PRELIMINARY STUDY ON THE EFFECT OF MOISTURE ON THE FLEXURAL BEHAVIOUR OF BEAM STRENGTHENING WITH CARBON COMPOSITE PLATE Piti Sukontasukkul Assistant Professor, King Mongkut’s Institute of Technology-North Bangkok, Thailand Adinan Lengwong Graduate Student, King Mongkut’s Institute of Technology-North Bangkok, Thailand Thawatchai Kongsaktragool Technical Manager, Sika (Thailand) Co., Ltd., Thailand Abstract The experimental results shown in this paper is part of the one-year study on the effect of moisture on the flexural property of reinforced concrete beam bonded with carbon composite plate. Both Conventional Reinforced Concrete beams (RC Beam) and RC beams bonded at the bottom with Carbon Composite plate (CC Beam) were fully submerged in water for 30, 60, 120, 180 and 270 days at all time. Results indicated that short-term submerging (30-120 days) does not affect the flexural behaviour of CCB. However, at the long term (>120 days), there were signs of strength drop, changes in behaviour and the pattern of failure. 1. Introduction The deterioration of concrete structures due to severe weather condition is one of the serious problems worldwide. Several techniques have been invented to find ways to repair and extend the service life of existing structures. One of the modern techniques is the external wrapping or bonding of composite materials to the concrete member. The technique is simple and quick. Composite plates can simply be wrapped around or bonded to structural members in several patterns to match different load types. Theoretically, with the perfect bond assumption, a composite plate is supposed to react to load in harmonized with the structural members it is bonded with. In practice, the perfect bond condition through out the loading is hard to achieve. Most composite plates are able