1. Introduction
In modern construction, concrete is no longer just a mixture of cement, aggregates, and water. As technology advances, concrete has evolved into a multifunctional material with customizable performance to suit various needs. One of the key elements in this evolution is the use of admixtures.
Admixtures are materials (other than water, cement, and aggregates) added to the concrete mixture either before or during the mixing process, to alter the properties of the fresh or hardened concrete. In other words, admixtures are used to improve concrete performance for specific purposes, such as enhancing workability, accelerating or slowing down setting time, improving durability, and reducing permeability.
The main goal of using admixtures is to optimize the quality of concrete, ensuring it meets the technical, aesthetic, and economic demands of various projects. For example, in hot climates, admixtures are used to delay the setting time so that the concrete does not harden too quickly. Conversely, in cold weather concreting, admixtures are used to accelerate setting to help the concrete reach initial strength more rapidly.
It is important to distinguish between admixtures and the main ingredients of concrete. The main ingredients are the fundamental components of concrete—cement, fine aggregates, coarse aggregates, and water. Admixtures, on the other hand, are supplementary materials added in relatively small amounts but having a significant impact on concrete properties.
The use of admixtures is regulated by national and international standards to ensure quality and safety. In Indonesia, the use of admixtures is governed by SNI 03-2847 and other related standards. Internationally, ASTM C494 is widely referenced for chemical admixtures, along with guidelines from the ACI (American Concrete Institute), particularly document ACI 212.3R on admixtures in concrete.
2. Classification of Admixtures
Admixtures in concrete vary widely in terms of function and method of use. Therefore, it is essential to understand how admixtures are classified to ensure their application meets technical needs in the field. Generally, admixtures are classified in two main approaches: based on their function and based on the time of addition.
2.1. Based on Function
Admixtures can be divided into two major groups based on their function:
- Chemical Admixtures: This group includes chemical substances added to the concrete mix to alter the properties of fresh or hardened concrete through chemical reactions. They are commonly used in projects with specific requirements, such as accelerating or slowing down setting time, improving workability, or increasing resistance to extreme environmental conditions.
- Mineral Admixtures: This group consists of mineral-based materials, typically in the form of fine powders, added to the concrete mix. The primary function of mineral admixtures is to improve strength, durability, and other properties through pozzolanic or hydraulic reactions. Additionally, using mineral admixtures can help reduce cement consumption, making it more economical and environmentally friendly.
2.2. Based on Time of Addition
Admixtures can also be classified based on when they are added to the concrete mix:
- Added During Mixing: Most admixtures, both chemical and mineral, are mixed with other main ingredients during the mixing process at a batching plant or at the construction site. This method ensures a more even distribution of the admixture in the mix.
- Added After Mixing: Some admixtures, especially those that adjust the properties of fresh concrete, are added after the initial mixing process is complete. For example, superplasticizers are added to the truck mixer before pouring to improve workability without adding more water. This method is often used to address changes in field conditions during pouring.
Now, let’s continue by discussing Chemical Admixtures in the next section.