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)

34 Properties of structural concrete
Table 2.5-5 Effect of duration of exposure
Duration of
exposure
Percentage of
long-term shrinkage
Duration of
exposure
Percentage of
long-term shrinkage
28days
6months
1 year
40
60
75
5 years
10 years
30 years
90
95
100
after a long time of exposure, say 30 years. Effects of the duration of
exposure can be allowed for using Table 2.5-5.
Using the shrinkage at 70% RH as the reference magnitude, shrinkage
can be assumed to increase at the approximate rate of 2% for each per cent
decrease in relative humidity down to about 40% RH, and decrease at the
approximate rate of 3% for each per cent increase in RH up to about 90%
RH [13, 14]. At 100% RH there is an expansion equal numerically to about
20% of the shrinkage at 70% RH. Shrinkages at 70% RH are given in
Table 2.5-4; for other humidities, shrinkage can be estimated from the
above general information.
There is insufficient information [8, 13] on the variation of shrinkage
with temperature. As an approximate guidance, it is suggested [14] that,
within the range of atmospheric temperatures in Great Britain, shrinkage
can be assumed to increase at the rate of 1% of the 15 oc shrinkage for
each degree Celsius rise in temperature and decrease at the same rate with
fall in temperature.
For reinforced and prestressed concrete work, the cement-paste content
by volume, calculated by the procedure shown in Table 2.5-3, generally
lies within the range 28-40%. Within this range shrinkage can be assumed
to increase at the approximate rate of 7% for each per cent increase in the
cement-paste content by volume, calculated as in Table 2.5-3.
The mix proportions of a concrete are completely defined by the cementpaste
content, the w/c ratio and the ratio of fine aggregate to coarse
aggregate. The effect of mix proportions on shrinkage is largely due to
their effect on the cement-paste content. Having allowed for the effect of
the cement-paste content, the additional effect of the w/c ratio is less
important (Fig. 2.5-5) while the effect of the ratio of fine aggregate to
coarse aggregate is in itself insignificant.
& 1·1
! c..
i 1·0
. J
............ ..........
I'
..........
............ ..........
....
J!
............
I o·9
0·7 0.6 0.5 0·4 0.3
water/cement ratio (by weight)
'
Fig. 2.5-5 Effect of water/cement ratio Qn shrinkage of concrete of given cementpaste
content