Short Answer:

Admixtures are special materials added to concrete during mixing to improve its properties or behavior. They are used in small amounts and help make concrete more workable, durable, stronger, or quicker/slower to set, depending on the need.

The function of admixtures in concrete is to modify specific characteristics such as setting time, strength, workability, water resistance, and durability. Common types of admixtures include plasticizers, retarders, accelerators, and air-entraining agents, each serving a unique purpose in enhancing concrete performance.

Detailed Explanation:

Function of Admixtures in Concrete

Concrete is a mixture of cement, water, sand, and aggregates. While this basic composition works well for general use, there are times when certain conditions require concrete to behave differently. For example, hot weather may cause concrete to set too fast, or high-rise construction may need concrete to be more workable without adding more water. In such situations, admixtures are used. These are materials added to the concrete mix in small quantities—usually less than 5%—to improve or modify its properties for specific requirements.

Main Functions of Admixtures in Concrete:

1. Improve Workability:
   One of the common reasons to use admixtures is to make the concrete easier to mix, place, and finish. This is especially helpful when the concrete needs to be pumped over long distances or placed in narrow or complex forms. Plasticizers and superplasticizers are admixtures that make concrete more fluid without adding extra water. This ensures the concrete maintains strength while becoming easier to handle.
2. Control Setting Time:
   In some cases, concrete may need to set faster or slower. For example, in cold weather, concrete may take too long to harden. Accelerators are admixtures that help concrete set and gain strength faster. In contrast, during hot weather or large pours, the setting time may need to be delayed to avoid cold joints or early hardening. Retarders slow down the setting process, giving more time for placing and finishing the concrete.
3. Increase Strength and Durability:
   Certain admixtures improve the long-term strength and resistance of concrete to environmental factors like water, chemicals, and temperature changes. Silica fume and fly ash are examples of mineral admixtures that react with cement to make the concrete denser and stronger over time. These admixtures also help fill tiny pores in concrete, reducing water absorption and increasing durability.
4. Reduce Water Content:
   Water-reducing admixtures allow the use of less water while keeping the concrete workable. This results in higher strength because lower water-cement ratios usually lead to stronger concrete. These admixtures are often used in high-strength concrete applications like bridges and tall buildings.
5. Enhance Resistance to Freezing and Thawing:
   In cold regions, water inside the concrete can freeze and expand, causing cracks. Air-entraining admixtures create tiny air bubbles within the concrete, which give the freezing water space to expand without damaging the structure. This makes the concrete more durable in freeze-thaw conditions.
6. Control Shrinkage and Cracking:
   Some admixtures help reduce shrinkage as the concrete dries and hardens. These are called shrinkage-reducing admixtures. They are particularly useful in slabs and large concrete surfaces where cracking due to shrinkage is a common issue.
7. Improve Bond and Surface Finish:
   Admixtures can also be used to improve the bond between old and new concrete or to give a smoother surface finish. These admixtures help make repairs and overlays stronger and longer-lasting.
8. Waterproofing and Chemical Resistance:
   Waterproofing admixtures make concrete less permeable, which is useful in basements, water tanks, and structures exposed to constant moisture. Other chemical admixtures improve the concrete’s resistance to aggressive environments like acids, salts, and sulfates.

Common Types of Admixtures:

• Plasticizers / Water Reducers
• Superplasticizers
• Retarders
• Accelerators
• Air-entraining agents
• Mineral admixtures (fly ash, silica fume, slag)

Conclusion:

Admixtures in concrete serve a wide range of functions that enhance the performance, durability, and workability of concrete. Whether the goal is to improve setting time, reduce water content, increase strength, or enhance durability against environmental conditions, admixtures provide engineers with the tools to meet specific construction demands. By carefully selecting and using the right admixture, concrete can be tailored for various applications while maintaining quality and strength.