ADVANCED CONCRETE TECHNOLOGY CONCRETE PROPERTIES

Advanced Concrete Technology

Concrete Properties

 

Introduction

Fresh concrete is a transient material with continuously changing properties. It is, however, essential that these are such that the concrete can be handled, transported, placed, compacted and finished to form a homogenous, usually void-free, solid mass that realizes the fullpotential hardened properties. A wide range of techniques and systems are available for these processes, and the concrete technologist, producer and user must ensure that the concrete is suitable for those proposed or favoured. Fresh concrete technology has advanced at a pace similar to many other aspects of concrete technology over the past three decades, and indeed many of these advances have been inter-dependent. For example, the availability of superplasticizers has enabled workable concrete to be produced at lower water/binder ratios thus increasing the in-situ strength. In this chapter, we will start by considering the property known as workability*, including its definition and common methods of measurement. We will point out the limitations of these, and show how this leads to the need for a more fundamental scientific description of the behaviour of fresh cement pastes and concrete. We will then describe how this has been achieved by applying the principles of rheology, and explain the development and use of test methods which give a more complete understanding of the behaviour. We will then discuss the effect on the rheological properties of a range of constituent materials, including admixtures and cement replacement materials, and how a knowledge of these properties can be used to advantage. The factors that influence the loss of workability before setting are then briefly considered.

*The alternative term ‘consistence’ is often used, particularly in specifications and standards.

We will not discuss the specific properties required for particular handling or placing techniques such as pumping, slipform construction, underwater concreting etc. These are covered in various chapters in Volume 3 of this series, but hopefully the more general description given in this chapter will be of value when reading these. We will, however, describe the principles of ensuring that the concrete is correctly placed and compacted to give a uniform, homogenous result. Finally, we will discuss the behaviour of the concrete after placing but before setting, with particular reference to segregation and bleed.