Study On Prestress Loss Of UHPC Thin Plate And Resistance Of RC Beam Strengthened

Ultra-high performance concrete（UHPC）is often used in combination with prestress technology in engineering due to its superior mechanical properties.Compared with the traditional reinforced concrete structure,the prestressed UHPC structure has higher bearing capacity and crack resistance,and can effectively reduce the section size of members and reduce the weight of the structure.At present,prestressed UHPC structures are mainly used in the field of bridge engineering,and most of the members are beam members,and the research focuses on their resistance performance,but there is little research on prestress loss in UHPC members.In this paper,the high-strength properties of UHPC and high-strength steel bars are fully utilized,and a prestressed UHPC thin plate is designed and prepared,which effectively broadens the component form of the prestressed UHPC structure.Taking UHPC thin plate as the research object,the whole process of prestress loss in UHPC components is studied.In addition,based on the excellent mechanical properties of thin plates,this paper puts forward a strengthening method of UHPC thin plates and applies it to the reinforcement of RC beams.The feasibility of the UHPC sheet strengthening method and the influence of UHPC sheet strengthening on the bending performance of RC beams are studied through bending tests.The main work and conclusions of this paper are as follows:（1）In this paper,a UHPC thin plate with a thickness of only 20mm is designed and prepared by combining UHPC with prestress technology.According to the pre-tensioning technology,a set of tensioning equipment suitable for construction in a laboratory environment is designed,which can meet the preparation requirements of the UHPC sheet.The traditional tensioning technology is improved,and the improved tensioning technology can reduce anchoring loss by 50%.Based on the ultra-high strength of UHPC,the tensioning time in UHPC components can be shortened to 3 days to save construction time.（2）The prestress loss of the UHPC sheet was monitored throughout the whole process,and it was found that there were four kinds of prestress losses in the preparation process of the UHPC sheet,namely,anchorage loss,stress relaxation loss of steel bar,elastic compression loss of concrete and shrinkage and creep loss of concrete.After 28 days of stretching,the measured total prestress loss rate of 0.75f_ptk sheet can reach 44.38%,and that of 0.65f_ptk sheet can reach46.61%.（3）Based on the preparation technology of the UHPC sheet,the calculation methods of prestress loss in GB 50010-2010 and JTG 3362-2018 are summarized.Combined with the measured data of the basic properties of raw materials,the prestress loss of the UHPC sheet is calculated and corrected theoretically.Comparing the calculated results of prestress loss with the measured results,it is found that the calculation of prestress loss in the two codes is too small,which underestimates the actual prestress loss of the UHPC sheet.Comparing the results of effective prestress,it is found that the error between the calculated results of GB 50010-2010and the measured results is smaller,and the error between the two kinds of tension control stresses is within 5%,while the calculated results of JTG 3362-2018 are larger,and the maximum error can reach 15.69%.Therefore,it is recommended to use GB 50010-2010 code to calculate prestress loss in UHPC components.（4）In this paper,a method of strengthening the UHPC sheet is proposed,and the feasibility of this method is verified by the bending test of the RC beam.By strengthening the normal section of the RC beam with the UHPC sheet,the influence of different sheet lengths on the flexural performance of the RC beam is studied.The test results show that both the unreinforced beam and the reinforced beam are damaged by suitable reinforcement,and the crack resistance and bending resistance of RC beams can be significantly improved by strengthening with UHPC thin plates.After strengthening with the UHPC sheet,the cracking load and ultimate bearing capacity of RC beams can be increased by 185.7%and 36.3%respectively.Using UHPC sheets to strengthen RC beams can effectively inhibit the generation and development of cracks,and greatly improve the flexural rigidity of RC beams,but it will cause some losses to the ductility of RC beams.Compared with the unreinforced beam,the ultimate deflection of the reinforced beam can be reduced by 57%,and the ductility of the reinforced beam can be reduced by 66.59%.（5）Based on the analysis of the test results in this paper,a theoretical formula for calculating the cracking load of RC beams strengthened with UHPC sheets is proposed,and it is found that the calculated values are in good agreement with the experimental values.The ultimate bearing capacity of strengthened beams is analyzed.Under the condition of normal section failure,the flexural bearing capacity of strengthened beams is theoretically deduced,and the corresponding theoretical calculation formula is put forward.