In construction of the offshore structures such as jetties, it is very important to use an economical light structure with high corrosion strength. The wood plastic composites (WPCs) are built from compact wood chips and resin for their low cost and great mechanical properties. One of the problems in WPC beams that are reinforced with fiber reinforcement polymer (FRP) sheets is the premature debonding of the sheet from WPC surface, which leads to premature and non-economic failure in members. In this study to avoid the debonding, longitudinal grooving was used in the beams. For this purpose, 45 beam specimens were cast, strengthened with one or two layers of FRP sheets and then subjected to four-point bending tests. Longitudinal grooves with various widths, depths and the number of reinforcement layers were determined for considering their effects on beam’s behaviour to determine its optimised width and depth. Results showed the groove with 3mm width and 2mm depth strengthened with one layer FRP sheet and the groove with 3mm width and 4mm depth strengthened with two layers of FRP sheet were obtained as optimised width and depth in reinforcement, leading to maximum strength without FRP debonding compared to those without surface preparation
