Research and development of alternative technologies to conventional reinforcement and exploration of printable building materials with excellent properties ECC prepared from polymer fibers has high ductility, excellent toughness, and superior strain-hardening energy dissipation properties. The effects of polyethylene (PE) fiber dosage (1.0%, 1.5%, and 2.0%, in volume) and length (6, 9 mm, and 12 mm) on the pre-strain-hardening early properties, post-strain-hardening mechanical properties, and microstructure of 3D-printed HS-ECC were investigated. This study provides new ideas for the design and performance optimization of 3D-printed HS-ECC, which further promotes the application of 3D-printed technology in construction.

In this study, we experimentally investigated the effect patterns of polyethylene (PE) fiber doping (1.0%, 1.5%, 2.0%) and length (6, 9, 12 mm) on the pre-hardening early properties, post-hardening mechanical properties, and microstructure of 3D-printed HS-ECC. The rheological properties and early mechanical properties were correlated by actual printing tests. The influence mechanism of mechanical virtual was explored by utilizing microscopic testing means SEM, NMR methods combined with experimental results.

The static and dynamic yield stresses of HS-ECC increased with increasing fiber doping and length, and the effect of fiber doping was more pronounced. With the increase in vertical strain, the stress of the specimens increased approximately linearly before yielding, and the increase in stress slowed down after yielding. Specifically, increasing fiber doping and fiber length increased the early strength of 33D-printed HS-ECC, and the bridging effect of fibers increased with increasing doping and length. The pattern of printable properties was consistent with the rheological properties and early mechanical strength by actual printing tests.

The post-hardening mechanical properties of the 3D-printed HS-ECC with 2.0% 12 mm PE fibers yielded a tensile strain capacity of more than 4%, an average crack width of 89.72 μm, and a compressive strength of more than 80 MPa. The degree of destruction of the PE fibers after detachment from the substrate increased with fiber doping and length. The 6 mm PE fibers were easier to pull out than 12 mm, and thus the ductility of the material increased with the increase in fiber doping and length. to pull out and thus the ductility of the material decreases. In addition, nozzle extrusion-based 3D-printed HS-ECC allows for effective directional fiber orientation, resulting in increased ductility of the 3D-printed HS-ECC.

The pore size distributions peaked in the range of 0.001 µm to 0.010 µm. Cast samples contained and the proportion of small pores in the casting samples is less than 90%, while the 3D-printed samples are 60% to 70%, which indicates that the 3D printed samples contain a higher number of large pores. The primary and secondary peaks of samples V1, V2, L1, and L2 are lower compared to those of samples V3 and L3, and the trend suggests that the increase of fibers raises the number of pores, which results in the decrease of compressive strength. Although the variation of the primary peaks of the 3D printed samples is consistent with the pouring trend, the pattern of the secondary peaks is complex, which may be related to the time between the printed layers and the pressure on the upper layer of the material, resulting in an irregular distribution of pores. The total porosity increases by about 21.75% and 11.75% with increasing fiber doping and fiber length, while the mechanical properties of 3D-printed HS-ECC specimens decrease at the same time. The comparative results show that the porosity of cast HS-ECC is 1.24 to 1.67 times higher than that of 3D-printed HS-ECC, but the harmful porosity of 3D-printed HS-ECC accounts for about 30%, which may lead to the decrease in the mechanical properties of large-scale components.

With the increase of fiber doping and fiber length, its dynamic and static yield stress increases. In addition, the early mechanical strength of 3D-printed HS-ECC increases. Through the actual printing test, the pattern of its printable properties is consistent with the rheological properties and early mechanical strength. The post-hardening mechanical property test results yielded that the 3D-printed HS-ECC containing 2.0% 12 mm PE fibers possessed a tensile strain capacity of more than 4%, an average crack width of 89.72 μm, and a compressive strength of more than 80 MPa. The degree of damage of PE fibers after separation from the matrix increased with fiber doping and length. 6 mm PE fibers were easier to pull out compared to 12 mm, resulting in a decrease in the ductility of the material. In addition, nozzle extrusion-based 3D-printed HS-ECC allows for effective directional fiber orientation, resulting in an increase in the ductility of 3D-printed HS-ECC. The increase in fiber doping and length increased the total porosity by 21.75% and 11.75%, respectively, which led to a decrease in the mechanical properties of 3D-printed HS-ECC specimens. 3D-printed HS-ECC showed a decrease in the intra-strip porosity compared to casting, and an increase in the interlayer and inter-strip porosity resulted in a decrease in strength.