Civil Engineering Reference
In-Depth Information
cantly affect the setting time. ASTM C 494 (AASHTO M
194) Type A water reducers can have little effect on setting,
while Type D admixtures provide water reduction with
retardation, and Type E admixtures provide water reduc-
tion with accelerated setting. Type D water-reducing
admixtures usually retard the setting time of concrete by
one to three hours (Fig. 6-5). Some water-reducing admix-
tures may also entrain some air in concrete. Lignin-based
admixtures can increase air contents by 1 to 2 percentage
points. Concretes with water reducers generally have
good air retention (Table 6-2).
The effectiveness of water reducers on concrete is a
function of their chemical composition, concrete tempera-
ture, cement composition and fineness, cement content,
and the presence of other admixtures. The classifications
and components of water reducers are listed in Table 6-1.
See
Whiting and Dziedzic (1992)
for more information on
the effects of water reducers on concrete properties.
MID-RANGE WATER REDUCING
ADMIXTURES
Mid-range water reducers were first introduced in 1984.
These admixtures provide significant water reduction
(between 6 and 12%) for concretes with slumps of 125 to
200 mm (5 to 8 in.) without the retardation associated with
high dosages of conventional (normal) water reducers.
Normal water reducers are intended for concretes with
slumps of 100 to 125 mm (4 to 5 in.). Mid-range water
reducers can be used to reduce stickiness and improve
finishability, pumpability, and placeability of concretes
containing silica fume and other supplementary cement-
ing materials. Some can also entrain air and be used in low
slump concretes (
Nmai, Schlagbaum, and Violetta 1998
).
HIGH-RANGE WATER REDUCING
ADMIXTURES
High-range water reducers, ASTM C 494 (AASHTO M
194) Types F (water reducing) and G (water reducing and
retarding), can be used to impart properties induced by
regular water reducers, only much more efficiently. They
can greatly reduce water demand and cement contents
and make low water-cement ratio, high-strength concrete
with normal or enhanced workability. A water reduction
of 12% to 30% can be obtained through the use of these
admixtures. The reduced water content and water-cement
ratio can produce concretes with (1) ultimate compressive
strengths in excess of 70 MPa (10,000 psi), (2) increased
early strength gain, (3) reduced chloride-ion penetration,
and (4) other beneficial properties associated with low
water-cement ratio concrete (Fig. 6-6).
Initial set
4
Cement 1
Cement 2
3
2
1
0
L
H
N
MB
X
Admixture
Final set
3
Cement 1
Cement 2
2
1
0
L
H
N
MB
X
Admixture
Fig. 6-5. Retardation of set in cement-reduced mixtures
relative to control mixture. Concretes L and H contain con-
ventional water reducer, concretes N, M, B, and X contain
high-range water reducer (
Whiting and Dziedzic 1992
).
Fig. 6-6. Low water to cement ratio concrete with low
chloride permeability—easily made with high-range water
reducers—is ideal for bridge decks. (69924)
