Rheological Measurement,Models And Their Applications For Fresh Concrete

The rheological property of fresh concrete has a remarkable influence on its workability and hardened properties.The rheological behavior of fresh concrete can be characterized by the rheological model,which is a function between shear rate and shear stress.Although the shear rate and shear stress cannot be measured directly,they can be obtained by converting a series of rotational speed and torque data measured with the coaxial cylinder rheometer.For a long time in the past,fresh concrete was considered as a linear Bingham fluid,the corresponding torque-speed function relationship(Reiner-Riwlin equation)was established,and the parameters of the rheological model could be obtained by fitting the test data with this equation.However,there are some problems in the research on the rheology of fresh concrete:(1)each torque and rotational speed data needs to be pre-processed by calculating the average value of multiple sample signals.The fluctuation of the signal has a great influence on the average value,and this data processing method is out of date.(2)With the extensive use of various admixtures and minerals,some fresh concrete shows nonlinear rheological behaviors.Therefore,two nonlinear rheological models,including Modified Bingham model and Herschel-Bulkley model,are also used to characterize the rheological properties of concrete.However,there are some problems in the derivation process of torque-speed equations of these two models,which made them only approximate solutions and deserve to be derived again.(3)The existing rheological models can only describe the rheological behavior of fresh concrete.No relationship between the rheological properties of fresh concrete and solid components was ever established,so it is difficult to guide the mix design of concrete and improve the workability of fresh concrete.In view of the above problems,the wavelet regression method was proposed to deal with the torque signal;the torque-rotation speed transformation equation of the Herschel-Bulkley model has been derived completely,two sets of H-B model parameters(one is for shear thickening,the other is for shear thinning)were applied to simulate different types of fresh concrete.The results of the theoretical calculation were compared with those of the CFD simulation to verify the correctness of the analytical equation.Based on the theoretical calculation,the torque data mixed with ?5% random error was used to simulate a set of test results of H-B fluid.Dozens of the fitting results of both methods were statistically analyzed to decide if the theoretical equation is better than the approximate one.Similarly,the torque-rotation speed equation of the modified Bingham model is modified;furthermore,the Robertson-Stiff model and Vocaldo model are proposed to characterize the rheological behavior of concrete.On this basis,a group of fresh concrete specimens is selected using the optimal mixing design method,and their rheological properties were tested.Then the fitting results of the above rheological models are compared.After that,the relationship between the parameters of rheological models and the constitutes of concrete is studied.The study shows that wavelet regression can improve the signal processing method and provide a new way for the torque measurements of concrete rheometer in the future.Herschel-Bulkley model,Modified Bingham model Robertson-Stiff model,and Vocaldo model can all be used to characterize the rheological properties of fresh concrete.The differences between the theoretical calculation results and CFD simulation results are trivial.The statistical analysis based on the fitting results of theoretical data with ?5% random error indicated that the theoretical equation is better than the approximate equation.After the comparison of the fitting results of the 16 fresh concrete samples with the above equations,it is found that the fitting accuracies of the Herschel-Bulkley model and Robertson-Stiff model are the highest.The functional relationship between the rheological model parameters of concrete and its solid volume fraction is studied,and functions can be found between every parameter of the Herschel-Bulkley model(Vocaldo model as well)and the volume fractions of concrete constitutes.