Contents lists available at ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct Experimental research on torsional strength of synthetic/steel fiber- reinforced hollow concrete beam Rafea F. Hassan a , Muna H. Jaber a , Nabeel H. Al-Salim a , Husam H. Hussein b, ⁎ a Civil Engineering Dept., College of Engineering, University of Babylon, Iraq b Dept. of Civil Engineering, Ohio University, Stocker Center, Athens, OH 45701, United States ARTICLE INFO Keywords: Syntactic fiber Fiber-reinforced concrete Steel fiber Torsion ABSTRACT Past research has used synthetic fiber (SY.F) and steel fiber (ST.F) for decades to advance concrete mechanical characteristics, including shear, tensile, and flexural strengths. Nevertheless, there is limited information con- cerning the torsional concrete reinforced with SY.F and ST.F performance. This research aims to explore the torsional performance of hollow reinforcement concrete beams reinforced with various fiber types. The fiber content of 1% with three different lengths of SY.F, 19, 38, and 57 mm, along with 13 mm of ST.F, was used. Two specimens were cast with normal concrete without using fibers as control beams and four hollow beams with ST.F and SY.F. To use a pure torsional load on the tested samples, an innovative test method was employed. The twisting angle of tested beams was calculated at each interval of the load as well as the first crack load and failure load. Outcomes illustrated that the utilizing of SY.F and ST.F in the reinforced concrete beams (RCB) enhanced the overall performance under torsional load compared to the control beam behaviors. This en- hancement in the performance was relative to the fiber type and length. For the first cracking load, the tested beams reinforced with the ST.F and SY.F of 19 and 37 mm lengths showed approximately the same value. Also, the beam reinforced with the SY.F length of 55 mm exhibited the highest first cracking load value, among other tested beams. As the fiber length increases of SY.F, the ultimate load capacity was raised by 4.7, 9.4, and 21.9% for beams cast with 19, 37, and 55 mm fiber length, respectively. For the ST.F reinforcement concrete beam, the ultimate load capacity was raised by 5.5%. Therefore, due to the significant impact of SY.F on the torsional performance, it is recommended using SY.F with normal concrete. 1. Introduction With the worldwide construction industry's rapid and enormous growth, concrete demand has considerably risen over the past twenty years. This is due to the appealing features of concrete, including its high compression capability but with very low-tension capacity. Concrete beams are elements of structure that produced from either steel, reinforced concrete, or composite materials designed for carrying external transverse loads. In addition to that, beams are supporting slabs, walls, columns, and other components, where those loads are transferred to the supporting columns. Nevertheless, one of the con- crete's severe flaws is its small ability to resist tensile stress. Therefore, past research has used synthetic fiber (SY.F) and steel fiber (ST.F) for the last decade to advance concrete mechanical characteristics, in- cluding shear, tensile, and flexural strengths [1–9]. Several researchers have concluded that adding the SY.F fiber to the concrete mix improves concrete mechanical performance. The enhancement size is mostly related to the amount of fiber, length, geometry, and mechanical characteristics. Research to date on using ST.F for reinforcing concrete beams has reported an enhancement in the structural performance. Nevertheless, there is no designing equation for reinforcement concrete beams that reinforced by SY.F under the torque load. Also, ACI, Eurocode, and BS (British Specifications) specifications do not have any details related to the hollow beam design with SY.F reinforced concrete under torsional load. The use of hollow beams decreases the structure dead load resulting in members with a small cross-section area. A comparison between the torsional performance of hollow and solid beams shows an impact of ST.F and SY.F with different lengths. Past research has explored the enhancement of adding ST.F on the performance of post-cracking torsional and the ultimate torsion strength [10–14]. Nevertheless, there are limited studies on the SY.F impact on the torsional features of the reinforced concrete beam (RCB) with fiber. The torsional performance of SY.F that reinforcing concrete under pure torsional pressure must, therefore, be tested utilizing https://doi.org/10.1016/j.engstruct.2020.110948 Received 22 December 2019; Received in revised form 29 May 2020; Accepted 8 June 2020 ⁎ Corresponding author. E-mail address: hh236310@ohio.edu (H.H. Hussein). Engineering Structures 220 (2020) 110948 0141-0296/ © 2020 Elsevier Ltd. All rights reserved. T