Research On Prestress Loss Of Full Prestressed Concrete Beam

It is known that the loss of the prestress force in tendons occurs due to elastic shortening, creep and shrinkage of concrete, steel relaxation, etc. in the condition of service of prestressed concrete. Existing prestress force and the actual stress in tendons have close connection with the active state of prestressed concrete structure, so the detection of prestress loss is the important part of the health monitoring and detection of prestressed concrete structure . The loss of prestress force is pre-estimated by the change of modal parameter from dynamic nature of construction in this paper.The relations between the frequency and prestress losse of unbonded prestressed concrete beam will be researched chiefly in this paper. Tow unbonded prestressed concrete beams and three bonded prestressed concrete beams were made. Prestress force was applied to these beams in grades, then their first four steps frequencies in every grade prestress force were measured. And relation of change between prestress force and frequencies which are used to analog the prestess loss is analysed in this paper. The test results indicate that the frequencies of these beams are increasing gradually with the increasing of the prestress force in the certain range, then the frequencies decreased slightly when the prestress force out of the range. Finite element model of Prestressed concrete beam was created according to the kinematic model of beam compression bar, then the frequencies and modals of test beam were calculated by it. The results of calculation indicate that the calculated frequencies of these beams is decreassing gradually with the increasing of the prestress force. This conclusion is opposite to the result of test. One of the reasons to this result is that the heterogeneous of the concrete was not considered, with the action of the compression force the minute crack close-up which is appeared when the concrete harden, which leads to the increasing of rigidity and natural frequency,the other factor is that natural frequency of the tendons is increasing when it is tensed, that is the tension in the prestressed tendons increase the whole rigidity of the beam. The finite element model was modified in this two aspects in this paper. Modal sensitivity coefficients (those of frequency, modal shape) of tendons tension force was deduced in the paper. Modal sensitivity coefficients of tendons tension force was calculated with the modifyed finite element model, which is used to predict the change of the tendons tension well with the change of test frequency.