Study On Performance-based Indicators Of Reinforced Concrete Durability Design Under Atmospheric Environment

The atmospheric environment is the most common service environment for reinforced concrete structures.The transport mechanisms of gas and moisture differ with respective environmental actions.Therefore,it is impossible to characterize the deterioration process of reinforced concrete with a single model.At present,there is a lack of in-depth study on the maintenance strategy and material selection of life-cycle durability design.Understanding the concrete deterioration mechanism corresponding to different environmental actions and establishing models capable of controlling the degradation dynamics process are important prerequisites for establishing a performance-based indicators system of concrete durability,and also the foundation of life-cycle durability design.In this thesis,two mechanisms for de-alkalinization of concrete in atmospheric environment were studied,which were the carbonation of CO2 in the atmosphere and the outward migration of alkaline materials under the action of drying-wetting cycles.The corresponding concrete de-alkalinization model was proposed.The existing carbonation model was modified for carbonation process and the drying-wetting cycles de-alkalinization model based on moisture transport was proposed for drying-wetting action.Then the reinforced concrete durability model considering both the concrete de-alkalinization and the internal steel corrosion was established.The experiments on the parameters of concrete de-alkalinization(accelerated carbonation test,gas permeability test,water vapor sorption isotherms test,etc.)were carried out and the experimental results made it possible to build the regression relationship between the performance-based indicators(diffusion coefficient and water sorptivity)of the durability model and the gas permeability coefficient.The performance-based indicators limits for various working conditions of the atmospheric environment were calculated using the durability model,and further converted to the limits of the gas permeability index that can be measured in-situ.The durability model was also used to study the life-cycle durability design from both economic and environmental aspects,in which the maintenance-free and limited maintenance strategies under various working conditions were compared and the optimal material selection for comprehensive evaluation were made.On the basis of above works,the following conclusions are reached:(1)both the carbonation of CO2 and the migration of alkaline substances under drying-wetting cycles lead to concrete de-alkalinization,but the two mechanisms are different and need to be modelled separately.The main concrete performance-based indicators for controlling carbonation process are CO2 diffusion coefficient and water sorptivity for drying-wetting action;(2)according to the classification of environmental actions of existing standards,the durability design indicators for different design life are obtained through the proposed model,and further converted to the requirements of the gas permeability coefficients for in-situ testing;(3)from the two aspects of economic cost and environmental cost of life-cycle,the optimal maintenance strategies depend on design life.The optimal materials choices change with design life,environmental action intensity,durability limit states and maintenance strategies.Concrete mix can be optimized by the comprehensive evaluation index at minimum economic-environmental costs.