Study On Shear Thickening Of Cementitious Mixture

Concrete is the most important building material in modern engineering.With the development of economy and society,modern buildings are developing towards super high rise,large span and complicated,and further requirements have been put forward for the workability of concrete.In the current project,slumps are commonly used to evaluate the workability of concrete.Although this method is easy to operate,the indicators are simple.The parameters such as yield stress,plastic viscosity and rheological index（shear thickening or shear thinning）in rheology make up for the defects of slump and can better describe the workability of cement-based materials.Rheological studies have found that shear-thickening behavior exists in cement-based material.In view of modern concrete with low water to cement ratio,large amount of cementitious material and large fluidity,the shear thickening property of cement-based materials is studied in this paper.With the effect of many factors,such as water to cement ratio,composition of cementitious materials,on the shear stress to shear rate（torque to rotation speed）curve of cement paste,mortar and concrete,Key factors affecting shear thickening of concrete are discussed.The link between the shear thickening properties of cement pastes,mortars,and concretes has been studied.For the defects of the H-B model in literature reading and experimental analysis,a modified H-B model was proposed.The main conclusions are as follows:Under the conditions of this experiment,changes in water to cement ratio,composition of cementitious materials and other factors have all found that the viscosity of cement-based material mixture increases to varying degrees as the shear rate increases.That means shear thickening.（1）The lower water to cement ratio of the cement-based material mixture（cement paste,mortar and concrete）,the greater shear thickening.The decrease of the water-binder ratio makes the water film thickness between the particles decrease,and the distance between the particles decreases.Therefore,the interaction forces between particles increase,and it is easy to form particle clusters which cause shear thickening.（2）The larger cement to aggregate ratio（volume ratio）of the cement-based material mixture,the greater shear thickening.The shear thickening theory,such as particle cluster theory and ordered disorder theory,and the data of this experiment all indicate that the shear thickening behavior of the mixture is mainly caused by the shear thickening of the paste.In cement paste,mortar and concrete,the shear thickening of cement paste is the most obvious,followed by mortar,and the shear thickening of concrete is the least obvious（rheological index n is the smallest）.In concrete,the shear thickening increases with the increase of the volume of the paste.（3）The more concentrated the particle size distribution in the cement-based material mixture,the greater shear thickening.When the difference in particle size is small,the flow space in the slurry system is small,and particle clusters are easily formed.Among them,in cement paste,the smaller the particle size difference between mineral admixtures and cement in the cement paste,the more obvious the degree of shear thickening.In concrete,the effect of particle size on shear thickening shows that the smaller the maximum aggregate size,the more obvious the degree of shear thickening.（4）For H-B model fitting,there is an overestimation of the yield stress.Combined with the Bingham model and the H-B model,a modified H-B model is obtained.Modified H-B model fixed yield stressτ₀ through Bingham model,with differential viscosity initial valueη₀ and rheological index n to evaluate the change trend of viscosity.This model compensates for the lack of physical significance of the viscosity parameter in the H-B model.The parameter n obtained has a comparability not lower than that of the H-B model at the same time avoiding errors ofτ₀.