AIR-ENTRAINED CONCRETE AND ITS AIR-VOID SYSTEM

INTRODUCTION

Air-entrained concrete is one of the most important developments in concrete technology. It was first used in the early 1930s, especially in the USA. Air-entrained concrete is a type of concrete that contains tiny air bubbles, of size ranging from 5 microns to 80 microns, distributed throughout the material. These air bubbles serve several important purposes in enhancing the properties and performance of concrete. It has to be noted that entrained air is often confused with entrapped air. Entrained air consists of microscopic air bubbles, intentionally introduced to improve the properties of the concrete, whereas entrapped air is voids of different shapes and bigger sizes (ranging from 10 to 1000 microns or more), which are not uniformly distributed throughout the mass of concrete, due to improper compaction. Entrapped air will result in the degradation of concrete over time, whereas entrained air will improve the quality of concrete. Due to the advantages offered by air-entrained concrete, about 85% of concrete manufacturing in the United States and Canada contain air-entraining agents, which are considered the ‘fifth ingredient’ in concrete manufacturing technology (Shetty, 2005).

In India, the use of air-entrained concrete is limited, primarily because frost damage to concrete is not a serious problem, till now. However, air-entrained concrete has been used in the construction of many dams such as the Hirakud Dam, Koyna Dam, Rihand Dam, and others dams (Shetty, 2005). Discovery by Accident

Air entrainment was discovered by accident in the mid-1930s. At that time, cement manufacturers were using a grinding aid to enhance the process of grinding cement. This grinding aid contained various chemicals, including salts of wood resin, which were added during the grinding process.

These chemicals unintentionally led to the creation of tiny air bubbles in the concrete mix, resulting in air entrainment. These bubbles improved the concrete’s workability during placement and increased its durability when hardened, especially in climates prone to freeze-thaw cycles.

T.C. Powers proposed a hypothesis in 1945 regarding the mechanism of frost damage in concrete. This hypothesis was later revised in 1949 to include requirements for the air void system, which remains the basis for many air entrainment practices today (Tunstall et al., 2021)

Foam Concrete vs Air-entrained Concrete

It has to be noted that?foam concrete is different from air-entrained concrete. Foam concrete is made by introducing stable air bubbles by using some kind of foam agent, resulting in a concrete that is lightweight, has a lower density, and is commonly used for insulation or filling voids. Air-entrained concrete, on the other hand, is produced by using admixtures, which produce evenly distributed tiny air bubbles in the concrete mass, which enhances durability, workability, and resistance to freeze-thaw cycles. Thus,air-entrained concrete will not affect the overall density or mechanical properties significantly. Another difference is in the manufacturing process: foam concrete involves the creation of a foam mixture separately, which is then mixed with cement, sand, and water to form the final product, whereas air-entrained concrete is produced by adding specialized admixtures directly into the concrete mix during mixing to create small air bubbles throughout the concrete volume.

The key points about air-entrained concrete are discussed below. Common types of air-entraining agents are briefly discussed. The advantages of air entrainment are listed. The effect of supplementary cementitious materials, the dosage of admixture, and the effects of production and construction variables on-air content are also briefly discussed. The air-void parameters such as total air content, air void size and distribution, spacing factor, and specific surface are explained. Finally, the methods of testing and their specifications are provided.

?KEY POINTS ABOUT AIR-ENTRAINED CONCRETE

Below are some key points about Air-entrained concrete:

Purpose of Air Entrainment: The primary purpose of air entrainment is to improve the durability of concrete, particularly in harsh environmental conditions. The presence of air voids helps to relieve internal pressure caused by freezing and thawing cycles, reducing the risk of cracking and damage.

Formation of Air Voids: Air entrainment is typically achieved by adding air-entraining agents to the concrete mix. These agents form microscopic air bubbles when mixed with water and cement paste during the hydration process. The air bubbles become dispersed throughout the concrete matrix.

Air-entraining Agents

Air-entraining agents are typically chemical additives that are added to concrete mixes to introduce microscopic air bubbles into the cement paste. These agents can be classified into several categories based on their chemical composition. Some common types of air-entraining agents include (Shetty, 2005, Tunstall et al., 2021):

1.????? Natural Organic Compounds: These agents are derived from natural sources and include substances such as natural wood resins?(Vinsol??resin, Darex, Pinova Inc.), animal and vegetable fats and oils, such as tallow, fatty acids such as stearic and oleic acids. Natural organic compounds have been historically used as air-entraining agents due to their ability to reduce surface tension and stabilize air bubbles in concrete.

2.????? Synthetic Surfactants: Synthetic surfactants are chemical compounds designed specifically for use as air-entraining agents. They are typically derived from petrochemicals and include various types of surfactants such as sulfonates, sulfates, and non-ionic surfactants. Synthetic surfactants offer precise control over air content and are widely used in modern concrete production.

3.????? Fatty Acid Esters: Fatty acid esters, such as calcium or sodium salts of fatty acids, are commonly used as air-entraining agents due to their ability to stabilize air bubbles in concrete mixes. These compounds are effective at low dosages and can provide excellent freeze-thaw resistance to concrete.

4.????? Alcohol-based Compounds: Alcohol-based compounds, such as glycerol and other alcohol derivatives, are sometimes used as air-entraining agents in concrete mixes. These compounds can help improve the workability of concrete and enhance its freeze-thaw resistance.

5.????? Sulfonated Agents: Sulfonated compounds, including lignosulfonates and naphthalene sulfonates, are commonly used as water-reducing agents in concrete mixes. In addition to their water-reducing properties, sulfonated agents can also act as air-entraining agents, providing improved durability to concrete.

6.????? Amphoteric Surfactants: Amphoteric surfactants are compounds that contain both hydrophilic (water-attracting) and hydrophobic (water-repelling) groups. These surfactants can stabilize air bubbles in concrete mixes and improve the freeze-thaw resistance of the hardened concrete.

7.????? Polymeric Additives: Certain polymeric additives, such as acrylic polymers and polyvinyl acetates, can also act as air-entraining agents when added to concrete mixes. These additives help improve the cohesion and workability of concrete while also providing enhanced freeze-thaw resistance.

American brand names include Aerosin-HRS, Rihand A.E.A., Koynaea, Ritha Powder, Hico, Sika? AIR (meets the requirements of ASTM C260/AASHTO M 154), Hostapur?, Genapol?, and Zeliquid. In India, commercial brand names include MC-Mischoel LP, MC-Michoel AEA, Complast AE 215, and Roff AEA 330(Shetty, 2005).

Air-entraining agents are added in an amount of about 0.001–0.1% weight, based on the weight of dry cement, to yield a desired level of air in a cement composition. The particular amount used will depend on materials, mix proportion, and mixing conditions. Different air-entraining agents produce different amounts of air entrainment. Similarly, different quantities of air-entraining agents will result in different amounts of air-entrainment. Polycarboxylate admixtures, especially when they are combined with synthetic air-entraining agents are known to cause air-entraining issues. The water/cement ratio (w/c ratio) is one of the important factors affecting the quantity of air; a w/c ratio of 0.4 to 0.6 is found to produce adequate air bubbles (note that a w/c ratio of about 0.38 is normally required for all the cement particles to hydrate). The hardness of the water may affect the air air-entraining agents just as it affects bath soap. Fluctuations in sand gradation will cause swings in air content. The No. 30 and No. 100 sieves are the most critical sieve sizes for consistent air entraining. If there are large changes in these sizes, it is difficult to control the air content. Sulfate content and fineness of cement will cause changes in the air content, along with loss of ignition (L.O.I.) and fineness of fly ash.

Effect of Supplementary Cementitious Materials and The Dosage of Admixture

Class F fly ash typically demands higher levels of admixture to maintain desired entrained air levels compared to Class C fly ash(Lallas, et al. 2023). Silica fume is another material that influences air-entrained concrete. Its fine particle size and smoothness necessitate higher dosages of air-entraining admixture than traditional concretes without silica fume-typically125% to 150% of the dosage used in traditional concrete (Lallas, et al. 2023).? Similarly, the use of slag cement and rice husk ash in concrete mixture designs will also require a higher air-entraining admixture dosage (Lallas, et al. 2023). Table 1 shows the effects of some of the concrete ingredients on the air content (PCI, 1998).

In addition, the way concrete is produced and handled can also have a significant effect on its air content and entrained air-void system. The following variables associated with concrete production that may affect the air content and the performance of concrete include the methods of batching, mixing procedures, and time and speed of mixing. Construction-related variables and field conditions include transport and delivery, retempering, placement, consolidation, finishing, and temperature. Table 2 shows the effect of some of these variables on the air content. Two mechanisms were found to be responsible for reducing air content in concrete during the pumping process, known as the dissolution mechanism and mechanical rupture on strike with a surface (Shah et al., 2021). Often air entraining admixture is applied by dispensing the admixture on the sand in the bins. This is a great way to apply, as it generates stable air content.

AIR-VOID PARAMETERS

?Air-void parameters play a crucial role in ensuring the durability and performance of concrete, both in its fresh state and after it has hardened. Once the concrete sets, the original air bubbles leave behind voids in the hardened concrete. This system is known as the air-void system. The major parameters for the air-void system (AVS) include the total air content, air void size and distribution, spacing factor, and specific surface. The shortest length between any point in cement paste and the nearest boundary of the air void is called the spacing factor.

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Total Air Content

The total air content refers to the volume of air present in the concrete mixture. It includes both entrained air (deliberately incorporated during mixing) and any entrapped air (air trapped unintentionally). Entrained air helps improve concrete’s resistance to freezing and thawing cycles by providing tiny air voids that act as reservoirs for freezing water. These voids relieve pressure and prevent damage to the concrete. Achieving the desired air content involves attention during the design, specification, and construction stages. General recommendations on total air content for concrete are shown in Table 3.

Spacing Factor

The spacing factor represents the relative distance between individual air voids within the concrete. Smaller spacing factors are preferable because they reduce the distance water would need to travel before entering an air void, thus minimizing pressure. A spacing factor of less than 0.2 mm (0.008 in.) is believed necessary to maintain freeze-thaw durability.

Specific Surface

The specific surface indicates the relative number and size of air bubbles within a given volume of air.A larger specific surface value is better because it corresponds to a greater number of small bubbles. A specific surface greater than 24 mm2/mm3 (600 in.2/in.3) is considered essential for freeze-thaw durability.

ADVANTAGES OF AIR-ENTRAINMENT

The following are some of the advantages of air entrainment:

• Freeze-Thaw Resistance: Air-entrained concrete is better able to withstand the expansion and contraction that occurs during freezing and thawing cycles, reducing the likelihood of surface scaling and spalling.
• Improved Workability: The presence of air voids enhances the workability of fresh concrete, making it easier to place and finish.
• Reduced Permeability: Air-entrained concrete typically exhibits lower permeability, which can help to minimize water penetration and enhance resistance to chemical attack and corrosion of reinforcement.
• Enhanced Cohesion: The air voids act as lubricants, reducing internal friction within the concrete mix and improving cohesion between particles.
• Other benefits: It also results in reduced bleeding and segregation.

TESTING AND SPECIFICATIONS

?The amount of air entrainment in concrete is typically measured as the air void content, expressed as a percentage of the total volume of the concrete. Various standards and specifications, such as ASTM C231 and ASTM C173 in the United States, govern the testing and requirements for air-entrained concrete.

The sample size for air content testing of fresh concrete should be a minimum of 0.028 m3 (1 ft3). The sample should not be taken from the very first or last portion of the batch. A sample should be obtained for every 75 m3 (100 yd3) of concrete, at least once per day (PCI, 1998). The gravimetric method is the oldest method of determining the air content of fresh concrete (ASTM C138M). The method used now to measure the air content in fresh concrete is the pressure methodor the volumetric air meter, as per ASTM C173M(PCI, 1998). When air content in fresh concrete is compared to air content in hardened concrete, differences can exist (PCI, 1998). These differences may be due to air bubble stability, and environmental factors like temperature and pressure, handling procedures including mixing, delivery, placement, and consolidation (Shah et al., 2021).

?The sequential air method or the SAM test method uses different pressures of 100, 207, and 310 kPa (14.5, 30, and 45 psi), and measures the air content and SAM number, which correlates with the spacing factor. The Sequential Air Method (SAM) meter, a type of ASTM C231M Type B pressure meter, with some modifications, is shown in Fig. 1. The SAM Number is calculated based on the readings from the SAM meter (Ley et al., 2017):

SAM Number =(Pc2-Pc1)/c

where Pc2 is the second equilibrium pressure at 310 kPa (45 psi) and Pc1 is the first equilibrium pressure at 207 kPa (30 psi). The value c is a constant that is 1.45 if the units are in kPa and 1.0 if the units are in psi. If a mixture had an air volume > 4% and SAM Number < 0.32 before pumping then its performance is considered as satisfactory as per ASTM C666. A new technique called the Air void analyzer has also been developed. Ley et al. (2017) observed that concrete with a SAM number of 0.32 showed a durability factor between 60 and 80% with 88% agreement, while a spacing factor of 200 mm correlates with the durability factor of 70% for 68% of data investigated. Moreover, they recommended the SAM number of 0.22 to ensure the satisfactory performance of concrete in rapid freeze and thaw testing.

The parameters of AVS in hardened concrete can be calculated using ASTM C457, RapidAir, flatbed scanner, and computerized tomography scan. More details of different types of testing procedures may be found in Shah et al., 2021. The Effect of air entrainment on fresh and hardened properties of cement-based materials such as workability, rheology, compressive strength (it is found to decrease by 4 to 6% with every 1% increase in air content), freeze-thaw resistance, and fire resistance may be found in Shaw et al., 2021.

APPLICATIONS

?Air-entrained concrete is commonly used in outdoor pavements, sidewalks, driveways, and other structures exposed to freeze-thaw cycles. It is also used in marine environments, bridge decks, and areas subjected to deicing salts or chemical exposure.

MIX DESIGN CONSIDERATIONS

?Proper mix design is essential for achieving the desired air void content and performance characteristics of air-entrained concrete. Factors such as aggregate type, cement content, water-cement ratio, and dosage of air-entraining admixture must be carefully controlled.

SUMMARY AND CONCLUSIONS

Air-entrained concrete offers significant benefits in terms of durability and performance, particularly in regions with varying weather conditions or exposure to harsh environments. The selection of appropriate air-entraining agents is crucial to ensure compatibility with the raw materials of concrete and the service environment. Factors such as desired air content, properties of cement and aggregates, environmental conditions, and specific application requirements influence the choice of air-entraining agents. Compatibility testing should be conducted to ensure compatibility with other chemical admixtures and materials in the concrete mix.

Controlling air void parameters is essential for achieving durable and resilient concrete structures. Understanding and managing air content is crucial for the long-term performance of pavements, buildings, and bridges. The Sequential Air Method (SAM) can be utilized to predict the spacing factor and SAM number of fresh concrete, aiding in achieving the desired air void system.

When incorporating super-plasticizers into air-entrained concrete, it is important to test their compatibility, determine the optimal addition sequence, and employ appropriate mixing methods. Increasing mixing time can affect both total air content and air loss, with the potential to optimize mixing time to achieve the desired air void system parameters.

Moreover, air entrainment can be utilized to enhance the fire resistance of concrete, offering an alternative to polypropylene fibers which may negatively impact concrete workability and rheology. Overall, careful consideration of air-entraining agents, mixing methods, and compatibility testing is essential to ensure the effectiveness and longevity of air-entrained concrete structures.

References

1.????? Lallas, Z.N., Gombeda, M.J., and Mendonca, F. (2023) "Review of supplementary cementitious materials with implications for age-dependent concrete properties affecting precast concrete production", PCI Journal, V. 68, No. 6, Nov.–Dec., pp. 46-64, https://doi.org/10.15554/pcij68.6-01

2.????? Ley, M.T., Welchel, D., Peery, J., Khatibmasjedi,S.? and LeFlore,J. (2017)“Determining the air-void distribution in fresh concrete with the Sequential Air Method”, Construction and Building Materials, Vol.150, pp. 723–737.

3.????? Shah, H.A., Yuan Q. and Zuo, S.(2021) "Air entrainment in fresh concrete and its effects on hardened concrete-a review", Construction and Building Materials, Vol. 274 (2021) 121835, 17 pp. https://doi.org/10.1016/j.conbuildmat.2020.121835

4.????? Shetty, M.S., Concrete Technology-Theory and Practice(2005) S. Chand & Company Ltd., New Delhi, 624 pp.

5.????? Tunstall, L. E., Ley, M. T and Scherer, G.W.(2021) "Air entraining admixtures: Mechanisms, evaluations, and interactions", Cement and Concrete Research,Vol. 150, Dec., 106557. https://doi.org/10.1016/j.cemconres.2021.106557

6.????? PCI (1998) “Control of Air Content in Concrete”, Concrete Technology Today, Vol.19, No.1, Apr., 4 pp.www.cement.org/docs/default-source/fc_concrete_technology/pl981.pdf

7.????? Price, B. (1996)“Measuring Air Voids In Fresh Concrete”, Concrete, The Concrete Society, U.K. Vol. 30, No. 4, pp. 29-31, https://trid.trb.org/view/468645

8.????? https://link.springer.com/article/10.1007/s11771-013-1590-z

9.???? https://www.sciencedirect.com/science/article/abs/pii/S0950061820338393