Civil Engineering Reference
In-Depth Information
containing supplementary cementing materials are similar
to the effects on concrete made with only portland cement;
however, the curing time may need to be longer for certain
materials with slow-early-strength gain.
High dosages of silica fume can make concrete highly
cohesive with very little aggregate segregation or bleeding.
With little or no bleed water available at the concrete surface
for evaporation, plastic cracking can readily develop, espe-
cially on hot, windy days if special precautions are not taken.
Proper curing of all concrete, especially concrete con-
taining supplementary cementing materials, should com-
mence immediately after finishing. A seven-day moist cure
or membrane cure should be adequate for concretes with
normal dosages of most supplementary cementitious
materials. As with portland cement concrete, low curing
temperatures can reduce early-strength gain (
Gebler and
Klieger 1986
).
120
ASTM Type I cement
100
70% slag
80
60
40
20
0
0
5
10
15
20
25
30
Time, days
Fig. 3-13. Effect of a slag on heat of hydration at 20°C (68°F)
compared to a Type I cement.
as the amount of portland cement, water requirement, the
type and reactivity of the slag or pozzolan dosage, and the
temperature of the concrete. Set retardation is an advan-
tage during hot weather, allowing more time to place and
finish the concrete. However, during cold weather, pro-
nounced retardation can occur with some materials, sig-
nificantly delaying finishing operations. Accelerating
admixtures can be used to decrease the setting time.
Calcined shale and clay have little effect on setting time.
EFFECTS ON HARDENED CONCRETE
Strength
Fly ash, ground granulated blast-furnace slag, calcined clay,
metakaolin, calcined shale, and silica fume contribute to the
strength gain of concrete. However, the strength of concrete
containing these materials can be higher or lower than the
strength of concrete using portland cement as the only
cementing material. Fig. 3-14 illustrates this for various fly
ashes. Tensile, flexural, torsional, and bond strength are
affected in the same manner as compressive strength.
Finishability
Concrete containing supplementary cementing materials
will generally have equal or improved finishability
compared to similar concrete mixes without them. Mixes
that contain high dosages of cementitious materials—and
especially silica fume—can be sticky and difficult to finish.
42
6
Pumpability
The use of supplementary cementing materials generally
aids the pumpability of concrete. Silica fume is the most
effective, especially in lean mixtures.
35
5
28
4
Plastic Shrinkage Cracking
Because of its low bleeding characteristics, concrete con-
taining silica fume may exhibit an increase in plastic shrink-
age cracking. The problem may be avoided by ensuring that
such concrete is protected against drying, both during and
after finishing. Other pozzolans and slag generally have
little effect on plastic shrinkage cracking. Supplementary
cementing materials that significantly increase set time can
increase the risk of plastic shrinkage cracking.
None
Fly ash A
Fly ash D
Fly ash E
Fly ash F
Fly ash G
Fly ash J
21
3
14
2
1
7
1
10
100
Age, days
Curing
Fig. 3-14. Compressive strength development at 1, 3, 7, 28,
and 90 days of concrete mixtures containing 307 kg/m
3
(517 lb/yd
3
) of cementitious materials with a fly ash dosage
of 25% of the cementitious materials (
Whiting 1989
).
The effects of temperature and moisture conditions on
setting properties and strength development of concretes
