Civil Engineering Reference
In-Depth Information
Table 6-1. Concrete Admixtures by Classification (Continued)
Type of admixture
Desired effect
Material
Superplasticizer* and
Increase flowability with retarded set
See superplasticizers and also water reducers
retarder (ASTM C 1017,
Reduce water-cement ratio
Type 2)
Water reducer
Reduce water content at least 5%
Lignosulfonates
(ASTM C 494 and
Hydroxylated carboxylic acids
AASHTO M 194, Type A)
Carbohydrates
(Also tend to retard set so accelerator is often added)
Water reducer and
Reduce water content (minimum 5%)
See water reducer, Type A (accelerator is added)
accelerator (ASTM C 494
and accelerate set
and AASHTO M 194,
Type E)
Water reducer and
Reduce water content (minimum 5%)
See water reducer, Type A (retarder is added)
retarder (ASTM C 494 and
and retard set
AASHTO M 194, Type D)
Water reducer—high
Reduce water content (minimum
See superplasticizers
range (ASTM C 494 and
12%)
AASHTO M 194, Type F)
Water reducer—high
Reduce water content (minimum
See superplasticizers and also water reducers
range—and retarder
12%) and retard set
(ASTM C 494 and
AASHTO M 194, Type G)
Water reducer—mid
Reduce water content (between
Lignosulfonates
range
6 and 12%) without retarding
Polycarboxylates
* Superplasticizers are also referred to as high-range water reducers or plasticizers. These admixtures often meet both ASTM C 494 (AASHTO
M 194) and ASTM C 1017 specifications.
AIR-ENTRAINING ADMIXTURES
ASTM C 260 and C 233 (AASHTO M 154 and T 157). Air-
entraining additions for use in the manufacture of air-
entraining cements must meet requirements of ASTM C
226. Applicable requirements for air-entraining cements
are given in ASTM C 150 and AASHTO M 85. See
Chapter
8
, Air-Entrained Concrete,
Klieger (1966)
, and
Whiting
and Nagi (1998)
for more information.
Air-entraining admixtures are used to purposely introduce
and stabilize microscopic air bubbles in concrete. Air-
entrainment will dramatically improve the durability of
concrete exposed to cycles of freezing and thawing (Fig.
6-2). Entrained air greatly improves concrete's resistance to
surface scaling caused by chemical deicers (Fig. 6-3). Fur-
thermore, the workability of fresh concrete is improved
significantly, and segregation and bleeding are reduced or
eliminated.
Air-entrained concrete contains minute air bubbles
that are distributed uniformly throughout the cement
paste. Entrained air can be produced in concrete by use of
an air-entraining cement, by introduction of an air-
entraining admixture, or by a combination of both meth-
ods. An air-entraining cement is a portland cement with
an air-entraining addition interground with the clinker
during manufacture. An air-entraining admixture, on the
other hand, is added directly to the concrete materials
either before or during mixing.
The primary ingredients used in air-entraining ad-
mixtures are listed in Table 6-1. Specifications and meth-
ods of testing air-entraining admixtures are given in
WATER-REDUCING ADMIXTURES
Water-reducing admixtures are used to reduce the quan-
tity of mixing water required to produce concrete of a
certain slump, reduce water-cement ratio, reduce cement
content, or increase slump. Typical water reducers reduce
the water content by approximately 5% to 10%. Adding a
water-reducing admixture to concrete without reducing
the water content can produce a mixture with a higher
slump. The rate of slump loss, however, is not reduced
and in most cases is increased (Fig. 6-4). Rapid slump loss
results in reduced workability and less time to place
concrete.
An increase in strength is generally obtained with
water-reducing admixtures as the water-cement ratio is
