Study On Mechanical Properties Of Corroded Steel Bars And The Time-spatial-varying Degradation Model Of RC Beams Resistance

Steel corrosion is an important reason to reduce the durability of concrete structures,although a number of studies have been carried out on the mechanical properties of corroded steel bar and its degradation,but there are some key issues need to be further research.Axial tensile test and scanning electron microscope(ESEM)test of steel bar specimens which was obtained by breaking the electrochemical accelerated corrosion RC beams to study the effect of corrosion on the performance index and the failure mode,and studies all aspects of the forms,characteristics and distribution of pitting corrosion.Finally,time-varying improved model of cross-sectional area of reinforcement and resistance time-spatial-varying regression model of RC beams are established,the main contents are as follows:(1)Based on tensile test results and different corrosion evaluation indexes,the mechanical properties of corroded steel bars were studied.With the increase of the degree of corrosion,the nominal strength of steel corrosion is approximately linearly decreased,and the pit corrosion strength remains unchanged;the mean value of elastic modulus E_s and nominal strength(α_y、α_u)did not reject the normal distribution;combined with scanning electron microscopy(SEM)test,the fracture morphology and fracture mechanism of steel bars are studied from the macroscopic and microscopic view.The limit corrosion rate of steel bar failure mode is discussed:when the maximum section loss rate C≤14.5%,all steel bars are all ductile failure;when C>14.5%,74%reinforced brittle failure;C>20.23%and all steel bars are brittle damage.(2)Shape and distribution of etch pits are analyzed:when the degree of corrosion is low,ellipsoidal pit has a large proportion;with the degree of corrosion increases,the distribution frequency generally showed a declining trend,while the saddle pit shape distribution just the opposite.The frequency distribution of pyramidal pits increased first and then decreased.(3)Statistical analysis of the three-dimensional size of the pit found:the depth and length of the corrosion pits do not reject the normal distribution,pit width does not reject the log normal distribution;comprehensively considering the influence of the three pit size on the nominal strength of steel bars show:the mean of corrosion pit depth has the greatest effect on the nominal strength,followed by the width,the length effect is negligible.Through linear fitting of the etch pits maximum dimension and the extent of corrosion show that the maximum depth of the pit growth is the fastest and not uniform,large discrete,followed by the largest width of the pit,the maximum length of the pit is relatively slow but relatively uniform,and the dispersion is small.(4)A new calculation model of sectional area loss is presented,and the reliability of the model is verified by the measured data;considering two kinds of corrosion state of corrosion and uniform corrosion,the improved model of steel bar section area is established,and carries on the statistical analysis to the corresponding weight parameter;finally based on the random field theory,considering the structure of the spatial variability of the parameters,establish a time-spatial-varying regression model of RC beams resistance and carries on the actual example analysis.