Effects of compaction on fiber distribution and orientation in fiber-reinforced concrete poles

Fiber-reinforced concrete (FRC) incorporates short, dispersed fibers to enhance strength, fatigue resistance, and impact resistance compared to plain concrete. In centrifugally cast elements, such as power poles, the effectiveness of reinforcement depends largely on the spatial arrangement and orientation of the fibers. This paper investigates how spinning affects the distribution of steel fibers in cylindrical FRC elements compared to compaction by vibration. Different fiber dosages are considered. The internal structure of the specimens is analyzed using non-destructive X-ray micro-computed tomography (µCT), and fiber orientation is quantified with a structure tensor approach. Results show that spun concrete exhibits higher density and lower porosity than vibrated concrete. Moreover, the compaction method is the decisive factor governing fiber distribution and orientation. In spun samples, centrifugal forces drive fibers outward, creating a reinforcement-free layer near the hollow core. Both compaction methods promote preferential circumferential alignment of fibers in the plane of samples’ circular cross-section. This effect is much stronger in spun concrete due to centrifugal force. In vibrated samples, this feature is attributed to “boundary effect”. Analysis of fiber orientation relative to the longitudinal axis shows that transverse alignment predominates in spun samples. This explains why lower fiber dosages fail to improve the mechanical performance of spun FRC. For structural elements such as poles, which primarily require longitudinally aligned fibers to resist bending, this effect is especially critical. These findings provide essential insights for interpreting the strength and deformation behavior of centrifugally cast FRC and highlight the need for further research in this field.