Properties of Aggregates for Concrete

What are the  Properties of Aggregates for Concrete?

Aggregates are an essential part of concrete. In simple words, aggregates are the sand, gravel, crushed stones, or other materials mixed with cement and water to make concrete. They help in giving strength, stability, and volume to concrete. Using good-quality aggregates ensures that the concrete lasts longer and performs well under different conditions.

In this blog, we will explore the key properties of aggregates, why they matter, and how they affect the quality of concrete. We’ll also cover types of aggregates and some frequently asked questions.

Why Are Aggregates Important in Concrete?

Concrete is a mix of cement, water, and aggregates. The aggregate makes up around 60% to 75% of the total volume of concrete. Good quality aggregates:

• Improve strength and durability
• Reduce shrinkage and cracking
• Lower the cost of concrete
• Provide better resistance to weather

Now let’s understand the key properties of aggregates that make them suitable for concrete.

Key Properties of Aggregates for Concrete

1. Composition

The chemical composition of the aggregate affects the concrete’s strength and durability. Aggregates should be chemically inert and should not contain materials that may react with cement or steel. For example, aggregates with high silica content can lead to alkali-silica reactions (ASR), causing cracking over time.

2. Size & Shape

• Size: Aggregates range from fine (sand) to coarse (gravel and stones). Typically, 20 mm is used for most concrete work.
• Shape: Angular aggregates offer better bonding with cement compared to rounded ones. Flaky or elongated aggregates reduce strength and workability.

3. Surface Texture

This refers to how smooth or rough the surface of the aggregate is. Rough-textured aggregates bond better with cement paste, which leads to stronger concrete. Smooth aggregates improve workability but may reduce strength.

4. Specific Gravity

Specific gravity is the ratio of the weight of aggregate to the weight of water with the same volume. It usually ranges from 2.4 to 2.9. Aggregates with high specific gravity give denser and stronger concrete.

5. Bulk Density

Bulk density is the weight of aggregates per unit volume, including the voids between the particles. It affects the overall volume of concrete mix and the proportions of ingredients used.

6. Voids

Voids are the spaces between aggregate particles. Excessive voids require more cement paste to fill the gaps, increasing cost and risk of shrinkage. Well-graded aggregates minimise voids.

7. Porosity & Absorption

Porosity is the volume of pores in the aggregate. High porosity means higher water absorption. Absorption should be under 2% to avoid affecting the water-cement ratio. Aggregates with high absorption can weaken concrete if water content is not adjusted.

8. Bulking of Sand

Bulking is the increase in volume of sand due to moisture. Damp sand forms a thin water film around particles, causing them to separate and swell. This is important when measuring sand by volume for mixing concrete.

9. Fineness Modulus of Aggregate

Fineness Modulus (FM) gives an idea of the average particle size of aggregates. A higher FM means coarser aggregate. For sand, FM typically ranges between 2.3 and 3.1. FM helps in deciding mix proportions.

10. Surface Index of Aggregate

This index measures the surface area of aggregates per unit volume. Higher surface index means more cement is needed to coat the particles, increasing cost but improving strength.

11. Deleterious Material

Deleterious materials include clay, silt, organic matter, and salts that may be present in aggregates. These can interfere with bonding, reduce strength, and cause expansion or corrosion. IS 383 specifies the limits for harmful substances.

12. Crushing Value of Aggregate

This value measures the resistance of aggregate to crushing under a gradually applied load. A lower crushing value indicates a stronger aggregate, suitable for use in heavy-load structures like roads and bridges. As per IS 2386, values below 30% are considered good.

13. Impact Value of Aggregate

Impact value shows the aggregate’s resistance to sudden shocks or loads. Lower values mean higher toughness. It’s especially important for road construction where vehicles apply sudden loads.

14. Abrasion Value of Aggregate

Abrasion value indicates how well the aggregate resists wear due to rubbing or friction. It’s critical for concrete used in floors, pavements, and industrial areas. The Los Angeles Abrasion Test is used to measure this.

Types of Aggregates

1. Fine Aggregate
   
   • Size: Less than 4.75 mm
   • Example: River sand, crushed sand
   • Role: Fills gaps between coarse aggregates and gives smooth finish.
2. Coarse Aggregate
   
   • Size: More than 4.75 mm
   • Example: Crushed stone, gravel
   • Role: Provides bulk, strength, and structure to concrete.
3. Natural Aggregates
   
   • Found in rivers, lakes, or quarries.
   • Less processing required.
4. Artificial Aggregates
   
   • Made from industrial waste like fly ash, blast furnace slag.
   • Used in special construction work.

How to Choose the Right Aggregate?

• Ensure aggregates are clean and free from impurities.
• Check crushing value, impact value, and water absorption in lab tests.
• Select aggregates based on project needs – for example, use harder aggregates for roads.
• Follow relevant IS Codes like IS 383:2016 for testing and quality standards.

IS Codes Related to Aggregates

Below are the key IS codes used for testing and specifying aggregates in concrete:

Conclusion🎯

Aggregates are more than just fillers – they play a major role in the strength, durability, and performance of concrete. Choosing the right aggregate with the correct properties can make a big difference in the quality of your construction. Always ensure that the aggregates are tested, clean, and well-graded before use.

FAQs

Q1. What is the ideal size of aggregate for concrete?
A: For general concrete work, coarse aggregates of size 20 mm are commonly used. Fine aggregate (sand) should pass through a 4.75 mm sieve.

Q2. Can we use sea sand as aggregate?
A: Sea sand contains salts which can cause corrosion in steel reinforcement. It should be avoided unless properly treated.

Q3. What happens if the aggregate is too smooth?
A: Smooth aggregates reduce the bond with cement paste, resulting in weaker concrete.

Q4. Are recycled aggregates safe to use?
A: Yes, recycled aggregates can be used if properly processed and tested. They are eco-friendly and cost-effective.

Q5. Why is shape of aggregate important?
A: Angular shapes give better bonding and strength. Rounded ones may improve workability but reduce strength.

Q6. Can we mix different types of aggregates?
A: Yes, mixing different sizes and types helps improve compaction and reduces voids in concrete.