Civil Engineering Reference
In-Depth Information
CHAPTER 8
Air-Entrained Concrete
One of the greatest advances in concrete technology was
the development of air-entrained concrete in the mid-
1930s. Today air entrainment is recommended for nearly
all concretes, principally to improve freeze-thaw resist-
ance when exposed to water and deicing chemicals.
However, there are other important benefits of entrained
air in both freshly mixed and hardened concrete.
Air-entrained concrete is produced by using either an
air-entraining cement or adding an air-entraining admix-
ture during batching. The air-entraining admixture stabi-
lizes bubbles formed during the mixing process,
enhances the incorporation of bubbles of various sizes by
lowering the surface tension of the mixing water,
impedes bubble coalescence, and anchors bubbles to
cement and aggregate particles.
Anionic air-entraining admixtures are hydrophobic
(repel water) and are electrically charged (nonionic or no-
charge admixtures are also available). The negative elec-
tric charge is attracted to positively charged cement
grains, which aids in stabilizing bubbles. The air-
entraining admixture forms a tough, water-repelling film,
similar to a soap film, with sufficient strength and elas-
ticity to contain and stabilize the air bubbles and prevent
them from coalescing. The hydrophobic film also keeps
water out of the bubbles. The stirring and kneading action
of mechanical mixing disperses the air bubbles. The fine
aggregate particles also act as a three-dimensional grid to
help hold the bubbles in the mixture.
Entrained air bubbles are not like entrapped air voids,
which occur in all concretes as a result of mixing, han-
dling, and placing and are largely a function of aggregate
characteristics. Intentionally entrained air bubbles are
extremely small in size, between 10 to 1000 µm in diam-
eter, while entrapped voids are usually 1000 µm (1 mm) or
larger. The majority of the entrained air voids in normal
concrete are between 10 µm and 100 µm in diameter. As
shown in Fig. 8-1, the bubbles are not interconnected; they
are well dispersed and randomly distributed. Non-air-
entrained concrete with a 25-mm (1-in.) maximum-size
aggregate has an air content of approximately 1
1
⁄
2
%. This
same mixture air entrained for severe frost exposure
would require a total air content of about 6%, made up of
both the coarser “entrapped” air voids and the finer
“entrained” air voids.
PROPERTIES OF AIR-ENTRAINED
CONCRETE
The primary concrete properties influenced by air entrain-
ment are presented in the following sections. A brief sum-
mary of other properties not discussed below is presented
in Table 8-1.
Freeze-Thaw Resistance
The resistance of hardened concrete to freezing and
thawing in a moist condition is significantly improved by
the use of intentionally entrained air, even when various
deicers are involved. Convincing proof of the improve-
Fig. 8-1. Polished section of air-entrained concrete as seen
through a microscope. (67840)
