Contents lists available at ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct Overloading impact on the flexural behavior of RC beams strengthened with FRP composites under fatigue: Experimental study Mohammad Al-Qaralleh a, ⁎ , Houssam Toutanji b , Tamer Eljufout c a Chair of the Dept. of Civil and Environmental Engineering, College of Engineering, Mutah University, Mutah, Karak, Jordan b Professor and Dean, College of Engineering and Computer Science, California State University, Northridge, CA 91330, USA c Dept. of Civil Engineering, College of Engineering, American University of Madaba, Madaba, Jordan ABSTRACT This study investigates the effect of periodic overloading on the fatigue life of Reinforced Concrete (RC) beams strengthened with externally bonded Fiber Reinforced Polymers (FRP). The study includes 6 RC beams with dimensions of 152.4 × 152.4 × 1500 mm strengthened with one layer of carbon fiber sheet attached to the soffits. One strengthened beam tested monotonically to serve as a control beam, and another one was tested under constant amplitude fatigue loading to serve as a reference for the fatigue testing. The rest of the beams were tested under fatigue loading with periodic overloading. The loading pattern was chosen to simulate the conditions of the traffic on bridges. Two sets of repetitive loading segments were applied with a ratio of (1:9), one overloading cycles to nine base loading cycles. When compared with the reference RC beam and the available data obtained from the literature; the results show that periodic overloading reduces the fatigue life of the strengthened beams. The Palmgren-Miner rule of linear cumulative damage overestimates the fatigue life of the strengthened beams. In addition, periodic overloading shifts the location of the failure in the strengthened beams outside the maximum moment zone. However, the tested beams had the same failure mode of the reference beam, which is the rupture of the primary reinforcing steel. In this study, a prediction model is presented of the fatigue life of strengthened RC beams subjected to overloading based on the applied stress ranges in the primary steel. 1. Introduction Bridges are usually designed to last for at least 75-years design period [1]. During such extended service life, reinforced concrete (RC) can exhibit some loss of integrity. Cracking of concrete due to service loading conditions, overloading, and other factors such as freezing and thawing cycles can expose the reinforcing steel to the harmful en- vironmental conditions. Reinforcing steel corrodes when exposed to moisture and deicing salts. This leads to concrete spalling and overall health deterioration of the RC components. The combination of old deteriorated bridges that were designed for lighter trucks and the heavier trucks that are travelling on them nowadays raises some safety concerns. For example, A girder dismantled from a bridge carrying Interstate 85 northbound over Cherokee Creek near Gaffney, S.C., be- tween Spartanburg, S.C., and Charlotte, N.C., designed in 1957 to carry H20 design load and erected in 1961, was unable to carry the HS25 design load when tested monotonically [2]. The combination of the increased truck loading and health deterioration may induce stress le- vels on existing bridges that exceed its fatigue design limits. An analysis of failure cases of bridges shows that 9% (i.e. 44 failure cases of bridges) of all the failure of bridges in the U.S., between 1989 and 2000, were due to overload, in addition, most of the failures happened during the service life and the average bridge life at failure was 52.5 years [3]. The Federal Highway Administration (FHWA) estimated the total cost of rehabilitation of bridges nationwide to be over $35 billion [4]. The superior properties of fiber reinforced polymer (FRP) materials, such as: high specific strength (i.e. strength to density ratio), high specific stiffness (i.e. modulus to density ratio), low density, corrosion re- sistance, long fatigue life, environmental stability, ease of installation and cost-effectiveness, make it a valuable alternative to the conven- tional strengthening techniques [5]. Hence, engineers started to utilize FRP materials for externally strengthening of RC bridges since the early 1980 s [6]. 1.1. Fatigue behavior of the constituent materials Each of the constituent materials of the RC beam strengthened with FRP has different behavior under fatigue loading. Hence, it is necessary to study the behavior of these materials to understand the behavior of the system comprised of them (i.e. RC beams strengthened with FRP sheets). 1.1.1. Concrete Concrete has fatigue strength of about 55% of its static load strength at 10 million cycles [7,8]. In addition, the fatigue life of concrete is independent of the properties of the mix such as: water/cement ratio https://doi.org/10.1016/j.engstruct.2020.111045 Received 28 November 2019; Received in revised form 27 May 2020; Accepted 29 June 2020 ⁎ Corresponding author. E-mail address: mohammad.alqaralleh@mutah.edu.jo (M. Al-Qaralleh). Engineering Structures 221 (2020) 111045 0141-0296/ © 2020 Elsevier Ltd. All rights reserved. T