Understanding the Rheology of Concrete



Introduction

Throughout its history, concrete has often been called ‘liquid stone’. Before turning into an artifi cial stone, concrete is indeed fl uid enough, for a short period, to fl ow and fi ll a mould. This property has given the construction industry the ability to cast structural elements such as slabs, beams and columns. Whereas stones are dressed, wood is sawn and steel is shaped at high temperatures, only concrete gives architects the freedom to shape buildings at their will at the building site.


‘Liquid’ is, however, a very simple word to describe the full complexity of the behaviour of concrete in the fresh state. First, unlike simple liquids such as water or oil, concrete is made of elements of many different sizes (from several nanometres to a couple of centimetres) and of various types (organic or mineral) suspended in water. Each of these elements brings its own contribution to the behaviour of concrete in the fresh state. This contribution is strongly affected not only by mix proportions but also by temperature and the casting process itself. The vast family of industrial cementitious materials present such a variety of behaviours that their classifi cation could seem unattainable. This is even more so for the potential to predict, if only qualitatively, their response in practical processing conditions.


These industrial processing conditions are also very complex. They are transient fl ows with complex boundary conditions. Lubricating layers may appear, for instance, at the surface of the mould. Moreover, formworks often contain a dense network of reinforcing steel bars, the size of which is close to the size of the coarsest particles in the material. Only a successful casting process is able to guarantee the mechanical bond between the reinforcement bars and concrete and, from a more general point of view, the adequate mechanical and durability performance requirements of the concrete structural element.


Moreover, unlike simple (Newtonian) liquids, the surface does not self-level when concrete is poured in a mould. The material is able to support an amount of the stress generated by gravity without fl owing similar to that of mayonnaise, paint or tomato ketchup. In the case of fi rm concretes, this property is so strong that vibration is often applied to the material to ease the casting process and allow for fi lling of the formwork. Only recently have concretes able to fl ow under their own weight without vibration while staying homogeneous (called ‘self-compacting concretes’ or ‘self-consolidating concretes’) been developed. This property may, however, prove useful in specifi c cases such as shotcrete where, after spraying, it is clearly desirable that the material stays on the wall in thick layers rather than fl ows down it.


Time has also a strong effect on the behaviour of concrete in the fresh state. When left at rest, material consistency increases signifi cantly with time. Some of these changes are reversible; their effects are erased by remixing in a truck for instance or by any type of strong shearing. Other changes, in particular the consequences of the hydration phenomenon, are irreversible and thus contribute to the long-term evolution of material properties (towards the solid state). No matter the origin of the evolution, it may strongly affect the way concrete is cast and the quality of the fi nal structural element. For instance, two successively poured layers of concrete may not form a homogeneous material if the resting time between the pouring of the layers is too long.


It is noticeable in the history of concrete technology that the development of processing methods has been faster than the development of the understanding of the material behaviour in the fresh state. Concrete has indeed been used extensively since the beginning of the twentieth century, whereas the modern science of rheology ( i.e. the study of the fl ow of matter) and the developments in technology which have made it possible have only occurred in the last 50 years.

What are the key objectives of research on the rheology of fresh concrete? Depending on scientifi c background, an academic answer could be ‘the ability to correctly measure and quantify the rheological properties of fresh concrete’ or ‘the understanding of the correlation between components’ proportions and rheological properties in the fresh state’ whereas a practitioner would probably answer: ‘the ability to predict whether or not a given concrete will correctly fi ll a given formwork’.


The continuing challenge of understanding particle interactions in cementitious materials in the fresh state up to the engineering prediction of the casting processes lies at the interface of three disciplines: physics, chemistry and fl uid mechanics. Physics and chemistry allow correlations to be made between mix design and macroscopic fresh properties. Fluid mechanics allows analysis of the tests used for the measurement of these properties and, in its most advanced form, for the prediction of industrial casting processes.


Keeping the above in mind, we have chosen in this book to gather international experts from the three disciplines. Although their approaches are different, they all have in common the dedication of some or most of their research activities to understanding the fresh properties of cementitious materials. We hope that combining these contributions will form a useful basis for the understanding of the rheology of fresh concrete from mix design to casting processes.


Download
http://s18.alxa.net/s18/srvs2/02/002...f.Concrete.rar