Civil Engineering Reference
In-Depth Information
methods. Regardless of the method used, adequate con-
trol and monitoring is required to ensure the proper air
content at all times.
Numerous commercial air-entraining admixtures,
manufactured from a variety of materials, are available.
Most air-entraining admixtures consist of one or more of
the following materials: wood resin (Vinsol resin), sul-
fonated hydrocarbons, fatty and resinous acids, and syn-
thetic materials. Chemical descriptions and performance
characteristics of common air-entraining agents are shown
in Table 8-3. Air-entraining admixtures are usually liquids
and should not be allowed to freeze. Admixtures added
at the mixer should conform to ASTM C 260 (AASHTO
M 154).
Air-entraining cements comply with ASTM C 150 and
C 595 (AASHTO M 85 and M 240). To produce such ce-
ments, air-entraining additions conforming to ASTM C 226
are interground with the cement clinker during manufac-
ture. Air-entraining cements generally provide an ade-
quate amount of entrained air to meet most job conditions;
however, a specified air content may not necessarily be
obtained in the concrete. If an insufficient volume of air is
entrained, it may also be necessary to add an air-
entraining admixture at the mixer.
Each method of entraining air has certain advantages.
On jobs where careful control is not practical, air-
entraining cements are especially useful to ensure that a
significant portion of the required air content will always
be obtained. They eliminate the possibility of human or
mechanical error that can occur when adding an admix-
ture during batching. With air-entraining admixtures, the
volume of entrained air can be readily adjusted to meet
job conditions by changing the amount of admixture
added at the mixer.
Variations in air content can be expected with varia-
tions in aggregate proportions and gradation, mixing time,
temperature, and slump. The order of batching and mixing
concrete ingredients when using an air-entraining admix-
ture has a significant influence on the amount of air
entrained; therefore, consistency in batching is needed to
maintain adequate control.
When entrained air is excessive, it can be reduced by
using one of the following defoaming (air-detraining)
agents: tributyl phosphate, dibutyl phthalate, octyl alcohol,
water-insoluble esters of carbonic acid and boric acid, and
silicones. Only the smallest possible dosage of defoaming
agent should be used to reduce the air content to the speci-
fied limit. An excessive amount might have adverse effects
on concrete properties (
Whiting and Stark 1983
).
FACTORS AFFECTING AIR CONTENT
Cement
As cement content increases, the air content decreases for a
set dosage of air-entraining admixture per unit of cement
within the normal range of cement contents (Fig. 8-16). In
going from 240 to 360 kilogram of cement per cubic meter
(400 to 600 lb of cement per cubic yard), the dosage rate
may have to be doubled to maintain a constant air content.
However, studies indicate that when this is done the air-
void spacing factor generally decreases with an increase in
Table 8-3. Classification and Performance Characteristics of Common Air-Entraining Admixtures
(
Whiting and Nagi 1998
)
Classification
Chemical description
Notes and performance characteristics
Wood derived acid salts
Alkali or alkanolamine salt of:
Vinsol® resin
A mixture of tricyclic acids,
Quick air generation. Minor air gain with initial mixing. Air
phenolics, and terpenes.
loss with prolonged mixing. Mid-sized air bubbles formed.
Compatible with most other admixtures.
Wood rosin
Tricyclic acids-major component.
Same as above.
Tricyclic acids-minor component.
Tall oil
Fatty acids-major component.
Slower air generation. Air may increase with prolonged
Tricyclic acids-minor component.
mixing. Smallest air bubbles of all agents. Compatible
with most other admixtures.
Vegetable oil acids
Coconut fatty acids, alkanolamine salt.
Slower air generation than wood rosins. Moderate air
loss with mixing. Coarser air bubbles relative to wood
rosins. Compatible with most other admixtures.
Synthetic detergents
Alkyl-aryl sulfonates and sulfates
Quick air generation. Minor air loss with mixing. Coarser
(e.g., sodium
bubbles. May be incompatible with some HRWR. Also
dodecylbenzenesulfonate).
applicable to cellular concretes.
Synthetic workability aids
Alkyl-aryl ethoxylates.
Primarily used in masonry mortars.
Miscellaneous
Alkali-alkanolamine acid salts
All these are rarely used as concrete air-entraining
of lignosulfonate.
agents in current practice.
Oxygenated petroleum residues.
Proteinaceous materials.
Animal tallows.
