A Method For Mixture Design Of Concrete Based On Rheological Behavior

The workability of fresh concrete affects the process of casting,forming and compacting,and even the properties of hardened concrete.The traditional evaluation of workability is often described by the single parameter such as slump,consistency,slump flow,and flow time.However,these terms are empirical,qualitative and subjective,and lack of quantitative basis.Furthermore,there is always a lack of corresponding theoretical guidance in the optimum design of mixture when the economy and concrete quality are satisfied.In this study,the rheological parameters such as yield stress and plastic viscosity were selected to describe the workability of concrete mixtures with different excess paste thickness,packing of aggregates and cement-fly ash-slag ternary cementitious system.The relationships between concrete components and rheological parameters were established and then the method for mixture design of concrete based on rheological behavior was proposed.The relationships between slump,rheological parameters and the excess paste thickness were established at the fixed water-to-cement ratio and sand-to-aggregate ratio.It was found that the flowability of fresh concrete with the excess paste thickness was primarily dominated by the yield stress.There existed a good y = axb function relation between the rheological parameters and the excess paste thickness.The yield stress and plastic viscosity decreased dramatically,and the slump significantly increased with the increase of the excess paste thickness.The slump,yield stress and plastic viscosity tended to be stable at higher excess paste thickness.According to the interrelationships between slump,yield stress,plastic viscosity and excess paste thickness,the optimum amount of cementitious materials of concrete with specific water-to-cement ratio and aggregate parameters can be determined.The rheological parameters of concrete with different sand-to-aggregate ratio were obtained at the fixed water-to-cement ratio and cement content.The results indicated that with the increase of sand-to-aggregate ratio,the yield stress and plastic viscosity decreased to a critical value and then increased,while the slump gradually increased at first and then remained unchanged.The optimum of proportion between aggregates can be obtained based on the relationships between sand-to-aggregate ratio and the rheological parameters.The coarse aggregate was divided into three particle sizes,i.e.5-10 mm,10-15 mm and 15-20 mm.The concretes with different combination of coarse aggregate were produced according to the simplex centroiddesign method.The optimum combination of coarse aggregate could be determined by the packing density and specific surface area of mixed coarse aggregates,and the slump,yield stress and plastic viscosity of fresh concrete.The simplex centroid design method was selected to design the concrete with ternary cementitious system at the fixed water-to-binder ratio,the total amount of cementitious materials and the sand-to-aggregate ratio.The slump,rheological parameters and compressive strength at 3 d and 28 d were tested.The ternary cementitious components were optimized based on the relationships between the three components and the concrete properties.Based on the materials used in this study,the optimum amount of fly ash and slag of mixtures at sufficient compressive strength,suitable flowability and best rheological properties were 15-20% and 15-25%,respectively.According to the above research results,the mix proportion design method of concrete based on rheological behavior was proposed,which could supply a reasonable control for aggregate proportions,and determination for content and composition of cementitious materials.The flowability,stability and uniformity of fresh concrete could be controlled,and the total content of cementitious materials and cement also could be reduced,and then the equality of concrete could be improved efficiently.