Impact of Double Hooked Steel-Fibers and Nano-Kaolin-Clay on Fresh Properties of 3D Printable Ultra-High-Performance Fiber-Reinforced Concrete (2024-10)¶

Due to their enhanced engineering properties, ultra-high-performance fiber-reinforced concrete (UHPFRC) mixtures are increasingly used in advanced construction techniques, including 3D concrete printing (3DCP). However, balancing workability, extrudability, buildability, and structural integrity remains challenging. This paper addresses this by investigating the fresh and hardened properties of UHPFRC mixtures containing (0.25 % and 0.5 %) nano-kaolin clay (NKC) and (1.5 % and 3 %) double-hooked end steel fibers (DHE-SFs). Rheological tests were conducted, including flowability, viscosity, yield stress, pumpability, and buildability trials at print speeds of 30, 60, and 120 mm/s. Results indicate that mixtures with higher NKC and DHE-SFs content, such as M9-SF3-NKC0.5, show significant improvements. Flowability tests reveal that while the control mixture M1-SF0-NKC0 shows a 32.5 % increase, M9-SF3-NKC0.5 shows a 24.1 % increase, indicating enhanced cohesion. Viscosity tests demonstrate shear-thinning behavior, with M9-SF3-NKC0.5 showing a 145.7 % increase, indicating better structural integrity. Yield stress tests reveal M9-SF3-NKC0.5 achieves a maximum initial yield stress of 0.405 kPa. Pumpability trials confirm all mixtures are pumpable at 30 mm/s, but only those with higher NKC and DHE-SFs maintain pumpability at 120 mm/s. Buildability trials show that M9-SF3-NKC0.5 maintains structural integrity without collapse at all speeds. Hardened property tests reveal compressive strengths increase from 110 MPa in the control to 150 MPa in M9-SF3-NKC0.5, while tensile strengths increase from 8 MPa to 12 MPa. These findings highlight the dual benefit of NKC and DHE-SFs in enhancing UHPFRC’s fresh workability and long-term strength, which is crucial for reliable 3DCP applications. The study concludes that optimizing NKC and DHE-SFs content in UHPC mixtures significantly improves the performance of 3D-printed concrete, ensuring enhanced durability and structural integrity, thus providing a firm foundation for future advancements in 3DCP technology.