Study Of Safety Evaluation On Existing Prestressed Concrete Beam Based On Damage Quantification

The insufficient bearing capacity of in-service prestressed concrete bridge is mainly caused by structural damages, which include reinforcement corrosion, concrete strength decreasing, section defect and crack. Among them, section defect and cracks can be measured directly. Concrete strength decreasing can also be measured by rebound method or core extraction. However, reinforcement corrosion cannot be measured directly in condition that in-service concrete beam is not destroyed. What’s more, prestress level decreasing can also not be measured directly without pre-embedded sensors.Based on the concrete damage theory, this paper takes reinforcement corrosion and prestress level decreasing damages as a starting point and studies the safety evaluation of in-service prestressed concrete beam under various damaging and erosive conditions by experiments and finite element method(FEM) simulation. The main contents are as follows:Firstly, based on the borehole releasing principle, prestress level decreasing can be converted into nominal corrosion rate of prestress reinforcement through defining nominial corrosion rate by FEM, borehole releasing experiments in lab and in in-service prestressed concrete beam, measuring concrete stress level and backstepping reinforcement prestress level and comparing with the initial value. The details are as follows:(1) primary laws about borehole releasing are simulated by elastoplastic damage theory of concrete and birth-death element method of ABAQUS. (2) stress/strain releasing laws, maximum releasing rate and factors are further discussed by in door and field experiments. Hereby, concrete stress level of measuring points and releasing process are given and these lay a good foundation for engineering applications. (3) nominal corrosion rate of prestressed reinforcement, derived from concrete stress level, is computed with provided reasonable data for safety evaluation.Secondly, corrosion-expansion can be classified into uniform corrosion-expansion and non-uniform corrosion-expansion according to corrosion characteristic of reinforcement concrete. For uniform corrosion-expansion, Bazant’s theory model is modified under the consideration of the filling effect of corrosion product in interface gap and cracks. Therefore, hoop stress in concrete cover is modified by introducing concept of damage and the damage model of corrosion-expansion force and critical corrosion rate of reinforcement is established at the moment when concrete cover starts cracking. For non-uniform corrosion-expansion, considering the filling effect of corrosion product in interface gap and cracks, FEM model is established for reinforcement concrete corrosion-expansion cracking based on USDFLD subroutine in ABAQUS. Reinforcement diameter, cover thickness, corrosion-expansion coefficient and some other factors on critical corrosion rate are discussed. Based on the two models, combining Rodriguez model and Vidal formula, the relationship between crack width and corrosion rate is built.Finally, the deflection is used as index for the discussion of reinforcement corrosion, concrete strength decrease, cross section defect and vertical cracks that have influences on deflection at different loads based on FEM model. Deflection correlation is discussed by combined analysis of each two damages. By theoretical analysis, it discusses the effects caused by various damages on flexural capacity and shear capacity of the beam, thus the weights of damages are given. According to the results, the mark-reducing evaluation method based on standard and damage correlation is established. Compared with the mark-reducing evaluation method based on standard, both advantages and disadvantages of these two methods are discussed. Besides, detection methods and evaluation methods are used in an in-service prestressed concrete bridge, Hou Dingxiang bridge, showing good results.