Corresponding author’s e-mail: clarissa@nist.gov Page 1 Corresponding author’s fax: +301-990 6891 NOVEL RHEOMETER TO MEASURE YIELD STRESS OF SUSPENSIONS Vincent Picandet (1), Chiara Ferraris (2), Daniel De Kee (3) (1) Université de Bretagne-Sud, LG2M, Centre de recherche de St Maudé, BP 92116, 56100 Lorient, France (2) National Institute of Standards and Technology (NIST), 100 Bureau Dr., Mail Stop 8621, Gaithersburg MD 20899, USA (3) Tulane University, New Orleans, LA 70118-5674 Abstract Slump has often been correlated with the yield stress of concrete as defined by the Bingham model. The discussion is still open as to what the yield stress value actually is and how to measure the yield stress of a suspension in general and for a cementitious material in particular. A plate rheometer is a recent development in the measurement of yield stress of suspensions that allows shear rates far below most rotational rheometers. This paper will present the rheometer and the modifications made so that it can be used with suspensions such as welan gum and bentonite in aqueous solution, as well as cement paste. A careful analysis of the yield stress measurements will be discussed. 1. INTRODUCTION Most suspensions, such as cement paste and concrete, do not behave as a Newtonian material for which the flow behaviour can be characterized entirely by the viscosity. A suspension flow should be characterized by at least two parameters [1, 2, 3] and the most commonly used are the Bingham parameters, yield stress and plastic viscosity. The slope of the shear stress vs. shear rate is the plastic viscosity and the intercept at zero shear rate is the yield stress. The measured yield stress depends on the range of shear rate selected and it is really an extrapolation more than a direct measurement. According to Hackley et al. [3], the physical definition of yield stress is the stress needed to initiate movement; therefore it should be measured by slowly increasing the shear stress until movement occurs. This direct method is not easy to implement in most rheometers because it implies that the shear stress is very well controlled and most rheometers cannot control the stress with step small enough to detect the yield stress. Also, most concrete rheometer are not stress controlled but only shear rate controlled. Other methods [4] were conceived to measure yield stress, such as stress growth. This method requires the material to be sheared at a very low shear rate. The lower the shear