Air entrainment sounds like a complicated and intricate term. But actually,
it is simply a creation of minute air bubbles in the concrete. Air entrainment
can be done at a batch plant or manually by a worker on site. The science
behind the process is pretty easy, but it plays a major role in increasing
the workability of the concrete in a plastic state as well as its durability
when it’s already hardened. On the other hand, concrete is one the
trickiest and most demanding materials in the world. It is hard to mix,
pour, smooth over and finish. Moreover, working with concrete is quite
costly. Especially, when it comes to making mistakes and fixing them.
But even if you managed to place the material perfectly, all your efforts
can go to waste in the winter; therefore it is very important to take
your climate into the account when you are dealing with concrete.
FREEZE-THAW DAMAGE
Concrete is a fair-weather friend, which means that it has low tolerance
for excessive moisture and sub-zero temperatures. Though concrete appears
completely solid, it contains microscopic pores. This is due to the water
needed to create the mix. The minimal water to cement ratio is approximately
0.25, which means that there are 25 parts of H2O for every 100 parts of
cement. Any extra water may be used to increase the plasticity of concrete.
Eventually, it will dry and leave small capillaries in its place. When
it rains or snows, moisture fills these voids. Later on, if the temperatures
fall below zero, this liquid will freeze, expanding and creating tiny
cracks in the concrete. The ice will ultimately melt, but gaps in the
material won’t go away. Moreover, they will further widen after
the next cycle of freeze-thaw; that can lead to permanent damage and cause
big chunks of concrete to break off. Also, if moisture reaches the reinforcing
steel, it will rust and expand creating even more cavities and subsequently
more pathways for the environmental water.
HOW AIR ENTRAINING WORKS
Fortunately scientists came up with air entraining technology to beat
the effects of the freeze-thaw. During the mixing process, constructors
add special agents containing a type of detergent to the paste. They create
air bubbles inside the concrete. Their size can vary from 10 to 500 micrometers
in diameter. The air pockets are spaced very closely and act as “ball-bearings”.
Bubbles compress when ice forms. They relieve the stress on the material,
acting as chambers for expansion. To achieve such results air bubbles
should have a distance of less than 0.25 mm between them.
MAIN PROPERTIES OF AIR-ENTRAINED CONCRETE
frost resilient
stress alleviating
crack preventative
resistant to sulfate and deicing chemicals
durable
higher workability
abrasion resistant
AIR ENTRAINING AGENTS
The technology of adding air bubbles to concrete paste was first used
in road making in 1930’s. At that time, construction workers applied
Vinsol resin as an admixture. But a lot has changed since then. Nowadays
there are three main types of modern air entraining agents:
fats and oils (both animal and vegetable) and fatty acids
wood resins
alkali salts of sulfate and sulfonated organic compounds
WHIPPED CREAM EFFECT
The workability of the concrete is measured by a slump test that is performed
on the construction site in order to check the mixture and make the necessary
adjustments. To carry out a slump test you need to take a metal cone and
fill it with concrete in four even layers. Each layer should be stroked
25 times with a tamping rod to make sure it is even. After you finish
stuffing the concrete, immediately raise the cone slowly and very carefully.
Then you need to measure the difference between the height of the concrete
and the height of the cone-mold. A slump of about 3 inches is considered
to be good. However, air entraining causes the whipped cream effect that
reduces the slump by 1-2 inches. Yet the workability stays the same. Thus,
air entraining can also be used to reduce the ration of water to cement
and therefore increase the strength of the concrete.
KEEP IN MIND
Though using air entrainment helps to eliminate the negative freeze-thaw
effect, you need to remember that the more air you add, the more the concrete
strength will decrease. The average amount of air in the paste should
be 4-7%, depending on the climate pattern. Take into account that some
air may be lost during transportation, so if you plan to mix it at the
batch plant, make some allowance to compensate. Air entrainment also reduces
the shelf-life of concrete.
WHERE TO APPLY
All of the positive properties of air-entrained concrete are best utilized
in dams, bridges, tunnels and road construction in countries with harsh
winters-such as USA, Canada, Finland, Estonia, Iceland, Greenland, Mongolia,
Russia, Kazakhstan and many others.
