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)

Shrinkage and its prediction 33
reader is referred to Reference 10. An authoritative and fuller account has
been given by Neville [11].
2.5(c)
Shrinkage and its prediction
The volume contraction which occurs as the concrete hardens and dries out
is called the drying shrinkage, or simply the shrinkage. Shrinkage is
thought to be due mainly to the loss of adsorbed water in the gel; its
nature, like that of creep, is thought to be primarily related to the physical
structure of the gel rather than its chemical composition [10, 12].
The shrinkage on first drying is partly irreversible and is called the initial
drying shrinkage. If dry concrete is resaturated with water, an expansion,
sometimes referred to as the moisture movement, of about 60% of the
initial drying shrinkage will occur. If this saturated concrete is redried, it
will shrink again, this shrinkage being substantially equal to the expansion.
The process of saturation and drying may be repeated indefinitely, and for
every cycle the expansion and shrinkage are very nearly equal, and will be
equal if the hydration of the cement is complete before the drying
shrinkage occurs. As hydration in concrete is rarely ever complete, the
shrinkage in each cycle is nearly always slightly larger than the expansion.
If, after casting, the concrete is continuously cured in saturated
atmosphere or under water, it undergoes an expansion. Such expansion is
mostly due to the adsorption of water by the gel, and is about 150 x 10- 6
for concrete of aggregate/cement ratio 6: 1 and w/c ratio 0.6. This
expansion increases with the cement content; for neat cement paste the
expansion will be 10 times as large. About 80% of this expansion occurs in
the first month after casting, 90% in 6 months and practically the full
amount in one year.
For practical design purposes, we are less concerned with the nature of
shrinkage than with the prediction of shrinkage values. An approximate
but simple procedure has been proposed for such purposes [14]. To
estimate the shrinkage of a particular concrete, start from the values in Table
2.5-4 and then successively allow for the effects of the various other factors
as explained below.
Table 2.5-4 shows some shrinkage values of specimen concrete mixes,
made of ordinary Portland cement and crushed-gravel aggregate and sand.
The concretes are wet cured for 28 days after casting and then exposed to
70% RH at 15 °C. These shrinkage values are the ultimate values reached
Table 2.5-4 Shrinkage of specimen mixes
Mix Ref.
Mix proportions
(by weight)
Shrinkage
Remarks
A
B
c
1 : 2: 4 w/c = 0.65
1: 1.5:3 w/c = 0.55
1 : 1 : 2 w/c = 0.40
400 x w- 6
500 x w- 6
600 x w-~>
Volume/surface
ratio of
specimens = 60 mm