Study On The Flexural Performance Of T Concrete Beams Strengthened With Near Surface Mounted FRP Bars

At present, there are many dangerous concrete bridges in our city, approximately 14 percent in the number of total bridges, proportion increasing year by year, which has much bad influence on the people's life, live and safety, so these bridges should be strengthened. Fiber Reinforced Polymer (FRP), light weight, high strength and corrosion resistance, is a new material, used in the strengthening of concrete structures widely. The method strengthened with near surface mounted FRP bars is a new strengthening method, grooving in the surface of concrete structure member (thickness of concrete cover), embedding FRP bar and bonding with binder (modified epoxy), to increase the capacity of member. It has many advantages, construction convenience, fire resistance, high temperature resistance and durability, comparing with traditional strengthening methods. Thereby, experimental investigate on T concrete beams strengthened with near surface mounted FRP bars was taken, studying the changing rules of cracks development, failure modes and flexural performance, studying the changing rules of flexural performance in the influencing of different concrete strength, reinforcement ratio, additional anchorage, surface characters, diameters and materials of FRP bars, providing and correcting the calculation formula of flexural capacity in T concrete beams strengthened with near surface mounted FRP bars based on the standards and the results of experiment. The main investigate contents as follows:To study and contrast the cracks development and failure modes in strengthened beams and unstrengthened beams,10 strengthened beams and 3 unstrengthened beams were investigated in flexural static experiment. The results show that more cracks with less spacing and narrower width,45 degree of axis and vertical cracks like root in the bottom of strengthened beams, and the speed of cracks development delay. The vertical stripping cracks are avoided by additional anchorage efficiently in GFRP strengthened beams; the cracks develop faster in plain GFRP strengthened beams; the larger the diameter of GFRP bars, the more the number of cracks, the less the spacing and width; vertical stripping cracks are more easily happening in CFRP strengthened beams. Bending failure happens in the strengthened beams with additional anchorage, and concrete stripping failure happens in the strengthened beams without additional anchorage. Bending failure happens in the strengthened beams with ribbed GFRP bars, and FRP-epoxy interface failure happens in the strengthened beams with plain GFRP bars. The diameter of FRP bars is no better larger than the half of deepth and width of the groove, or else FRP-epoxy interface failure would happen easily. Concrete stripping failure happens easily in CFRP strengthened beams.To study and contrast the changing rules of stiffness, capacity, mid-span displacement, ductility, strain in mid-span steel bar in strengthened beams and unstrengthened beams, the results of experiment are investigated. The results show that stiffness of strengthened beams in crack stage has no increase, but yield and ultimate stage have increase, especially significant increasing in ultimate stage. The crack load of strengthened beams has no increase, and yield load increases 15.32%-67.07%, and ultimate load increases 16.3%-76.17%, especially significant increasing in ultimate load. The mid-span displacement of strengthened beams in crack stage has no decrease, but yield and ultimate stage have decrease, especially significant decreasing in ultimate stage. The ductility of all strengthened beams has decrease. The mid-span steel bar of strengthened beams in crack stage has no delay, but yield and ultimate stage have delay, especially significant delay in ultimate stage.To study the influencing rules of concrete strength, reinforcement ratio, additional anchorage, surface characters, diameters and materials of FRP bars on stiffness, capacity, mid-span displacement, ductility, strain in mid-span steel and FRP bar in strengthened beams, the experiment results of 10 strengthened beams are investigated. The results show that concrete strength has significant influence on the increase rate of yield and ultimate load, but no influence on others. The larger the reinforcement ratio of strengthened beams, the larger stiffness, yield load, ultimate load and mid-span yield displacement, the lower the increase rate of yield and ultimate load and ductility, and no different in others. Additional anchorage has important effect in failure stage, the strain in mid-span steel bar of strengthened beams with additional anchorage is less, but others are larger. The stiffness, ultimate load, the increase rate of ultimate load, the strain in mid-span and coefficient of utilization in FRP bar of beams strengthened with ribbed GFRP bars are larger, but the less mid-span yield and ultimate displacement, ductility and strain in mid-span steel, and no different in others. The larger the diameter of GFRP bars, the larger stiffness, ultimate load and the increase rate of ultimate load of strengthened beams, but the less the strain in mid-span steel bar, and no different in others; but if the diameter of GFRP bar is too large, though the mid-span yield and ultimate displacement are larger, the strain in mid-span and coefficient of utilization in FRP bar is lower. The stiffness, yield and ultimate load, the increase rate of yield and ultimate load are larger in the beams strengthened with CFRP bars, but the mid-span ultimate displacement, ductility, the strain in mid-span and coefficient of utilization in FRP bar are less, and no different in others.To verify the calculation formula of flexural capacity in T concrete beams strengthened with near surface mounted FRP bars applying or not, at first, the calculation formula of flexural capacity in members strengthened with bonding FRP in Design Code for Strengthening Concrete Structure GB 50367-2006 is based, and the calculation formula of flexural capacity in T members in Code for Design of Concrete Structures GB 50010-2010 is also based, and the calculation formula of flexural capacity in T concrete beams strengthened with near surface mounted FRP bars is provided. Then through the method of binary linear regression to make sure the reduction coefficient km in the calculation formula of flexural capacity, based on the results in contrast of test and theory in flexural capacity in 4 strengthened beams with bending failure. The corrected calculation formula of flexural capacity in beams, strengthening with near surface mounted FRP bars, corresponds with the experiment results well.