Experimental Study On Long Term Prestress Losses With Reinforced Concrete Beams Strengthened With Prestressed CFRP Plates

The reinforced concrete beams strengthened with prestressed carbon fiber plates(CFRP)can effectively exert the advantages of high strength and light weight of carbon fiber materials,and can also improve the utilization ratio of materials.Compared with non prestressed CFRP plates reinforced concrete beams,the prestressed CFRP plates can significantly improve the flexural rigidity and bearing capacity of reinforced concrete beams,reduce cracks and improve the working performance.When the prestressed CFRP plates is strengthened,the prestress loss is an important index that cannot be neglected.The size of the prestress loss will directly affect the reinforcement effect.In this paper,fiber Bragg Grating(FBG)is used to monitor the prestress loss of CFRP plates with 12 reinforced concrete beams,and the composition of prestress loss is analyzed theoretically.The experimental results are compared with the theoretical calculations.At the same time,according to the form of anchorage(anchor plate anchor and plate anchor),beam type(T beam and small size rectangular beam),prestress loss is comparatively studied.In addition,the load test of reinforced beams is designed to monitor the behavior of reinforced beams under different loading levels and different environments(dry and wet).The deflection of reinforced beams,the strain distribution of CFRP plates and the strain law of concrete are discussed.The main research contents and conclusions are as follows:Firstly,the experimental results show that the prestress loss of the flat-plate reinforced beams is about 3%,and the loss is concentrated in the end of the carbon fiber sheet near the tensioning end.The cumulative loss of the rectangular beam strengthened by Lica anchorage system is 6.07% ~ 10.96% after 7 days of reinforcement.The results also show that the prestress loss is closely related to the fastening effect of the nut,and the plate anchorage clamping plate should be set in the engineering practice,Loosen the front nut.Secondly,the prestress loss of two large size T beam is monitored,and the prestress loss is less than 5% after 90 d.The theoretical calculation formula of prestress loss is used to verify the long-term prestress loss of T beam,the theoretical loss ratio of TP20% is smaller,and the theoretical loss ratio of TP40% is larger than the experimental value.The results show that the accumulative loss of T beam in 7d is less than 5%,and the average loss of rectangular beam is about 8% in the short term test of T beam and rectangular beam.Thirdly,the design of reinforcement to load test beams,studied under sustaining load beam strengthened by CFRP plate deflection,strain,strain variation of concrete The changes of the parameters under the condition of different holding capacity,prestress level and dry and wet environment of carbon fiber sheet were analyzed.The test results show that the stiffness of reinforced concrete beam can be improved by increasing the prestress level of CFRP plate,and the deflection of the strengthened beam can be further reduced.The influence of the carbon plate on the deflection of the beam is small.Fourthly,the number,length and width of cracks in reinforced beams are analyzed quantitatively by means of the study on the bearing capacity.The variation of strain distribution of CFRP plate under load is analyzed with the time,and it is found that the concrete strain tends to be stable after 4 months.The creep deflection of reinforced concrete beam is calculated with the time,and the experimental results are compared with the experimental values.