Civil Engineering Reference
In-Depth Information
Table 6-1 lists the primary components and specifica-
tions for plasticizing (superplasticizer) admixtures.
less than that obtained with a Type A water reducer. Type
D admixtures are designated to provide both water reduc-
tion and retardation.
In general, some reduction in strength at early ages
(one to three days) accompanies the use of retarders. The
effects of these materials on the other properties of con-
crete, such as shrinkage, may not be predictable. There-
fore, acceptance tests of retarders should be made with
actual job materials under anticipated job conditions. The
classifications and components of retarders are listed in
Table 6-1.
RETARDING ADMIXTURES
Retarding admixtures are used to delay the rate of setting
of concrete. High temperatures of fresh concrete (30°C
[86°F]) are often the cause of an increased rate of harden-
ing that makes placing and finishing difficult. One of the
most practical methods of counteracting this effect is to
reduce the temperature of the concrete by cooling the
mixing water and/or the aggregates. Retarders do not
decrease the initial temperature of concrete. The bleeding
rate and bleeding capacity of concrete is increased with
retarders.
Retarding admixtures are useful in extending the set-
ting time of concrete, but they are often also used in
attempts to decrease slump loss and extend workability,
especially prior to placement at elevated temperatures.
The fallacy of this approach is shown in Fig. 6-15, where
the addition of a retarder resulted in an increased rate of
slump loss compared to the control mixtures (
Whiting and
Dziedzic 1992
).
Retarders are sometimes used to: (1) offset the accel-
erating effect of hot weather on the setting of concrete;
(2) delay the initial set of concrete or grout when difficult
or unusual conditions of placement occur, such as placing
concrete in large piers and foundations, cementing oil
wells, or pumping grout or concrete over considerable
distances; or (3) delay the set for special finishing tech-
niques, such as an exposed aggregate surface.
The amount of water reduction for an ASTM C 494
(AASHTO M 194) Type B retarding admixture is normally
HYDRATION-CONTROL ADMIXTURES
Hydration controlling admixtures became available in the
late 1980s. They consist of a two-part chemical system:
(1) a stabilizer or retarder that essentially stops the hydra-
tion of cementing materials, and (2) an activator that
reestablishes normal hydration and setting when added to
the stabilized concrete. The stabilizer can suspend hydra-
tion for 72 hours and the activator is added to the mixture
just before the concrete is used. These admixtures make
it possible to reuse concrete returned in a ready-mix truck
by suspending setting overnight. The admixture is also
useful in maintaining concrete in a stabilized non-hard-
ened state during long hauls. The concrete is reactivated
when it arrives at the project. This admixture presently
does not have a standard specification (
Kinney 1989
).
ACCELERATING ADMIXTURES
An accelerating admixture is used to accelerate the rate of
hydration (setting) and strength development of concrete
at an early age. The strength development of concrete can
also be accelerated by other methods: (1) using Type III or
Type HE high-early-strength cement, (2) lowering the
water-cement ratio by adding 60 to 120 kg/m
3
(100 to 200
lb/yd
3
) of additional cement to the concrete, (3) using a
water reducer, or (4) curing at higher temperatures.
Accelerators are designated as Type C admixtures under
ASTM C 494 (AASHTO M 194).
Calcium chloride (CaCl
2
) is the chemical most com-
monly used in accelerating admixtures, especially for non-
reinforced concrete. It should conform to the requirements
of ASTM D 98 (AASHTO M 144) and should be sampled
and tested in accordance with ASTM D 345.
The widespread use of calcium chloride as an acceler-
ating admixtures has provided much data and experience
on the effect of this chemical on the properties of concrete.
Besides accelerating strength gain, calcium chloride
causes an increase in drying shrinkage, potential rein-
forcement corrosion, discoloration (a darkening of con-
crete), and an increase in the potential for scaling.
Calcium chloride is not an antifreeze agent.
When used in
allowable amounts, it will not reduce the freezing point of
concrete by more than a few degrees. Attempts to protect
5
125
No admixture, 23
°
C (73
°
F)
No admixture, 32
°
C (90
°
F)
Retarder, 32
°
C (90
°
F)
4
100
3
75
2
50
1
25
0
0
0
20
40
60
80
100
120
140
Elapsed time, minutes
Fig. 6-15. Slump loss at various temperatures for conven-
tional concretes prepared with and without set-retarding
admixture (
Whiting and Dziedzic 1992
).
