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• Pore distribution in the interlayer analyzed using X-ray Computed Tomography. • Relationship between pore fraction and position of tensile bond fracture determined. • Tensile bond strength of 6.27% of compressive strength obtained. • Weakest layer occurred when tensile stress is perpendicular to printing direction. Recently, researches on additive manufacturing (AM) method have been actively carried out as the latest technique for building concrete structures in the construction field. It is known that the additive manufacturing method, also called 3D printing technique, is a method of constructing a structure by printing layers, and the adhesion strength in the interlayer between the layers plays a dominant role in the performance of the structure. In this study, we focused on the formation of interlayer of concrete structure built using additive manufacturing method. In this study, the position of the interlayer was analyzed using the computed tomography (CT) method and the correlation between porosity and tensile bond strength in the analyzed interlayer was tried. As a result of the CT analysis of 13 specimens extracted from the printed specimens, it was confirmed that the porosity was formed high in the interlayer. The porosity of the interlayer was at least 2.15% and 6.66% higher than the average porosity. After analyzing the porosity by CT, the tensile bond strength of the specimens was measured to confirm the tensile bond strength and location of the fracture surface. Tensile bond strengths were 2.58–3.77 MPa with an average of 2.80 MPa using 10 specimens. It was confirmed that all of the fracture surfaces occurred along the interlayer. It was confirmed that there was no correlation between the tensile bond strength and porosity of the test specimens used in this study. Six of the ten specimens failed in the other interlayer, but four specimens failed in the interlayer with the highest porosity. As a result of analysis of the fracture surfaces of six specimens without fracture at the highest porosity, the porosity at the fracture surface was 5.73–9.18%, which was higher by 0.6–3.3% than the average porosity. However, defects occurred during layer output were confirmed from the failure of six specimens. Through this study, we confirmed that the interlayer is the weakest when tensile stress is applied in the perpendicular direction of printing, and that it is necessary to review the defects when applying the printing method. [ABSTRACT FROM AUTHOR]

The mechanical properties of 3D-printed mortars are determined in terms of their compressive and direct tensile bond strengths. To determine such properties using existing methods, a preliminary experiment was conducted. The compressive strength of the printed mortar was compared to mold-casted specimens and it was found that the compressive strength decreased by ~30%. Among the fabrication variables, an increase in nozzle height negatively influenced the direct tensile bond strength. For the same conditions and age, the direct tensile strength decreased by as much as 16–29% when the number of layers increased from 2 to 6. When the specimens were fabricated using a specially designed stainless steel frame and core drill, followed by extraction and the application of physical impact, the 28 days compressive strength of the specimen decreased by ~50%. [ABSTRACT FROM AUTHOR]

Highlights: [•] The final goal is to develop a fiber reinforced concrete for containment buildings. [•] High rate tensile behavior of FRC was investigated. [•] Strain energy frame impact machine was used for tensile impact tests. [•] Different rate sensitivity of FRC was found according to the type fiber. [•] Adding more fibers by increasing S/a is positive for higher impact resistance of FRC. [ABSTRACT FROM AUTHOR]