Flexural Experimental Study Of T-shaped Concrete Beams Strengthened With Near Surface Mounted FRP Under Two-stage Loading

Recently, researchers have proposed a strengthening technique which is called Near-surface mounted FRP reinforcement. The FRP bars or laminates are bonded into slits which are cut on the concrete cover previously using bonding materials. The bonding materials such as epoxy adhesive can make the FRP bars or laminates work together with the primary structure, with the aim of increasing the flexural capacity and shear capacity of reinforced concrete structures. And this is a promising technique using FRP for retrofitting reinforced concrete structures.The experimental study is composed of5T-shaped reinforced concrete beams, which are strengthened with CFRP laminates under sustaining load, after strengthening secondary loading was applied, with the aim of examining the flexural behavior. The main experimental parameters include different levels of sustaining load at the time of strengthening, load history and amounts of CFRP laminates. Based on the research results of FRP initial delayed strain and bonding performance between FRP and bonding materials, a flexural capacity model is proposed for the debonding failure between FRP and epoxy adhesive. The results obtained by the flexural capacity model are compared with the experimental results, and we find that they are in agreement with each other.The experimental results show that, the flexural stiffness, the steel yielding strength and the ultimate strength of the T-shaped reinforced concrete beams can be enhanced obviously by using this technique, the ultimate strength is increased by29.4%-64.7%. Considering the FRP’s initial strain or not doesn’t have an important influence on the flexural capacity of the experimental beams. Although accounting for the FRP’s initial strain could make the error between calculating values and experimental values smaller, the error of the flexural capacity is only-2%～4%between consider the FRP’s initial strain and neglecting it. The preloading level will affect the strengthed beam’s flexural capacity when the beams are strengthend under the sustaining load which is smaller than the steel yielding strength. It shows a trend that the ultimate strength increases with the levels of preloading upgraded at the same amounts of CFRP. When the preloading is at the same level, the ultimate strength increases as the amounts of CFRP laminates increase.