Study of rheological behavior of high performance concrete

Abstract

High performance concrete has become indispensable in construction of modern high rise buildings, long span bridges, nuclear structures, off shore structures and in many other applications. This is generally preferred for high strength and high durability. To achieve these objectives, production of concrete should be given due consideration in terms of proper workability. Principles of fluid rheology have been found to describe workability of high performance concrete in a more scientific way than conventional empirical rules do. Rheology of fresh concrete is recognized as difficult subject due to the fact that concrete is a complex material with time dependent properties and includes wide range of particle sizes. It has been found that concrete can be considered as Bingham fluid with good accuracy where flow is described by two parameters: yield stress and plastic viscosity. Two types of rheometers viz. coaxial and parallel plate type are in use to determine rheological parameters of cementitious materials and concrete. While the rheometers for cement pastes and cementitious material are more or less established in terms of its functioning, concrete rheometers still today present challenge in design and operation because of large size of coarse aggregate. The present study undertakes the design and construction of a new rheometer with parallel plate geometry. Frictional resistance between concrete and vertical wall of cylindrical container has been taken into account while deriving expression for total torque. Measures have been taken to prevent wall slip by providing ribs in the cylindrical container of the rheometer. Concrete is subjected to torsion where the shear rate is not uniform in all areas of material. This non-uniform variation of shear rate has been taken in to account while deriving the expression for the torque. For practical use, shear stress versus torque and overall shear strain rate versus rotational frequency relationships have been established for the given geometry of the rheometer. Calibration of torque in the present rheometer has been performed by rotor blocking method. Calibration of torque was validated by testing a magneto-rheological fluid (MRF 132DG) with the present rheometer and comparing these test results with the results independently obtained by HAAKE RS1 rheometer. It was observed that measured values of MRF properties by the present rheometer and HAAKE RS1 are comparable. Repeatability of the tests has been judged by statistical approach. The presently developed rheometer has been used to investigate rheology of high performance concrete without using any mineral admixtures and then using different mineral admixtures and steel fibers. In concrete without mineral admixtures, experiments have been conducted to examine the influence of some basic parameters such as percentage sand, aggregate gradation, chemical admixture dosage and elapsed time on rheological properties of high performance concrete (HPC). It was observed that different parameters affected rheological parameters in different manners. Optimum values exist for percent sand, sand zone and HRWRA dose. It was also observed that, in addition to water-cement ratio, cement paste volume/aggregate volume ratio affect the rheological parameters of concrete.