What Is Air Entrained Concrete?
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.
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 WORKSFortunately 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
- higher workability
- abrasion resistant
AIR ENTRAINING AGENTSThe 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