Precast concrete materials, manufacture, properties and usage - Chapter 2 pot

2

ADMIXTURES

This chapter covers the range of admixtures used in precast concrete

products and mortars used with precast masonry. They are all used at

relatively low concentrations (0·01–5% w/w cement) for the purpose of

modifying the properties of the fresh and/or the hardened material. The

chapter excludes additives such as ground blast furnace slag and

pulverised fuel ash (fly ash), the latter being covered in Chapter 4.

Pigments, although in the admixtures group, are covered in the next

chapter.

It is often stated that well-designed, compacted and cured concrete

should not need an admixture. However, when all the practical aspects of

a situation are considered such dogmatism can seldom be justified. The

author’s opinion is that, with the complexity of castings, the chance to

reduce water contents for the same workability, and with the many other

advantages that admixtures can bestow, there are few cases where their

non-use can be justified. However, one point that needs to be emphasised

is that admixtures are used at relatively low concentrations and over or

under dosage can lead to potentially disastrous results. Control of the

basic ingredients of aggregate, cement and water must be strict because

if there is any doubt about this there is no point in using admixtures. The

admixtures are used to make good concrete products better, not to make

poor or mediocre concrete good.

Having said all this, one can now proceed to discuss admixtures under

performance headings.

2.1 ACCELERATORS

The purpose of these is to accelerate the setting and hardening rates of

cement for the main purpose in precast work of getting a faster turnover

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