On The Rheology And Thixotropy Of Fresh Concrete By Discrete Element Method

Concrete is a widely used building materials. With the development ofmechanization construction, it is strict to the quality of fresh concrete which is notonly related to its components, but also to the rheology. The accurate measure of freshconcrete rheology is precondition for normal mechanization construction andhardening properties improving. Except theoretical and experimental research method,numerical simulation is another way to study the fresh concrete rheology. Comparedwith other numerical method, the discrete element method regard every singleparticles as research object, so that it is more suitable to simulation of fresh concrete.With experiment and simulation, the main work is as follows:(1) At job site, the slump test is used to measure the rheology of fresh concrete,the slump value just represent the rheology before flow. With same slump value,ordinary concrete and high performance will show different flow performance. Yieldstress and plastic viscosity have been used to describe the fresh concrete rheology, soa concrete rheometer is developed to measure them. And lateral pressure also can bemeasured.(2) With the contact model in PFC3D, a general DEM(Discrete Element method)Model is established, which can describe the microstructure of fresh concrete. AL-Box tests is carried out to verify the feasibility of the general model. Then the microcontact parameters are calibrated by slump test and concrete rheometer test. Based onthe general DEM model, the effect of coarse aggregate volume on concrete rheologyand pumping process are discussed.(3) A thixotropy DEM model is building by inserting a time parameter into thegeneral DEM model. The lateral pressure of the steady fresh concrete on the wall isinvestigated where the changing characteristic of the pressure with time is studied andthe influence of thixotropy on yield stress is also addressed. The numerical results isin agreement with physical results, by which the accuracy of the thixotropy model isverified. Based on the thixotropy model, the influence of thixotropy on mixingtransportation process was studied.