control were
eliminated.
To understand the extent of sustainability improvement by the abovementioned
strategies, a detailed quantitative assessment of the environmental and
economic impact of the 3D printable mixtures developed in this study for the
production of one cubic meter volume was performed. 3D printable concrete
mixtures satisfying a set of performance criteria were evaluated. Increasing the
aggregate content decreases the environmental impact. However, incorporating
natural and recycled coarse aggregates does not significantly reduce the
environmental impact at a lower replacement level. The study provides insights
into the environmental and economic impact of extrusion-based 3D concrete
printing materials satisfying the same functional requirements. It was observed
that the calcium sulfoaluminate-limestone binder systems have significantly lower
global warming potential. However, the depletion of fossil resources indicator is
much higher than the Portland cement-based mixtures. A significant portion of the
material cost (even up to half of the cost) is contributed by the chemical admixtures
in the 3D printable concrete mixtures, while the cost contribution by the chemical
admixtures was much lower in the case of conventional mould-cast concrete
mixtures.
The thesis also focuses on the performance assessment of 3D printable
concrete mixtures in fresh and hardened states. To assess the drying shrinkage a
novel rheometer-based shrinkage measurement technique was developed. The
method enables the measurement of shrinkage strains from the onset of
preparation of the mixtures. It was found that the incorporation of about 3 wt%
shrinkage-reducing agents significantly reduces the early-age shrinkage strain.
The effect of early-age drying-induced pore structure changes was investigated
using mercury intrusion porosimetry (MIP), loss on ignition and electrical
resistivity measurements. When subjected to early-age drying, the porosity of the
samples was found to increase significantly, and the assessment of chemically
bound water revealed that the degree of hydration was not uniform but varied as a
gradient, with the minimum occurring at the drying surface. Also, other shrinkage
mitigating strategies were explored, which include the use of polypropylene fibers
and the use of coarse aggregates. The effect of polypropylene fibers, shrinkagereducing
agent and incorporation of natural or recycled coarse aggregates on the
shrinkage cracking potential was assessed by restrained ring tests. The mixtures
with the shrinkage-reducing agent and coarse aggregates prolong the time for
cracking significantly due to the reduced shrinkage strain, while the addition of
fibers increases the time to cracking marginally by enhancing the tensile strength
of the mixtures.
The study also focused on the porosity and pore structure of 3D-printed
concrete elements, as the durability of the concrete is closely linked to its pore
structure. The porosity characterisation was done with MIP and X-ray computed
micro-tomography (X-ray μCT). Using the MIP data, surface fractal dimension
and tortuosity parameters were calculated. It was observed that the CSA-limestone
blended system has higher pore complexity and tortuosity than the Portland
cement-slag blended mixture. The study revealed that the interlayer region
contains larger and interconnected pores with low tortuosity, which could lead to
enhanced transport of ions. Significantly higher open porosity and the presence of
elongated pores with a high aspect ratio were observed in the interlayer compared
to the bulk region. Also, the durability performance of the 3D printable mixtures
was assessed by measuring the resistance against chloride migration and salt
scaling. It was observed that the chloride migration coefficients were higher due
to interconnected pores at the interlayer region and the application of fresh cement
paste between the layers enhances the resistance to chloride migration. 3D-printed
concrete elements showed much higher resistance to salt scaling as compared to
mould-cast concrete due to the compensation of the glue spall stress by suction
created from the ice formation at the interlayers.
The practical feasibility of one of the most promising 3D printable
mixtures developed was demonstrated by 3D printing a 1:4 scale breakwater
element (8 kN weight). From the static mechanical tests, it was observed that the
peak load at failure of the 3D printed breakwater element was lower than that of a
mould-cast breakwater element with the same shape and dimensions. Also, the
hydraulic stability of the 3D printed breakwater units was assessed and compared
with a practically used breakwater element (ACCROBERMTM II) with wave flume
tests at a 1:45 scale. The hydraulic performance of 3D printed breakwater elements
was similar to that of the ACCROBERMTM II.}},
  author       = {{K Mohan, Manu}},
  isbn         = {{9789463556866}},
  language     = {{eng}},
  pages        = {{XVIII, 300}},
  publisher    = {{Ghent University. Faculty of Engineering and Architecture}},
  school       = {{Ghent University}},
  title        = {{Towards sustainable concrete 3D printing for marine structural applications}},
  year         = {{2023}},
}