Construction and design faults, damage from natural disasters, destructive environmental effects, changes in the building user type and changes in the design codes requirements are among the factors that a building may require to be rehabilitated/retrofitted at service load levels. One of the most well-known methods, in this regard, to strengthen an RC flexural member is the external bonded reinforcement (EBR) technique in which the fiber reinforcement polymer (FRP) sheets bond to the beam/slab tension face. The widely utilization of FRP sheets is certainly attributed to the unique mechanical properties of FRP materials over that of steel plates including high strength to weight ratio, high durability, great resistance to corrosion and ease of installation. One of the most challenging problems in the field of FRP strengthening using EBR method is the premature debonding of FRP sheets from the concrete substrate. Although increase of FRP bond length has a substantial effect on the FRP-to-concrete bond strength, however there is a specific length of FRP sheet, beyond which no significantly increase in the bond strength is obtained. Considering the presence of coarse aggregates adjacent to the interface between FRP and concrete substrate, the volume ratio and geometry specifications of these aggregates definitely are of the great importance to control the bond strength/effective bond length of FRP-to-concrete joints. Hence the main objective of the current study is to investigate the effect of both the maximum size and volume of coarse aggregates on the FRP-to-concrete bond behavior including the bond strength and the effective bond length. To reach this goal, 68 concrete prism specimens with dimensions of 350 × 350 × 150 mm were cast in 5 distinct groups each to study one special parameter. The investigated parameters included concrete compressive strength, the maximum size and volume of coarse aggregate; while all the other parameters remained constant. The aggregate maximum size in groups I, II-III and IV-V is 4.75, 9.5 and 19 mm respectively. The amount of coarse aggregate for the tested specimens in groups II through V is not the same. Moreover, the concrete specimens in this study, were cast all to reach 28-day compressive strength of either 23 or 35 MPa. To externally strengthen the specimens using EBR method 50 mm wide FRP sheets having bond length of 200 mm were selected and adhered to the concrete substrate. The specimens all were subjected to the single lap shear test after completion of the strengthening process items. These items are included in preparation of concrete substrate, bonding FRP sheet using epoxy resin and finally curing the fresh epoxy in laboratory conditions aimed to utilize the epoxy resin maximum capacity. Full field deformation measurements of the specimens’ faces undergoing loading were conducted using digital image correlation (DIC) method, specially known as particle image velocimetry (PIV). To do so, following images were taken and recorded from the deformation field besides the corresponding load reading. Experimental results explicitly correlate both the FRP-to-concrete bond strength/effective bond length to the coarse aggregate maximum size and its volume ratio in the concrete mixture design. The concrete compressive strength, though, was proved not to have a significant effect on the FRP-to-concrete bond strength. The experimental results as well indicate the poor performance of guideline models to exactly predict the FRP-to-concrete bond strength/effective bond length. Keywords: FRP composites, Bond strength, Effective bond length, Particle image velocimetry, Single-shear test, Mix design