| dc.description.abstract | To achieve the 2050 net-zero carbon emission target, the civil engineering sector must gradually reduce its carbon emissions. As a highly intelligent ＂green construction method,＂ 3D-printed concrete can reduce greenhouse gas emissions during the construction process. However, to meet rheological performance requirements, current printable concrete mix designs often conflict with net-zero emission principles.
This study aims to develop ＂fiber-reinforced 3D-printed sustainable concrete (3DFPC)＂ tailored for 3D printing and to conduct a comprehensive life cycle assessment (LCA). Through 3D printing tests, the effects of various sustainable materials on the performance of 3D-printed concrete are explored, a recommended mix design for 3DFPC is proposed, and application recommendations are provided based on the LCA results.
The study first evaluates the fresh and hardened properties of 3D-printed concrete matrices using nine sustainable materials (including natural minerals, industrial by-products, and agricultural residues) and various chemical admixtures. It is found that silica fume (SF), fly ash (FA), and ground granulated blast furnace slag (BFS) exhibit significant potential when used to replace a high proportion of Portland cement (PC). Additionally, superplasticizer D (SPD) combined with a composite viscosity-modifying admixture (CVMA) is identified as the optimal chemical admixture combination for maintaining fresh properties.
Further testing reveals that during the fresh state, shape retention is more effective than flowability in assessing the printability of concrete materials. A minimum shape retention of 87% is recommended to ensure high printability potential. In the hardening stage, among the influences of aggregate parameters, the performance of manufactured sand (MS) can be slightly better than quartz sand (QS) due to its particle shape advantage. An increase in the sand-to-binder ratio can effectively improve the drying shrinkage of the material, but raising it to 1.5 has a significant negative impact on both the compressive and flexural strength of 3D-printed concrete. Among binders, SF achieves higher strength development in binary systems due to its particle size and pozzolanic activity but leads to significant drying shrinkage at high replacement levels. FA and BFS result in strength reduction at high replacement levels, which is mitigated in ternary systems. Overall, using MS with an A/B ratio of 1.25 and replacing 50% of PC volume with a binary or ternary binder system balances mechanical performance and sustainability.
The study also introduces fiber reinforcement techniques to test the applicability of various fibers (e.g., polyoxymethylene fibers (POMF), polypropylene fibers (PPF), carbon fibers (CF), basalt fibers (BF), and calcium sulfate whiskers (CSW)). The impact of fiber length and dosage on printability and hardened properties is analyzed. Results indicate that BF causes nozzle blockage in the fresh state and is unsuitable for 3D printing. POMF and PPF enhance toughness with increased dosage but reduce strength. Conversely, CF and CSW improve strength but do not enhance toughness. Through optimization, a combination of 0.5% PPF for toughness improvement and 1.0% CSW for strength enhancement is identified, achieving a balance of high strength and toughness.
Based on these findings, the recommended mix design for 3DFPC includes MS with an A/B ratio of 1.25 for aggregate design, a ternary binder system with SF and FA replacing 50% of PC volume, and the aforementioned composite fiber reinforcement strategy. This design achieves compressive and flexural strengths of up to 88.9 MPa and 13.6 MPa, respectively, with a toughness modulus in the Y-axis direction of 6,174 J for 3D printing. Compared to other 3D-printed concrete materials, the compressive and flexural strengths show average improvements of approximately 50.4% and 38.1%, respectively. Strength loss in 3D-printed structures is attributed primarily to increased macroporosity, as well as limitations of printing equipment and layer size variations. The anisotropy of mechanical properties is influenced by the number and type of interlayer interfaces, with compressive strength higher in the loading direction parallel to the printing direction, while flexural strength is higher in the perpendicular direction. The unique fiber alignment exacerbates flexural strength anisotropy. Regarding the compression-to-bending ratio, 3DPC shows higher performance in the parallel loading direction, whereas no significant difference is observed in 3DFPC.
From an environmental perspective, the study employs carbon emission quantification and LCA to examine the environmental impact of 3DFPC in raw material usage and construction processes. Results calculated using the carbon coefficient method indicate that the proposed 3DFPC mix design reduces raw material carbon emissions by approximately 40% compared to conventional 3D-printed concrete. For constructing equivalent functional components, this material combined with 3D printing reduces carbon emissions by approximately 6.6% compared to traditional casting methods. LCA results show that adopting the proposed 3DFPC mix design in 3D printing achieves carbon emission reductions of approximately 22%, 41%, and 11% during material production, transportation, and construction stages, respectively, compared to traditional casting methods. The overall reduction in emissions is about 26%, with further potential for optimization through structural design.
In conclusion, the proposed 3DFPC integrates mechanical performance and environmental benefits, providing theoretical support for promoting 3D printing technology in civil engineering and offering an effective solution for achieving the 2050 net-zero carbon emission target. | en_US |