Life-cycle Design And Optimization For Concrete Structures Based On Reliability And Sustainability

The life-cycle design(LCD)and optimization of concrete structures refer to evaluate and predict the development of the structural performance deterioration by considering environment and load effects in service life;and to optimize the initial design variables and maintenance schemes with the goals of maximum time-varying reliability and minimum life-cycle cost(LCC)in the early stage of structural design,so that the environmental contamination and social disruption of the construction,operation,maintenance and demolition activities can be deduced.However,the current design method does not pay enough attention to the long-term performance of concrete structures,ignores the LCC control and the sustainability evaluation,which cannot meet the users’ multi-objective and detailed design requirements for the life-cycle performance.Considering the abovementioned problems,this dissertation conducts theoretical research and case analyses on the time-varying reliability and sustainability evaluation,as well as their combination of reinforced concrete(RC)structures.The main research work is as follows:(1)The uncertainty characterizations of the random variables in design theories and methods are analysed,and the development process of LCD objective and index systems from the binary independent system to the comprehensive correlation system is introduced.The life-cycle performance and cost modules are modified by using the improved economic,environmental,and social cost indicators.To improve the reliability of multi-objective optimization design and decision-making results,the risk indicator is joined in the cost module and a new LCD system of engineering structures is proposed.(2)Based on the ion transport,corrosion current density and steel pitting development models,the specific meanings of the limit state functions including corrosion,cracking,serviceability,and ultimate limit states are clarified,and the Monte Carlo Simulation(MCS)is used to calculate the time-varying reliability indexes of RC structures under the combined effects of chloride ion erosion and vehicle loads.Then,the structural service life of durability,serviceability and safety are predicted,and the sensitivities of initial design parameters and environmental parameters that may affect the service life and reliability indexes are analysed.(3)Based on the sustainability assessment theory,the system boundary,material and energy flow,and stakeholder categories of the structural life-cycle sustainability assessment(LCSA)are analysed.The assessment objectives and scope of environmental and social impacts,inventory lists,and assessment methods are compiled.The frequencies of social impact assessment contents and social cost categories in previous literature are counted,and the social costs are listed based on the stakeholders and impact categories.Then the unified monetization indicators are used to integrate the three dimensions of LCSA.(4)The direct and indirect cost assessment methods are adopted to incorporate the environmental cost and social cost into the life-cycle cost analysis(LCCA).The environmental cost models,including building materials,mechanical fuels,and vehicle delays,are supplemented.The calculation process is explained by the case study of initial bridge design comparison.Based on the social cost list,quantitative social cost models of engineering activities for construction workers,traffic users,local communities and public organizations are proposed,and the values of key social cost parameters applicable to the Zhejiang Province are summarized.The economic and social costs of the bridge maintenance project are calculated and compared.(5)The durability maintenance and improvement measures applicable to RC structures in different deterioration stages are clarified.The life extension effect on durability,serviceability,and safety of epoxy coating,crack sealing,electrochemical chloride extraction(ECE),bi-directional electro-migration rehabilitation(BIEM)and bonding steel plates on the concrete surface are simulated by MCS.The economic cost and environmental cost of various maintenance measures are calculated.Then,the life-cycle sustainability cost of epoxy coating and ECE maintenance plans of the case bridge are calculated and compared.(6)Applying the life-cycle management(LCM)framework to the 200-year long-life design and optimization analysis of bridge structures,two workable initial design schemes(a.using stainless steel bars,b.expanding the cross-sectional area of steel bars and improving concrete strength)and a combined maintenance scenarios(with epoxy coating,BIEM,and bonding steel plates)are proposed.Their life-cycle sustainability costs are calculated and compared with the structural rebuilding scheme.The uncertainty sources of sustainability costs are analysed,and the cost statistical parameters are given.The utility theory is used to calculate the multi-attribute utility values under different risk attitudes and to evaluate the comprehensive sustainability of different long-life design and maintenance schemes.(7)Based on the multi-objective optimization design theory,the cost objective function including economic,environmental,social and failure costs is proposed.The initial design parameters and the maintenance time of bonding steel plates of the corroded RC structure are analysed by using the Genetic Algorithm(GA)in MATLAB.The single-objective optimization(minimum life-cycle sustainability cost),bi-objective optimization(minimum life-cycle sustainability cost and maximum reliability index)and tri-objective optimization(minimum life-cycle economic,environmental,and social costs)analyses are carried out to help designers conduct a comprehensive life-cycle design and management considering reliability,service life,economy,environment,and society.