Page 3 - Concrete-Technology-Interacted_Book_Prof-MIK_Neat

1/4 Fresh concrete

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.

1.2 Workability

1.2.1 Terminology and definitions

Problems of terminology and definition are immediately encountered in any discussion
of the fresh properties of concrete. Every experienced concrete technologist, producer
and handler has an understanding of the nature and properties of the material, and can
choose from a wide variety of terms and expressions to describe it; examples include
harsh, cohesive, lean, stiff, rich, etc. Unfortunately, all these terms, and many others, are
both subjective and qualitative, and even those that purport to be quantitative, e.g. slump,
give a very limited and sometimes misleading picture, as we will see. This is not to say
that such terms and values should not be used, but that they must be used with caution,
particularly when trying to describe or specify the properties unambiguously.

A satisfactory definition of workability is by no means straightforward. Over 50 years
ago, Glanville, et al. (1947), after an extensive study of fresh concrete properties, defined
workability as ‘the amount of work needed to produce full compaction’, thereby relating
it to the placing rather than the handling process. A more recent ACI definition has
encompassed other operations; it is ‘that property of freshly mixed concrete or mortar
which determines the ease and homogeneity with which it can be mixed, placed, consolidated
and finished’ (ACI, 1990). This makes no attempt to define how the workability can be
measured or specified. A similar criticism applies to the ASTM definition of ‘that property
determining the effort required to manipulate a freshly mixed quantity of concrete with
minimum loss of homogeneity’ (ASTM, 1993).

Such definitions are clearly inadequate for the description, specification and quality
control of fresh concrete, and many attempts have been to provide a more satisfactory
definition which includes quantitative measurements. These are sometimes more restrictive,
for example the ACI (1990) definition of consistency as ‘the relative mobility or ability
of freshly mixed concrete to flow’, which is measured by the slump test. This difficulty
illustrates that no single test or measurement can properly describe all of the required
properties of the fresh concrete.

(Tattersall 1991) has proposed a division of the terminology relating to workability
into three classes:

Class 1: Qualitative, to be used in a general descriptive way without any attempt to
Class 2: quantify, e.g. workability, flowability, compactability, stability, pumpability.
Quantitative empirical, to be used as a simple quantitative statement of behaviour
in a particular set of circumstances, e.g. slump, flow table spread.

3