Types of Admixtures Used in Concrete | Properties and Uses

This article is about uses of admixtures, types of admixtures used in concrete pdf, best admixture for concrete, properties of accelerating admixtures, additives and admixtures in concrete, disadvantages of admixtures in concrete and chemical admixtures used in concrete.

Types of Admixtures Used in Concrete

What are Concrete Admixtures?

Concrete admixtures are additional components used in the production of concrete to enhance specific properties of the material. These admixtures are added to the mixture, which consists of cement, water, and aggregates, in order to modify the properties of either the fresh or hardened concrete.

While cement, water, and aggregates are the primary components of concrete, admixtures are used to impart specific qualities to the material. These additives are utilized to alter the behavior of the concrete during its production or application. Admixtures can be used to improve the workability of the concrete or to reduce the amount of water needed to create a desired consistency.

ASTM C496 defines concrete admixtures as ingredients other than cement, water, and aggregates that are added to the concrete mixture. These ingredients can be used to improve the strength, durability, or appearance of the concrete. Admixtures can also be used to enhance the concrete’s resistance to chemical attack or to reduce the amount of time it takes for the concrete to set and harden.

In summary, concrete admixtures are additional components that are added to the mixture of cement, water, and aggregates in order to modify the properties of the fresh or hardened concrete. These additives can enhance the strength, durability, workability, appearance, and other characteristics of the material, making it more suitable for a wide range of applications.

Why use Concrete Admixtures?

To reduce the cost of concrete construction, various measures can be taken, such as using locally sourced materials, optimizing the mix design, and minimizing waste during production. These steps can help to reduce the overall cost of materials and labor, making concrete construction more affordable.

Achieving specific concrete properties can be a challenge, but there are several ways to do it more effectively. Using high-quality raw materials, precise mixing techniques, and accurate testing methods can help ensure that the desired properties are achieved consistently. Additionally, using advanced admixtures and additives can enhance specific properties such as strength, durability, and workability.

Ensuring the quality of concrete during mixing, transporting, placing, and curing can be challenging, especially in adverse weather conditions. However, measures such as using temperature-controlled mixing equipment, covering the concrete during transportation, and providing proper curing conditions can help to ensure the quality of the final product.

In the event of emergencies during concrete construction operations, quick and effective response is crucial. Having contingency plans in place and providing adequate training to workers can help to minimize the impact of emergencies such as equipment failure, accidents, and extreme weather events. Additionally, using durable and high-quality materials can help to prevent emergencies from occurring in the first place.

Classification of Concrete Admixtures:

Admixtures are essential components in modern concrete technology as they can modify the properties of concrete to meet the specific requirements of a project. Air entraining agents are a type of admixture that can introduce microscopic air bubbles into the concrete mix, making it more resistant to freeze-thaw cycles. Water reducers, on the other hand, are admixtures that reduce the amount of water required to achieve a given consistency of concrete. High-range water reducers, also known as superplasticizers, are a type of water reducer that can significantly reduce the water content in concrete without compromising its workability.

Retarders and accelerators are two types of admixtures that modify the setting time of concrete. Retarders slow down the setting time, making it easier to work with concrete in hot weather conditions, while accelerators speed up the setting time, making it possible to work with concrete in cold weather conditions. Fine minerals such as silica fume and fly ash are also used as admixtures to improve the strength and durability of concrete.

Lastly, there are specialty admixtures that are tailored to specific applications. These include corrosion inhibitors, which protect steel reinforcements from rusting, and shrinkage reducers, which can reduce the amount of shrinkage in concrete as it cures. Other specialty admixtures include viscosity modifiers, bonding agents, and waterproofing agents, all of which can improve the performance of concrete in different ways.

Air Entraining Concrete Admixture

Air entraining concrete admixtures are a type of material added to concrete during its production process. These admixtures have the ability to produce small air bubbles within the hardened concrete structure. The purpose of these bubbles is to create space within the concrete for water to expand into when it freezes.

The process of adding air entraining admixtures to concrete is an important one, especially in areas that experience freezing temperatures. When water freezes, it expands, which can cause significant damage to concrete structures. By incorporating air bubbles into the concrete mix, the water has space to expand into without causing damage to the overall structure.

The use of air entraining concrete admixtures has become increasingly common in the construction industry due to their ability to improve the durability and longevity of concrete structures. This type of admixture is typically added during the mixing stage of concrete production, and the amount used can vary depending on the specific requirements of the project.

Overall, air entraining concrete admixtures play an important role in the production of concrete structures that are able to withstand the effects of freezing temperatures. By creating tiny air bubbles within the concrete, these admixtures provide space for water to expand into without causing damage to the structure, ultimately improving the durability and longevity of the final product.

How do they work?

Anionic agents are substances that do not mix well with water. They have a unique property of creating bubbles that are durable, pliable, and filled with air. These bubbles play a crucial role in mitigating the impact of stresses arising from the movement or freezing of water.

They accomplish this by providing an increased volume for expansion and a shorter flow path, which helps to reduce the effects of stress. The bubbles created by anionic agents are highly effective in addressing the issues related to water movement and freezing, making them an indispensable tool in various industrial applications.

Benefits of Air-Entraining Admixture

The addition of air entrainers to fresh concrete can improve its workability, while also increasing its durability and resistance to various environmental factors such as freezing thaw cycles, de-icers, salts, sulfates, and alkali-silica reactivity. However, one downside is that it can decrease the strength of the concrete.

To mitigate this effect, it is recommended to lower the water cement ratio and increase the cement factor in moderate strength concrete. Air entrainers typically consist of various salts such as those derived from wood resins (Vinsol resin), sulfonated lignin (a paper production by-product), petroleum acids, proteinaceous material, fatty and resinous acids, alkylbenzene sulfonates, and sulfonated hydrocarbons. These substances are typically in liquid form and meet the specifications outlined in ASTM C260.

Water Reducing Concrete Admixture

Water reducers have the ability to produce three significant effects. Firstly, they can result in an increased slump at a constant water-cement ratio. Secondly, they can increase the strength of the mixture by reducing the water content. Thirdly, they can help in reducing the overall cost of cement.

Water reducers function by enhancing the mobility of cement particles in the plastic mix. This property enables the same level of workability to be achieved with lower water contents. As a result, water reducers can decrease the amount of water required to achieve a particular consistency of the cement mixture, leading to an increase in the strength of the resulting concrete.

Additionally, this reduction in water content can also help lower the cost of cement production.

Superplasticizers

Superplasticizers are a type of water reducer that are commonly referred to as “high-range” water reducers. These additives are specifically designed to improve the workability and flowability of concrete mixes in situations where consolidation of the concrete is difficult.

One such situation is when pouring concrete into thin sections or around tightly spaced reinforcing steel. In these cases, the use of superplasticizers can greatly improve the ability of the concrete to flow and fill the intended space without the need for excessive vibration or compaction. This can help to ensure that the resulting concrete is of high quality and free from defects.

Another situation where superplasticizers are commonly used is when placing concrete underwater. In these scenarios, the high water-reducing properties of these additives can help to reduce the amount of water needed in the mix, making it easier to place and consolidate the concrete in a wet environment.

Finally, superplasticizers are often used when pumping concrete into place. The high fluidity provided by these additives can help to reduce the resistance to flow and prevent blockages from occurring in the pumping equipment. This can help to ensure that the concrete is placed quickly and efficiently, while also minimizing the risk of any quality issues.

Note:

Superplasticizers are a type of chemical additive that are commonly used in the production of concrete. These additives are known for their ability to increase the workability of concrete by reducing its viscosity, which makes it easier to pour and mold into various shapes.

However, one of the drawbacks of using superplasticizers is that they can significantly reduce the length of time that the fresh concrete remains workable. In fact, when these additives are used, the fresh concrete typically only stays workable for a relatively short period of time, usually between 30 minutes to an hour.

Because of this limitation, superplasticizers are typically added to concrete at the construction site, rather than during the initial mixing process. This allows construction workers to more easily manipulate the concrete as needed, without having to worry about it becoming too stiff or difficult to work with.

Overall, the use of superplasticizers can help to improve the efficiency and effectiveness of concrete construction projects, by making it easier to work with and achieve the desired end result.

Retarding Concrete Admixture

Retarders are commonly used in concrete construction to delay the initial setting of concrete. This is particularly useful in hot weather conditions, where the setting time is accelerated due to the high temperatures. Retarders allow for the concrete to remain workable for a longer period of time, enabling the concrete to be placed and finished properly despite the heat.

Retarders are also beneficial when there are unusual placement requirements or long haul distances involved in the construction project. In such cases, the use of retarders can help extend the setting time of the concrete and allow for proper placement and finishing.

However, the use of retarders can also have some adverse effects. They can reduce the early age strength of the concrete and decrease the time between initial and final set. Therefore, it is important to evaluate the use of retarders experimentally before incorporating them into the mix design.

On the other hand, there are also some potential advantages of using retarders in concrete construction. For instance, they can help in air entrainment and increase workability. Additionally, they can also reduce the time between the initial and final set, allowing for better curing and improved overall performance of the concrete.

Accelerating concrete admixture:

Accelerators are commonly used in the construction industry to speed up the development of final strength characteristics in concrete. There are several reasons for using accelerators, including reducing the amount of time required before finishing operations can begin, decreasing the curing time, and increasing the rate of strength gain.

In addition to these benefits, accelerators can also be used to plug leaks under hydraulic pressure more efficiently and offset the effects of cold weather. Among the different types of accelerators, Calcium Chloride is the most widely used. This compound is known to reduce both the initial and final set times of concrete, making it an ideal option for projects that require a fast turnaround time.

Overall, the use of accelerators in concrete construction offers numerous advantages, including time savings and improved efficiency. By reducing the time required to achieve the desired strength characteristics in concrete, accelerators can help construction teams complete projects more quickly and with greater ease.

CaCl2 by weight	Initial Set Time in Hrs.
0%	6
1%	3
2%	2

The Portland Cement Association advises against using calcium chloride in certain situations, including when concrete is prestressed, contains embedded aluminum such as conduits, is subjected to alkali-aggregate reaction, is in contact with water or soils containing sulfates, is placed during hot weather, or when mass application of concrete is involved.

Instead of using calcium chloride, there are several alternatives that can be considered. One option is to use high early strength cement (type III), which can provide faster setting and increased strength development. Another option is to increase the cement content in the mixture, which can also contribute to higher strength.

If feasible, curing the concrete at a higher temperature can also accelerate the setting and strength development process. In addition, there are other chemical admixtures that can be used in place of calcium chloride, such as triethanolamine, sodium thiocyanate, calcium formate, calcium nitrite, or calcium nitrate. These admixtures can provide similar benefits to calcium chloride without the potential negative effects in certain situations.

Fine Minerals as Concrete Admixtures

Fine mineral admixtures are used in concrete to improve its properties, both in the plastic state and when hardened. These admixtures are added in large amounts, typically ranging from 20% to 100% of the weight of cement used in the concrete mixture.

These mineral admixtures are classified based on their chemical and physical properties. This classification helps to determine the appropriate admixture to use for a specific concrete mix. The properties of the admixture also determine the degree to which the concrete properties can be improved.

When added to the concrete mix, these fine mineral admixtures can significantly enhance the characteristics of both plastic and hardened concrete. The use of mineral admixtures in concrete can improve its durability, workability, strength, and other desirable properties. The choice of admixture is influenced by factors such as the type of project, environmental conditions, and specific properties required for the concrete.

1. Cementitious

Hydraulic cementing properties can be found in various materials such as blast furnace slag, natural cement, and hydraulic hydrated lime. These substances are capable of solidifying and hardening when mixed with water, making them useful for construction purposes.

Blast furnace slag is a byproduct of iron production that is finely ground and mixed with water to create a hydraulic cement. This type of cement is known for its durability, high strength, and resistance to chemical erosion.

Natural cement is a type of cement that is made by burning limestone containing clay or shale. This produces a material that hardens quickly and is capable of setting underwater. Natural cement was widely used in construction during the 19th century.

Hydraulic hydrated lime is made by combining quicklime with water. This mixture can solidify and harden even when submerged in water, making it ideal for use in structures such as bridges and dams. It is also useful for soil stabilization and can be used as a coating for walls and floors.

Overall, these materials with hydraulic cementing properties have played a significant role in the development of modern construction methods and have contributed to the durability and strength of many structures.

2. Pozzolanic

The materials in question are siliceous and aluminous in nature, and they possess little or no inherent cementitious value. However, when exposed to moisture, they are capable of reacting with calcium hydroxide to produce compounds that possess cementitious properties. To aid in this process, hydrated lime is added to the mix, typically at a weight of 15% relative to the weight of Portland cement.

The addition of pozzolan to the mix can further enhance the cementitious properties of the hydrated lime. This is because pozzolans are naturally occurring or synthetic materials that contain silica or alumina in a reactive form. When mixed with hydrated lime and calcium hydroxide, pozzolans react to form additional cementitious compounds, further strengthening the overall structure of the material.

Overall, while these siliceous and aluminous materials may not possess inherent cementitious properties, their ability to react with calcium hydroxide and the addition of hydrated lime and pozzolan can transform them into materials with significant cementitious value.

Classification of Fly Ash and Natural Pozzolans

In civil engineering, pozzolans are materials that can be added to concrete to improve its strength and durability. There are several classes of pozzolans, including Class N, which consists of raw or calcined natural materials like diatomaceous earth, opaline cherts and shales, ruffs and volcanic ashes, and some calcined clays and shales.

Another class of pozzolans is Class F, which refers specifically to fly ash that possesses pozzolanic properties. Fly ash is a byproduct of burning coal in power plants, and it is the most commonly used pozzolan in civil engineering structures. When mixed with concrete, it reacts with the calcium hydroxide in the cement to form additional cementitious materials, resulting in a stronger and more durable final product.

Class C pozzolans, on the other hand, are fly ashes that have both pozzolanic and cementitious properties. These pozzolans have a greater ability to strengthen and harden concrete compared to Class F fly ash.

Overall, pozzolans are important additives in concrete because they can improve the durability and longevity of civil engineering structures. By understanding the different classes of pozzolans and their properties, engineers can select the most appropriate pozzolan for a given application, leading to better performance and longer-lasting infrastructure.

Special Concrete Admixtures

Concrete is a versatile material that can be improved through the use of various admixtures. These admixtures serve different purposes and can enhance the quality of concrete in a number of ways.

For example, workability agents are admixtures that can make the concrete easier to mix, place, and finish, while maintaining its desired properties. These agents can improve the flowability of the concrete, making it easier to work with and reducing the need for excessive water that can weaken the final product.

Another type of admixture that can improve concrete quality is bonding agents. These admixtures can improve the bond between the concrete and other materials, such as steel or existing concrete surfaces. This can be especially useful in repairs or construction projects where the concrete needs to adhere to other materials, or when there is a need for increased durability.

In addition to workability and bonding agents, there are many other types of admixtures that can be used to enhance concrete properties. For example, air-entraining agents can improve the freeze-thaw resistance of concrete, while accelerating agents can speed up the setting and hardening process. Retarding agents, on the other hand, can slow down the setting process, allowing more time for finishing and placement.

Overall, the use of admixtures can significantly improve the quality and performance of concrete, making it a more durable and versatile material for a wide range of applications.

Table: Effect of Mineral Admixtures on Fresh Concrete

Quality Measure	Effect
Water Requirements	Fly ash reduces water requirements Silica fume increases water requirements
Air Content	Fly ash and silica fume can decrease the air content in concrete. This can be mitigated by increasing the use of air-entraining agents.
Workability	Fly ash, slag, and inert materials increase workability, while silica fume reduces it, requiring the use of super-plasticizers to compensate.
Hydration	Fly ash reduces hydration heat, while silica fume may not have an effect. Superplasticizers used with silica fume can increase heat.
Set Time	Set time of concrete is increased by fly ash, natural pozzolans, and blast furnace slag. This effect can be counteracted by using an accelerator.

Table: Effect of Mineral Admixtures on Hardened Concrete

Quality Measure	Effect
Strength	Fly ash can increase the ultimate strength of concrete, but may slow down the rate of strength gain. On the other hand, silica fume has a lesser impact on the rate of strength gain compared to pozzolans.
Drying shrinkage and creep	Drying shrinkage and creep: Ground slag and fly ash typically have a minor effect on drying shrinkage and creep at low concentrations, but high concentrations may increase shrinkage. In contrast, silica fume can reduce shrinkage.
Permeability and absorption	Permeability and absorption are generally reduced by the addition of mineral admixtures, with silica fume being particularly effective.
Alkali-aggregate reactivity	The use of mineral admixtures generally reduces alkali-aggregate reactivity, but the extent of improvement varies depending on the type of admixture.
Sulfate resistance	The sulfate resistance of concrete is generally improved due to the reduced permeability that results from the use of mineral admixtures such as fly ash, ground slag, and silica fume. This reduced permeability can limit the ingress of harmful sulfates into the concrete, thereby reducing the risk of sulfate attack and improving the durability of the structure over time.

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