Research On The Rheology Of Fresh Concrete And Its Influence On Segregation And Bleeding

Concrete maintains homogeneity and compactness, which are directly related to its mechanical properties and durability, on the premise that fresh concrete has good workability, during mixing, transporting, irrigating, vibrating, molding, etc. The research on the rheology of concrete and its influence on segregation and bleeding, using the quantitative description of flow states with rheological parameters instead of the analysis on workability by experience, is significant to assess the workability of fresh concrete objectively, avoid or reduce segregation and bleeding, and guarantee the quality of concrete construction and long-term durability.In the present study, concrete was considered as a two- phase material, consisting of coarse aggregate and mortar.First of all, this paper determined the flow curves of the mortar phase with different proportions of mixture, obtained the rheological parameters by the fitting curves based on Bingham model, and studied on the influence of proportions on the rheology of fresh concrete, including the water-binder ratio, sand ratio, dosage of superplasticizer and fly ash, and cementitious material consumption. The results showed that within a certain range, the increase of the water-binder ratio decreased the yield stress and plastic viscosity; the increase of sand ratio improved the yield stress, but impacted on the plastic viscosity limitedly; increasing the dosage of superplasticizer significantly reduced yield stress, but impacted on the plastic viscosity limitedly; the addition of fly ash improved the plastic viscosity while reduced the viscosity; the increase of cementitious material consumption reduced the plastic viscosity but impacted on the yield stress little.Secondly, this paper determined the segregation rate and bleeding rate, respectively by washing and sieving coarse aggregate in different heights and by testing the displacements of water level and settlement with high-precision laser sensors, and studied the influence of rheological parameters, yield stress and plastic viscosity, on the segregation and bleeding of fresh concrete. The results showed that fresh concrete with low yield stress and plastic viscosity had more segregation and bleeding. The τ0-μ-S1 image showed that when fresh concrete had less than 400 Pa yield stress and 13Pa·s plastic viscosity, the degree of segregation was more than 25%; however, when fresh concrete had more than 1000 Pa yield stress and 26Pa·s plastic viscosity, the degree of segregation was less than 10%.Finally, this paper studied the influence of segregation and bleeding on the homogeneity of concrete, including the distribution of coarse aggregate by image processing, chlorine ion penetration resistance in different heights by testing concrete electric flux, and pore structure of concrete in different heights by MIP. The results showed that under the influence of segregation and bleeding, some of coarse aggregate gathered at the bottom of concrete; the permeability and porosity of concrete decreased from bottom to top; The pore size distribution essentially unchanged.