Contents lists available at ScienceDirect Composite Structures journal homepage: www.elsevier.com/locate/compstruct Performance of preloaded CFRP-strengthened fiber reinforced concrete beams Kareem Helal a , Sherif Yehia b, ⁎ , Rami Hawileh b , Jamal Abdalla b a Project Engineer, SPME Middle East General Contracting LLC, United Arab Emirates b Department of Civil Engineering and Materials Science and Engineering Institute (MSERI), American University of Sharjah, Sharjah, United Arab Emirates ARTICLE INFO Keywords: Strengthening CFRP Fiber reinforced concrete Preloading Steel fiber Synthetic fiber ABSTRACT Extensive experimental investigation was conducted to evaluate the performance of fiber reinforced concrete (FRC) beams that have been externally strengthened with and without preloading in flexure using carbon fiber reinforced polymer sheets. Steel, synthetic and hybrid fibers (mix of steel and synthetic fibers) with a volume fraction of 0.5% were added to the concrete matrix to prepare 15 beams. Test results show that the addition of all fibers types improved the flexural capacity, crack initiation and propagation, stiffness, post cracking behavior, deflection and ductility of the beams. Hybrid fibers was found to improve the ductility of the FRC beams by 4 times when compared to the control specimen without fibers. Furthermore, preloading before strengthening of concrete beams without fibers led to increase of the ultimate capacity by about 120%. Preloading caused im- provement in elastic stiffness of the beams by 33.4%, 23.5% 17.6 for steel, synthetic and hybrid fibers, re- spectively. While strengthening increased the elastic stiffness for all beams and up to 65.3%. However, for the FRC beams there was no significant improvement in the ultimate capacity from those strengthened without preloading. Applicability of ACI 440 equations to predict the capacity of fiber reinforced concrete beams was also investigated. 1. Introduction and background Carbon fiber reinforced polymers (CFRP) are increasingly being used as external strengthening materials to strengthen and compensate for structural deficiencies in reinforced concrete (RC) members [1]. Some causes of these structural deficiencies are building use mod- ification that causes an increase in live load, severe environmental exposure, degradation in concrete strength, corrosion of steel re- inforcement, and imperfections during design and construction stages [2–7]. The ongoing efforts to strengthen and rehabilitate existing RC structures are enhanced by the advances in such FRP composite mate- rials development. These advanced composite materials allow FRP’s to be bonded to the exterior surface of different RC members via epoxy adhesives to provide additional flexural, shear and compression capa- cities [3,8–15]. In addition CFRP laminates are becoming the materials of choice in RC strengthening applications due to their superior ad- vantages over the conventional steel plating. Some of these advantages are high-strength-to-weight ratio, ease of use and installation, corrosion resistance, and lightweight [14–17]. Research efforts have focused on the stress-strain behavior of different types of FRP materials in addition to their overall behavior at yield or failure of beam specimens. Most of the research conducted during the three decades has focused on testing RC beams and slabs externally strengthened with FRP laminates in shear and flexure [18–21]. The tested beam specimens in most of these studies were not preloaded. Limited studies have been conducted to examine the effect of external FRP reinforcement on the flexural per- formance of preloaded RC beams and slabs. Among those limited re- search studies, A. El-Shihy 2005 [22] tested under 4-point loading RC beams strengthened with glass (GFRP) laminates to failure and con- cluded that the presence of stresses in the member at the time of repair impacts the ultimate capacity of the RC member [22]. Recent studies focused on evaluation of flexural performance of preloaded RC beams using prestressed CFRP laminates [23] and near-surface mounted (NSM) CFRP strips as strengthening techniques [24,25]. Furthermore, Seręga et al. 2018 [26] numerically investigated the effect of preloading on strengthening efficiency of CFRP prestressed beams by developing a model that simulated and accurately predicted their behavior under a wide range of pre-loading. Other studies have concluded that exposure to initial loading affects the efficiency of the FRP strengthening by re- ducing the stiffness of the RC member [27]. Many research findings showed that the addition of steel or syn- thetic fibers transforms concrete into a pseudo-ductile material, thus https://doi.org/10.1016/j.compstruct.2020.112262 Received 22 October 2019; Accepted 23 March 2020 ⁎ Corresponding author. E-mail addresses: khelal@structural.net (K. Helal), syehia@aus.edu (S. Yehia), rhaweeleh@aus.edu (R. Hawileh), jabdalla@aus.edu (J. Abdalla). Composite Structures 244 (2020) 112262 Available online 26 March 2020 0263-8223/ © 2020 Elsevier Ltd. All rights reserved. T