Civil Engineering Reference
In-Depth Information
60
100
w/c = 0.40
w/c = 0.53
w/c = 0.71
14000
8
Moist-cured entire time
50
80
12000
In air after 28 days moist curing
10000
6
40
60
In air after 7 days moist curing
8000
In laboratory air entire time
30
40
6000
4
4000
20
20
Outdoor exposure - Skokie, Illinois
150-mm (6-in.) modified cubes
Type I cement
2000
2
10
0
0
3d 7d 28d 3m 1y
3y 5y 10y 20y
Age at Test
0
0
0 7
28
91
365
Fig. 1-14. Concrete strength gain versus time for concrete
exposed to outdoor conditions. Concrete continues to gain
strength for many years when moisture is provided by
rainfall and other environmental sources
(
Wood 1992
)
.
Age at test, days
Fig. 1-12. Concrete strength increases with age as long as
moisture and a favorable temperature are present for
hydration of cement
(
Gonnerman and Shuman 1928
)
.
50
7000
For example, as mentioned, concrete must continue to hold
enough moisture throughout the curing period for the
cement to hydrate to the extent that desired properties are
achieved. Freshly cast concrete usually has an abundance
of water, but as drying progresses from the surface inward,
strength gain will continue at each depth only as long as
the relative humidity at that point remains above 80%.
A common illustration of this is the surface of a
concrete floor that has not had sufficient moist curing.
Because it has dried quickly, concrete at the surface is weak
and traffic on it creates dusting. Also, when concrete dries,
it shrinks as it loses water (Fig. 1-15), just as wood and clay
do (though not as much). Drying shrinkage is a primary
cause of cracking, and the width of cracks is a function of
the degree of drying, spacing or frequency of cracks, and
the age at which the cracks occur.
While the surface of a concrete element will dry quite
rapidly, it takes a much longer time for concrete in the inte-
rior to dry. Fig. 1-15 (top) illustrates the rate of drying at
various depths within concrete cylinders exposed to labo-
ratory air. Field concrete elements would have different
drying profiles due to environmental conditions, size
effects, and concrete properties.
The moisture content of concrete depends on the
concrete's constituents, original water content, drying
conditions, and the size of the concrete element
(
Heden-
blad 1997
and
1998
)
. After several months of drying in air
with a relative humidity of 50% to 90%, moisture content is
about 1% to 2% by mass of the concrete. Fig. 1-15 illustrates
moisture loss and resulting shrinkage.
Size and shape of a concrete member have an impor-
tant bearing on the rate of drying. Concrete elements with
large surface area in relation to volume (such as floor slabs)
6000
40
5000
30
4000
3000
20
Casting/curing temperature,
°
C
°
(F)
10/10 (50/50)
23/10 (73/50)
23/23 (73/73)
32/32 (90/90)
2000
10
1000
0
0
0
10
20
30
Age, days
Fig. 1-13. Effect of casting and curing temperature on
strength development. Note that cooler temperatures result
in lower early strength and higher later strength
(
Burg,
1996
)
.
Drying Rate of Concrete
Concrete does not harden or cure by drying. Concrete (or more
precisely, the cement in it) needs moisture to hydrate and
harden. When concrete dries out, it ceases to gain strength; the
fact that it is dry is no indication that it has undergone sufficient
hydration to achieve the desired physical properties.
Knowledge of the rate of drying is helpful in under-
standing the properties or physical condition of concrete.
