Civil Engineering Reference
In-Depth Information
5
5
25%
replacement
Moist cure
50%
replacement
Moist cure
4
4
Cementitious
material
3
3
Cementitious
material
250 kg/m
3
(417 lb/yd
3
)
305 kg/m
3
(508 lb/yd
3
)
335 kg/m
3
(588 lb/yd
3
)
250 kg/m
3
(417 lb/yd
3
)
305 kg/m
3
(508 lb/yd
3
)
335 kg/m
3
(588 lb/yd
3
)
2
2
1
1
0
0
None
A
D
E
F
G
J
None
A
D
E
F
G
J
Fly ash
Fly ash
Fig. 3-20. Relationship between deicer-scaling resistance and dosage of fly ash for air-entrained concretes made with
moderate to high water-cementitious materials ratios. Replacement of portland cement with fly ash: (left) 25% and (right)
50%. A scale rating of 0 is no scaling and 5 is severe scaling (
Whiting 1989
).
Permeability and Absorption
Alkali-Aggregate Reactivity
With adequate curing, fly ash, ground slag, and natural
pozzolans generally reduce the permeability and absorption
of concrete. Silica fume and metakaolin are especially effec-
tive in this regard. Silica fume and calcined clay can provide
concrete with a chloride resistance of under 1000 coulombs
using the ASTM C 1202 rapid chloride permeability test
(
Barger and others 1997
). Tests show that the permeability of
concrete decreases as the quantity of hydrated cementing
materials increases and the water-cementitious materials
ratio decreases. The absorption of fly-ash concrete is about
the same as concrete without ash, although some ashes can
reduce absorption by 20% or more.
Alkali-silica reactivity can be controlled through the use of
certain supplementary cementitious materials. Silica fume,
fly ash, ground granulated blast-furnace slag, calcined clay,
calcined shale, and other pozzolans have been reported to
significantly reduce alkali-silica reactivity (Figs. 3-21 and
3-22). Low calcium Class F ashes have reduced reactivity
0.7
No pozzolan
0.6
0.25
0.5
0.20
ASTM C 441
Cement: 0.92% alkali
Pyrex glass
0.4
0.15
0.10
0.3
0.05
0.2
20% Calcined shale
0
15% 25%
15% 25%
15% 25%
35% 50%
Control
Low-CaO
fly ash
Medium-CaO
fly ash
High-CaO
fly ash
Slag
20% Calcined clay
0.1
Fig. 3-21. Effect of different fly ashes and slag on alkali-silica
reactivity. Note that some ashes are more effective than
others in controlling the reaction and that dosage of the ash
or slag is critical. A highly reactive natural aggregate was
used in this test. A less reactive aggregate would require less
ash or slag to control the reaction. A common limit for eval-
uating the effectiveness of pozzolans or slags is 0.10%
expansion using this rapid mortar bar test (
Detwiler 2002
).
0
0
2
4
6
8
10
12
Age, months
Fig. 3-22. Reduction of alkali-silica reactivity by calcined
clay and calcined shale (
Lerch 1950
).
