Doctoral Degrees (Civil Engineering)

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Browsing Doctoral Degrees (Civil Engineering) by Author "Cho, Seung"

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    Rheo-mechanics and foam stability of 3D printable foamed concrete with nanoparticle Incorporation
    (2021-03) Cho, Seung; Van Zijl, Gideon P. A. G.; Van Rooyen, Algurnon S.
    ENGLISH ABSTRACT: The imaginative concept of digital construction (DC), through the 3D concrete printing (3DCP) technique, is emerging, driven by the ceaseless efforts of academia and the dynamic collaboration with pioneering start-up companies. The endeavours towards automation in construction are accelerated by the great demand for construction time- and cost-savings in economic and sustainability aspects, as well as acquiring design flexibility. he most renowned lightweight building material, namely lightweight foamed concrete (LWFC) – consists of a concrete paste that is aerated with a natural protein-based foaming agent, is considered as energy-efficient due to its native prominent thermal and acoustic insulative properties, and reduced embodied energy of a concrete structure due to the reduced solid content and low weight. The evolution of LWFC began 50 years ago, and many applications of LWFC were developed for the building and construction (B&C) industry. From the overarching benefits of 3DCP and LWFC, the harmonisation of those techniques is expected to bring promising outcomes for the B&C industry. In this research, the mix design development of LWFC for 3D printing application (3DP-LWFC) through rheological modification without compromising foam stability is presented and investigated through various theoretical discussions and experimental programmes. As a preliminary work, normal weight 3D printable concrete mix design was developed, and rheological and mechanical properties were characterised. As a validation for the printability – a term expressing suitable pumpability, buildability and shape retention – several objects were constructed with the designed material and 3D printing technique. Rheological modification of LWFC raises concern for foam stability, since foam stability received significant attention in literature on conventional LWFC which is highly flowable, and has low shearing yield stress in the order of tens of Pascals. The effect of fresh stiffness of the base mix on foam stability is initially investigated here, and an optimal slump flow range, 185 mm – 195 mm, acquired by considering both foam stability and buildability. Based on the optimal base mix, three different LWFC mixes were designed for 3D printing application, namely 700, 1000 and 1400 kg/m3 denoted as TD-7, TD-10 and TD-14. Foam stability tests with the designed samples were conducted under static and dynamic environments. In particular, the identical 3D printer and concrete pump setup were used to investigate the actual foam stability behaviour of 3DP-LWFC. In a static environment, no foam destabilisation was observed for all samples. The post-pumped TD-10 sample showed slight densification after 3 minutes of continuous 3D printing extrusion, whereas TD-14 showed a higher degree of densification immediately. The TD-7 sample showed the most stable behaviour for both pre- and post-pumped samples, i.e. no densification was found. Void structures of three samples were investigated through X-ray computed tomography scan images, and a directly proportional relation between porosity (19.94% - 52.77% depending on the density of the sample) and foam volume fraction confirmed. The analysis also showed that pumping pressure, in most cases, created confining pressure which led to reduced void sphericity and increased smaller sized pore populations. Rheological characterisation is presented for the same LWFC density range, from tests performed with a rotary concrete rheometer. Two types of tests, i.e. a controlled shear-rate (CSR) test and a flow curve test, were performed to identify yield stress, thixotropic parameters (Rthix and Athix), elastic shear modulus, plastic and kinematic viscosity. A direct proportional relation is found between density and yield stress, thixotropic behaviour and shear modulus. With the rheological characteristics, buildability was investigated for the three densities. As result, 17 printed layers were constructed with TD-14 in continuous high-rate 3DPC before plastic collapse. A potential practical 3DP-LWFC application is demonstrated by constructing modular buoyant façade elements, i.e. with density less than 1000 kg/m3. Finally, the effect of nanoparticle inclusion in LWFC for both rheological and mechanical behaviour is studied. The yield stress of nanoparticle infused LWFC is increased by roughly two orders up to 1170 Pa, compared to conventional LWFC. Nanoparticle inclusion also introduced significant improvement in mechanical strength – compressive strength 31.1 MPa, flexural strength 3.8 MPa and elastic modulus 13.8 GPa at 28 days– compared to the conventional LWFC – compressive strength 11.6 MPa, flexural strength 1.4 MPa and elastic modulus 7.1 GPa at 28 days. This research unlocks structural and innovative application of LWFC, and introduces the engineered material to the B&C industry so that they can bring innovative ideas and new thinking regarding structural and architectural design.