Developing Falling-Ball Viscometer for Highly Flowable Cement-Based Materials

A falling-ball viscometer that is composed of a spring, a suspended steel ball, and a scale was designed and developed at the Center for Advanced Cement-Based Materials (ACBM) at Northwestern University for measuring the rheological properties of cement-based materials. With the designed viscometer, the plastic viscosity and the yield stress of a fluid can be determined by investigating the relationship between the drag force that is exerted on the surface of the ball and the velocity of the falling ball. The study was conducted in two steps. In the first step, the properties of pastes with the same water-cement ratio (w/c) but various high-range water-reducing admixtures (HRWRA) and viscosity-modifying admixture (VMA) contents were investigated. In the next step, the research was moved further to mortars with various sand contents but the same paste matrix. It was found that the developed falling-ball viscometer is sensitive enough to measure changes from varying the mixture composition. The measured yield stress corresponds very well to the slump flow diameter that was determined with the mini-cone slump test. The research shows that this field-friendly viscometer has a high potential to be applied to high fluidity concretes, such as self-consolidating concrete (SCC).

Keywords: slump; viscometer; viscosity.

INTRODUCTION

During the past decades, important developments in concrete technology have taken place, especially in the field of material processing. The development of self-consolidating concrete (SCC) is an example. SCC is characterized by its flowability, segregation resistance, passing ability, and mold-surface finishability. These properties are closely related to the rheological properties of the material.1,2 Thus, the need for a better knowledge of rheology, for methods to measure viscosity, and for designing equipment for both lab and field testing are raised.3 Two aspects should be considered when discussing the rheological properties of fresh concrete. First, what are the plastic viscosity and yield stress of the paste matrix because the properties of the paste matrix govern the stability of the suspended aggregates. Second, what are the shape, size distribution, and volume fraction of the aggregates.

Considerable research has been conducted during the past two decades to evaluate the rheological properties of concrete. Determination of the plastic viscosity and yield stress of concrete can be obtained by a number of indirect and direct techniques. One of the most common techniques is the slump test. It was found that the plastic viscosity is related to the time when the flow spread reaches 50 cm (1.64 ft) in diameter T^sub 50^,4 and the yield stress is best correlated to the final flow diameter of the concrete.5-7 The slump flow test is a simple and convenient test; however, the relationships between these parameters were based on empirical correlation. The theoretical supports to these correlations are needed.

Direct measurements of the rheological properties of concrete are attainable by using rheometers or viscometers. These instruments have various geometries, such as parallel plates, coaxial cylinder, and vane configuration.8 Commercial rheometers were originally designed for polymer systems but have been used for cementitious material. The limitation of the sample size and the low torque capacity limit their application to cement pastes. Hence, there was a need to develop specific rheometers for mortar and concrete. BML9 and CEMAGREF-IMG10 are two examples of concrete rheometers with coaxial cylinder configuration. BTRHEOM,11 the rheometer used at the University of Illinois at Urbana-Champaign,12 and the recently developed viscometer at Northwestern University13 for fiber-reinforced concrete are examples of parallel plate rheometers. A comparison of different rheometers was conducted in two international round-robins partially sponsored by ACI.14,15 The study showed that, for the same concrete mixture, different rheometers yielded very different results. The reasons could be attributed to various geometries of the sensors and rheometers, testing procedure, and the inhomogeneity of the material itself. It should be noted that the existing commercial concrete rheometers cannot be used to evaluate the viscosity of cement paste due to the different requirements on the sample size and the sensitivity of the sensors. Similarly, the existing commercial paste rheometer cannot be used for testing concrete.

Another type of viscometer is based on the falling/pullingball concept. The viscosity calculation is based on the velocity of the moving ball and the equilibrium of forces. The external forces are normally traced with a pulley system16,17 or the intensity of a magnetic field,18,19 neither of which is suitable for field measurement. The objective of this research is to design a field-friendly falling-ball viscometer, which can be used under both lab and field conditions. Fieldfriendly means the equipment should be lightweight, relatively inexpensive, and easy to maintain.