Introduction

The purpose of this paper is to present a new approach for predicting the rheological properties of concrete. The relationship between cement paste rheology and concrete rheology has never been completely established. It is clear that changes in the rheology of cement paste affect the concrete. For instance, to increase workability, water or high range water reducing admixtures (HRWRA) are added. These materials mainly affect the cement paste rheology as they have almost no effect on the aggregates. Therefore, it is assumed that a correlation between cement paste and concrete rheology should be possible. The main reason that such a correlation has never been successful is that cement paste rheology is always measured under conditions that are never experienced by the cement paste in concrete. The values usually reported in the literature for cement paste correspond to bulk values and do not take into account the contribution of the aggregates [1]. A method is being developed to predict concrete rheology from constituent properties that will include measurements on cement paste using a novel procedure and a simulation model to simulate the concrete being sheared. The cement paste measurement methodology will be presented here. The details of the simulation model, which is under development, will be presented elsewhere [2].

Determining the correct method for measuring cement paste rheology requires examination of the conditions that cement paste experiences in concrete. Various factors were addressed in designing the proposed test procedure:

• Cement paste is "squeezed" between the aggregates. The distance between the aggregates, the gap, depends on the paste content of the concrete considered [3]. Therefore, the rheometer geometry must be a variable geometry in which the gap can be changed. A parallel plate rheometer is a suitable device.
• The mixing of cement paste must imitate the shear stresses experienced in concrete. Portland Cement Association (PCA) [4] developed a methodology and identified the hardware required.
• As the purpose is to correlate paste rheology with concrete, the parameters of the cement paste tests should be selected to correspond to the parameters of the concrete tests [5].

This paper presents the methodology to measure cement paste rheology and give some results that show the influence of the gap and the addition of admixtures on the rheological properties of cement paste.