| The heavy haul railway is a key facility of improving the railway cargo transport capacity. And its economic and social benefits are distinct for its strong transport capacity. With rapid development of national economy, improving the axle load of vehicle is a development trend of heavy haulage in the future. Increasing axle load can enhance the shock and vibration, and cause some damage as same as cracking in the beam body and so on, which may seriously endanger the safety of railway train operation for the existing railway lines. The external prestressing technique is used widely, for it can greatly improve bearing capacity among all the ways of bridge strengthening. Due to the long construction period in constructing the anchor devices and the tedious maintenance of the bareness steel bar in traditional external prestressing technology, discovering and researching new reinforce devices for external prestressing technology is significant to guarantee the safety of railway traffic and realize the sustainable development of the overloaded railway transport.With support of a subtopic of the National High-tech Research and Development Projects (863)(Project code:2009AA11Z102-2), in view of structural style of heavy load bridge, a reasonable layout pattern of external prestressing system was put forward, and a set external prestressing strengthening device of CFRP (carbon fibre reinforced plastic) tendon were developed. To systematic study flexural performance of bridges strengthed by externally prestressed CFRP tendons, tests along with numerical simulation and theories were carried out in this paper. The main work and achievements are as follows:(1) A novel external prestressing technique for strengthening of concrete bridges was proposed, which includes anchor devices and tension devices, and experimental study of tension and anchor performance and finite-element analysis simulating the geometry had been carried out. Tension strength tests were conducted repeatedly on clamping-type anchorage assemblies of CFRP tendon which was preloaded by means of designed devices. The results show that the designed device plays an effective role in preloading. CFRP tendon can cooperate with beam well during the whole loading process of strengthened beams, and the finite element calculation results agree with the test results well.The anchorage devices show good elastic behavior in the whole loading process and the maximum principal stress of concrete near the web plate is less than the standard value of tensile strength. The designed externally prestressed strengthening device can satisfy the requirements of the heavy haul railway bridge strengthening, which is fast, uninterrupted transportation and greatly improving the bridges' bearing capacity.(2) To study the flexural behavior of beams strengthened by external prestressing CFRP tendons, a static experimental study on eleven RC strengthened T-beams were conducted.The influences of tension control stress, the ratio of longitudinal bar, concrete strength grade and the original crack width on flexural behavior were analyzed. The results shows that the loading process of strengthened beams is the same as common beams and it can be divided into three stages which are flexible stage, fracture stage and failure stage. Compared with the common beams, using external prestressing CFRP tendon can significantly improve flexural capacity, flexural rigidity and ultimate deformation of beams, and restrain the crack development. The effects of tension control stress and the ratio of longitudinal bar on flexural bearing capacity are prominent,but the influence of concrete grade is much less. Original fracture has distinct impacts on the cracking load of strengthened beams, but the influence on the yield load and ultimate load is little. The external prestressing can make the original crack closed. The average strains of concrete and CFRP tendon of strengthened beams is basically a linear change along with the depth of midspan section before cracking, and the average strain of CFRP tendon is obviously less than the strain of bottom concrete after cracking. The deformability of strengthened beams is decreasing with concrete strength grade, the ratio of longitudinal bar and the initial tension stress, but increasing with the original crack's width.(3) The tension-anchorage test of CFRP tendon and long-term stress observation test of two strengthened beams with different control stress were conducted, based on the existing prestress loss calculation method, the reasearch on prestressing loss of CFRP tendons was done. The results show that the prestressing losses of CFRP tendon of strengthened beams is caused by five main factors, as follows, the elastic compression of concrete, retraction of anchorage plates, temperature change, shrinkage and creep of concrete and the relaxation of CFRP tendon and steel plate. The higher initial stress level of CFRP tendon, the more prestressing losses. The two testing beams' prestressing losses of CFRP tendon observed in100days have a percentage in the initial tension stress, which are12.3%and19.5%respectively. The short-term prestressing losses are2.1%and4.2%of the initial tension stress, and the long-term losses are10%and16.0%of the initial effective stress. When the CFRP tendon has a higher initial stress, the losses are caused primarily by relaxation of CFRP tendon and long-term deformation of steel plate. The result can provide a reference for practical application of anchoring device.(4) According to the test results and the characteristic of anchorage device itself, the ultimate stress increment of CFRP tendon was got by means of calculation theory of equivalent plastic hinge length. On the basis of considering the quadratic effect, the calculation formula of flexural bearing capacity on concrete T-beams strengthened by external prestressing CFRP tendons was established. Finally, finite element software-ABAQUS was used to analysis the main factors which effect the reinforced beam's flexural capacity and the CFRP tendon's stress increment. The research result can provide a reference for practical engineering design.(5) Based on the test results and combining theoretical analysis, the calculation method of short-time deflection on concrete simply supported T-beams strengthened by external prestressing CFRP tendons were studied. The results shows that the flexural stiffness of concrete beams with incipient fracture should be divided into two stages to consider in the process of applying externally prestressed CFRP tendon, which are cracking stage and non-cracking stage. The calculation formula of short-time deflection on beams strengthened with externally prestressed CFRP tendons was established using the method of effective inertia considering the influence of second order effects, which can provide a reference for practical engineering design.Finally, on the basis of summarizing the full text, and further research Suggestions are put forward. |
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