Interlayer Bond Strength of 3D Printed Concrete Members with Ultra High Performance Concrete (UHPC) Mix

In structures manufactured using 3D concrete printing, cracks can easily propagate along the interface between printed layers. Therefore, it was necessary to determine the interlayer bond strength. In this study, direct shear and tensile tests were performed to determine the interlayer bond stability of the 3DCP members. To confirm the appropriateness of the mix proportion used to fabricate the specimens, the open time available for printing was identified via a mixing test, and the extrudability and buildability were verified via a printing test. In addition, direct shear and tensile tests were performed using the specimen manufacturing method (i.e., mold casting and 3D printing) and printing time gap (PTG) between the laminated layers as key test variables. The interlayer bond strengths of the specimens, according to the variables obtained from the test results, were compared and analyzed based on the interfacial shear strength standards presented in the current structural codes. In the direct shear test, failure occurred at the interlayers of all the specimens, and the interlayer bond strength tended to decrease with increasing PTG. In addition, the interlayer bond strength of the direct shear specimens exceeded the interfacial shear strength suggested by current structural codes. In contrast, in the direct tensile test, interlayer surface failure occurred only in some specimens, and there was no distinct change in the interlayer bond strength owing to PTG.