Analysis of UHPC Fiber Alignment in Two and Three Dimensions

Abstract

The quantity and orientation of steel fibers in a structural UHPC element will impact its ability to carry and distribute tensile stresses. Because UHPC is typically placed as a highly flowable material, fibers tend to align along the direction of flow, thereby influencing the local tensile behavior of the material. Quality assurance and quality control (QA/QC) methods for assessing the quantity and alignment of fibers in UHPC elements are therefore desirable for understanding the potential impact of placement methods on the structural performance of a UHPC element. Accordingly, two techniques have been independently developed by the authors to characterize fiber distribution and alignment in UHPC cores extracted from structural members: (1) three-dimensional imaging of concrete samples by x-ray microtomography and (2) two-dimensional imaging of individual cut surfaces. This paper presents a collaborative research effort in which the fiber quantity and fiber orientation of ten samples of UHPC, representing seven different batches of material, were characterized using both the three-dimensional and two-dimensional imaging techniques. Parallel analyses were performed to quantify the total fiber content and "fiber alignment factors" for each sample using each method and the results were compared. The results indicate that while three-dimensional imaging by x-ray microtomography of fiber alignment provides a comprehensive assessment of fiber alignment, two-dimensional analysis of cut planes can provide a reasonable estimate for use as a QA/QC method or as a tool to investigate or explain UHPC performance.