CONCRETE IN HOT ENVIRONMENTS - CHAPTER 7 ppt

Chapter 7

Drying Shrinkage

7.1. INTRODUCTION

It was explained earlier that hardened cement is characterised by a porous

structure, with a minimum porosity of some 28%, which is reached when all

the capillary pores become completely filled with the cement gel (see section

2.4). This may occur, theoretically at least, in a well-cured paste made with a

water to cement (W/C) ratio of about 0·40 or less. Otherwise, the porosity of

the paste is much higher due to incomplete hydration and the use of higher W/

C ratios. In practice, and under normal conditions, this is usually the case, and

a porosity in the order of some 50%, and more, is to be expected.

The moisture content of a porous solid, including that of the hardened

cement, depends on environmental factors, such as relative humidity etc., and

varies due to moisture exchange with the surroundings. The variations in

moisture content, generally referred to as ‘moisture movement’, involve

volume changes. More specifically, a decrease in moisture content (i.e. drying)

involves volume decrease commonly known as ‘drying shrinkage’, or simply

‘shrinkage’. Similarly, an increase in moisture content (i.e. absorption) involves

a volume increase known as ‘swelling’. In practice, the shrinkage aspect is

rather important because it may cause cracking (see section 7.5), and thereby

affect concrete performance and durability. Swelling, on the other hand, is

hardly of any practical importance. Hence, the following discussion is mainly

limited to the shrinkage aspect of the problem. In this respect it should be

pointed out that, although shrinkage constitutes a bulk property, it is usually

Copyright 1993 E & FN Spon

measured by the associated length changes and is expressed quantitatively by

the corresponding linear strains,
l/l0.

7.2. THE PHENOMENA

A schematic description of volume changes in concrete, subjected to alternate

cycles of drying and wetting, is given in Fig. 7.1. It may be noted that

maximum shrinkage occurs on first drying, and a considerable part of this

shrinkage is irreversible, i.e some part of the volume decrease is not recovered

on subsequent wetting. Further cycles of drying and wetting result in

additional, usually smaller, irreversible shrinkage. Ultimately, however, the

process becomes more or less completely reversible. Hence, the distinction

between ‘reversible’ and ‘irreversible’ shrinkage. In practice, however, such a

distinction is hardly of any importance and the term ‘shrinkage’ usually refers

to the maximum which occurs on first drying.

7.3. SHRINKAGE AND SWELLING MECHANISMS

As mentioned earlier, shrinkage is brought about by drying and the associated

decrease in the moisture content in the hardened cement. A few mechanisms

Fig. 7.1. Schematic description of volume changes in concrete exposed to

alternate cycles of drying and wetting.

Copyright 1993 E & FN Spon