1 air-entraining agent research and application status
      Concrete air-entraining agent is one of the most ancient admixture, forty years of engineering began to be used in concrete frost. Air-entraining agents in concrete applications in foreign countries has been recognized in many countries. In other countries, particularly Japan, almost most of the concrete, including high-strength high performance concrete (HPC) should be mixed with air-entraining agent, causing water-reducing agent, and will not only air-entraining agents or other additives mixed with the concrete as special concrete. Specification requirements in Japan, the concrete must be mixed with air-entraining agent (AE agent) or air-entraining water-reducing agent (AE water-reducing agent). The northern United States and Canada, provided all open to the use of concrete mixed with air-entraining agents, mainly on account of improving the durability of concrete. The fifties began to develop our air-entraining agent, used to improve the frost resistance of hydraulic concrete. At present, China and Hong Kong only in hydraulic engineering concrete design procedures are clearly mixed with air-entraining agent requirements. However, with the needs of the development of concrete technology, construction equipment, especially the introduction of foreign and foreign engineering companies to enter China's construction market, air-entraining agents also began in other occasions. For example, high-rise buildings in Shanghai are required concrete pumping, extensive use of air-entraining agent. In the second phase of Qinshan Nuclear Power Station, also called the foreign must be mixed in the concrete air-entraining agent. Professor Huang Shiyuan China has repeatedly called for writing papers using air-entraining agent, indicating that air-entraining agent is really good to improve the durability of concrete admixtures.
      Air-entraining agent for concrete work in our use is not common, concrete air-entraining agent mixed concrete accounted for only a few percent of the total. The reason, in addition to worry about air-entraining agent to reduce the concrete strength, the lack of more high performance, affordable air-entraining agents to choose from and because improper use of air-entraining agent resulted in no significant effect on improving the impermeability of concrete, insufficient attention to durability, but also a major factor.
2 Mechanism
    Most air-entraining agent is anionic surfactant in water ---- air interface, hydrophobic adsorption to the air side of the orientation, in the cement ---- water interface, or the hydration of cement particles and hydrophilic phase adsorption, and hydrophobic hydration of cement particles away from the formation of hydrophobic adsorption layer, and try to close the air surface, because the particles near the surface to air and air-entraining agent molecules in the air - water interface adsorption, significantly reduce the surface tension of water, the concrete in the mixing process produces a large number of micro-bubbles, these bubbles have the same charge with a directional adsorption layer, so mutually exclusive and can be evenly distributed; the other hand, many air-entraining agent in the calcium ion high levels of calcium precipitation in cement solution, adsorbed on the bubble membrane, can effectively prevent the bubble burst, the introduction of uniform small bubbles can be stable within a certain time. Major role in air-entraining agent is the introduction of the first bubble, followed by dispersing and wetting. Air-entraining agent was first used as improve concrete impermeability, frost resistance, salt and freezing erosion of an admixture use. The main principle is air-entraining agent into small bubbles off the capillary channel, reducing the capillary action, to improve the impermeability of concrete. These micro-pores in the freezing process to release the expansion of ice crystals inside the capillary pressure, thus avoiding damage to generate pressure to reduce and prevent freeze-thaw damage, and improve frost resistance of concrete.

3 air-entraining agent for concrete properties
     3.1 air-entraining agent on properties of fresh concrete
     Air-entraining agents can significantly alter the performance of fresh concrete.
     Air-entraining agents in concrete because the introduction of a large number of small and independent bubbles, these bubbles as spherical ball workability of the concrete as a greater degree of improvement. As the bubbles increase the paste volume and mixing material for lubrication, and increased the slurry viscosity and yield stress, so the air-entraining concrete workability, plasticity and cohesion has been significantly improved, significantly higher than non-air-entrained concrete the good. Air-entraining agent introduced by the micro-bubble like a fine aggregate, the gradation poor, especially the fine particles of fine aggregate with a lack of compensation, can make concrete look mortar surplus; these tiny bubbles results and support the cement particles, filling the the gap between the cement particles, thereby blocking or reducing the cement and aggregate particles surrounding water, reducing the bleeding of concrete, settlement and segregation. Also, because these bubbles "poke" or "dispersion" effect, which greatly increase the cement or fine aggregate free (non-condensing) surface area, enhancing mixture viscosity and workability.
       The proportion of raw materials under the same conditions, air-entraining concrete can improve the mobility, while at the same slump, the air-entraining agent mixed concrete, the paste workability, flow, plastic, cast of, pound solid measure is non-air-entraining agent mixed concrete slurry can not be compared, bleed air can reduce the mixing water. In addition, the bleed air can reduce the slump loss of fresh concrete.
      With the construction techniques and the development needs of high-rise buildings, concrete pump performance can become increasingly important. In fact, the pumpability of concrete work is good for a special form, due to increased air-entrained concrete cohesion and lubrication between materials and reduce the swelling stream, so that excessive pumping without segregation and bleeding water, so the air can lead to improve the availability of fresh concrete pump performance. However, pumping the air content of concrete should not be too high, because of excessive gas content caused by the compressibility of the air enough to produce saturation improved to increase the pumping losses when the pump pressure and reduce the efficiency of pumping concrete. Such concrete air content generally less than 6% as well. Because of these reasons, resulting in pumping concrete and mass concrete, and road and other civil construction widely used in air-entrained concrete, even if there is no frost or anti-icing salt is so demanding environments. In the dry lean concrete and hard concrete or roller compacted concrete, and lightweight aggregate concrete, the largest air-entraining can improve their performance and maximum air-entraining agent to play these good things.
        Pumpability of concrete slump is not only relevant, but also with the secretion of water stress on the concrete, under the same conditions, reducing the bleeding of concrete means to improve pumpability. Therefore, in foreign countries and China's Shanghai almost all of the pumping of air-entraining agents contain ingredients.
   3.2 air-entraining agent on the properties of hardened concrete
         3.2.1 Air-entraining agents on the strength of
        Air-entrained concrete will reduce the strength, especially when the content is large (> 4%) decreased more significantly, this is the biggest drawback of air-entraining agents, air entraining agent is restricted in our large-scale application of the main reasons, because In many of our design specifications are based on concrete strength as an indicator. Has long been that the mixed air-entraining agent will certainly reduce the strength of the concrete view is incomplete.
        (1) strength of concrete and concrete not only the amount of air introduced, but also with the introduction of air bubbles on the structure and pore size and so on. In a sense, pore structure and pore size than porosity effects on the macroscopic properties of concrete is more important. Pores of different pore size effects on concrete performance is not consistent, some foreign scholars, 50nm diameter holes are harmless to the following hole. Different varieties of air-entraining agent for concrete pore structure formation is different, thus the concrete strength, permeability of the impact is different and can not be generalized.
         (2) the use of high performance air-entraining agent can typically reduce water consumption of concrete 8% to 10%, which can compensate for the increase in porosity caused by the intensity decreased.
        (3) the application of air-entraining agent to improve the workability of the concrete work and greatly reduce the concrete bleeding, segregation, so that the concrete interface characteristics can be improved. In the ordinary high water-cement ratio concrete, the interface characteristics of the concrete compressive strength of less than C70, but the interface properties of high strength concrete compressive strength on the larger, so the low water-cement ratio of high-strength concrete mixed with the right amount of air entraining agent to improve the interface properties, interface defects are often caused because the concrete strength, especially flexural strength loss can be compensated. Ministry of Communications Highway Research Institute found that the concrete road, under certain conditions, but can improve the air-entraining flexural strength of concrete compressive strength. Air-entrained concrete flexural strength due to lower rates much smaller than the compressive strength of the lower rate, that is air-entrained concrete compression ratio can be improved or toughness. This is particularly important for road works and practical significance.
       (4) in the concrete mixture in a separate mixing air-entraining agent is minimal. Usually air-entraining agent and superplasticizer used simultaneously, or in the amount of superplasticizer in the complex air-entraining agents, water-reducing agent such because of the high water reducing ratio and improve the concrete strength, fully compensate for the use of air-entraining agent porosity of the concrete strength due to increased losses.
       (5) There are many research data show that for dry lean concrete and hard concrete or roller compacted concrete, and lightweight aggregate concrete, not only did not reduce the amount of air-entraining strength, but increase the intensity, mainly with air-entraining concrete slurry can be increased volume, workability, plasticity and cohesion related.
    3.2.2 Air-entraining agent for concrete Durability
        3.2.2.1 frost resistance
        As we all know, air-entraining agent mixed with the main role is to improve the durability of concrete frost. Currently, air-entraining agents as frost resistance of concrete to improve the most important technical measures have been widely used in engineering practice (such as hydraulic and port workers and other concrete work), the effect has also been recognized.
       General non-air-entrained concrete air content less than 1.8%, and more for this part of the air bubbles trapped large bubbles, the performance of concrete has a negative impact. However, air-entraining agent to produce more uniform bubbles tiny bubbles, which has many beneficial properties of concrete role, compared with the retention of large bubbles are very different.
      The best general introduction air content of concrete is 3% to 6%, the mortar is 9% to 10%. Within the control of the above, with the gas content increases, a gradual increase in frost resistance of concrete, the maximum can be increased several times. Air-entraining agents in concrete due to the introduction of many uniformly distributed in the tiny bubbles, completely changed the original gas content of only 1.8% in the internal structure of concrete. Air-entraining agents in concrete at all levels with the introduction of air bubbles in the concrete settlement of various materials can inhibit secretion of water and to prevent uneven configuration of the role of concrete materials, thus preventing large bubbles blank concrete frost resistance of concrete unfavorable situation from occurring. Tests showed that the introduction of 2.2% air content, so that the total air content of concrete to reach 4%, the durability of concrete up to 120% of the index DF, contains only 1.8% of the original gas content of the concrete durability index was only 15%.
       However, the bigger the better is not the gas content, gas content exceeds a certain limit, the performance degradation of concrete frost, such as air content increased by 1%, the strength of concrete fell 3% to 5%.
    3.2.2.2 salt freezing
       Now a lot of concrete highway and city roads, often spreading de-icing salt in winter (mainly NaCl) to clear the road surface ice and snow. Salt and frozen together, particularly serious damage to the concrete. Concrete erosion damage is frozen salt de-icing salt in the presence of the concrete due to frost damage produced by a special form, mainly as a serious erosion surface, aggregate exposure. So far more than, in Beijing, Tianjin and other cities overpass, and Heilongjiang, northeast high-grade concrete pavement found on a number of serious erosion damage cold instances of salt. The most important reason is the destruction of the concrete examples did not take action to prevent freeze damage to the salt of technical measures, such as the mixed air-entraining agent. As the salt is essentially frozen erosion damage due to water freezing expansion damage, so the air-entraining agents also improve the performance of concrete salt freezing very good results.
      As the winter snow in northern road, therefore, from the first year of the second year in October to March, sprinkle about 2 to 3 times the de-icing salt, in order to ensure the roads clear of snow, but salt and frozen in together, is particularly devastating for concrete. Concrete erosion of salt damage is frozen in the presence of de-icing salt conditions, the concrete due to cold damage resulting from a special form, mainly as surface erosion, exposed aggregate, concrete surface layer of the shell layer of cement mortar All eroded away. Incorporation of air-entraining agent can effectively prevent the bleeding of concrete up the process, the introduction of many tiny air bubbles and prevent the erosion of salt damage to the ice.
3.2.2.3 impermeability
     Studies have shown that the introduction of concrete air content of 4%, can increase the permeability of concrete about 15% or more, because the air-entraining agent to change the pore structure of concrete systems, closed a number of pores channels, while the cement particles water-repellent film formed on the surface, thereby reducing the capillary suction effect.
      Many water into closed pores can not be fully filled, the excess down to slow the physical expansion - organic salt crystals or mineral crystals; or chemical expansion - aggregate reaction or response to damage caused by sulfate to improve impermeability.
       Water-cement ratio is the main factor affecting permeability. Higher water-cement ratio of cement particles surrounded by waterproof layer thicker, and many in the cement mixing water connection was formed between the pores channels without rules, to increase the permeability of concrete, air-entraining admixture superplasticizer agents concrete water-cement ratio is below 0.5, even up to 0.3 below, the change in porosity of concrete, combined with the small air-entraining agent produces uniform independent and interlinked bubbles, effectively cut off the pores in the concrete channel to prevent water permeability. Testing using a variety of methods that air-entraining agent mixed with the concrete impermeability grade of up to more than S12, that is not under pressure 2MPa more permeable. Engineering Practice has proved that air-entraining agents and water-reducing agent no longer prepared to use other waterproof concrete measures to achieve the purpose of water impermeability, economic benefits are substantial.
    3.2.2.4 Winter Concrete Construction
      After the concrete in the shape and condensation, such as early subjected to freezing, the concrete in unhydrated cold water swelling, damage to the internal cohesion of the concrete structure to form the internal structure of hardened concrete damage, which is most easily applied concrete in winter occurred quality of the accident, in order to ensure the internal structure of concrete is not caused by an early freeze damage, must be reached before the minimum intensity of cold, also known as the critical strength. At present, the concrete extent of the damage caused by early frost is generally used to indicate loss of compressive strength, the laboratory used by the freezing mark early 28-day strength and did not raise the standard curing 8 days cold strength for comparison, many domestic and foreign Test data indicate that the critical strength in 3.5 ~ 7.0MPa. In order to ensure the quality of concrete, the strength of the cold front must be greater than the critical strength.
      In recent years, many articles suggest use of antifreeze in the winter when the applied composite air-entraining agents, they think of concrete air-entraining agent can reduce the early freezing of the frost heaving force. Air-entraining agent mixed early freezing of the strength loss of concrete is not mixed with air-entraining agent than ordinary concrete to reduce the critical intensity can be reduced to 2 ~ 4MPa. Air-entrained concrete to resist the early capacity by freeze-thaw cycle is particularly significant.
      Thus, the concrete admixture applied in the winter and air-entraining agent, both to improve the durability of structures, but also improve the ability of concrete to resist early freezing, reducing the critical strength, improve safety facilities in winter. Taking into account the mixed antifreeze salt for the adverse effects of frost resistance of concrete to be applied in winter to promote mixing in the air-entraining agent, even in the standard should be specified, must be applied in the winter mixed with air-entraining agent.
      3.2.2.5 erosion resistance
      Improve the erosion resistance of concrete lies in the density and improve the impermeability of concrete. Concrete admixture superplasticizer, you can significantly improve the density of concrete, and then mixing the right amount of air-entraining agents, the introduction of micro-bubbles, can improve the impermeability of concrete, so it has better anti-corrosion performance.
    3.2.2.6 anti-carbonation properties
Experimental data show that air-entraining agents and water reducer mixed concrete porosity is small, dense concrete, combined with tiny bubbles, can reduce the rate of concrete carbonation, carbonation resistance of concrete to improve performance and reduce the steel corrosion damage.
    3.2.3 Air-entraining agent for concrete shrinkage and creep of
     Increase in air content of concrete will have shrinkage values ​​increased, but the air-entraining water-reducing agent role and can make the shrinkage down, so when the concrete strength, workability phase-while, within a certain range of air-entraining not creep of concrete will have an impact.
4, air-entraining agent application problems and solutions
       Promote the use of air-entraining agents affect mainly due to a number of engineering and technical personnel in there in an old idea that mixed with air-entraining agent will reduce the concrete strength and increase the air content will affect the volume stability of concrete, even in high-strength concrete not mixed with air-entraining agents, these ideas are far from comprehensive. Superplasticizer mixed with an appropriate air-entraining agent, the preparation of C70 can even grade of HPC, the preparation of the main technical performance of concrete can be mixed with superplasticizer beyond a single concrete.
      Air-entraining agent in the promotion process, in addition to change the old concept of air-entraining agent addition, is to develop high-quality air-entraining agent. An excellent air-entraining agent, first in itself must have the ability and good foam stability, bubble structure also has good strength and small loss rate; Second, we must adapt to a strong performance, if they can adapt to different varieties of cement and Mixing of construction methods, temperature and environment; the third, and other additives combined with a strong performance; Finally, convenient and safe to use, reasonably priced.
     Therefore, the new high-performance research and development of air-entraining agent is placed in front of us an important issue. In addition, with the continuous development of concrete technology and mature set of excellent work, mechanical properties and durability of concrete in one of the high-performance concrete technology has become the main direction, how the application of air-entraining agent will be extended to the field of high performance concrete to improve the durability of concrete and work at the same time to warrant a large degree of its intensity caused by the negative effect of high-performance air-entraining agent is a new research and development to be of major concern.
5, high-performance air-entraining agent and concrete technology
      High-performance concrete (HPC) technology is the development of a concrete technology in recent years, different schools have different definitions of its concepts, mainly in the intensity of the debate. However, they agreed that the high-performance concrete must reflect the "high performance", that must have good work and good durability, as strength, although strength is also required, but not limited strength or excessive pursuit of high-strength.
      Durability and workability in high-performance concrete has a very important position. Through the above described analysis can be seen, mixing concrete air-entraining agent, and its performance has been significantly improved, reducing the secretion of concrete. Water, stratification and segregation, increase the flow of concrete and pumpability; also hardened body the technical performance indicators have been significantly improved, to improve the durability of concrete. Improve the performance of concrete, making the performance of the concrete itself, has the incomparable advantage of ordinary concrete. Use air-entraining agent to adapt to current and future requirements of high performance concrete, therefore, we believe that air-entraining agent in the promotion of high-performance concrete application process should play an important role, high-performance air-entraining agent should be added to concrete to improve concrete performance.
6 Summary
     6.1 used in concrete works a certain amount of air-entraining agent to improve the mobility of fresh concrete, workability, pumpability, reduce the mixture to segregation and bleeding, to improve the uniformity of concrete, durability (permeability , frost, etc.) are very useful.
     6.2 Goods-performance concrete using air-entraining agent, can significantly increase the plasticity of the concrete and hardened concrete properties, but also reduce the cost of admixtures and concrete.
     6.3 good air-entraining agent research and application of concrete admixtures and their development needs and necessity.
    6.4 The use of air-entraining agents follow the trend of development of concrete technology, adapt to the needs of the development of high performance concrete, so the air-entraining agent development, promotion and application of far-reaching, it should be essential for the future development of high performance concrete admixtures