Prediction And Assessment Of The Instantaneous Stiffness For Cracked Reinforced And Prestressed Concrete Beams

Due to low tensile strength of concrete,reinforced concrete（RC）bridge,prestressed concrete（PC）bridge,and other compression-flexure concrete member in service are always cracked.Flexural crack is one of the typical cracks for reinforced concrete members.Either the reduction in cross-section by flexural cracks or the degradation of the bond between concrete and steel reinforcing bars by the interfacial microcracks leads to the decrement in the stiffness of cracked reinforced concrete beam affecting the comfort of driving,even resulting in the safety problem.In this thesis,evaluation method on the time-dependent instantaneous stiffness of cracked reinforced concrete beams by using residual cracks and prediction method on the instantaneous stiffness of cracked prestressed concrete members were proposed.With regard to evaluate stiffness of cracked reinforced concrete beams by using residual cracks,damage parameters D_ccccc under the residual loads can be calculated by using crack information（including the type,the crack width,number and spacing,etc.）with the concrete shrinkage and creep effect.Then,the instantaneous stiffness of cracked reinforced concrete beams can be calculated by using the average inertia model.The results show that the theoretical results are in good agreement with the experimental results,and the theoretical results reflect the stiffness distribution and development of the cracked concrete beams accurately.With regard to predict stiffness of cracked prestressed concrete bridges,the instantaneous stiffness can be calculated by damage parameters D_ccccc based on the axial tensile strain criterion,strain relationship between concrete and steel after bond damage,the depth of pressure zone,etc.The results show that the stiffness prediction mothed can predict the stiffness of cracked prestressed concrete bridges accurately,while the distribution and width of the crack are unknown.