Effect Of Steel Strands Corrosion On Prestressed Concrete Structures Under Chloride Environment

The problem of prestressed concrete structures durability based on corrosion of tendons is becoming increasingly remarkable, but the investigations are very poor. So, as the basic and instant problem, the corrosion effects of prestressed concrete structures with steel strands in chloride environment are studied in this dissertation.The experimental research shows that typical pitting forms appears on steel strands in concrete contaminated with chloride, and the configurations of pits include 3 modes as pyramid, spheroidicity and saddle. The statistics of geometrical size of pits shows a fractal characteristic in the distribution of pits depth and pits length, and there are many shallow and short and moderate wide pits.The theoretical analyse based on the microstructures of material and the long pe-riod experimental research by simulating actual conditions show that it is not susceptive to the stress corrosion cracking with both anodic dissolution and hydrogen induced cracking of steel strands in concrete contaminated with chloride. The tests indicate that the flexural fatigue fracture of steel wires of steel strands shows a typical anisotropic property.A new understanding about the corrosion mechanism of steel bars in concrete is put forward, namely, both the cathodic process and anodic process take place in active zones; polarizing resistance is the only dominant resistance of corrosion. Oxygen diffu-sion control and water supplying control are the only 2 control modes of steel bars cor-rosion.Based on the new understanding about the corrosion mechanism, the basic theo-retical models and the applied models of corrosion rate of steel bars in concrete under the 2 control conditions are derived with corrosion polarization theory and test data and oxygen diffusion coefficient from literature. Both the experimental results and the trial examples show that the corrosion rate of steel strands of prestressed structures in normal service state increases 27% than it in stress of 0.The static tension tests show that, to the steel strands with pits, the embrittlement increases and the ultimate tensile capacity and ultimate mean strain decline. The frac-tures of wires of steel strands include 3 basic modes and their combined modes, the 3 basic modes are cleavage-mode, fraise-mode and cup-mode. With SEM technique and metal fracture theory, and considering the microstructures characteristic, the mi-cro-fracture mechanism of corroded wires is put forward which based on the orientation of pearlite colonies in pits.The fracture criterion of corroded steel strands is put forward based on the fracture mechanism in which the Mises total strain is used as fracture parameter. The distribution regularities and models of fracture resistance of corroded steel strands are obtained by tensile testing cooperated with finite element simulation and mathematical statistics. The distribution regularities and models of fracture effect of corroded steel strands are obtained through finite element simulation and mathematical statistics of 4 corroded steel strands. Based on the above, the model of nonserviceable probability in fracture varying with corrosion rate is obtained through reliability theory of structures.The experimental research shows that, when corrosion cracks width is within the range of 0 to 0.7mm, the behaviors of bond between corroded steel strands and concrete do not appear significantly decreasing, and furthermore, the ultimate bond strength and stiffness increase a little.The experimental research shows that 2 typical failure modes are occurred in cor-roded prestressed concrete beams. The 2 typical failure modes include the traditional fit-reinforcement-failure mode and the wire-breakage failure mode put forward in this dissertation. Under the condition of low corrosion rate (less than 2.87%), the effects of corrosion are not remarkable to the cracking moment, the yielding moment, the ultimate moment, the cracking deflection, and the yielding deflection, but are remarkable to the ultimate deflection.It is put forward about a research idea that the design and evaluation of carrying capacity of prestressed concrete beams with corroded steel strands may be carried out in terms of the applied design expressions of probabilistic limit states used in the code, and in which the variation of probabilistic values of tensile strength indexes of steel strands caused by corrosion is considered. Based on the former model of nonserviceable prob-ability in fracture, the models of characteristic value and design value of tensile strength of corroded steel strands in prestressed concrete beams are derived by combining the crack theory of concrete structures.