JOURNAL OF COMPOSITE MATERIALS Article Alkali and temperature long-term effect on the bond strength of fiber reinforced polymer-to-concrete interface Mahdie Mohammadi 1 , Majid Barghian 1 , Davood Mostofinejad 2 and Adel Rafieyan 3 Abstract The effects of such environmental conditions as alkali media at temperatures of 23  C, 40  C, and 60  C were investigated on the fiber reinforced polymer-to-concrete bond strength. For this purpose, 42 specimens were strengthened via the externally bonded reinforcement and the externally bonded reinforcement on grooves techniques. The specimens were later subjected to the single-shear test after the specified durations of exposure to an alkaline medium. The particle image velocimetry technique was used to investigate such bond characteristics of the strengthened specimens as load-slip behavior, strain profiles, and strain fields along the fiber reinforced polymer-to-concrete bond. Experimental results showed that the specimens strengthened via the externally bonded reinforcement on grooves method exhibited ultimate bond loads by up to 50% higher than those strengthened via the externally bonded reinforcement method. Keywords Externally bonded reinforcement, externally bonded reinforcement on grooves, alkaline environment, temperature, surface preparation Introduction Much research has been devoted to the investigation of the effects of harsh environmental conditions on the bond strength of the fiber reinforced polymer (FRP)- to-concrete interface. In most studies, the externally bonded reinforcement (EBR) technique has been used as a conventional method to strengthen the specimens. In this method, the surface of the concrete specimen is prepared initially by sand blasting and cleaning before a layer of epoxy is applied uniformly on the concrete surface. FRP sheets are then installed on the surface and saturated with the epoxy. 1–3 Although the EBR technique is being used widely, in certain conditions, it may suffer from premature debonding of the FRP off the concrete. This drawback has led to development of externally bonded reinforce- ment on grooves (EBROG) technique, originally introduced by Mostofinejad and Mahmoudabadi, 4 as an attempt to improve the performance of FRP-to- concrete interface. In this method grooves with proper length, width, and depth are made on the con- crete surface. The concrete surface and the grooves are then cleaned using an air jet before they are filled with an appropriate epoxy. Finally, FRP sheets or strips are installed using proper amounts of epoxy on the concrete surface. 4–8 In the study of Mostofinejad and Mahmoudabadi, 4 longitudinal grooves 10-mm deep caused an increase of 80% in ultimate rupture strength as compared to spe- cimens with only EBR surface preparation. Furthermore, Hosseini and Mostofinejad 5 obtained that cutting two 5  10  75 mm (width  depth  length) grooves into the concrete surface made the bond of FRP-to-concrete strong enough that the FRP composite reached its full tensile capacity under single-shear test. However, specimens strength- ened via this new technique have not yet been subjected 1 Dept. of Civil Engineering, Univ. of Tabriz, Iran 2 Dept. of Civil Engineering, Isfahan University of Technology, Iran 3 Dept. of Civil Engineering, Najafabad Branch, Islamic Azad University, Iran Corresponding author: Mahdie Mohammadi, University of Tabriz, Tabriz, Iran. Email: Mahdie.mohammadi67@yahoo.com Journal of Composite Materials 0(0) 1–12 ! The Author(s) 2017 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0021998317740201 journals.sagepub.com/home/jcm