Steel--Fatigue

One of the major problems the construction industry faces today is low corrosion resistance of reinforcing and prestressing steel, which significantly affects the durability of concrete structures. Theoretically Advanced Composite Materials (ACM) can successfully be used in concrete structures, in lieu of steel, as reinforcing and/or prestressing elements, owing to high tensile strength, immunity towards corrosion, low Young's modulus, light weight and high fatigue resistance. Very little experimental and performance data are available on the properties of ACM and their application in concrete structures. Thus, to ensure safety of the structures, accurate assessment and continuous performance monitoring of the ACM together with the structure have to be made with an option of active and/or passive structural control. This investigation is aimed to establish the feasibility of using Aramid Fiber Reinforced Plastic (AFRP) cables as reinforcing/prestressing elements in concrete bridge structures. Besides investigating the durability of the AFRP cables in adverse environments (alkali and seawater), static and ultimate load tests were performed on a Double-Tee beam and three rectangular beams together with static, fatigue and ultimate load tests on a half scale model Double-Tee bridge system prestressed with AFRP. The AFRP specimens exposed to alkali and seawater for 900 hours retained 88% of the average failure strength of control specimens. Large deformations at ultimate conditions and good fatigue resistance were observed in the experimental studies. A computer code, FRPFLEX, was developed to perform flexural analysis of beams prestressed/reinforced with the ACM. An incremental, stiffness augmented non-linear analysis was performed using grillage analogy to assess static flexural behavior of Double-Tee bridge system. Analytical results showed good correlation with experimental findings. An active deformation/vibration control model is suggested, which can be incorporated in prototype bridges for safety and performance data evaluation. Feasibility of the use of the AFRP cables in bridge structures is assessed based on the experimental and analytical parameters such as deflections, strains, crack distributions, crack widths and energy considerations.

Near-threshold fatigue crack growth rate data were generated for six high strength steels at stress ratios of 0.5 and 0.8 under different environmental conditions, which included air, freely corroding in seawater, and cathodically polarized in seawater to -0.80v, -0.95v and -1.10v (vs SCE). The influence of stress ratio, loading frequency, magnitude of cathodic polarization and initial DeltaK on fatigue crack growth behavior for these high strength steels was experimentally characterized. The results disclosed general trends of fatigue crack growth under conditions relevant to offshore structure applications and circumscribed variables that are primarily influential with regard to service performance. The rate controlling processes and cracking mechanisms of near-threshold fatigue crack growth, particularly under cathodic polarization in seawater, were also studied in conjunction with crack mouth opening displacement measurements, fractographic observation and crack profile examination. The data and observations suggest that calcareous deposit induced closure was the rate-controlling factor for near-threshold fatigue crack growth under cathodic polarization in seawater although the cracking involved a brittle failure mechanism as a result of hydrogen embrittlement. With different FCGR determining procedures which included natural K-decreasing, enhanced K-decreasing, natural K-increasing and constant DeltaK, the FCGR under cathodic polarization in seawater was found to be time-dependent and, as such, was not a unique function of DeltaK but depended upon K-history or experimental procedures. These observations were consistent with results of a theoretical analysis which involved kinetics modeling of the interaction between fatigue crack growth and calcareous deposit thickening. The model also predicted the existence of an upper limit transition DeltaK, DeltaK Upp/T, between the Paris and threshold regions and it was found that the predicted DeltaK Upp/T was in agreement with the experimental one. The basis for applying this critical parameter (DeltaK Upp/T) to offshore structure designs was addressed in conjunction with experimental results.