Analytical And Experimental Study On Ductility Of Reinforced High Strength Concrete T-section Beams Retrofitted With FRP

In virtue of the advantages of fiber reinforced polymer (FRP), light-weight, high-strength, nice environmental and fatigue resistibility, it has been broadly used in the field of retrofitting reinforced concrete structures. High strength concrete (HSC) offers many advantages such as high strength, low shrinkage and creep, and high durability over conventional concrete. It is increasingly applied as the development of high-rise structures and long-span bridges all around the globe. However, high strength concrete has the disadvantages of low ductility and high fragility of resisting accidental impact, overloading and earthquake. The question whether the ductility of high strength concrete structure can be improved by FRP has become a very attractive topic in the field of structural engineering. Therefore, combining with the research programs of National Natural Science Foundation of China, analytical and experimental study on ductility of reinforced high strength concrete T-section beams retrofitted with CFRP were carried out in this thesis. The main contents of the thesis include:(1) Studies were carried out on the ductility of T-section beams externally bonded with carbon fiber reinforced polymer (CFRP) sheet, and how such retrofitting affects deflection, curvature and energy ductility of strengthened T-section beams were analyzed. A total of nine T-section beams were constructed and tested under flexural load. The effect of longitudinal CFRP amount, type of cross-section of specimen, and the location and configuration of U-anchorage on ductility was studied. Experimental results show that it is necessary to assign U-anchorages in the position of mid-span and loading point to limit the development of crack for the purpose of improving ductility of the strengthened beam. The quantitative relationship between CFRP amount and displacement ductility index by means of theoretic calculation method is proposed on the basis of the relevant research of stiffness. And relevant suggestions for its design and application are also presented.(2) The formulas of calcuating the section curvature of T-beams externally bonded with FRP when they come to the yielding state or the ultimate state are derived based on the definition of curvature ductility index, the plane-section assumption, the assumption of no-bond-slip in the interface and any other basic assumptions. The failure modes of crush of concrete and abruption of FRP are considerable. Furthermore, a simplified calculation method of curvature ductility index is presented. The experimental results and analytical calculation results have a good agreement. (3) On the basis of the improving strengthened methods provided in the experiments of Deifalla and Anil, analytical and experimental investigation of flexural ductility of twelve reinforced HSC T-section beams confined by hybridized CFRP method were carried out. The width of CFRP, the thickness of CFRP and the type of cross-section of T-section beam were considerable. A modified formula of calculating the rotation capacity of reinforced HSC T-section beam confined by hybridized CFRP method is presented based on the methods proposed by Mattock and Baker. The author also gives some construction suggestions for ductile strengthened designing of beam-slab system in the cast-in-place framework structures.(4) A new retrofitted method for improving the mechanical behavior of high strength concrete prisms is invented in which the confined CFRP sheet, embedded steel bolt bar and clamping steel plate are used. Ten groups of confined high strength concrete prisms were axially tested to examine the effect of wrapped FRP sheet, embedded steel bolt bar with clamping steel plate and the hybridized method on the mechanical performance. The effect of these retrofitted methods on improving ductility behavior is analyzed by the theory of strain energy accumulated in the element. This research can also offer a theoretical reference for the mechanism of the improvement of ductility of reinforced HSC T-section beam confined by hybridized FRP method as well.