Characteristics Of Chloride Transport In Interfacial Zones Of Full-span Precast Prestressed Concrete Bridges

The combined action of environmental erosion and load over a long period of time is a typical mode of action for durability problems of bridge structures.In the case of assembled prestressed concrete bridge structures,the interfacial zone between adjacent sections has a more prominent durability problem as a physically weak zone.In view of the fact that whole-span assembled prestressed concrete bridges have become an important form for the development of new precast bridges,and there is a lack of scientific research on the special durability problems in the interfacial zone.In this thesis,whole-span precast prestressed concrete bridges are targeted to the de-icing salt and marine related chloride salt environment as the background,and the complex spatiotemporal evolution characteristics of the initial physical state inhomogeneity,stresses and environmental media in the connection interfacial zone that further cause chloride transport are considered.By means of experimental studies,theoretical analyses and finite element simulations,the special chloride transport behavior in the interfacial zone of new-old concrete is investigated.The specific research contents and innovative results are as follows:(1)The spatial and temporal distribution characteristics and evolution of chloride concentration in the interfacial zone were obtained by long-term wet-dry cycle tests of chloride salt solutions under four exposure periods(450d,540 d,630d and 720d).The results show that the chloride concentration in the interfacial zone is higher than that in the new-old concrete on both sides,showing the “Interfacial Zone Effect(IZE)”.The mathematical description of the IZE shows that the IZE increases with the increase of the eroding depth and decreases with the increase of the exposure period.Based on the non-uniform distribution of the mesoscopic composition of the interfacial zone of newold concrete,the distribution model of the chloride diffusion coefficient in the interfacial zone was established.(2)The spatial and temporal distribution characteristics and evolution of chloride concentration in the interfacial zone under constant compressive stress(2MPa,5MPa and 8MPa)were investigated through three sets of long-term wet-dry cycle of chloride erosion tests on the concrete specimens with four exposure cycles(450d,540 d,630d and 720d).Based on the distribution model of chloride diffusion coefficient in the interfacial zone without stress,the main factors affecting the chloride transport behaviorconcrete porosity and microcracks as the main starting point,the relationship between constant compressive stress and porosity(elastic effect)and strain and specific crack area(damage effect)were established.Therefore,a revised model of the chloride diffusion coefficient in the interfacial zone under constant compressive stress was proposed.(3)A long-term(360d)erosion fatigue test study of specimens in the interfacial zone was carried out by designing a multi-range and multi-temporal fatigue loading and chloride salt erosion long-term combined action system.The results show that the physicaldamage in the interfacial zone always precedes the new-old concrete bodies on both sides under the fatigue stress.With the increase of fatigue stress range,the IZE first increases and then decreases.Taking the main factors affecting chloride transport behavior-concrete porosity(before new microcracks are generated)and microcracks as the main entry point,based on the relationship between volumetric strain and porosity,strain and specific crack area(tensile microcracks perpendicular to the main stress direction),a revised model of chloride diffusion coefficient in the interfacial zone under fatigue stress was established.(4)In view of the structural durability control factors,a total-probability method is used to calibrate the sub-factors and consider the structural durability action parameters such as different chloride salt erosion environments and different loading conditions,as well as the structural durability resistance parameters such as concrete strength grade and protective layer thickness.Based on the durability limit state of the rusting of the main reinforcement in the interfacial zone,a prediction model of the chloride erosion life of the whole-span precast prestressed concrete bridges was obtained.