Fatigue Behavior Analysis, Safety And Durability Assessment Of Corroded RC Beam Bridges In Coastal Area

The aging of reinforced concrete (RC) bridge structures, the coastal environment with high content of chlorine ion, the extensive use of snow-melting salt in winter, the rapid growth of traffic as well as the widespread overload of freight cause the simultaneous increase of corrosion degree, stress amplitude and fatigue cycles of steel bars in concrete. A large number of corroded RC bridge structures are in urgent need of fatigue performance analysis, fatigue life prediction and safety and durability assessment. In order to solve the above problems, this paper has done the following research:(1) The electrochemical accelerated corrosion tests of RC beam specimens were carried out. The corrosion mechanisms of steel bar in concrete under natural climate condition and by accelerated corrosion were analyzed. The control equation for the corrosion degree of steel bar was derived. And the control technique of accelerated corrosion test was summarized.(2) Fatigue loading tests of corroded RC beams were carried out to study the failure mode, fatigue life, crack, concrete strain and mid-span deflection of corroded RC beams under repeated loading. Test results show:Brittle fatigue failure occurred in all the corroded beam specimens after cyclic loading to a certain number of times, which was dangerous and without any warning. And the higher the corrosion degree of steel bar was, the shorter the fatigue life of RC beam was. The occurrence of corrosion has greatly reduced the fatigue life of RC beams. Fatigue failure did not occur in the uncorroded beam specimen after 2 million times of cyclic loading. And the uncorroded beam specimen exhibited good ductile failure characteristics in subsequent monotonic static loading test. It is showed that fatigue effect has no substantial influence on the static performance of RC beam without corrosion. Under repeated loading, with the increase of fatigue cycles, flexural stiffness change of the corroded beam specimens had two obvious features of stages, namely slowly decreasing stage and stable stage. The modified calculation formula for flexural stiffness of corroded RC beam under repeated loading was quantitatively presented. The formula was verified by comparing the calculated values with the measured values, which can provide reference for calculation and evaluation of deflection for corroded RC bridge structures under long-term live load.(3) Through the static tensile test of reinforcement specimens that were removed from the beam specimens after fatigue loading test, mechanical behavior of steel bars after corrosion fatigue were studied. Test results show:After 2 million times of cyclic loading, the uncorroded steel bars like the steel bars of original state without corrosion fatigue, showed good ductile failure characteristics. Fatigue effect has no substantial influence on mechanical properties of uncorroded steel bars. Characteristics of stress-strain curves for steel bars after corrosion fatigue changed obviously:the yield strength decreased, the ultimate tensile strain decreased, and features of yield plateau changed. Material properties of steel bars after corrosion fatigue changed from initial mild steel into exhibiting a certain degree of hardening. Degree of the change was related to the degree of corrosion fatigue. The emergence of fatigue crack in the steel bar makes the static failure mode change:the yield strength decreased significantly, and tensile failure suddenly occurred in extremely low percentage of elongation. Ductility is almost completely lost with the risk of brittle failure. Constitutive relation of the steel bars after corrosion fatigue changed correspondingly. Combined with the experimentally obtained stress-strain curves for the steel bars after corrosion fatigue, constitutive relation model for the steel bars in concrete after corrosion fatigue was quantitatively presented. And the model was verified by comparing the calculated values with the measured values, which can provide reference for calculation and evaluation of corroded RC bridge structures under long-term live load.(4) Fatigue life evaluation of corroded RC beams was studied. The method of compiling fatigue load spectrum was introduced. Based on the fatigue test results, correction coefficient was introduced considering the effect of corrosion. And the S-N curve equation of corroded RC beam was fitted out. Based on the S-N curve of corroded RC beam and Miner’law of fatigue cumulative damage, the fatigue life evaluation method of corroded RC beam bridge was presented. The evaluation procedure was described and evaluation example was given, which can provide technical support for the scientific maintenance decision of bridge managers.(5) The basic theory and method of bridge assessment were studied. A piecewise linear dimensionless normalized evaluation model was established, and the weight calculation method of evaluation index was presented. The variable weight theory was introduced so that, those individual assessment results with severe damage can be reflected in the overall assessment results.(6) Taking medium and small span RC beam bridges as the research object and considering the features and influence of coastal enviromment, the durability and safety assessment of RC beam bridges in coastal areas was studied. The durability evaluation system of RC beam bridges was established. Combined with the evaluation method proposed in this paper, a method of evaluating the durability of RC beam bridges in coastal areas was presented. Based on the evaluation system of JTG/T H21-2011, by improving the evaluation criteria of some indexes in the specification and replacing constant right with variable right, a method of evaluating the technical status of RC beam bridges in coastal areas was presented. Based on the load-bearing capacity evaluation method of JTG/T J21-2011, by modifying the values of comprehensive modification coefficients in the specification, quantitative expressing the section-reduction coefficient of steel bar that is qualitative described in the specification, and introducing the strength-reduction coefficient of steel bar considering the effect of corrosion fatigue, a method of evaluating the safety of RC beam bridges in coastal areas was presented. Finally, through comparative analysis of three bridge evaluation examples in different areas, validity and applicability of the evaluation methods proposed in this paper are proved. Our proposed methods are necessary supplement and improvement to the current standard.