This article, the first of a series of three on using rheology to measure fresh concrete workability, focuses on the scientific basics of rheology.
When you study our industry closely, there's evidence of a switch from the historical view of mix design based on proportioning to design that includes the science of fresh concrete workability.
In terms of field performance, the slump test is still used as the standard measurement. However, any field professional will tell you six different mix designs with the same slump measurement can perform six different ways in the field in terms of their workability and placement.
This problem is further compounded by the increase in technologies from chemical and mineral admixture companies. There is no debate that the slump test indicates numerous other variables such as strength and water-cement ratio. Also, higher slump values generally indicate easier placement. Higher slumps obtained by the addition of water indicate decreases in strength and other performance properties. However, with the use of relatively new technologies, such as water reducers, this is not the case, and in most cases sufficient data supports the claim that the performance properties of concrete can actually increase with increasing slump.
Finally, this whole issue becomes even more complex as we add the time-sensitive nature of fresh concrete to the picture. When properly performed, the slump test has proven to be not only an acceptable method for measuring how fresh concrete moves under its own weight, but also a predictive tool of concrete quality.
A single slump test is only a good indicator of fresh concrete movement under its own weight at a single point in time. What other options exist to give a complete workability picture for the concrete, keep pace with technology, and generate a meaningful field measurement? The answer is rheology.
Rheology, the study and science of matter flow, uses fundamental engineering principles to closely examine, describe, and predict the relationship between solids and fluids as they move. Rheologically based measurement may be better than the slump test because:
- The results are based on fundamental physical properties.
- The results have numerically similar values, i.e., similar values translate into similar material properties.
- The results are reproducible.
In advocating a new method for workability measurement, rheology researchers began from a different viewpoint in measuring how fresh concrete behaves. In a world that recognizes only liquids and solids, fresh concrete fits both descriptions. First, when sufficient pressure is applied to fresh concrete, it behaves much like a fluid. When not enough pressure is applied, the fresh concrete does not move and behaves much like a solid.
Although rheology may be a new term to the concrete production industry, it has been an accepted science for a long time. From the work of F.C. Bingham in 1922, researchers developed a Bingham Material Model. The model states that shear stress equals the yield stress plus that apparent viscosity multiplied by the shear rate.
From a field perspective, this means yield stress is the amount of energy or force necessary to make fresh concrete move, and as long as that pressure is maintained (yield stress) or exceeded, the concrete will move at a certain speed (plastic viscosity). To this end, two different concretes can have the same yield stress, but different viscosity values, or vice versa.
Although trying to apply the Bingham Material Model may seem very complex and theoretical, several researchers using rheology have their sights set on practical industry implications. Dr. Leslie Struble, University of Illinois associate professor, and associate director of Advanced Cement Based Materials (ACBM), says, "The use of rheology is driven by the fact that the slump test does not adequately describe the flow behavior. To that end, rheological parameters can be used to model concrete behavior in terms of other, measurable responses. In 10 years we might see a mix design procedure in ACI based on rheology."
Another leading researcher in the field, Dr. Clarissa Ferraris of the National Institute for Standards and Testing, adds, "The main improvement in fresh concrete technology will occur in the areas of admixtures and production equipment that might be improved and modified to accommodate the production of concrete." To that end, others within the industry have already taken a step in that direction by making measurements and designing their products based on rheological properties.
With advanced research like this, the slump test could very well become unnecessary. Today a driver may take end-of-the-chute samples for slump measurement. But in the future, using rheological measurement, drum-mounted monitors could accurately assess workability for more efficient delivery and improved ready-mixed concrete quality.