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Air-Entrained Concrete
Concrete Technology
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Construction >Finishing Air-Entrained Concrete
Finishing Air-Entrained Concrete
 Used
in many applications, air-entrained concrete uses a chemical admixture
(or sometimes, air-entraining cement) to produce a system of small
voids during the mixing process. These voids are stabilized by the
air-entraining admixture and remain in the hardened concrete paste.
The primary use of air-entraining concrete is for freeze-thaw resistance.
The air voids provide pressure relief sites during a freeze event,
allowing the water inside the concrete to freeze without inducing
large internal stresses. Another related use is for deicer-scaling
resistance. The air voids again provide relief sites for the buildup
of salt concentrations and the pressures that result due to concentration
gradients. Other uses of air-entrained concrete include sulfate
resistance, resistance to alkali-silica reactivity, and improved
workability.
Hard troweling is a process by which a finisher uses a steel trowel
to densify the surface of the concrete. This finish is optional
and produces a hard, smooth surface. Repeated passes of steel troweling
will result in a burnished finish, which has a mirror-like appearance.
Hard-troweled surfaces are not recommended for exterior concrete
slabs, because the smooth finish becomes slippery when wet.
Hard troweling is also not recommended for air-entrained concrete
for several reasons. The primary reason is densification. Densification
pushes air out of the surface, leading to a decrease in the air
content. This increases the probability of freeze-thaw damage and
deicer-scaling damage. Additionally, the densification can push
the air down and rupture the voids, leading to a large void just
below the surface mortar. This will cause delaminations and may
cover a significant portion of the placement. Once the surface has
been opened, through scaling or delamination, the densified surface
gained through hard troweling is lost.
Many alternatives to hard-troweled surfaces exist for concrete.
A simple broom texture is the easiest method. A broom texture provides
a slip resistant and durable surface. Other options include decorative
finishes, such as stamped or colored concrete.
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