Civil Engineering Reference
In-Depth Information
Table 3-3. Effect of Fly Ash on Mixing Water Requirements for
Air-Entrained Concrete
bleed water is primarily due to the reduced
water demand in fly ash concretes.
Gebler
and Klieger (1986)
correlate reduced bleed-
ing of concrete to the reduced water
requirement of fly ash mortar.
Concretes containing ground slags of
comparable fineness to that of the cement
tend to show an increased rate and amount
of bleeding than plain concretes, but this
appears to have no adverse effect on segre-
gation. Slags ground finer than cement
reduce bleeding.
Silica fume is very effective in reduc-
ing both bleeding and segregation; as a
result, higher slumps may be used. Cal-
cined clays, calcined shales, and metakao-
lin have little effect on bleeding.
Fly ash content,
Change in mixing
Fly ash mix
Class of
% by mass of
water requirement
identification
fly ash
cementitious material
compared to control, %
C1A
C
25
-6
C1D
F
25
-2
C1E
F
25
-6
C1F
C
25
-8
C1G
C
25
-6
C1J
F
25
-6
C2A
C
50
-18
C2D
F
50
-6
C2E
F
50
-14
C2F
C
50
-16
C2G
C
50
-12
C2J
F
50
-10
All mixtures had cementitious materials contents of 335 kg/m
3
(564 lb/yd
3
), a slump of
125 ± 25 mm (5 ± 1 in.), and an air content of 6 ± 1%. Water to cement plus fly ash ratios
varied from 0.40 to 0.48 (
Whiting 1989
).
Air Content
The amount of air-entraining admixture
required to obtain a specified air content is
normally greater when fly ash is used. Class C ash requires
less air-entraining admixture than Class F ash and tends to
lose less air during mixing (Table 3-5). Ground slags have
variable effects on the required dosage rate of air-entrain-
ing admixtures. Silica fume has a marked influence on the
air-entraining admixture requirement, which in most cases
rapidly increases with an increase in the amount of silica
fume used in the concrete. The inclusion of both fly ash
and silica fume in non-air-entrained concrete will generally
reduce the amount of entrapped air.
crease water demand up to 5% (
Gebler and Klieger 1986
).
Fly ash reduces water demand in a manner similar to
liquid chemical water reducers (
Helmuth 1987
). Ground
slag usually decreases water demand by 1% to 10%,
depending on dosage.
The water demand of concrete containing silica fume
increases with increasing amounts of silica fume, unless a
water reducer or plasticizer is used. Some lean mixes may
not experience an increase in water demand when only a
small amount (less than 5%) of silica fume is present.
Calcined clays and calcined shales
generally have little effect on water de-
mand at normal dosages; however, other
natural pozzolans can significantly in-
crease or decrease water demand.
Table 3-4. Effect of Fly Ash on Bleeding of Concrete
(ASTM C 232, AASHTO T 158)*
Fly ash mixtures
Bleeding
Identification
Class of fly ash
Percent
mL/cm
2
**
Workability
A
C
0.22
0.007
Fly ash, slag, and calcined clay and shale
generally improve the workability of
concretes of equal slump. Silica fume may
contribute to stickiness of a concrete
mixture; adjustments, including the use of
high-range water reducers, may be re-
quired to maintain workability and permit
proper compaction and finishing.
B
F
1.11
0.036
C
F
1.61
0.053
D
F
1.88
0.067
E
F
1.18
0.035
F
C
0.13
0.004
G
C
0.89
0.028
H
F
0.58
0.022
I
C
0.12
0.004
J
F
1.48
0.051
Average of:
Class C
Bleeding and Segregation
Concretes using fly ash generally exhibit
less bleeding and segregation than plain
concretes (Table 3-4). This effect makes the
use of fly ash particularly valuable in
concrete mixtures made with aggregates
that are deficient in fines. The reduction in
0.34
0.011
Class F
1.31
0.044
Control mixture
1.75
0.059
* All mixtures had cementitious materials contents of 307 kg/m
3
(517 lb/yd
3
), a slump
of 75 ± 25 mm (3 ± 1 in.), and an air content of 6 ± 1%. Fly ash mixtures contained
25% ash by mass of cementitious material (
Gebler and Klieger 1986
).
** Volume of bleed water per surface area.
