Direct Tensile Behavior of Three-Dimensional-Printable Steel Fiber-Reinforced Ultrahigh Performance Concrete

To investigate the tensile behavior of 3D-printed ultrahigh performance concrete (UHPC), this study developed a 3D-printable steel fiber-reinforced UHPC with a cube compressive strength of up to 115 MPa. Through direct tension tests on dogbone specimens and computed tomography scanning, the influence of the 3D printing methods, specimen thickness and number of specimen layers on the failure mode, cracking strength, peak strength, energy absorption, and orientation of steel fibers in 3D-printed UHPC was analyzed in detail. The results indicate that under direct tension, the failure mode of the 3D-printed UHPC specimens remained in the tensile test section, with no significant difference from those of the cast UHPC specimens. After cracking, a large number of steel fibers were observed in the cracks of the 3D-printed UHPC specimens, forming the effect of a "steel fiber bridge" and exhibiting a trend of directional orientation along the printing direction. This was notably different from the relatively random orientation of steel fibers in the cast specimens. The experimental results show that with 3D printing preparation, the orderly arrangement of steel fibers could significantly improve the tensile performance of UHPC. Moreover, by reducing the nozzle thickness, the steel fibers can be distributed in a wide range along the printing direction. Therefore, the peak tensile stress, peak strain, and energy absorption values of the 3D-printed UHPC specimens could be significantly increased.