Aerogenesis mechanism
Mainly, concrete always contains a certain volume of air bubbles that are formed during concrete mixing. In the absence of aerating agents or other air-containing material, the total volume of air entrapped in concrete is usually between 1 and 3 percent. In order to increase the amount of air in concrete, it is necessary to add an aerating agent to concrete. In general, the creation of very small bubbles (about 10 to 1000 micrometers) uniformly in concrete improves some properties of fresh concrete as well as hardened concrete.
During mixing, eddies are created that trap air bubbles in the paste and mortar. In the absence of aerating agents, the air bubbles trapped in the paste or mortar are unstable and coalesce to form larger bubbles. These large bubbles are transferred to the upper surface of concrete and removed. Also, vibration facilitates the elimination of the last entrapped air bubbles, so that the final volume of air entrapped in the concrete is very small (typically less than 3%). These trapped air bubbles have a diameter of 0.3-5 mm, which is close to the diameter of sand grains. These bubbles cannot effectively protect the concrete against the attack of freezing and thawing cycles.
During the mixing of concrete, the addition of aerating agent causes the formation of numerous finer air bubbles that are very stable and do not coalesce. These tiny air bubbles that are deliberately introduced into the concrete mix are usually 5-100 microns in diameter, like cement particles. When formulating a concrete mixture, the volume of entrained air and the structure of the bubble network can be controlled by selecting the type of aerator and the appropriate dosage.
Aerating agents are organic molecules with different chemical compositions known as surfactants. These compounds usually have a hydrophilic end attached to a hydrophobic chain. The hydrophobic and hydrophilic ends of the air are absorbed in the air-water or cement-water interface, which reduces the surface tension in the air-water interface. The higher the concentration of the surfactant, the lower the surface tension of the solution, so that the aerator helps to stabilize the small bubbles due to the reduction of the surface tension at the air-water interface.
Choosing the right air conditioner
Since increasing the amount of aerator outside the optimal range can lead to a decrease in concrete strength and a sudden increase in slump, in order to choose the type of aerator and adjust its dosage, it should be noted that two general categories of factors affecting the performance of the aerator should be carefully examined.
1- The ingredients of the mixing plan
Cement:
In general, at a fixed amount of aerating cement, the amount of air created decreases with the increase of the amount of cement. Also, reducing the size of cement particles will lead to reducing the amount of air bubbles. In addition, increasing the alkalinity of cement increases the stability of the created bubbles, as a result, a lower dose of aerating agent is needed.
Aggregate:
Aggregate particle shape and texture, grain size distribution, and maximum coarse aggregate size may reduce entrained air volume. For a fixed dose of cement and aerator, the volume of air introduced increases with the increase of fine aggregate dose. Sand particles with a diameter between 160 and 630 mm help to absorb air, and on the contrary, increasing the proportion of particles with a diameter of less than 160 mm significantly reduces the volume of air taken in. Some aggregates contaminated with oil or organic matter may cause large changes in entrained air.
Water:
Increasing the mixing water makes more water available for the formation of air bubbles in the environment and therefore increases the amount of air. Adding a small amount of water to low-slump concrete, which contains large amounts of water-reducing and air-entraining admixtures, can greatly increase the air content and slump of the concrete. On the other hand, adding water to very runny mixes (with a slump of 200 to 250 mm) may reduce the air content of the concrete.
Pozzolanic materials:
the use of silica fume with a dose of 5-10% of cement mass has a very small effect on the production of air bubbles. In some cases, silica fume may reduce the surface area (smaller bubbles) and slightly increase the air volume. This increase in air volume is only observed when a super-lubricant is used to reduce the water content of the mixture.
Using large amounts of slag or silica fume in concrete may double the amount of admixture required to achieve a given amount of air.
Chemical additives:
Retardant additives and water reducers increase the efficiency of aerating additives by stabilizing air bubbles. Therefore, when these materials are used, smaller amounts of aerating additives usually provide the desired amount of air. Also, the time of adding water-reducing additives or retarders into the mixture affects the amount of air created. Generally, the later these additives are added to the mixture, the amount of air increases.
Tests conducted on aerated concretes containing polycarboxylate superplasticizer have shown that these concretes have good durability against successive cycles of freezing and thawing. This issue may be caused by the reduction of water-cement ratio in concretes with super-lubricants.
2- Mixing conditions
How to mix, mixing temperature, how to move and pump concrete are other important factors that affect the performance of aerating additives.
Mixing:
The type of mixer affects the air entrainment mechanism as well as the distribution of air bubbles in the paste and mortar (fracture and consolidation). Mixing parameters (eg, material addition sequence, duration, speed, torque, shear) affect bubble network generation. The volume of concrete in the mixer is also important: a lot A little or a lot of concrete in the mixer affects the shear rate of the mixture and the formation of vortex. The wear of the paddle as well as its cleanliness may lead to a significant reduction of air bubbles and their dispersion, so that sometimes it is necessary to increase the mixing time to trap the required air volume.
Temperature:
increasing the temperature of concrete reduces the volume of air released. In winter, when hot water is used to heat the concrete, it loses its air permeability. In such a situation, it is better to add the aerating agent to the mixture after the hot water increases the temperature of the concrete. Otherwise, it will be necessary to increase the dose of aerogen.
Transfer Mode:
Generally, some concrete air, approximately 1 to 2 percent, is lost during the transfer of concrete from the mixer to the concreting site. The amount of air in concrete during transportation is influenced by some factors such as transportation time, the amount of stirring or vibration during transportation, temperature, slump, the amount of water that is added again, and the components of concrete. The amount of air in the concrete remains almost constant at the concreting site and during concreting through discharge with a shot, crane and cup (bucket), ferghun, motorized cart and shovel. Moving concrete with pumps and long conveyor belts can reduce the air content of concrete. Pumping concrete reduces the amount of air to about 2.5%. Air loss in flowable concrete during mixing and handling is about 1.5%.
Advantages of using air conditioners
1- Increased resistance to freezing and thawing cycles
2- Reducing the permeability and as a result increasing the waterproofing property
3- Reducing separation and shedding
4- Mental improvement and workability
