Study On The Rheology Of Fresh Concrete In Truck Mixer And Its Application

Fresh concrete as the main materials of engineering constructions is widely applied to some infrastructures such as bridges,roads,buildings and so on,and others special structures as nuclear reactor.Fresh concrete is a kind of multi-component composite material.Its mechanical characteristic is very complex and has an obvious randomness and time-dependence.Comprehensive performance of fresh concrete behaves as workability(flowability,viscidity and water-keeping ability,etc)which is evaluated through its rheological properties.The rheological behavior of fresh concrete is studied using the rheological theory,which can better guide its construction process.The rheological properties of fresh concrete affect the difficulty,efficiency and continuity of construction,the strength and durability of hardened concrete structure.Moreover,it has a significant effect on the quality engineering and construction.Therefore,a comprehensive understand to the rheological properties of fresh concrete has the special and important significance to the subsequent construction.The Nation requires that the large construction must use the commercial concrete,which is produced in nerby mixing plant and transported to the construction site relying on the truck mixer.Therefore,the concrete truck mixer as the main equipment of concrete transport has been widely used in high-rise building,harbor construction and the construction of some national major construction projects such as the nuclear power station.In transport process,the change of the fresh concrete rheological property will affect the uniformity of the mixture and then decide to the smooth of subsequent construction,having important influence on the occurrence of the blocking pipe in pumping process and the compressive strength,durability and other characteristics of hardened concrete.In order to maintain a good homogeneity of the fresh concrete and avoid the occurrence of segregation after it is transported,the fresh concrete should be further mixed in the mixing transportation process.Therefore,the studies on the rheology of fresh concrete in truck mixer and optimizing the technology parameters of transport process and structure parameters of blade have important economic and practical significance.This paper first and developed a new type and pressurized rheometer to measure the effect of loading pressure on the rheological parameters of fresh concrete accorging to its rheological properties;Second,combining with the advantages and disadvantages of the discrete element model of fresh concrete at home and abroad,the hybrid discrete element model of fresh concrete was established.It is more suitable to simulate the flow behavior and homogeneity of fresh concrete in mix trucker.The contact parameters of discrete element model on fresh concrete were calibrated through its workability and rheological tests.Then,the characterization methods on the homogeneity of fresh concrete in truck mixer corresponding to different time and different studied methods were put forward.These characterization methods were applied to the experimental and numerical study of mixing and discharging process on granular materials in experimental truck mixer prototype.It was used to verify the correctness of the simulation results and analyze the occurrence and transition of segregation during the whole process;finally,the mixing and discharging processes of fresh concrete in truck mixer mixer was numerically simulated,the variation of homogeneity of mixture and its mixing mechanism were analyzed.Through the orthogonal test and single factor numerical simulation,the influence trend of each factor on the discharging homogeneity and efficiency and the optimal parameter group can be obtained.The studied results also have been applied to the producter of the mix trucker of Zoomlion co.,LTD.This thesis was divided into seven chapters,and the main research objective and content of each chapter is described as follows:In chapter one,an introduction of the background of this research was given.The research status of fresh concrete rheology and its research methods and the transport process were introduced.First,the rheological research status of fresh concrete was summarized from the rheological model,the flow and rheological test,the research methods of rheology.Then the research status of fresh concrete using numerical simulation was introduced mainly containing single fluid method,discrete particle group method and suspended particle method.The works using the discrete element method(DEM)to study the flow and rheological behavior of fresh concrete were detailed described from the research status of the discrete element model and discrete element simulation;Finally,the research status of the concrete truck mixer was reviewed,including the development history of fresh concrete truck mixer,mixing mechanism,the flow of fresh concrete in truck mixer,the homogeneity characterization of mixture,parameters optimization etc.The research contents and significance of this thesis were outlined.In chapter two,based on the rheological properties of fresh concrete,the pressurized condition of fresh concrete in the operational process as well as traits,the pressurized and ratational rheometer was developed,which could measure the rheological parameter values of the fresh concrete under pressure.In chapter three,combining the rheology theory of the fresh concrete with fresh concrete‘s flow characteristics and rheological behavior,the discrete element model of fresh concrete was established.The micro contact parameters of discrete element model of fresh concrete were calibrated by the experiment and numerical simulation of rheological test,such as slump test,L-box test and rheometer test.The flow behavior and rheological characteristics of the fresh concrete in these tests were analyzed.The effect of coarse aggregate content on the rheological property was studied.In chapter four,the homogeneity characterization of fresh concrete in truck mixer was put forward some different methods.It should be to choose an appropriate characterization meture thod according to the research object,problem and research method and so on.These characterization methods were applied to the experiment and numerical simulation of granular materials in experimental truck mixer prototype and the correctness of the simulation results was verified;The effect of motion parameters on mixing homogeneity,discharging homogeneity and discharging efficiency were studied;The mechanism of the occurrence of axial segregation and transition in mixing drum,the influence of initial loading profiles on the axial and radial segregation,and the size effect of the mixing drum model were studied.In Chapter five,the flow behavior of the fresh concrete in the process of transportation of 12m3 truck mixer was studied.First,the similarity of the numerical model of the fresh concrete in truck mixer was analyzed.Then,the mixing and discharging processes of fresh concrete in truck mixer was simulated as the whole movement morphology of concrete particles in the mixing drum being obtained.The simulation was verified to be correct through the homogeneity of discharging samples comparing with the simulation results with the experimental results.The homogeneity distribution phenomena of the discharging samples,the variation diagram of the discharged whole weight with the discharging time were obtained.Finally,through the overall flow pattern,tracking the motion trajectory of single particle and cluster particle,the mixing mechanism and function of fresh concrete in truck mixer were studied and analyzed.In Chapter six,the optimization of movement and blade structure parameters on fresh concrete in the transport process of truck mixer was simulated.First,the single factor method was used to study the influence of the movement parameters,such as the mixing speed,mixing time,discharging speed and so on,on the discharging homogeneity and discharging efficiency.Then,the numerical simulation of the orthogonal test method was used to study the role of primary and secondary,the effect tendency of the blade structure parameters on the discharging efficiency and discharging homogeneity,to get the optimal blade model.Finally,using the orthogonal test to simulate the influence of the blade model and movement parameters on the discharging homogeneity and the discharging efficiency,the optimal blade type and technological parameters group getting the better discharge homogeneity and discharging efficiency can be obtained.In chapter seven,the contents of this thesis were briefly summarized,and the future researches were discussed.