PORTLAND CEMENT - Materials for Civil and Construction Engineers - page 220

3. to ensure quality of concrete during the stages of mixing, transporting,
placing, and curing in adverse weather conditions
4. to overcome certain emergencies during concrete operations

■ 6.11.1 Air Entrainers

Air entrainers produce tiny air bubbles in the hardened concrete to provide
space for water to expand upon freezing. As moisture within the concrete
pore structure freezes, three mechanisms contribute to the development of
internal stresses in the concrete:

1. Critical saturation—Upon freezing, water expands in volume by 9%. If
the percent saturation exceeds 91.7%, the volume increase generates
stress in the concrete.
2. Hydraulic pressure—Freezing water draws unfrozen water to it. The
unfrozen water moving throughout the concrete pores generates stress,
depending on length of flow path, rate of freezing, permeability, and
concentration of salt in pores.
3. Osmotic pressure—Water moves from the gel to capillaries to satisfy ther-
modynamic equilibrium and to equalize alkali concentrations. Voids per-
mit water to flow from the interlayer hydration space and capillaries into
the air voids, where it has room to freeze without damaging the parts.

Internal stresses reduce the durability of hardened concrete, especially when
cycles of freeze and thaw are repeated many times. The impact of each of
these mechanisms is mitigated by providing a network of tiny air voids in the
hardened concrete using air entrainers. In the late 1930s, the introduction
of air entrainment in concrete represented a major advance in concrete
technology. Air entrainment is recommended for all concrete exposed to
freezing.
All concrete contains entrapped air voids, which have diameters of 1 mm
or larger and which represent approximately 0.2% to 3% of the concrete
volume. Entrained air voids have diameters that range from 0.01 mm to 1
mm, with the majority being less than 0.1 mm. The entrained air voids are
not connected and have a total volume between 1% and 7.5% of the con-
crete volume. Concrete mixed for severe frost conditions should contain ap-
proximately 14 billion bubbles per cubic meter. Frost resistance improves
with decreasing void size, and small voids reduce strength less than large
ones. The fineness of air voids is measured by the specific surface index,
equal to the total surface area of voids in a unit volume of paste. The spe-
cific surface index should exceed 23,600 m 2 /m 3 600 in. 2 /in. 3 for frost

1 2

resistance.
In addition to improving durability, air entrainment provides other im-
portant benefits to both freshly mixed and hardened concrete. Air entrain-
ment improves concrete's resistance to several destructive factors, including
freeze-thaw cycles, deicers and salts, sulfates, and alkali-silica reactivity.
Air entrainment also increases the workability of fresh concrete. Air entrain-
ment decreases the strength of concrete, as shown in Figure 6.7; however,