Understanding the Rheology of Concrete

N Roussel

• Publisher: Woodhead Publishing
• Year: 2011
• Print ISBN: 9780857090287
• eBook ISBN: 9780857095282
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

Estimating, modelling, controlling and monitoring the flow of concrete is a vital part of the construction process, as the properties of concrete before it has set can have a significant impact on performance. This book provides a detailed overview of the rheological behaviour of concrete, including measurement techniques, the impact of mix design, and casting.Part one begins with two introductory chapters dealing with the rheology and rheometry of complex fluids, followed by chapters that examine specific measurement and testing techniques for concrete. The focus of part two is the impact of mix design on the rheological behaviour of concrete, looking at additives including superplasticizers and viscosity agents. Finally, chapters in part three cover topics related to casting, such as thixotropy and formwork pressure.With its distinguished editor and expert team of contributors, Understanding the rheology of concrete is an essential reference for researchers, materials specifiers, architects and designers in any section of the construction industry that makes use of concrete, and will also benefit graduate and undergraduate students of civil engineering, materials and construction.- Provides a detailed overview of the rheological behaviour of concrete, including measurement techniques, casting and the impact of mix design- The estimating, modelling, controlling and monitoring of concrete flow is comprehensively discussed- Chapters examine specific measurement and testing techniques for concrete, the impact of mix design on the rheological behaviour of concrete, particle packaging and viscosity-enhancing admixtures

Part I

Measuring the rheological behaviour of concrete

1

Introduction to the rheology of complex fluids

P. Coussot, Université Paris-Est, France

Abstract:

This chapter considers the main types of rheological behaviour. The basic ways of modelling them (essentially in simple shear) and the physical origins of the behaviour of different material types are explained and typical real materials behaving in that way are mentioned. The most important behaviour types (yielding and thixotropy) for cementitious or concrete materials, the importance of flow regime (or ‘physical state’) with regards to the time scale of observation and the flow rate for predicting the behaviour type, the importance of distinguishing the main interaction types within the material for evaluating the physical origin of its behaviour are emphasised. Solids, simple fluids, suspensions, slightly non-Newtonian materials, yield stress fluids, thixotropy and viscoelasticity are reviewed.

Key words:

solid

liquid

suspension

yield stress fluid

thixotropy

1.1 Solids

1.1.1 Apparent behaviour

We live in a solid environment made of ground, furniture, cars, etc, which fortunately keep their shape when we use them. If however we apply a force, larger than a critical value, to one of these objects, we reach a situation when they start to strongly deform or even break (actually for a force smaller than this critical value they deform slightly). For two different objects made of the same material this critical force is different, which means it is not an appropriate variable for describing the material’s behaviour as it is not intrinsic to it. In fact the appropriate variable is the stress, which is the ratio of the force to its surface of application. Finally the mechanical behaviour of solid materials is basically described in terms of the deformation induced by the stress applied and the critical deformation and critical stress at which the material flows or breaks.

To better understand these concepts of stress and deformation it is useful to consider one of the most frequent and simple situations, namely the ‘simple shear’. In this case a piece of material is contained between two parallel solid planes which move relative to each other along one of their directions. The resulting deformation corresponds to the relative motion of parallel planes of materials (Fig. 1.1). The shear stress, τ is the ratio of the force (F) applied to the section of the piece of material (S): τ = F/S. The deformation, ε. is defined as the ratio of the relative displacement of the solid planes ∆x to the thickness H of the piece of material: ε = ∆x/H.

1.1 Principle of a simple shear: relative motion of material planes.

In mechanics we are interested in the constitutive equation of the material, which is a relationship describing the intrinsic behaviour of the material, i.e. independently of the specific conditions to which it is submitted. For a solid undergoing a simple shear, the constitutive equation is mainly described in terms of the relationship between τ and ε, but in some cases the rate of displacement must also be taken into account (see Section 1.7). The simplest solid behaviour is the linear elastic behaviour, in which τ is simply proportional to ε:

[1.1]

In simple shear the coefficient of proportionality G in this relation is the elastic shear modulus.

For a sufficiently large stress, the material no longer follows this type of behaviour but deforms widely. Here we can distinguish two main types of behaviour: for plastic materials the deformation tends to localise in some specific region within the material; for fragile materials there is a breakage into several parts (Fig. 1.2).

1.2 Two different possible (idealised) behaviour types of solid materials: elastic then fracture or plastic behaviour beyond a critical deformation. The drawings illustrate the different aspects of the material successively in the initial state, for a homogeneous deformation in the elastic regime, then either for a ductile or brittle behaviour.

1.1.2 Microstructural origin

Often we do not see what a material is made of and how it is organised at a local scale. Indeed most materials appear smooth or homogeneous at our scale of observation. The elements of materials that are of basic interest for our understanding of their mechanical properties are the largest elements which, by their distribution in space, play a critical role in the behaviour. These elements may be atoms, molecules, polymer chains, cells, clay particles or cement grains, which means that the size of the constitutive elements may range from few nanometres to several centimetres. The basic structure at the origin of a solid’s behaviour is the distribution of elements either linked to each other or jammed in a given space. An illustrative example is a vacuum pack of coffee beans: it is apparently solid because the beans are jammed in a limited volume, and its properties depend only on the properties of the beans and their distribution in space.

The simplest material of this type is a solid crystal made of identical atoms or molecules distributed along periodic and symmetrical positions x_i. The specificity of such a system is that an atom (situated in x) interacts with its neighbours via strong van der Waals, covalent or ionic forces F_i, and as a function of the distance between the elements x – x_i, so that this atom has a potential energy (E) defined by

. In the crystalline state each atom is in a position of force equilibrium (Fig. 1.3), ∑ F_i = 0, which corresponds to a minimum of the potential energy. As a consequence each atom is in a well of energy from which it can escape only if a sufficiently large force (or a stress) is applied to it. In addition each element is submitted to a thermal agitation which in general induces small amplitude motions of this element around its equilibrium position.

1.3 Different types of material structures associated with solid behaviour: (a) crystalline solid; (b) glass; (c) reticulated polymer; soft solid behaviour; (d) colloidal aggregate; (e) concentrated foam or emulsion. Different types of material structures associated with liquid behaviour: (f) simple molecular liquid; (g) polymer melt or solution; and (h) dilute or semi-dilute suspension in a liquid. For systems (a), (b) and (f) the arrows illustrate the thermal agitation...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• introduction
• Part I: Measuring the rheological behaviour of concrete
• Part II: Mix design and the rheological behaviour of concrete
• Part III: Casting and the rheological behaviour of concrete
• Index