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)

30 Properties of structural concrete
Table 2.5-l Creep of specimen mixes
Mix proportions Creep
Mix Ref (by weight) (per N/mm 2 ) Remarks
A 1:2:4 3 w/c = 0.65 120 x w-6 Cement:
ordinary Portland
B 1: 1.5:3 w/c = 0.55 100 x w-6 Fine aggregate:
c 1:1:2 w/c = 0.40 10 x w- 6
sand
Coarse aggregate:
crushed gravel
• Cement: fine aggregate: coarse aggregate (by weight).
taken as roughly proportional to the stress. Hence Mix C, say, in Table
2.5-1 will have a limiting creep of (70 x 10- 6 ) x 10 = 700 x 10- 6 if the
sustained stress is 10 N/mm 2 • For different concretes of the same cementpaste
content, creep is approximately proportional to the stress/strength
ratio, i.e. the ratio of the applied stress to the cube strength of the
concrete,at the time of loading. Hence, a concrete of 45 N/mm 2 cube
strength stressed to 15 N/mm 2 would have approximately the same creep
as another concrete of 30 N/mm 2 stressed to 10 N/mm 2 , provided the two
concretes have the same cement-paste content.
Creep depends on the duration of loading, as shown in Table 2.5-2.
The effect on creep of the age at loading is mainly due to the increase in
strength of concrete with age. Since, for a given stress, creep is inversely
proportional to strength, the effect of age at loading can be estimated
provided the strength-age relation is known. For example, if the oneyear
strength of a concrete is 1.5 times the 28-day strength, then by
applying the load at age one year instead of at age 28 days, the long-term
creep will be reduced by the ratio 111.5. As pointed out earlier, strength is
related to the maturity, and a sample calculation is given in Reference 9.
If concrete is wet cured to an age of 28 days and then loaded, the limiting
creeps in air at 50% relative humidity (RH) and at 100% RH are respectively
about 1j and! times that for storage in air at 70% RH. Creeps at
70% RH are given in Table 2.5-1; for other relative humidities, creeps can
be estimated from the above general information.
If concrete is stored for a sufficiently long time, say a year, at a particular
Table 2.5-2 Effect of duration of loading
Duration of loading
28days
6months
1 years
5 years
lOyears
30years
Percentage of
long-term creep
40
60
75
90·
95
100