Concrete Admixtures: Types, Uses, Functions & Advantages (ACI & IS 9103)

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Concrete admixtures are materials added to concrete before or during mixing to improve its properties in the fresh or hardened state. They are widely used in modern construction to enhance workability, reduce water demand, control setting time, increase strength, improve durability, and provide better resistance to harsh environmental conditions. With the growing demand for high-performance and long-lasting concrete, admixtures have become an essential part of concrete technology.

According to American Concrete Institute ACI and Indian standards such as IS 9103 (commonly cited for concrete admixtures in India), admixtures are classified into different categories based on their function, such as water-reducing admixtures, retarders, accelerators, air-entraining agents, and mineral admixtures. Each type is selected depending on the project requirements, weather conditions, construction speed, and desired concrete performance.

In this article, you will learn about concrete admixtures, their types, uses, functions, advantages, disadvantages, and their importance in modern construction practices.

Concrete admixtures can alter the physical and chemical properties of concrete or mortar for specific functions. With several types of admixtures available, this article aims to discuss their application in concrete works.

What Are Concrete Admixtures?

Concrete admixtures are substances other than cement, aggregates, water and reinforcement to enhance performance based on specific requirements of the concrete. Typically, the amount used does not exceed 5% of the cement mass, except in special cases. Admixtures are classified according to their primary functions and generally include water-reducing agents, air-entraining agents, hardening accelerators, set retarders, flash-setting agents, expanding agents, antifreeze agents, rust-resistant agents, and others.

As per ACI, it is defined as “a material other than water, aggregates, cementitious materials, and fibre reinforcement, used as an ingredient of a cementitious mixture to modify its freshly mixed, setting, or hardened properties and that is added to the batch before or during its mixing.”

Why Are Admixtures Used in Concrete?

Admixtures are used in concrete because they improve the quality of fresh concrete and enhance its performance at later stages after hardening.

• Improve workability
• Reduce water demand
• Increase strength
• Accelerate setting time
• Delay setting in hot weather
• Improve durability
• Waterproofing
• Reduce shrinkage

Classification of Admixture

Admixtures used in concrete are generally classified into two categories: chemical admixtures and mineral admixtures.

Common Types of Chemical Admixtures

According to ASTM International ASTM C494/C494M, chemical admixtures are ingredients added to concrete before or during mixing to modify the properties of fresh or hardened concrete. The chemical admixture include:

1. Water Reducing Admixtures
2. Retarding Admixtures
3. Accelerating Admixtures or Accelerator
4. Air-entraining concrete admixture
5. Air Detraining Admixtures
6. Grouting Admixtures

1. Water Reducing Admixtures

They are used to minimise the water demand in a concrete mix. Water-reducing admixtures increase the workability and strength of concrete, provide a good bond between concrete and steel, and prevent cracking, segregation, honeycombing and bleeding. Based on raw materials and chemical composition, it can be divided into lignosulphonate, alkyl aryl sulphonate (known as coal tar water-reducing admixture), melamine water-reducer, molasses and humic aid water reducer, etc.

Based on the performances and functions, water-reducing admixtures can be divided into ordinary water-reducer, effective water-reducer, hardening water-reducer, retarder water-reducer and air entraining water-reducer. It is a type of surfactant that has a hydrophilic group and a hydrophobic group of molecules.

When concrete is mixed with water, the cement paste forms a flocculated structure due to the cohesion between cement particles [see Figure 1(a)]. Some of the mixing water becomes trapped within this structure, reducing the mobility of the concrete mix. However, when a water-reducing admixture is added, its hydrophobic groups attach to the surface of the cement particles, giving them the same charge. This causes the particles to repel each other due to electrical repulsion [see Figure 1(b)], releasing trapped water from the flocs and increasing mobility without adding extra water. Additionally, the water-reducing admixture forms a layer of solvent water film [see Figure 1(c)], acting as a lubricant between the cement particles, further improving mobility.

Plasticizers and superplasticizers are commonly used water-reducing admixtures that improve concrete workability while reducing water demand.

2. Retarding Admixtures or Set Retarder

Retarding admixtures are used to slow down the initial setting time of concrete. Due to this, they are generally used in hot weather or high-temperature conditions, and long transport distances to prevent the loss caused by the early slump of the concrete mix. They don’t have a bad impact on the development of concrete strength at the latter stage.

There are two types of retarding admixtures, which are organic retarders and inorganic/chemical retarders.

3. Accelerating Admixtures

According to IS 9103, accelerating admixtures, also known as accelerators, are admixtures used to shorten the setting time of concrete and increase the rate of early strength development. They act by accelerating the hydration of hydraulic cement, allowing concrete to gain strength faster at early ages. Accelerating admixtures are commonly used in cold weather concreting, rigid pavement construction, precast or prefabricated concrete works, and emergency repair works. They are especially useful during winter construction where low temperatures slow down cement hydration.

Generally, accelerating admixtures are of three categories that can be divided into inorganic, organic and inorganic-organic compounds.

4. Air-Entraining Admixtures

Air-entraining admixtures are admixtures that cause the development of microscopic air bubbles in concrete while mixing. This increases its resistance to freezing and improves the workability and durability of concrete. It is a kind of surfactant which can influence the concrete as follows:

1. Improve the workability of concrete mixtures: Closed bubbles act like small balls, reducing friction between cement particles, which improves the concrete’s mobility. At the same time, the bubble film helps conserve water, aiding in better hydration of the cement.
2. It can enhance impermeability and frost resistance: It leads to closed stomata in concrete, thereby cutting off the capillary porosity ducts and reducing pores, thereby enhancing impermeability.
3. It can reduce the strength of concrete: If the air content in concrete is increased by 1%, its compressive strength can decrease by 4% to 6%.

2. Mineral Admixtures

Mineral admixtures are also known as Pozzolanic Admixtures since they are derived from other substances and are not chemically manufactured. Pozzolanic Admixtures affect the properties of concrete in both the plastic and hardened state. One property of pozzolanic admixtures is that they react/hydrate with cement; therefore, they are also called supplementary cementing materials.

Mineral admixtures are naturally occurring materials or industrial by-products added to concrete to improve strength, durability, and workability. They include:

• Fly ash
• Ground Granulated Blast furnace Slag (GGBS)
• Silica fume
• Rice husk ash

Types of Concrete Admixtures and Their Purpose

Superplasticiser Admixture

When added to concrete or mortar, it imparts very high workability or allows a significant decrease in water content for a given workability.

Advantages of Concrete Admixtures

• lower water-cement ratio
• improved strength
• better placement
• faster construction
• durability

Disadvantages of Concrete Admixtures

Very important and often missing.

• wrong dosage causes segregation
• delayed set
• compatibility issues
• higher cost
• testing required

How to Select the Right Admixture?

Selecting the right admixture is crucial in concrete works, and they are selected based on:

• weather
• required strength
• placement method
• durability exposure
• project schedule

Frequently Asked Questions