This study developed a novel ultra-high ductile concrete (UHDC) for 3D concrete printing, which was modified using crumb rubber to possess high ductility. Flowability tests were conducted to determine optimal open time range for continuous printing. A series of mechanical tests, including uniaxial tensile test, compressive test, flexural test and double shear test, were carried out to investigate the anisotropic-mechanical properties of the printed UHDC. The results indicate that the tensile strength, flexural strength and shear strength of the printed specimens were slightly lower than those of mold-cast specimens, while reverse tendency was observed in compressive strength. It is of interest that the deformability and energy dissipation of the printed UHDC at some directions are higher than those of mold-cast UHDC. Additionally, it is found that the printed UHDC performed minimal anisotropy in flexural strength, but significant anisotropy in flexural deformability, compressive and flexural energy dissipation capacity.

Fresh and anisotropic-mechanical properties of 3D printable ultra-high ductile concrete with crumb rubber

Recent trends in ultra-high performance concrete (UHPC): Current status, challenges, and future prospects

Fibers are essential in reinforcing the mechanical properties of ultra-high performance concrete (UHPC), particularly the tensile and flexural strength. This paper conducts a systematic review on the effect of fibers with different textures and geometry on the properties of UHPC from the following aspects: (1) the bond mechanism of steel fibers with the UHPC matrix; (2) the effect of fiber shape, fiber orientation and hybridization of steel fibers on the microstructure, failure mode, mechanical properties, autogenous shrinkage and durability of UHPC; (3) the reinforcing mechanism of synthetic fibers (polyvinyl alcohol fibers (PVA), polypropylene fiber (PP), polyethylene fibers (PE)), mineral fibers (basalt fibers, wollastonite fibers) and carbon fibers in UHPC and their effect on the performance of UHPC; (4) the use of hybrid fibers and their synergetic effect on the mechanical performance and shrinkage of UHPC. Finally, this paper discussed the further research trends of fibers in UHPC. 

Utilization of fibers in ultra-high performance concrete: A review

The unparalleled attributes of ultra-high-performance concrete (UHPC) reported over the last three decades sum up the development of this material in two phases: UHPC and ultra-high-performance fiber-reinforced concrete (UHP-FRC)/ultra-high-performance hybrid fiber-reinforced concrete (UHP-HFRC). The integration of scientific knowledge pertaining to material interactions and technical observations of developed UHPC has laid the foundation for further research and development. Microstructural research has revealed the contribution of finer sizes mineral admixtures along with lower water/binder ratios in the range of 0.16–0.24 to the development of high-density calcium silicate hydrate with a very low possibility of ettringite formation and alkali silica reactions in UHPC matrix. The addition of fibers is challenging when seeking to maintain the rheology of UHPC; therefore, technically sound concrete experts are required for the field applications. The challenges preventing the widespread use of UHPC are the higher initial cost, the requirement of special skilled labor for execution, and the lack of open discussions about various UHPC standards available worldwide to reach an agreement on minimum strength achievements and test standards. The hurdles hindering the use of UHPC in recent emerging 3D printing technology are also discussed in the present study. The vast literature compilation also reveals that less research has concentrated on UHP-HFRC as compared to UHPC and UHP-FRC. Therefore, a comprehensive review regarding the effects of mineral admixtures and fibers on the microstructural characteristics , fresh properties, and mechanical properties overall to address the upcoming challenges preventing the widespread use of UHPC and UHP-FRC/UHP-HFRC is essential.

A comprehensive review on effects of mineral admixtures and fibers on engineering properties of ultra-high-performance concrete

3D-printable alkali-activated concretes for building applications: A critical review

Effect of polyethylene fiber content on workability and mechanical-anisotropic properties of 3D printed ultra-high ductile concrete

Ultra-high ductile concrete (UHDC) is known for its excellent tensile strain capacity. However, investigations on the size effect of UHDC still remain scarce. This study aims to investigate the size effect on the flexural behavior of UHDC with various levels of specimen size, initial notch and ductility. A series of three-point bending tests were conducted on normal concrete beams and UHDC beams with different fiber volume fractions (1% and 2%) and various sizes of artificial notch (notch/beam depth ratio, 0.10 and 0.23). The results indicate that, with the help of strong fiber bridging, the fracture toughness and processing zone diameter of materials are enhanced by orders of magnitude, subsequently size effect becomes less significant and disappeared when materials change from brittle to ductile. Finally, the variation observed in the fracture processing zone (FPZ) of materials was used to explain the negligible size effect of UHDC.

Flexural size effect of ultra-high ductile concrete under different damage and ductility levels

Circular defects are widely distributed in porous rock materials, and the defects greatly affect the mechanical behavior and crack evolution of rock masses. In this paper, numerical models containing random circular defects are constructed based on discrete element method. Then, the uniaxial compressions are numerically performed to reveal the influence of the porosity or size homogeneity of the defects on the mechanical behavior, crack evolution, and acoustic emission (AE) events of the models. The results suggest that a univariant increase in porosity leads to a nonlinear decrease in the peak strength and a linear decrease in the elastic modulus. The number of cracks and AE events decrease with increasing porosity. As the size homogeneity coefficient increases, the peak strengths show a slight linear rise, while the elastic modulus values show a minimal linear downward trend, and the number of cracks and AE events show wave-like increases. The cracks first appear at the location with dense defects, and the cracks initiate from the top and bottom of the circular holes. The crack propagation and intersection modes between two adjacent defects are affected by their positions. These findings provide a reference for the fracture mechanism of rock with random circular defects.

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Can Cui

Papers

Fresh and anisotropic-mechanical properties of 3D printable ultra-high ductile concrete with crumb rubber

Effect of polyethylene fiber content on workability and mechanical-anisotropic properties of 3D printed ultra-high ductile concrete

Flexural size effect of ultra-high ductile concrete under different damage and ductility levels

Mechanical and cracking behavior of porous rock models containing random circular defects under uniaxial compression