Concrete is one of the most commonly used materials in the construction of modern homes, especially in India. Since its invention in the early 19th century, there have been many technological advances that have helped in increasing its quality. Admixtures constitute one such technology that is frequently used in home construction and help enhance specific concrete properties like durability, workability and setting time. Read on to learn more about what an admixture is and what its types are.
What is an Admixture?
Concrete is generally composed of cement, sand and aggregates in different proportions to create different grades. An admixture is an element other than these, which is added to concrete to enhance its properties such as workability, increase or decrease in setting time, durability, etc. Inclusion of an admixture in a concrete mix allows builders to customise it based on specific site requirements. It can be added before mixing concrete or while mixing it or even during the setting stage, depending on the function it is required to perform. The quantity of an admixture in a concrete mix is generally small and typically ranges from 0.5% to 5% of the cement mass in the mix.
Types of Admixtures in Concrete
Different types of admixtures are used to provide different benefits and ways to enhance concrete quality, workability and durability, based on specific requirements. A concrete admixture can be broadly categorised into the following categories.
- Chemical Admixtures
- Mineral Admixtures
Chemical Admixtures for Concrete
These are water-soluble elements that can be in powdered or liquid forms and are generally added before or during the preparation of a concrete mix. A chemical admixture is usually used in smaller quantities that make up a very small percentage of the overall concrete mix design. The following are the various types of chemical admixtures available in the market today, which can be used for different purposes as required.
- Retarders
A retarder is an admixture included in the concrete mix to delay the time taken by concrete to set. In hotter climates, concrete undergoes a faster setting. Retarders aid in cooling down the concrete temperature, thus increasing its setting time and ensuring better finishing.
- Accelerators
Accelerators are used to decrease the setting time of concrete, especially in colder climates where the process can take longer. They help in speeding up the construction process with higher early concrete strength. However, these must be used in moderation, in order to avoid shrinkage and loss in concrete strength.
- Water Reducing Admixtures/ Plasticisers
As the name suggests, water reducing admixtures are utilised to reduce the amount of water in a cement mix or the water-cement ratio. The amount of water required can generally be reduced by 5% to 10%, but higher range admixtures or superplasticisers can also reduce this quantity by 30%. This is especially beneficial in areas with low water availability and can also aid in increasing concrete strength.
- Air-entraining Admixtures
Addition of an air-entraining admixture in a concrete mix ensures the entrapment of uniform, stable air bubbles. This reduces friction between cement particles, thus increasing the workability of concrete. It also ensures impermeability of concrete, leading to increased durability. However, inclusion of this admixture in higher quantities can lead to drastic decrease in concrete strength.
- Miscellaneous Admixtures
Apart from the above categories, there are also other admixtures available in the market today, which can be used for purposes such as:
- Hydration control
- Reduction of shrinkage
- Colouring
- Inhibition of corrosion
- Damp-proofing
Mineral Admixtures for Concrete
Mineral admixtures are available in fine powdered forms and are generally made up of waste materials from different industries, which are insoluble in water. These types of admixtures for concrete are added in larger quantities to the concrete mix as compared to chemical admixtures and can sometimes even be used to partially replace cement in the mix. These are also available in different types, some of which are explained below.
- Fly Ash
Fly ash is a by-product of thermal power plants, consisting of Silica, Alumina and Calcium oxides. The use of fly ash in concrete as an admixture increases its workability and setting time. It can also replace a certain quantity of cement in the mix, helping the production of greener concrete with lower shrinkage, cracking and creep.
- Silica Fume
This is a by-product of electric arc furnaces that produce silicon-based compounds. As an admixture, it increases the flexural and compressive strength of concrete, while enhancing its resistance to erosion and abrasion. However, its use in a concrete mix highly increases water requirement, which is why it’s almost always used along with a plasticiser.
- Metakaolin
Thermally activated ordinary and kaolin clay is called metakaolin. This mineral admixture reduces the permeability of concrete and increases its resistance to chemical attacks, thus ensuring increased durability. It also helps in the early strengthening and setting of concrete, leading to faster construction.
Benefits and Uses of Concrete Admixtures
As explained in the above sections, admixtures provide various benefits in the use of concrete, which can be summarised as follows:
- Early/ late setting and hardening
- Increased durability
- Enhanced workability
- Increased compressive and flexural strength
- Resistance to environmental factors such as corrosion, chemical attacks, shrinkage and cracking.
- Enhanced finishing
- Speedy construction and lower costs
- Providing colour options for finishing
Factors Affecting the Performance of a Concrete Admixture
Type and Composition of Cement
Different types of cement (OPC, PPC, etc.) contain varying proportions of compounds such as tricalcium silicate (C3S) and tricalcium aluminate (C3A). These compounds interact differently with admixtures. High C3A content can increase the rate of reaction with some admixtures, especially water reducers or retarders. The fineness of cement particles also affects how quickly admixtures disperse and react.
Water-Cement Ratio
The water-cement ratio determines the consistency of the mix and influences how admixtures perform. For water-reducing admixtures, a lower w/c ratio enhances strength, but may reduce workability if dosage is not adjusted properly. High w/c ratios can dilute the admixture’s effectiveness, leading to inconsistent results.
Mix Proportions and Aggregate Characteristics
The type, size, shape, and gradation of aggregates affect the efficiency of admixtures:
Angular or rough-textured aggregates increase water demand, requiring more admixtures for the same level of workability. Moisture content and absorption capacity of aggregates also influence admixture dosage and performance.
Dosage and Method of Addition
The amount and timing of admixture addition play a critical role. Overdosing can lead to adverse effects such as delayed setting, segregation and excessive air entrainment. Under-dosing may result in incomplete dispersion and limited performance benefits. Adding admixtures directly to dry cement or after partial mixing can reduce effectiveness, thus making proper sequencing essential.
Temperature and Environmental Conditions
Temperature affects both chemical reactions and admixture efficiency. High temperatures accelerate cement hydration, reducing the working time of retarders and increasing the water demand. Low temperatures can slow down reaction rates, diminishing the effect of accelerators and potentially causing delayed setting. Humidity and wind speed can also affect evaporation and admixture behaviour in fresh concrete.
Mixing Time and Uniformity
Proper mixing ensures uniform distribution of the admixture throughout the concrete. Insufficient mixing leads to uneven workability and setting times. Over-mixing, particularly with air-entraining agents, can cause excessive loss of air content.
Compatibility of Materials
Admixtures must be compatible with the specific cement, supplementary materials, and other admixtures in use. Incompatible combinations can cause flocculation, inconsistent setting, or loss of strength. For example, certain superplasticizers may not perform well with high-alumina cements or fly ash–blended concretes.
Age and Storage Conditions of the Admixture
Admixtures can degrade over time if not stored properly. Freezing, overheating, or contamination can alter chemical composition and reduce effectiveness. Expired admixtures may cause unpredictable performance.
Limitations of Concrete Admixtures
Certain intrinsic properties of admixtures limit their frequent usage in the production of concrete elements, such as:
- Increased cost
- Increased probability of cracks
- Uncalculated quantities of admixtures can result in decreased strength
- Use of poor quality admixtures results in structural defects
Therefore, the inclusion of admixtures in a concrete mix requires careful consideration and study of the properties of each element, along with the environmental conditions of the site. Even slight changes in the quantity of admixtures used can have drastic effects on a structure. Thus, it is advisable to consult experts such as structural engineers to determine the type, quantity and proportion of concrete admixtures to be used. Contact Brick & Bolt to ensure the usage of the right admixtures to build your dream home today!
Frequently Asked Questions (FAQs)
- What is concrete admixture?
An admixture is an element added to concrete to enhance specific properties such as workability, increase or decrease in setting time, durability, etc.
- Which admixture is best for concrete?
The best admixture for concrete can be selected based on the specific properties required: quick-setting/ slow-setting, water reducing, enhanced workability, etc.
- How many admixtures are used in concrete?
For normal construction, concrete admixtures are typically not used. When specific enhanced properties are required, a maximum of 1-2 admixtures are generally included in the concrete mix.
- When to add admixture in concrete?
Admixtures are generally added to concrete during the mixing process. This is done when the normal properties of concrete need to be enhanced.
- Can admixtures reduce concrete cracking?
Yes, there are specific concrete admixtures available today to reduce cracking once concrete sets.
Apoorva H P Achar