Materials: Chemical Admixtures
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Air-Entraining Admixtures
 Air-entraining
admixtures are used to stabilize microscopic air bubbles in concrete.
Proper air-entrainment, with appropriate volume and spacing factor,
will dramatically improve the durability of concrete exposed to
moisture during cycles of freezing and thawing. Entrained air also
improves concrete’s resistance to surface scaling caused by
chemical deicers.
 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 methods. 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.
Specifications and methods of testing air-entraining admixtures
are given in ASTM C 260 and C 233 (AASHTO M 154 and T 157). Both
the total volume of air and the spacing factor (average distance
between a point in the paste and the edge of an air void) are important
to providing resistance to freezing and thawing. Air-entrainment
is affected by concrete materials selection, mixing, and placement
methods as summarized in the following table. Applicable requirements
for air-entraining cements are given in ASTM C 150 and AASHTO M
85. See Manual
on Control of Air Content in Concrete for more information.
| Effects
of Concrete Materials and Production Practices on Air Entrainment |
Material/Practice |
Change |
Effect |
Cement |
Increase in cement content |
|
| Increase in fineness |
|
| Increase in alkali content |
|
SCMs |
Fly Ash (especially with high carbon) |
|
| Silica Fume |
|
| Slag with increasing fineness |
|
| Metakaolin |
|
Aggregates |
Increase in maximum size |
|
| Sand content |
|
Chemical Admixtures |
Water reducers |
|
| Retarders |
|
| Accelerators |
|
| High-range water reducers |
|
W/CM |
Increase W/CM |
|
Slump |
Increase in slump up to 150 mm (6 in.) |
|
| High slump (> 150 mm or 6 in.) |
|
| Low slump concrete (< 75 mm or 3 in.) |
|
Production |
Batching |
|
| Increased mixer capacity |
|
| Mixer speeds to 20 rpm |
|
| Mixer time |
|
Transport and
delivery
|
Transport |
|
| Long hauls |
|
| Retempering |
|
Placing
and
finishing
|
Belt conveyors |
|
| Pumping |
|
| Wet-mix shotcrete |
|
| Prolonged internal vibration |
|
| Excessive finishing |
|
Key:  decrease
in air content  increase
in air content  no
significant change |
| (Source: Admixtures
for Use in Concrete, Thomas and Wilson 2002) |
Mechanisms
Air-entraining agents stabilize air bubbles in concrete by reducing
the surface tension at the air-water interface. The shearing action
of the mixer and aggregates will entrain and divide air into finely
divided bubbles surrounded by the stabilizing agent. It is critical
that sufficient mixing time be allowed for the air bubbles to be
generated and stabilized. Trial mixtures or prior experience with
the job materials and job equipment is necessary to determine the
proper dosage and minimum mixing time. Fresh concrete testing (ASTM
C 231 or C 173; AASHTO T 152 or T 196)) during construction will
confirm the proper volume of air. A relatively new technique, known
as the air-void analyzer (AVA), shows promise as a tool for determining
the spacing factor in fresh concrete. (For
AVA references, click here.)
 Air
voids in concrete improve durability by reducing stresses associated
with freezing water in pores. The expansion as water converts from
liquid to solid upon freezing creates a pressure on the remaining
liquid. Entrained air provides relief for this pressure, by providing
space for the water to flow into. Without air voids, the pressure
creates stress on the concrete, crreating cracks which cumulatively
begin disrupting the concrete.
Other Effects
Generally, for every 1% entrained air, concrete loses about 5% of
its compressive strength (Whiting
and Nagi, 1998). However, in addition to providing saturated
freeze-thaw protection and improved salt scaling resistance, entrained
air provides several other benefits: increased workability, reduced
water demand, decreased segregation and bleeding, and reduced permeability.
Additional information on air in concrete:
More on the Air Void Analyzer (AVA):
|