Experimental Study On Wave-shaped Gear System And Flexural Behavior Of Reinforced Concrete T-beams Strengthened With External Prestressed CFRP Belt Based On Its System

Carbon fiber-reinforced polymer (CFRP) comes from carbon fiber bonded with resin. Due to its high strength, light weight, anti-fatigue, corrosion resistance and other advantages, CFRP material is very prospective for the development and application in the aerospace, aviation and civil engineering. Therefore, CFRP material has been widely used for strengthening structures. The non-prestressed CFRP material strengthen technique fails to utilize the high strength of CFRP materials because it is easy to peel off and to suffer stress lag. At present, most of the prestressed CFRP strengthen technique is that one layer CFRP cloth is stretched and bonded once. This strengthen technique can not rapidly complete strengthen construction because the construction cycle is long due to layer by layer bonding CFRP and polishing the surface of structure. In order to overcome the above problem and achieve rapid strengthen, our research team invent a wave-shape gear (WSG). A series of experiments show that the WSG is both an anchor for CFRP belt and a clamp for realizing transverse tension CFRP belt. The design parameters of WSG anchor system and WSG clamp system and external unbonded prestressed CFRP strengthen techniques based on the WSG system is calibrated by three types of tests designed as following:①Tensile tests for one meter long CFRP belts with WSG or flat steel anchors at two ends in the universal testing machine.②The construction technology that WSG clamps stretched transversely CFRP belt is simulated in steel beam pedestal.③The unbonded external prestressed strengthen technique is that CFRP belt is not affixed to the beam and is transversely pulled by WSG clamp at two sides or bottom of the beam. Five same 7m long reinforced concrete T-beams are designed. Three beams are strengthened at two sides of the beam with above strengthen technique. One beam was strengthened at bottom of the beam with above strengthen technique. But another beam is strengthened with regular CFRP materials strengthen technique. These five beams are tested after being strengthened.Using general purpose three-dimensional finite element (FE) analysis program ABAQUS, nonlinear analyses of four unbonded external prestressed strengthen beams are carried out in this thesis. By comparing the data of above three type experiments and FE analysis, this paper reached the following conclusions. ①The data of WSG anchors in tensile tests and strengthen concrete T-beams are analyzed and the following conclusions are drawn.1) Based on data of WSG anchors in tensile tests, the parameters of the anchor length formula are quantified.2) Test results of the WSG anchors with bond and without bond to anchor CFRP belt indicate that the friction force resultsing from the axial force increment of the first row bolt in the WSG anchor can firmly anchor CFRP belt. For the bolts of WSG anchor system design, the design axial fore and shear fore of the first row bolts can be set as the 80 percent of ultimate tensile of CFRP belt. The design axial fore and shear fore of the remaining row bolts can be set as the 70 percent of ultimate tensile of CFRP belt.3) WSG anchor can reliably anchors CFRP belt until the CFRP belt fracture without slide.②The data of WSG clamps in steel beam pedestal and strengthen concrete T-beams are analyzed and the following conclusions are made.1) When CFRP belts are transversely pulled by WSG clamp, the prestress of CFRP belt can meet the required level of strengthen.2) The reduction factor can be used to evaluate the correction of theoretical elongation in CFRP belt under laterally pulling with WSG clamp. The reduction factor is 0.45 when the folded layer of CFRP belt is within 12 layers. If there is no straightening measurements or the gap of WSG is big, the reduction factor may correspondingly decrease.3) When the bolts of WSG clamp system are designed, the total axial force of the bolts is 4 times pre-tension value of CFRP belt and bolts evenly share this total axial force at the tensile stage. During the loading stage, the total axial force of the bolts is 1.5 times of ultimate tensile of CFRP belt subtracting pre-tension value of CFRP belt and two row bolts at both ends of WSG clamp share this total axial force. The total shear force of the bolts is 20% of pre-tension value of CFRP belt. During the loading stage, the total shear force of the bolts is that the ultimate tensile of CFRP belt subtracts 80% of pre-tension value of CFRP belt and first row bolts at large pulling side share 80% of total shear force, the rest bolts evenly share rest shear force.4) when the CFRP belts are laterally pulled with WSG clamp, the ratio of the axial force of the screw bolts to the pulling force of CFRP belt is about 2, which can be used to monitor the pulling forces of the CFRP belt in existing strengthening methods with straightened measures. 5) According to stretching process, the strengthen technique that CFRP belt is laterally pulled by WSG clamp only requires portable equipment and simple operation, futher it can short construction period compared to stretching and sticking CFRP fabric layer by layer. At the same time, this strengthen technique can achieve requirements of fast construction.③Based on test data of strengthen beams at two sides or bottom of the beam under no initial load and certain amount of initial load, the following conclusions are drawn.1) This external prestressed CFRP sheets strengthen technique can significantly improve the cracking load, yield load, ultimate load and stiffness of RC beams and the width of flexural cracks in concrete beams is effectively controlled.2) The section area of CFRP belt is changed according to moment of beam along the length of beam(the CFRP belt is divided into several sections with four WSG after the CFRP belt is stretched)and the strength capacity of the CFRP belts can be fully utilized. The high strength and deformation performance are achieved after strengthening.3) Initial load has no influence on yield load, ultimate load and deformability of strengthen beam.4) CFRP belt is anchored in multiple-point with WSG because CFRP belt does not slip at the end WSG and middle WSG at damage stage. The pressure of concrete at WSG is small, thus local concrete crushing is avoided at the end anchor.④Stress losses of CFRP belt laterally pulled by WSG clamp are caused by relaxation of CFRP belt and component material, temperature change and the gap of bolts in WSG clamp and components. Suggestion design value of pre-stress losses has been made.⑤The formulas for calculating bearing capacity and the mid-span deflection (before steel yields) used in this thesis is consistent with the experimental data and these formulas are available for design reference.⑥In this thesis, the commercial FE program ABAQUS is used to develop an analysis model for reinforced concrete beams strengthened by external unbonded prestressed CFRP belts under no initial load and the initial load condition. The FE model includes two types of main elements: concrete solid element and external unbonded prestressed CFRP belts and steel truss element. Steels are embedded in concrete solid element by embedded constraints. External unbonded prestressed CFRP belts and concrete solid element are connected with tie constraints in WSG clamp.The reliability of the analysis model is verified by compare analytical results to experimental tests. The computational results of the four-point anchoring model and the two-point anchoring model show that the multi-point anchoring prestressed CFRP belt can change CFRP belt section based on component moment, reduce strengthen cost under same load capacity and deformation capacity condition, and avoid concrete local compression crushing at end anchorage.