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 31
humidity and then loaded and stored under the same humidity, it will be
found that the value of this humidity does not have a significant effect on
the creep. This indicates that when the concrete has reached moisture
equilibrium with the surrounding atmosphere creep becomes almost
independent of the relative humidity of the surrounding air.
Up to about 100 oc the general shape of the creep/time curve is similar
to that at normal temperatures, and the relation between creep and the
stress/strength ratio remains linear. The rate of creep increases with an
increase in temperature up to a maximum at about 70 oc, thereafter
decreasing somewhat up to 100 °C. For general design purposes, it seems
sufficient to assume that creep increases linearly with temperature at a rate
of H% of the 15 oc creep for each degree Celsius.
For reinforced and prestressed concrete work, the cement-paste content
by volume, calculated by the procedure shown in Table 2.5-3, nearly
always lies within the range 28-40%. Within this range, creep (for the
same stress/strength ratio) can be assumed to increase at the approximate
rate of 5% for each per cent increase in the cement-paste content by
volume. In calculating this cement-paste content, the speciic gravity of
Portland cements may be taken as 3.15 and that of common gravel and
stone aggregates as 2.60. Table 2.5-3 shows sample calculations for two
mixes.
Within the limits of concrete mixes used in reinforced and prestressed
concrete work, the effect of the w/c ratio is indirect. Consider an increase
in the w/c ratio. First, it causes a reduction in strength (Fig. 2.5-2),
thereby increasing the stress/strength ratio for a given stress. Second, for a
given aggregate/cement ratio, an increase in the w/c ratio increases the
cement-paste content. By allowing for these two effects, the influence of a
change in the w/c ratio is automatically taken care of [9, 10].
Within the range of practical mixes, the aggregate/cement ratio,
aggregate content, cement content and water content do not by themselves
have important effects on creep. Their effects are mainly due to their
influence on the w/c ratio and the cement-paste content, and can be
allowed for accordingly.
Table 2.5-3 Calculation for cement-paste content
Mix
I: I~: 3 wlc = 0.55
Cementlagg. = I: 5 w/c = 0.55
Ingredient
Weights
Volumes
Weights
Volumes
Cement
Water
Aggregate
1
0.55
4.5
113.15 = 0.31
0.55/1 = 0.55
4.5/2.6 = 1.73
2.59
1
0.55
5.0
1/3.15 = 0.31
0.55/1 = 0.55
5.0/2.6 = 1.92
2.78
Cement-paste content:
(0.31 + 0.55)/2.59 = 0.333
Cement-paste content:
(0.31 + 0.55)/2.78 = 0.309