Civil Engineering Reference
In-Depth Information
Table 3-5. Effect of Fly Ash on Air-Entraining Admixture Dosage and Air Retention
The air-entraining dos-
age and air retention char-
acteristics of concretes con-
taining ground slag or
natural pozzolans are sim-
ilar to mixtures made only
with portland cement.
Percent of
air-entraining
admixture
Fly ash mixtures
relative to control
Air content, %
Minutes after initial mixing
Identification
Class of fly ash
0
30
60
90
A
C
126
7.2
6.0
6.0
5.8
Heat of Hydration
B
F
209
5.3
4.1
3.4
3.1
C
F
553
7.0
4.7
3.8
2.9
Fly ash, natural pozzolans,
and ground slag have a
lower heat of hydration
than portland cement; con-
sequently their use will re-
duce the amount of heat
built up in a concrete struc-
ture (Fig. 3-13). Calcined
clay imparts a heat of hy-
dration similar to mod-
erate heat cement (
Barger
and others 1997
). Some
pozzolans have a heat of hydration of only 40% that of
Type I portland cement. This reduction in temperature rise
is especially beneficial in concrete used for massive struc-
tures. Silica fume may or may not reduce the heat of hydra-
tion.
Detwiler and others (1996)
provide a review of the
effect of pozzolans and slags on heat generation.
D
F
239
6.6
5.4
4.2
4.1
E
F
190
5.6
4.6
4.3
3.8
F
C
173
6.8
6.5
6.3
6.4
G
C
158
5.5
4.8
4.5
4.2
H
F
170
7.6
6.9
6.5
6.6
I
C
149
6.6
6.5
6.5
6.8
J
F
434
5.5
4.2
3.8
3.4
Control mixture
100
6.6
6.0
5.6
5.3
Concretes had a cementitious materials content of 307 kg/m
3
(517 lb/yd
3
) and a slump of 75 ± 25 mm
(3 ± 1 in.). Fly ash mixtures contained 25% ash by mass of cementitious material (
Gebler and Klieger 1983
).
The amount of air-entraining admixture required for a
certain air content in the concrete is a function of the fine-
ness, carbon content, alkali content, organic material
content, loss on ignition, and presence of impurities in the
fly ash. Increases in alkalies decrease air-entraining agent
dosages, while increases in the other properties increase
dosage requirements. The Foam Index test provides an
indication of the required dosage of air-entraining admix-
ture for various fly ashes relative to non-ash mixtures; it can
also be used to anticipate the need to increase or decrease
the dosage based on changes in the foam index (
Gebler and
Klieger 1983
).
Setting Time
The use of fly ash and ground granulated blast-furnace
slag will generally retard the setting time of concrete (Table
3-6). The degree of set retardation depends on factors such
Table 3-6. Effect of Fly Ash on Setting Time of Concrete
Fly ash test mixtures
Setting time, hr:min
Retardation relative to control, hr:min
Identification
Class of fly ash
Initial
Final
Initial
Final
A
C
4:30
5:35
0:15
0:05
B
F
4:40
6:15
0:25
0:45
C
F
4:25
6:15
0:10
0:45
D
F
5:05
7:15
0:50
1:45
E
F
4:25
5:50
0:10
0:20
F
C
4:25
6:00
0:10
0:30
G
C
4:55
6:30
0:40
1:00
H
F
5:10
7:10
0:55
1:40
I
C
5:00
6:50
0:45
1:20
J
F
5:10
7:40
0:55
2:10
Average of:
Class C
4:40
6:15
0:30
0:45
Class F
4:50
6:45
0:35
1:15
Control mixture
4:15
5:30
—
—
Concretes had a cementitious materials content of 307 kg/m
3
(517 lb/yd
3
). Fly ash mixtures contained 25% ash by mass of cementitious mate-
rial. Water to cement plus fly ash ratio = 0.40 to 0.45. Tested at 23°C (73°F) (
Gebler and Klieger 1986
).
