Characterizing the Properties of Fresh Concrete: BML-Viscometer and BTRHEOM Rheometer Comparative Experiments
During the last years, the characterization of the workability of fresh concrete gained increasing interest in concrete technology and building industry. For present-days concretes, made with addition of mineral and chemical admixtures, the classical slump test does not give sufficient information about such an essential property of the concrete mixtures as workability. The development of the two-point workability test by Tattersall (1976) marked the first attempt to quantify the rheological behaviour of fresh concrete, in particular the workability. The implementation of the two-point workability test for concrete is recognition of its Bingham-material characteristics in the case of low shear rates, which usually occur in practice. Wallevik and Gjørv (1990) developed a computerized coaxial cylinders viscometer (BML-Viscometer) and proposed a method to calculate the yield value (τ0) and the plastic viscosity (μ) from the measured flow resistance (g) and relative viscosity (h) and the dimensions of the apparatus. De Larrard, Hu (1997) and co-workers designed a computerised plane-to-plane rheometer (Btrheom), which can also be implemented for field-testing. A drawback of most of the invented two-point tests is that the measured g-value (flow resistance) and h-value (relative viscosity), cannot be related directly (also not easily) to the characteristics of the material (in terms of the Bingham model): yield value of the shear stress and plastic viscosity. The aim of the ‘live, in real time” experiments executed with the BML-Viscometer and the Btrheom was to compare the rheological characteristics (τ0 and μ) of different concrete mixes (soft-to-fluid consistency) measured by the two rheometers of different principle with concrete of the same batch.
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