Contents lists available at ScienceDirect Cement and Concrete Research journal homepage: www.elsevier.com/locate/cemconres Gradient-based fbre detection method on 3D micro-CT tomographic image for defning fbre orientation bias in ultra-high-performance concrete Marta Miletić a, ⁎ , Lakshminarayanan Mohana Kumar b , Ji-Youn Arns c , Ankit Agarwal d , Stephen J. Foster e , Christoph Arns f , Dunja Perić g a Department of Civil, Construction, and Environmental Engineering, San Diego State University, San Diego, CA, USA b School of Civil and Environmental Engineering, Centre for Infrastructure Engineering and Safety, The University of New South Wales, Sydney, Australia c CJEL Digital Imaging Education Solution Pty. Ltd, Sydney, Australia d School of Civil and Environmental Engineering, Centre for Infrastructure Engineering and Safety, The University of New South Wales, Sydney, Australia e School of Civil and Environmental Engineering, Centre for Infrastructure Engineering and Safety, The University of New South Wales, Sydney, Australia f School of Minerals and Energy Resources Environmental Engineering, The University of New South Wales, Sydney, Australia g Department of Civil Engineering, Kansas State University, Manhattan, KS, USA ARTICLE INFO Keywords: UHPC Micro-CT tomography Steel fbre orientation Segmentation Sobel ABSTRACT Ultra-high-performance fbre reinforced concrete (UHPC) is a class of advanced cementitious composites char- acterized by its high compressive and fexural strengths, toughness and enhanced durability. The mechanical properties of the UHPC are to a great extent dependent on fbre volume fraction, orientation and distribution within the cementitious matrix. However, determination of the true three-dimensional fbre orientation and distribution is challenging. In this paper micro-computed tomography (micro-CT) is used to determine these parameters. Cylindrical samples of UHPC were extracted from a dogbone tension test specimen and from a pretensioned bridge girder, and high-resolution micro-CT images were then acquired. Using the scanned data, and following three-dimensional image reconstruction and image processing, quantitative fbre information was obtained via a novel image post-processing technique based on local-intensity gradient signifcantly improving cross-fbre detection compared to existing techniques. The estimated fbre volume fraction is close to design and experimentally measured values. Straight and hooked end fbres in UHPC sample were successfully identifed and segmented using this novel technique. The test results show the fbre arrangement to be highly anisotropic with fbres aligned predominantly in one direction, which is attributed to the casting processes and fow. 1. Introduction Ultra-high performance fbre reinforced concrete is a class of ad- vanced concrete materials characterized by its high strengths in fexure and compression, high toughness and long-term durability, which is due to homogeneous and densely packed microstructure [1]. It has a low water-to-binder ratio, high cement content, high silica fume con- tent, high dosage of a third-generation polymer-based superplasticizer, and small but hard aggregates such as quartz sand. Steel fbre re- inforced UHPC has an improved ductility and can exhibit a strain hardening behaviour accompanied with multiple cracking under tensile loading due to an incorporation of small-sized high strength steel fbres to the matrix [2]. Since the efciency of fbres in a cement matrix depends mainly on the volumetric fbre content, fbre orientation relative to the direction of applied stress, as well as fbre spatial distribution within the matrix [3], these parameters signifcantly infuence the mechanical properties of UHPC. Yet there is lack of reliable technology to assess these para- meters for a given hardened UHPC specimen. While an isotropic distribution of fbres in all directions within UHPC is expected to result in isotropic mechanical properties, a bias in orientation will lead to diferent properties along diferent structural axes. Fibre orientation has been found to depend on several factors such as specimen's size and shape, type and length of fbres, fowability of concrete, production process, reinforcement layout, and casting method [4–12]. Furthermore, fbre orientation and distribution within the ce- mentitious matrix have diferent efects on diferent mechanical prop- erties of the concrete. For example, experimental study on the efect of fbre orientation on the compressive properties of UHPC by Stiel et al. https://doi.org/10.1016/j.cemconres.2019.105962 Received 28 April 2019; Received in revised form 10 December 2019; Accepted 20 December 2019 ⁎ Corresponding author. E-mail address: mmiletic@sdsu.edu (M. Miletić). Cement and Concrete Research 129 (2020) 105962 0008-8846/ © 2019 Elsevier Ltd. All rights reserved. T