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INTRODUCTION

The ease of placement of concrete is usually refered to as workability. This word is usually ill defined, hiding at least two characteristics: the yield stress and the plastic viscosity. These two parameters were first used in concrete by Tattersall [10] using the definition given by the Bingham equation as follows:

 =  ₀ +  µ                       (1)

where

 = shear stress (in Pa) applied to fresh concrete,
is the shear strain rate (also called the strain gradient, in s^-1),
₀ =  is the yield stress (in Pa), and
µ is the plastic viscosity (in Pa·s). The yield stress and the plastic viscosity (the Bingham parameters) characterize the flow properties of the fresh concrete (Figure 1)

Figure 1 - Yield stress and plastic viscosity of fresh concrete

Tattersall proposed using an instrumented mixer to obtain a more complete characterization of the flow behavior of fresh concrete (Tattersall 1991 [10] ) than was previously done. Recently an improved rheometer has been developed by the Laboratoire Central des Ponts et Chausseèes (LCPC), the BTRHEOM rheometer (de Larrard et al. 1997, de Larrard et al. 1996, and Hu et al. 1995 [2, 3, 7] ). This rheometer, which was used in the present study, allows the quantitative determination of the yield stress and the viscosity of plastic concretes. The major difference between this instrument and its predecessors is the parallel plate geometry, while others have a vane or bob rotating in a cylindrical container. The parallel plate geometry allows a mathematical description of the velocity field, which permits analytical calculations of the yield stress and plastic viscosity in terms of rheometer measurements.

While the standard slump test Standard Test Method for Slump of Hydraulic Concrete Cement (ASTM C143-95), the most widely used of all field tests of fresh concrete, provides an indication that is reasonably well correlated to the yield stress (de Larrard 1997 [2] ), other tests, such as the DIN flow table and VEBE apparatus (de Larrard et al. 1994 [4] ), provide results that are not very useful in terms of quality insurance in the field. In most of these tests, the concrete flows under the effect of a dynamic sollicitation. Thus, the behavior of the concrete under a single, apparently arbitrary, level of vibration is examined, with no possibility of determining fundamental rheological parameters and no obvious relationship to fresh concrete in the field.

A survey of the state of the art (Ferraris 1996 [5] ) shows that none of the current field tests (in distinction to rheometers) is able to assess the plastic viscosity of the fresh concrete. However, this parameter is of increasing importance in modern concretes. For high-performance concretes, it frequently constitutes the critical parameter that controls pumpability (de Larrard 1997 [2] ) and ease of finishing.

Concerning the standard slump cone test, (Tanigawa et al. 1989, 1991 [8], [9] ) performed measurements of the slump as a function of time. They found that the slump-time curve could be simulated by finite element analysis of the fresh concrete assuming that the concrete is a Bingham material. The slump-time curve depends on both the yield stress and the plastic viscosity. Since the final slump is related directly to the yield stress, it is reasonable to assume that the time-dependence of slump is likely to be controlled by the plastic viscosity. Considering that the slump test is currently the only field test in the world for most practitioners, the test procedure was modified slightly to make possible measurements of both the yield stress and the plastic viscosity of fresh concrete. This paper describes the modification made to the standard slump test apparatus, test procedure and calculation to determine both the yield stress and the plastic viscosity.