CONCRETE IN HOT ENVIRONMENTS - CHAPTER 4 docx

Chapter 4

Workability

4.1. INTRODUCTION

The ‘workability’ of concrete may be defined as ‘the property determining the

effort required to manipulate a freshly mixed quantity of concrete with a

minimum loss homogeneity’ (ASTM C125). In this definition the term

‘manipulate’ is meant to include all the operations involved in handling the fresh

concrete, namely, transporting, placing, compacting and also, in some cases,

finishing. In other words, workability is that property which makes the fresh

concrete easy to handle and compact without an appreciable risk of segregation.

The workability may be defined somewhat differently and, indeed, other

definitions have been suggested. Nevertheless, and regardless of the exact

definition adopted, it may be realised that the workability is a composite

property and, as such, cannot be determined quantitatively by a single

parameter. In practice, however, such a determination is required and, strictly

speaking, common test methods (slump, Vebe apparatus) actually determine

the ‘consistency’ or the ‘compactability’ of the fresh concrete rather than its

‘workability’. In practice, however, workability and consistency are usually

not differentiated.

Generally, the workability is essentially determined by the consistency and

cohesiveness of the fresh concrete. That is, in order to give the fresh concrete

the desired workability, both its consistency and cohesiveness must be

controlled. The sought-after cohesiveness is attained by proper selection of

mix proportions using one of the available mix-design procedures [4.1, 4.2].

Copyright 1993 E & FN Spon

In other words, once cohesiveness is attained, the workability is further

controlled by the consistency alone. This is usually the case and in practice,

indeed, workability is controlled by controlling the consistency of the mix.

Hence, the sometimes indiscriminate reference to ‘consistency’ and

‘workability’, as well as the use of consistency tests such as the slump, or the

Vebe tests to control workability (BS 1881, Parts 102, 103 and 104). In this

respect it is further assumed that a stiffer mix is less workable than a more

fluid one, and vice versa. This assumption, however, is not always true,

because a very wet mix may exhibit a marked tendency to segregate, and as

such is, therefore, of a poor workability.

4.2. FACTORS AFFECTING WATER DEMAND

4.2.1. Aggregate Properties

The consistency of the fresh concrete is controlled by the amount of water which

is added to the mix. The amount of water required (i.e. the ‘water demand’ or

‘water requirement’) to produce a given consistency depends on many factors

such as aggregate size and grading, its surface texture and angularity, as well as

on the cement content and its fineness, and on the possible presence of

admixtures. The water wets the surface of the solids, separates the particles, and

thereby acts as a lubricant. Hence, the greater the surface area of the particles,

the greater the amount of water which is required for the desired consistency,

and vice versa. Similarly, when a greater amount of mixing water is used, the

separation between the solid particles is increased, friction is thereby reduced,

and the mix becomes more fluid. The opposite occurs when a smaller amount

of water is added, i.e. friction is increased bringing about a stiffer mix. Hence,

the sometimes synonymous use of ‘wet’ and ‘fluid’ mixes on the one hand, and

the use of ‘dry’ and ‘stiff’ mixes, on the other.

It must be realised, however, that quantitatively the relation between the

consistency and the amount of mixing water is not linear, but rather of an

exponential nature. It can be generally expressed mathematically by the

following expression:

y=CWn

where y is the consistency value (e.g. slump etc.); W is the water content of

Copyright 1993 E & FN Spon

the fresh concrete; C is a constant which depends on the composition of the

mix, on the one hand, and the method of determining the consistency, on the

other; n is also a constant which depends, again, on the method of determining

the consistency but not on concrete composition. A graphical representation of

this equation is given in Fig. 4.1 for n=10.

It is clearly evident from Fig. 4.1 that the slump of the wetter mixes is more

sensitive to changes in the amount of mixing water than the slump of the

stiffer ones. In other words, a given change in the amount of mixing water

(
W1=
W2) causes a greater change in the slump of the wetter mixes than in

the slump of the stiffer ones (
S1>
S2).

Generally, the aggregate comprises some 70% by volume of the concrete,

whereas the cement comprises only some 10%. Moreover, usually, the specific

surfaces of the cements used in daily practice are more or less the same. Hence,

in practice, excluding the effect of admixtures, the amount of water required

to give the fresh concrete the desired consistency (usually specified by the

slump), is estimated with respect to the aggregate properties only, i.e. with

respect to aggregate size and shape. Size is usually measured by the parameter

known as ‘maximum size of aggregate’, which is the size of the sieve greater

than the sieve on which 15% or more of the aggregate particles are retained

for the first time on sieving. In considering shape and texture, a distinction is

made between ‘crushed’ and ‘uncrushed’ (gravel) aggregate. The particles of

crushed aggregate are angular and of a rough texture whereas those of gravel

aggregate, are round and smooth. Hence, the latter are characterised by a

smaller surface area, and require less water than the crushed aggregate to

produce a mix of a given consistency.

Fig. 4.1. Schematic representation of the

relation between slump and the amount

of mixing water. (Adapted from Ref. 4.3.)

Copyright 1993 E & FN Spon