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The Faculty of Engineering at Stellenbosch University is one of South Africa's major producers of top quality engineers. Established in 1944, it currently has five Engineering Departments.

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Browsing Faculty of Engineering by Subject "3D concrete printing"

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    Rheo-mechanical, hardened mechanical characterisation, and tensile creep of limestone calcined clay cement fibre-reinforced printed concrete (LC3-FRPC)
    (Stellenbosch : Stellenbosch University, 2023-10) Ibrahim, Kamoru Ademola; Babafemi, Adewumi John; Van Zijl, Gideon P. A. G.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering
    ENGLISH ABSTRACT: Concrete is a widely used and acceptable civil engineering construction material. It contributes significantly to infrastructural development and global economic growth. However, its major challenge is its vulnerability to environmental degradation, pollution, carbon emissions, and cracking, which have detrimental influence on the sustainability of its applications under high demand, low recycling rates, and loading. The scarcity of raw materials, caused by wastage, overuse, and environmental issues, threatens infrastructural development. Hence, research on an emerging technology named 3D printed concrete (3DPC) to reduce waste, but also time and cost associated with the construction of concrete infrastructure is imperative. 3DPC is an evolving construction method and has been proposed as an alternative and environmental-friendly construction method to traditional construction method. The high cement content required for the 3D extrusion process is being reduced by partial replacement with supplementary cementitious materials (SCMs) such as fly ash, slag, and silica fume. However, these SCMs are limited in volume and do not have global spread, implying an urgent need for alternatives, but also care for the longevity of infrastructure. Limestone calcined clay cement (LC3) is a suitable SCM for sustainable concrete because of its global availability and comparable early and later strength gain, as recommended by previous studies. When polypropylene fibres are added to the mix, control of plastic shrinkage cracks, increased toughness, and reduced brittleness of 3DPC can be achieved. The main goal of this study is to investigate the rheology, hardened mechanical properties, and tensile creep of fibre-reinforced 3DPC containing LC3 (LC3-FRPC) by quantifying its layer deformation, but also the orthogonal interlayer bond deformation at different stress levels under sustained loadings. To achieve the goal stated above, this research develops the material mix design satisfying 3DPC requirements for early strength and stiffness, shrinkage cracking resistance and mechanical properties, including interfacial bond. The rheology and hardened mechanical properties of LC3-FRPC were compared to that of the fly ash-based counterpart (FA-FRPC). The strategy for strengthening the interfacial bond of LC3-FRPC with effective microorganisms (EM), which enhances not only the bond strength but also improves mechanical capacities are presented. The mechanical responses were verified by microstructural analysis through scanning electron microscopy augmented by energy-dispersive X-ray spectroscopy and X-ray computed tomography to assess the hydration products of the blended binders and the self-healing action of LC3 and EM in FRPC. Then, the creep and shrinkage deformations in two orthogonal directions, and other parameters associated with creep responses, including creep fracture are also conducted experimentally on LC3-FRPC. The creep specimens were subjected to sustained stresses of 40, 60, and 80% of the direct tensile strength, and 40% of the flexural strength results obtained from the quasi-static tests. The results revealed that FRPC mixtures tested showed good rheological properties, with LC3-FRPC showing improved the workability, open time, and buildability by 1.7%, 15.4%, and 19%, respectively, compared to FA-FRPC. FA-FRPC outperformed LC3-FRPC in compression, tension, and flexure because of its lower water demand, but the bond strength between the interfacial layers is higher in LC3-FRPC than in FA-FRPC, with an increase of 8.1% for tension and 9.8% for flexure. EM-enhanced LC3-FRPC had significantly higher bond strengths than the reference LC3-FRPC in both direct tension (26.1%) and flexure (33.7%), thereby implying a lower level of anisotropy. The effects of the binders, particularly the LC3 and the EM on the strengths of FRPC and the macropores at the interfaces of printed concrete, improved the material by forming more calcite crystals. Finally, for the creep response under sustained loadings, none of the LC3-FRPC specimens fractured in tension and flexure. Instead, higher direct tensile and flexural strengths were recorded for the creep specimens after 225 days loaded at different stress levels.
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    Rheo-mechanics, durability and microstructural characterisation of slag-nodified metakaolin-based geopolymer concrete for extrusion-based 3D printing applications
    (Stellenbosch : Stellenbosch University, 2023-11) Jaji, Mustapha Bamidele; Babafemi, Adewumi John; Van Zijl, Gideon P. A. G.; Stellenbosch University. Faculty of Engineering. Dept. of Civil Engineering
    ENGLISH ABSTRACT: Extrusion-based 3D-printed geopolymer concrete (3DPGPC) is a potential alternative to Portland cement concrete (PCC). Research is sparse on the use of metakaolin (MK) for extrusion-based 3D concrete printing applications. The widespread adoption of 3DPGPC is limited due to the unknown durability properties and the long setting time of a two-part geopolymer system. To address the long setting time, this study modified MK-based 3DPGPC with slag up to 30% for 3D printing due to its high Ca2+ ion content. The printable mixture developed comprises M1 (100% MK-3DPGPC) and M2 (95% MK and 5% slag), beyond 5% slag inclusion; the mixtures stiffened with inadequate open time for printing. To address the stiffening, sodium phosphate is incorporated to achieve sufficient open time for constructability of the new mixtures and to improve structural build-up in the mixtures containing slag, M-S10 (90% MK and 10% slag), M-S20 (80% MK and 20% slag) and M-S30 (70% MK and 30% slag), while the mixture without slag, M-S0 (100% MK), is the control. The slump obtained using a mini-slump cone is in the range of 3–5.5 mm and the slump flow using a slump flow table is between 148–157 mm. The setting time using the Vicat apparatus depicts an open time of 6.8 hours for the control (M-S0), and 1.2–1.3 hours for slag-modified mixtures. Rheology tests using an ICAR rheometer reveal that the initial static yield shear stress (𝜏𝑠,𝑖 ) increased from 1898–1900 Pa and initial dynamic yield shear stress (𝜏𝐷,𝑖 ) evolve from 1452–1482 Pa due to 5% slag inclusion. Also, re-floccution (Rthix) and structuration (Athix) rates improved from 5.16 and 0.2 Pa/s to 5.2 and 0.4 Pa/s, respectively. After 28 days of curing age, 70 mm × 140 mm cored cylindrical-3DPGPC specimens exhibited compressive strength of 23.7–33.13 MPa and splitting tensile strength of 1.79–2.43 MPa. Saw-cut 40 mm × 40 mm × 160 mm beam specimens attained flexural strength of 5.48– 7.29 MPa and an interlayer bond strength of 5.40–6.90 MPa. The durability of 3DPGPC is investigated using the water absorption test, capillary and gel porosity test, oxygen permeability index (OPI), and drying shrinkage tests. After 90 days of curing, the drying shrinkages in the vertical direction are 2.98 and 2.86% for the control specimen (M1) and the slag-modified specimen (M2), respectively. In the horizontal direction, the drying shrinkages are 1.14 and 1.1%, respectively. The vertical strain obtained during drying includes plastic shrinkage, drying shrinkage, and vertical creep due to the sustained weight of the upper layers in the fresh state. Drying shrinkage varied along and across the layers of 3DPGPC, depicting anisotropic behaviour. After 90 days of curing, water absorption decreases to 7.33% and 5.2% in M1 and M2 specimens, respectively. The total porosity of 3DPGPC decreases from 20.5–14.5% after 90 days of curing, while mould cast decreasesfrom 15 to 10% in M1 specimens. Slag inclusion further reduce the porosity of 3DPGPC, and mould cast from 17–10.9% and 11.5–8%, respectively. After 90 days of curing, 3DPGPC specimens cored perpendicular to the printing direction (vertical) exhibits (OPI) of 11.07–11.86 kPa, and specimens cored perpendicular to the printing direction (horizontal) exhibits OPI in the range of 10.99–11.74 kPa, while mould cast specimens exhibit OPI of 11.23–11.92 kPa. CT-scan shows that mould-cast specimens have a total porosity of 4.07% and exhibit spherical pores, while 3DPGPC have a total porosity of 1.81% and exhibit elongated pores due to pumping. CT-scan also reveals that porosity is position-dependent in 3DPGPC due to the presence of voids between 0.1–1.7 mm at the interlayer, whereas mould-cast specimens exhibit randomly distributed voids in the range of 0.1–2.5 mm in diameter. Backscattered electron images show increasing C-S-H, N-A-S-H and C-A-S-H gel formation due to the presence of alumina, silica, sodium in MK and high Ca2+ ion as slag content increases. The BrunauerEmmett-Teller (BET) surface area increases with an increase in slag content from 5–23 m2 /g, resulting in the densified 3DPGPC matrix, thereby improving buildability from 27 layers to 42 layers and enhancing mechanical performance. Nitrogen physisorption test shows that the adsorption and desorption isotherms and the hysteresis loops are within the IUPAC Class IV and H3 types, indicating the presence of mesopores (2–50 nm) and macropores (>50 nm). This research demonstrates that MK-based 3DPGPC can be successfully 3D printed and modified with slag to improve the fresh properties, rheology, mechanical properties, microstructural morphology, pore characteristics, and long-term durability performance. It also reveals that 3DPGPC exhibits anisotropy in orthogonal directions. The results obtained from this study are recommended for numerical modelling strategies.