Prestress Loss Recognition Method Of Concrete Beam Based On The New Constitutive

The remaining prestress in concrete beam is an important indicator of prestressedstructural’s safety. Therefore, the prestress-loss testing of structures is particularlyimportant. Except for some large bridge structureswhich were embedded sensors in thestructure at the beginning of construction to realize real-time detection,a large numberof prestressed concrete bridges have entered the mid and old period of their service.How to assess the prestress loss of the old bridges becomes the key of structuredetection.Theoretical studies show that vibration frequencies of unbonded prestressed beamsdon notchange with the prestress. This does not comply with the experimental resultsthat the natural frequencies of the beam increase with the prestress. Existing researchesusuallyprovidean artificial increasing coefficient of the beams’ rigidity, whichis lack ofa theoretical basis and the universal applicability.In order to find the main reason that the natural frequencies of concrete beamchange with the prestress,a large number of measuredstress-strain curves of concretewere analyzed.The new concrete constitutive model, based on tangent modulus of thestressed concrete, is proposed which is suitable for the analysis of prestressed structural.This new model indicates that the compressive tangent modulus of concrete increaseslinearly at first and then decreases in accordance with the quadratic parabolic law whenthe compressive strain increases, showing a two-stage law.The tensile tangent modulusdecreases linearly and monotonically in different phase with the increasing tensile strain,showing three-stage law. This constitutive relation is reasonable and accurate for a lotof experiments. This new constitutive relations can explain the static and dynamiccharacteristics ofthe prestressed beams.The nonlinear characteristics of the material can not be taken into accoutautomatically in the commercial software ANSYS when conducting modal analysis.Aparametric APDL program to consider the non-linear characteristics of concretematerials is programmed in this paper. Elements’ stress is extractedto correct tangentmodulus of the material. The calculated and experimental results are in good agreementwith each other in the overall trend when the new constitutive relations are applied tothe program under different experimental conditions. To analyze the effect of the new constitutive on the static and dynamiccharacteristics of prestressed concrete beams, the form of constraints, beam cracks,tendon eccentricity, span to height ratio of the beam, the standard initial modulus ofconcrete, standard inflection strain and many other factors areconsidered to calculate thenatural modes. The natural frequency increases first and then decreases in the overalltrend when the prestress increases. The static stiffness is also affected by theprestress.Before strain reaches the inflection point, the static stiffness of prestressedbeams increases with the increasingprestress.The non-destructive method used in actualproject to detect the prestress loss by the characteristics that the prestress can change thedynamic stiffness and static stiffness of the beams at the same time is also proposed.