Air-entraining agent is an admixture that introduces a large number of evenly distributed tiny closed bubbles during the mixing process of concrete or mortar. Its basic functions are shown in the following aspects.
(1) Improve the workability of fresh concrete mixtures
Most air-entraining agents are anionic surfactants. Generally, the water reduction rate of air-entraining agents is 6%. ~9%, although not as large as water reducing agent, it can also improve the fluidity of fresh concrete a lot.
(2) Improve the durability of hardened concrete
The tiny bubbles in hardened concrete can buffer the expansion pressure caused by the freezing of water and reduce freezing damage, which improves the durability of concrete. Frost resistance; tiny bubbles can cut off capillaries in hardened concrete and reduce penetration due to capillary action, thus improving the impermeability of concrete.
(3) The strength of concrete is weakened
The size of the weakening is related to the amount of bubbles produced by the air-entraining agent and the size of the bubbles. If the bubbles generated are mainly small holes, The weakening is relatively small. Of course, the water-reducing effect of the air-entraining agent will compensate for the strength weakening caused by the air-entraining agent.
What is the water-cement ratio of air-entraining agent waterproof concrete?
The water-cement ratio affects the consistency of the concrete mixture. The lower the water-cement ratio, the thicker the mixture; The compound consistency determines the generation, quantity and quality of bubbles, so the water-cement ratio will have an important impact on the quality of air-entraining agent waterproof concrete. Under different water-cement ratios, the recommended limit dosage of air-entraining agent is as follows: when the water-cement ratio is 0.50, the dosage of air-entraining agent is 0.01%~0.05%; the water-cement ratio is 0. .55, the dosage is 0.005%~0.03%; when the water-cement ratio is 0.60, the dosage is 0.005%~0.01%.
The volume expansion of water freezing and the migration of supercooled water will generate high pressure, which is the main reason why concrete suffers freeze-thaw damage in the atmosphere. When water freezes, its volume expands by up to 9%
If the moisture content in the capillary pores in concrete exceeds a certain critical value (91.7%), a great pressure will be generated when it freezes. In addition, when the capillary water freezes, the gel pore water is in a supercooled state. The vapor pressure of the supercooled water is higher than the vapor pressure of ice at the same temperature. Therefore, the gel water will penetrate into the ice interface in the capillary pores. Since it takes a certain amount of time for infiltration to reach an equilibrium state, even if the cement stone maintains a certain freezing temperature, the expansion of the cement stone caused by the osmotic pressure will continue to occur for a certain period of time. This is the expansion characteristic caused by the migration of supercooled water.
Research shows that the freeze-thaw resistance of concrete is mainly restricted by the two factors of water-cement ratio and air content when other conditions are the same. After the water-reducing agent is added to the concrete, due to its water-reducing effect, the water-cement ratio is reduced, so that the free water that can be frozen in the concrete structure is significantly reduced; due to its air-entraining effect, a certain number of independent micro-bubbles can be introduced into the concrete. Alleviating the concentration of expansion pressure caused by freezing and supercooled water migration; in addition, the water reduction-air entrainment effect improves the mesoporous structure of the concrete, the capillary pore size becomes finer, and the freezing point of the aqueous solution will drop further. It can be seen that water reducing agent can improve the freeze-thaw resistance of concrete.
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