Experimental And Theoretical Study On RC Beams Strengthened With CFRP

CFRP(Carbon Fiber Reinforced Polymer) was widely utilized in structural retrofit and reinforcement field because of its lightweight, high strength, good corrosion resistance and fatigue durability in the last ten years. A series of researches on beams strengthened with CFRP have been conducted. Most of these researches focused on simply surpported beams, but little of these researches on continuous beams. In fact continuous beams were widely used in existing concrete structures. On most cases the high strength of CFRP which is nonprestressed cann't be used fully. The problem can be sovled by applying prestress to CFRP. This paper focues on the mechanical properties of continuous beams strenghened with CFRP and simply surpported beams strenghened with prestressed CFRP.In this paper, two groups of tests of 8 T continuous beams strengthened with CFRP are carried out. CFRP is adhered in the hogging region in Group A but in the sagging region in Group B. The mechnical properties including failure models, enhancement of load and moment capacities, strains of materials, moment redistribution and the effectivity of CFRP adhered on the flange of the specimens in group A are investigated.The moment distribution of continuous beam strengthened with CFRP is analysed. Neutral axis depth approch which is adopted by the current code is not suitable for the continuous beam strengthened with CFRP based on the results of the tests and theoretical analysis. The moment distribution factor of a two spans continuous beams strengthened with CFRP is deduced by plastic hinge approch. A practical method is established based on some reasonable simplifications.In order to achieve the performance of the specimens under ultimate load, non-linear analysis is carried out with general FEA software ANSYS. The results of experiment and calculation are compared. The process how the CFRP stress on flange transfers to web plate of continuous beam is studied. The design method about rehabilitate of continuous beam with CFRP on flange of part in tension is set forth. The foreign and domestic published papers provide various construction technologies of prestressed CFRP strengthening beams. The prestressed CFRP is adhered to the tension surface of the beam in these construction technologies. The disvantages of these technologies are those CFRP cann't be prestressed ideal stress level before the resin impregation and the process of prompting CFRP results in a considerable prestress loss. To overcome these disvantages a new technology in which CFRP needn't be adhered to the tension surface of the beam is developed. In this paper, the core of the new construction technology and the prestress loss are studied.The diference between mechenical properties of the beams strengthened with bonded and unbonded prestressed CFRP is the stress increment in CFRP. And the eccentricity of the prestressed CFRP is changeable with the defelction of the beam increcing. This phenomenon is called second-order effect. The stress increment in CFRP can be caculated by force method, plastic hinge method and variable flexual rigitity method. The second-order effect decrease the ultimate moment capacity of beam strengthened with unbonded prestressed CFRP. Through a case, the diference of the stress increment in CFRP between force method, plastic hinge method and variable flexual rigitity method is about 20 percent. The ultimate moment capacity when we consider second-order effect is 93 percent of that when we neglect second-order effect. And 50 percent material can be saved when beam is strengthened with unbonded prestressed CFRP in place of nonprestressed CFRP.In the finality, the problems requiring further studies are discussed.