Sustainable Design Of Fly Ash RC Beams Based On Life Expectancy Under Climate Change Scenarios

As the world’s largest bridge powerhouse,China has constructed more than 900,000highway bridges,with a total length of more than 70 million metres.With the increase in service life,the structural performance of China’s bridges in service has deteriorated and the phenomenon of"ageing"has become increasingly prominent.Nevertheless,traditional bridge design is based on the limit state design method of time-invariant reliability theory,which mainly focuses on the safety,serviceability and durability of the bridge at the initial stage of construction,ignoring the impacts of structural deterioration over decades of service.Furthermore,the deterioration of the structural performance of a bridge depends not only on the construction process and materials used,but also on environmental factors such as atmospheric CO₂concentration,temperature and humidity.It is therefore necessary to consider the effects of durability degradation at the outset of structural design and to develop a whole life cycle design concept.According to the sixth report of the Intergovernmental Panel on Climate Change（IPCC）and the latest climate bulletin issued by the World Meteorological Organisation（WMO）,global temperature increases will continue,with warming expected to reach or exceed 1.5°C,pushing ecosystems to a critical level.Such a severe climate change situation will pose unprecedented challenges to bridge structures.Global warming is closely linked to greenhouse gas emissions from human activities.The construction and maintenance of bridges consumes a large amount of resources and energy and generates a large amount of CO₂.According to studies,every ton of cement produced emits0.65-0.95 ton of CO₂into the atmosphere.Therefore,in order to implement the"3060"double carbon goals to combat climate change,it is necessary to incorporate"carbon emissions"as an environmental attribute into the life cycle management of bridges.In this thesis,we analyzed the impacts of climate change on carbonation of fly ash concrete and the impacts on the time-dependent reliability of fly ash reinforced concrete（RC）beams in the context of global warming.Then,two solutions were proposed to cope with climate change for reinforced concrete beams with expected life:improving the initial design reliability index（for new beams）and optimising the life-cycle maintenance schemes（for existing beams）.The two solutions were evaluated from both environmental and economic aspects,taking into account the theories of life cycle assessment（LCA）and life cycle cost（LCC）.The research work of this paper mainly includes the following aspects:（1）A probabilistic model for the carbonation depth of fly ash concrete based on Bayesian theory was developed.Based on the four core climate prediction scenarios of the Coupled Model Intercomparison Project Phase 6,the average of predicted data from high-resolution climate models of five countries was introduced into the probabilistic model for carbonation,and the effects of time-dependent atmospheric CO₂concentration,temperature and relative humidity on the carbonation depth of concrete were analyzed.Then,the standard values of the carbon sink of fly ash concrete under climate change scenarios were proposed.（2）Initial corrosion probability,corrosion damage probability and time-varying reliability models for fly ash RC beams were established in five building climate zones under four climate scenarios.Based on the expected service life of 100 years and the magnitude of reliability degradation,a climate correction factor was proposed to ensure that the reliability of reinforced concrete beams under climate change scenarios always meets the current code requirements（for new beams）.（3）Based on four climate change scenarios,the maintenance effectiveness of essential maintenance measures for fly ash reinforced concrete beams in five climate zones and their variation over time,as well as the effects of fly ash admixture on maintenance effectiveness,were analyzed.Quantitative models of the costs,carbon emissions and social impacts of the maintenance stage of the fly ash RC beams were developed.（4）Taking the cumulative economic risk,cumulative environmental risk and cumulative failure probability as the optimization goals,using the non-dominated sorting genetic algorithm-III,dynamic optimal maintenance schemes for the fly ash reinforced concrete beams under four climate scenarios were proposed（for existing beams）.（5）Life cycle carbon emissions and life cycle costs quantification models for the fly ash RC beams were established considering the time factor and the development level of the civil construction industry.The carbon emissions and costs of new and existing RC beams in response to climate change in five climate zones under four climate scenarios were statistically analysed,and the economic feasibility and sustainability of adopting climate correction factors at the stage of design was demonstrated.