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)

Creep and its prediction 29
..
E
E
........ -6
z 40x10
c.
~
Age (months)
14
Fig. 2.5-3 Typical strain/time curve for concrete subjected to constant load
followed by load removal
elastic recovery and the delayed recovery the creep recovery. The elastic
recovery is less than the elastic strain; the creep recovery is much less than
the creep.
The mechanism of creep is still subject to controversy [8-12]. Though it
is likely that creep is related to the structure of the gel, it is hard to suggest
definite conclusions on the mechanism of creep; the difficulty is that a
satisfactory theory must explain in a unified way the behaviour of concrete
under various environmental and stress conditions. Perhaps the only noncontroversial
statement that can be made is that the presence of some
evaporable water is essential to creep [12]. Methods for estimating creep
values have been suggested by various authors [8-12] including the CEB
[13]. One thing is common to all these methods: the accuracy is not high
and an error of ±30% or more is possible. An account is given below of an
approximate but simple method previously proposed for engineers in
practice [9, 10]. To estimate the creep of a particular concrete, start from the
creep values in Table 2.5-1 and then successively allow for the effects of
various other factors as explained in the subsequent paragraphs.
Table 2.5-1 shows some creep values of specimen concrete mixes, loaded
to within one-third of the cube strength, assuming loading starts after 28
days wet curing and service condition of 70% relative humidity and 15 OC.
These are limiting-creep values and are reached when the stresses are
sustained for a very long period, say 30 years. Creeps corresponding to
shorter periods of loading, as well as the effects of various factors are
estimated as explained below. In practical structures, where the concrete
is unlikely to be stressed beyond one-half of the cube strength, the
creep of a given concrete under a given period of sustained stress may be