###### Abstract  
Three-dimensional printing concrete (3 DPC) has garnered significant attention for its construction efficiency and complex geometric capabilities. However, its mechanical properties differ markedly from cast-in-place concrete due to interlayer/intralayer interface weakening and pore anisotropy. Flowability directly regulates printability and pore distribution, thereby influencing mechanical properties. This study systematically examines flowability's effects on 3 DPC mechanical properties through compressive, flexural, and interfacial splitting tensile tests, integrated with [PERSON]'s fracture theory and stress intensity factor calculations. The key findings are as follows: (1) 3 DPC exhibits pronounced anisotropy--compressive strength (X \(>\) Y \(>\) Z), flexural strength (Y \(\approx\) Z \(>\) X2 \(>\) X1), and splitting tensile strength (C \(>\) T). Increased flowability enables compressive and Y/Z direction flexural strengths to approach cast concrete levels. (2) The anisotropy coefficient \(I_{\mathrm{a}}\) decreases significantly with flowability (66.7% for compressive, 66.8% for flexural strength). Flexural strength shows greater sensitivity to interfacial defects than compressive strength. (3) The aspect ratio of ellipsoidal pores directly influences the anisotropic compressive behavior. Increased flowability promotes morphological transformation of elliptical pores toward more spherical geometries with reduced flattening, thereby mitigating the anisotropy in compressive performance. These results establish a theoretical framework for optimizing and predicting 3 DPC mechanical performance, supporting its practical application in construction. flowability; 3D printing concrete; mechanical properties; strength prediction; anisotropy +
Footnote †: journal: Applied sciences  
0000-0001-5000-5000-0000]X.X. Flowability, a critical workability parameter for 3 DPC, directly governs material extrudability, interlayer bond strength, and pore structure distribution [5; 6; 7]. Insufficient flowability leads to excessively high yield stress and plastic viscosity of cementitious materials, impairing filament extrudability and preventing dense packing during layer deposition. This results in detrimental interfacial pores [8], adversely affecting mechanical performance [9]. Moreover, 3 DPC exhibits pronounced mechanical anisotropy along three axes, significantly compromising structural load uniformity and stability. Existing studies on flowability's influence remain limited, predominantly focusing on shape retention [10; 11], while systematic investigations into unfavorable pore structures under low flowability and their mechanical implications are scarce. Existing studies on 3 DPC's compressive behavior generally report the highest strength along the X-axis (printing direction), though conclusions on Y- and Z-axis strengths remain inconsistent [12; 13; 14; 15; 16; 17; 18; 19; 20]. Some researchers have analyzed this anisotropy: For instance, [PERSON] [21] employed simulation and theoretical modeling to reveal stress concentration effects in homogenized single-pore 3 DPC specimens, explaining the X \(>\) Y \(>\) Z strength hierarchy. [PERSON] [22] investigated the correlation between pore geometry and interlayer interfacial bond strength. In parallel, [PERSON] [23] investigated the influence of interfacial bonding properties on the anisotropic mechanical behavior of 3 DPC through FE modeling incorporating traction-separation laws at interlayer interfaces. Flexural and splitting tensile strengths follow a clearer trend [7; 24; 25], with interfacial loading yielding minimum strength due to high interfacial porosity-induced defects [26]. As demonstrated by [PERSON] [27], the flexural strength of 3 DPC is the lowest along the interlayer direction and the highest in the direction perpendicular to the layers.