Available online at www.CivileJournal.org Civil Engineering Journal (E-ISSN: 2476-3055; ISSN: 2676-6957) Vol. 8, No. 07, July, 2022 1507 Behavior of Post-Tensioned Concrete Girders Subject to Partially Strand Damage and Strengthened by NSM-CFRP Composites Abbas Jalil 1* , Alaa Hussein Al-Zuhairi 1 1 Department of Civil Engineering, Faculty of Engineering, University of Baghdad, 10071 Baghdad, Iraq. Received 05 April 2022; Revised 13 June 2022; Accepted 19 June 2022; Published 01 July 2022 Abstract Studies on the flexural behavior of post-tensioned beams subjected to strand damage and strengthened with near-surface mounted (NSM) technique using carbon fiber-reinforced polymer (CFRP) are limited and fail to examine the effect of CFRP laminates on strand strain and strengthening efficiency systematically. Furthermore, a design approach for UPC structures in existing design guidelines for FRP strengthening techniques is lacking. Hence, the behavior of post-tensioned beams strengthened with NSM-CFRP laminates after partial strand damage is investigated in this study. The testing program consists of seven post-tensioned beams strengthened by NSM-CFRP laminates with three partial strand damage ratios (14.3% symmetrical damage, 14.3% asymmetric damage, and 28.6% symmetrical damage). The experimental results showed that the use of CFRP laminates significantly increases the flexural capacity by up to 17.4 to 20.4%, corresponding to a strand damage ratio of 14.3 and 28.6%, respectively, enhances the stiffness, and reduces strand strain by up to 15.8 to 22.2%. However, the flexural stiffness of strengthened beams during serviceability phases is critical as strand damage ratios increase. Additionally, semi-empirical equations were proposed to predict the actual strain of unbonded strands whilst considering the effects of CFRP laminates. The suggested equations provide accurate predictions with little variance. Keywords: NSM-CFRP Laminates; Post-Tensioned Concrete; Strand Damages; Strengthening. 1. Introduction Post-tensioned concrete (PT) girders are widely used in bridges and large-span structures. Some of the strands and surrounding concrete may be damaged as a result of exposure to harsh weather conditions, high-vehicle collisions, etc. Consequently, partial loss of their structural capacity to resist stresses and an increase in deformation are expected. The process of replacing these members within sensitive facilities is often expensive and complicated. Therefore, the repair and strengthening of members are ideal options to ensure their return to the equivalent design capacity. The Washington State Department of Transport (WSDOT) developed a set of criteria to describe damaged girder situations that would necessitate replacement without recourse to strengthening techniques. One of these criteria is that the strand damage rate exceeds 25%, and this requires researching the behavior of the members that have a higher strand damage rate using innovative strengthening methods [1]. Several field investigations have shown that CFRP materials may be applied to strengthen damaged PS bridge girders after severe concrete cross-section losses and a limited number of strand members have ruptured [2, 3]. External bonding (EB) and Near-Surface mounted (NSM) techniques are two common methods of strengthening and repairing concrete members using fiber-reinforced polymer (FRP) materials. The FRP material is characterized by * Corresponding author: abbas19asdi@gmail.com http://dx.doi.org/10.28991/CEJ-2022-08-07-013 © 2022 by the authors. Licensee C.E.J, Tehran, Iran. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC-BY) license (http://creativecommons.org/licenses/by/4.0/).