F. K. Kong MA, MSc, PhD, CEng, FICE, FIStructE, R. H. Evans CBE, DSc, D ès Sc, DTech, PhD, CEng, FICE, FIMechE, FIStructE (auth.)-Reinforced and Prestressed Concrete-Springer US (1987)

28 Properties of structural concrete
principally by affecting the w/c ratio required for workability. If the w/c
ratio is the same and the concrete mixes are plastic and workable, then
considerable changes in the characteristics of the aggregate will have only a
small effect on the strength of the concrete. The effects of aggregate
characteristics on workability play an important role in concrete mix
design, as we shall see later.
The long-term strength of concrete is not much dependent on the type of
cement used. Basically, the behaviours of all Portland cements are similar.
It is true that some cements gain their strength more rapidly than others,
but, for a given w/c ratio, the differences in long-term strengths are only
about 10%. These differences in concrete strenght are, very roughly, of the
same order as those due to the variations in strength of cements of
nominally the same type. For example, it has been found that, for
concretes of average strength of 35 N/mm 2 or more, such variations in
cement quality correspond to a standard deviation of 3.6 N/mm 2 when the
cement comes from many works, and of 3 N/mm 2 when it comes from a
single works [7].
The strength of concrete improves with proper curing after it is cast. By
curing is meant the provision of moisture and a favourable temperature for
the cement to continue to hydrate, thereby increasing the strength of the
concrete. In practice, shortly after the fresh concrete is placed, it is covered
with absorbent materials which are kept moist or with polythene or other
impervious sheets to prevent the evaporation of water. It is difficult to be
precise about the effect of temperature on concrete [8], but generally
speaking it is good practice to maintain a temperature between about 5 and
20 oc during the first half day or so after placing the concrete. During this
initial period, a much higher temperature might retard the later development
in strength, while a much lower temperature (such as would cause the
fresh concrete to freeze) might permanently impair the strength. After this
initial period, the strength development increases with the maturity, which
is the product of age and temperature. The age is measured from the time
of mixing and the temperature from a datum, which is often taken as
-11.7 °C. Of course this assumes the presence of moisture for the cement
to continue to hydrate. It should be pointed out that even when the
concrete is not specially protected against drying, the evaporable water is
not immediately lost under ordinary climatic conditions, and hence the
strength will continue to increase for some time. However, in the absence
of evaporable water in the concrete, there can be no strength increase.
2.5(b)
Creep and its prediction
When a stress is applied to a concrete specimen and kept constant, the
specimen shows an immediate strain followed by a further deformation
which, progressing at a diminishing rate, may become several times the
original immediate strain (Fig. 2.5-3). The immediate strain is often
referred to as the elastic strain and the subsequent time-dependent strain
referred to as the creep strain, or simply the creep. That part of -the strain
which is immediately recoverable upon removal of the stress is called the