| Fresh concrete is a fluid material composed of multi-phase,and the working behavior of fresh concrete involves fluidity,stability,viscidity,time variation,and so on.The working behavior of fresh concrete has a vital influence on the load capacity,durability,and surface morphology of its hardened structure.Whereas the material compositions of fresh concrete are significantly different,the physical and mechanical behavior is complex,and the construction processes are diverse and various.In the construction of fresh concrete,many engineering quality problems are caused by low passing ability,segregation,and improper vibration.Therefore,to optimize the design theory of fresh concrete,improve the quality of concrete construction,and make the working behavior of fresh concrete meet the requirements of various constructions,it is necessary to study the working behavior of fresh concrete under different construction processes and influence factors.Based on the discrete element method(DEM)and fluid-solid coupling method,the topics of flow blocking in the pouring process,static segregation after storage,dynamic response during vibration,and diffusion in underwater pouring of fresh concrete are explored in this thesis.The main contributions of this study are as follows:(1)A mesoscopic simulation method for evaluating the working behavior of fresh concrete is proposed to solve the issues of the inaccuracy of the existing discrete element model to describe the rheological behavior of fresh concrete and the limitations of applicable research.Due to the inaccuracy of the existing discrete element model to describe the rheological behavior of fresh concrete and the limitations of applicable research.The fluid-solid coupling methods(Immersed Boundary Method,Average Volume Method)based on the discrete element method are developed.Additionally,the generation and random placement of irregular aggregate particles and the development of an improved Bingham fluid discrete element contact constitutive model are realized to establish a mesoscopic simulation method that can fully reflect the working behavior of fresh concrete.This method can be used to describe the complex interaction between aggregates and mortar,provide an efficient and accurate technical means for studying the motion and distribution of aggregates in mortar,study interaction between fresh concrete and structures,and further realize the comprehensive evaluation of the working behavior of fresh concrete during different construction processes.(2)The quantitative influence mechanism of multiple factors on the working behavior of fresh concrete in different construction processes is clarified,considering that the factors affecting the working behavior of fresh concrete are multiple and the mechanisms of how they affect concrete are not yet clear.In view of the factors affecting the working behavior of fresh concrete are multiple,and the mechanisms of these factors affecting concrete are not yet clear.Considering the internal factors such as concrete material composition and physical and mechanical properties(aggregate shape,gradation,mortar rheological properties,etc.),and external factors such as casting methods and construction environment(rebar distribution,vibration parameters,water flow velocity,etc.),the motion mode and distribution law of aggregates in fresh concrete are evaluated.The main factors affecting the working behavior of fresh concrete in the construction process of casting,storage,vibration,and underwater casting are quantitatively analyzed,which provide an essential theoretical basis for guiding the construction of fresh concrete.(3)The mechanisms of concrete segregation in different construction processes are revealed because the motion and distribution of coarse aggregate in mortar is the primary cause of static and dynamic segregation of fresh concrete.The motion and distribution of coarse aggregate in mortar is the primary cause of static and dynamic segregation of fresh concrete.Considering the diversity of aggregate shape and gradation,the complexity of mortar rheological behavior,and the universality of the interaction between fresh concrete and structures,a combined experimental(aggregate image recognition test,improved L-box test)and mesoscopic simulation(discrete element method,fluid-solid coupling method)study is conducted,and the motion mode and distribution law of aggregates in different concrete construction processses such as casting,storage,vibration,and underwater casting are studied.This study could reveal the generation mechanism of fresh concrete flow blocking,static segregation,vibration compaction,and underwater material loss from a mesoscopic perspective,and further provide critical technical support for the better construction practices of fresh concrete. |
PDF Full Text Request