Interlayer adhesion is one of the most pivotal aspects related to mechanical performance and durability of 3D printed concrete. However, due to continuous printing of multi-layered structures, large-scale buildings require long time intervals between layer deposition, with interlayer bond strength decreasing as time intervals between printed layers increase. This study evaluates comprehensively the effects of time intervals between layers: 1 h, 1.5 h, 2 h, 3 h, and 4.5 h as well as two different height to span ratios (). In order to assess the properties of interlayer regions microstructural examinations using techniques such as X-ray micro-computed tomography (micro-CT), scanning electron microscopy (SEM) were applied. In addition destructive testing of compressive, splitting tensile and flexural strengths along with crack opening analysis using the Digital Image Correlation (DIC) method was performed and supported with FEM analysis. The results show that interlayer adhesion strength decreased significantly for time intervals of up to 2 h, with a linearly decreasing trend. The reduction in strength the interlayer bond for a 2-h interval was 56.6%, compared to the reference specimen. Afterwards, the decrease in strength was minimal (up to 68.5% after 4.5 h time interval). However, microstructural investigations revealed that specimens with printing interval beyond 2 h exhibited noticeable continuous cracking in interfacial zone with widths reaching of up to 100 μm (SEM). At the same time micro-CT analysis confirmed creation of longitudinal pores which in turn was reflected in different cracking phenomena of specimens with longer printing intervals (>2 h). This phenomenon was further explained through FEM modeling. As an outcome it was proven through DIC that that increasing the time interval between the deposit of successive layers not only affects the strength of the entire beam, but also affects its cracking mode. The discontinuities in the interlayer contact zone leads to destruction characteristic of composite structures, thus the structure comes to be destroyed layer by layer, resulting from insufficient interlayer adhesion. The presented study is the first to fully evaluate the failure behavior of 3D printed specimens with long time intervals.

中文翻译：

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层间粘附力对 3D 打印梁单元应力分布的影响


层间附着力是与 3D 打印混凝土的机械性能和耐久性相关的最关键的方面之一。然而，由于多层结构的连续打印，大型建筑需要较长的层沉积时间间隔，层间结合强度随着打印层之间的时间间隔的增加而降低。本研究综合评估了层间时间间隔的影响：1 h、1.5 h、2 h、3 h 和 4.5 h 以及两种不同的高跨比 ()。为了评估层间区域的特性，使用 X 射线微型计算机断层扫描 (micro-CT)、扫描电子显微镜 (SEM) 等技术进行微观结构检查。此外，还使用数字图像相关 (DIC) 方法进行压缩、劈裂拉伸和弯曲强度的破坏性测试以及裂纹张开分析，并得到有限元分析的支持。结果表明，在长达 2 h 的时间间隔内，层间粘合强度显着下降，且呈线性下降趋势。与参考样本相比，2 小时间隔后层间粘合强度降低了 56.6%。此后，强度下降幅度很小（4.5 小时时间间隔后下降达 68.5%）。然而，微观结构研究表明，打印间隔超过 2 小时的样品在界面区域表现出明显的连续裂纹，宽度达到 100 μm（SEM）。同时，显微 CT 分析证实了纵向孔隙的产生，这反过来又反映在具有较长打印间隔（> 2 小时）的样本的不同裂纹现象中。通过有限元建模进一步解释了这一现象。 结果通过DIC证明，增加连续层沉积之间的时间间隔不仅会影响整个梁的强度，还会影响其开裂模式。层间接触区的不连续性导致复合材料结构的破坏特性，因此层间粘合力不足导致结构逐层被破坏。本研究首次全面评估了长时间间隔 3D 打印样本的失效行为。