INTRODUCTION

The rheology of fresh concrete is a relatively young science. There have been many attempts to characterize the consistency of fresh concrete by a variety of technological tests [1, 2], but few researchers have applied continuous media mechanics to the rheological behavior of fresh concrete.

Tatersall was probably the first to carry out systematic investigations in this field, which have been summarized in a book [3 ]. Pointing out that testing fresh concrete in a classical coaxial viscometer would require a gigantic apparatus, he suggested the use of a more modest device (the 'Two-point test') where the concrete is mixed in a sort of instrumented mixer. Finding a linear relationship between the increase of the torque and the rotation speed of the impeller, Tatersall stated that fresh concrete had a Bingham behavior, which can be expressed by the relationship:

=  0   +  µ

(1)

where is the shear stress, the strain gradient (or shear rate), ₀ the yield stress, and µ the plastic viscosity. The two last parameters are assumed to be material constants.

However, this relationship was only suggested by an analogy between fundamental parameters (stress, strain gradient) and macroscopic measurements (torque, speed of rotation). More recently, de Larrard et al. developed a parallel-plate rheometer called BTRHEOM¹ [4], where the strain field is imposed by the geometry (see Fig. 1). From the relation between the torque and the rotation speed, one can deduce the material law of behavior. Therefore, for the first time, characteristics in fundamental units can be measured on fresh concrete. This rheometer was developed taking into account of a comprehensive series of specifications, encompassing both engineering and scientific requirements. The measurements were validated by finite-element calculations, and comparisons with most existing types of concrete rheometers [5, 6]. The use of this rheometer for characterizing a series of concretes, mostly high-performance concretes, led to the conclusion that the Bingham behavior was an acceptable description of the flow behavior of most fresh concretes [5, 7]. However, some of our recent measurements suggest that this is not always a good approximation and that a more general description of the flow curves seems necessary. A possible model is similar to the one used by Coussot and Piau, who studied the rheology of coarse aggregate-mud suspensions with a large coaxial rheometer [8].

Figure 1. Velocity field in BTRHEOM [3, 4].

¹ The names of manufacturers are identified in this report to adequately describe the experimental procedure. Such an identification does not imply recommendation or endorsement by the National Institute of Standards and Technology, nor does it imply that the material identified is necessarily the best available for this purpose.