Aggregates

Classification of Aggregates According to Size and Shape

Aggregates are granular materials such as sand, gravel, and crushed stone used in concrete. They are classified according to their size and shape, which influence the workability, strength, and durability of concrete.

1. Classification According to Size

a) Fine Aggregate:
Fine aggregate consists of smaller particles that generally pass through the 4.75 mm IS sieve. Natural sand, manufactured sand, and crushed stone sand are common examples. Fine aggregate fills the voids between coarse aggregate particles.

b) Coarse Aggregate:
Coarse aggregate consists of particles that are retained on the 4.75 mm IS sieve. Examples include gravel and crushed stone. It forms the major portion of concrete and provides strength and stability.

c) All-in Aggregate:
All-in aggregate contains a suitable combination of fine and coarse aggregate in one material.

2. Classification According to Shape

a) Rounded Aggregate: Smooth and rounded particles, commonly obtained from river deposits.

b) Irregular Aggregate: Particles having irregular surfaces and shapes.

c) Angular Aggregate: Sharp-edged particles, generally produced by crushing rocks.

d) Flaky Aggregate: Particles whose thickness is small compared with their length and width.

e) Elongated Aggregate: Particles whose length is considerably greater than their other dimensions.

f) Flaky and Elongated Aggregate: Particles that are both thin and elongated.

Characteristics of Aggregates

Aggregates are granular materials such as sand, gravel, crushed stone and slag used in concrete, mortar, road construction and other civil engineering works. Their characteristics strongly influence the strength, durability, workability and economy of concrete.

1. Particle Size and Shape

· Particle size determines the grading and packing of aggregates.

· Properly graded aggregates contain different sizes that fit closely together, reducing voids and cement requirement.

· Rounded particles provide better workability but generally have less interlocking.

· Angular and cubical particles provide better interlocking and strength but may require more water.

· Flaky and elongated particles are generally undesirable.

2. Surface Texture

The surface may be smooth, rough, porous or glassy.

· Smooth aggregates improve workability and require less paste.

· Rough-textured aggregates provide better bond with cement paste and improve mechanical strength.

· Excessively porous aggregates can absorb more water.

3. Specific Gravity

Specific gravity is the ratio of the mass of a given volume of aggregate

to the mass of an equal volume of water.

It is useful for mix design, calculating quantities and assessing

aggregate quality. Normal-weight aggregates generally have a

specific gravity around 2.6–2.7.

4. Bulk Density

Bulk density is the mass of aggregate occupying a unit volume, including the spaces between particles.

It depends on:

· Particle size and grading

· Shape and texture

· Degree of compaction

· Moisture content

It is commonly expressed in kg/m³.

5. Water Absorption

Aggregates contain pores that can absorb water. Water absorption is usually expressed as a percentage of the dry mass of aggregate.

High absorption indicates a more porous aggregate and can affect:

· Concrete workability

· Effective water-cement ratio

· Durability

6. Surface Moisture

Aggregates may carry moisture on their surfaces. Depending on their moisture condition, they can either add water to or absorb water from the concrete mix

Therefore, aggregate moisture must be considered when determining the actual mixing water.

7. Bulking of Sand

Bulking is the increase in the volume of sand due to the presence of a thin film of moisture around the particles.

· It is particularly significant in fine sand.

· Bulking increases with moisture up to a certain point.

· With further addition of water, the apparent volume decreases toward the saturated condition.

This is important when measuring sand by volume at construction sites.

8. Deleterious Materials

Deleterious materials are harmful substances present in aggregates, such as:

· Clay and silt

· Organic impurities

· Coal and lignite

· Mica

· Soft particles

· Chlorides and sulphates

They can reduce strength, bond and durability and may cause cracking or other deterioration.

9. Soundness

Soundness is the ability of an aggregate to resist deterioration caused by repeated wetting and drying, freezing and thawing, or temperature changes.

A sound aggregate should remain stable without excessive:

· Cracking

· Disintegration

· Expansion or contraction

In short: A good aggregate should have suitable size and shape, proper grading, adequate specific gravity and density, low harmful absorption, limited deleterious materials, and good soundness. These properties ensure durable and strong concrete.

Grading of Aggregates

1. Introduction

Aggregates are granular materials such as sand, gravel, crushed stone and stone chips that form the major portion of concrete and many other construction materials. The particles of an aggregate are not all of the same size. The distribution of different particle sizes in an aggregate is known as grading of aggregates.

Proper grading is important because it affects the workability, strength, durability, permeability, density and economy of concrete. A well-graded aggregate contains particles of different sizes that fit together efficiently, reducing the voids between particles and consequently reducing the quantity of cement paste required.

2. Objectives of Grading

The main objectives of aggregate grading are:

1. To obtain a dense and compact arrangement of aggregate particles.

2. To reduce the amount of voids between particles.

3. To reduce the quantity of cement paste required.

4. To improve the workability of concrete.

5. To improve the strength and durability of concrete.

6. To achieve an economical concrete mix.

7. To control segregation and bleeding.

3. Types of Aggregates According to Size

Aggregates are generally classified into:

A. Coarse Aggregate

Coarse aggregate consists of particles that are retained on the 4.75 mm IS sieve.

Examples include:

· Gravel

· Crushed stone

· Stone chips

· Broken brick aggregate

Common nominal sizes include 10 mm, 20 mm and 40 mm.

Coarse aggregate provides the main skeletal framework of concrete and contributes significantly to its strength and stability.

Characteristics of Good Coarse Aggregate

A good coarse aggregate should:

· Have suitable particle size and grading.

· Be hard and strong.

· Be durable and sound.

· Be free from harmful materials.

· Have suitable shape and surface texture.

· Have good bonding characteristics with cement paste.

4. Fine Aggregate

Fine aggregate consists of particles that pass through the 4.75 mm IS sieve and are generally retained on the 75-micron sieve.

The most common fine aggregate is natural sand, although manufactured sand is also widely used.

Fine aggregate fills the spaces between coarse aggregate particles and contributes to the workability and cohesiveness of concrete.

Characteristics of Good Fine Aggregate

Good fine aggregate should:

· Have suitable grading.

· Be clean and free from excessive silt and clay.

· Be hard and durable.

· Contain minimum harmful organic matter.

· Have appropriate fineness modulus.

· Produce a workable and cohesive concrete mix.

5. All-in-One Aggregate

All-in-one aggregate, also called combined aggregate, contains both coarse and fine aggregate particles in appropriate proportions.

Thus, it consists of a mixture of:

· Fine particles, and

· Coarse particles.

The purpose is to obtain a suitable overall grading that provides good packing and reduces voids.

Advantages

· Reduces segregation when properly proportioned.

· Can provide good particle packing.

· May reduce the requirement for separate proportioning of fine and coarse aggregates.

· Can be useful in certain concrete and construction applications.

However, the grading must be properly controlled to obtain the required concrete properties.

6. Fineness Modulus

The Fineness Modulus (FM) is an empirical numerical index that indicates the relative fineness or coarseness of an aggregate.

It is calculated from the cumulative percentage retained on a specified set of standard sieves.

Formula

4.75 mm, 2.36 mm, 1.18 mm, 600 μm, 300 μm and 150 μm.

Interpretation

· Higher FM → coarser aggregate

· Lower FM → finer aggregate

Fineness modulus is useful for:

· Comparing aggregates.

· Controlling uniformity.

· Adjusting concrete mix proportions.

· Selecting suitable sand for concrete.

Important: Fineness modulus does not describe the complete grading of an aggregate. Two aggregates can have the same FM but different particle-size distributions.

7. Grading Curve

The grading of an aggregate is commonly represented using a grading curve obtained from sieve analysis.

In a typical grading chart:

Horizontal axis: Particle size or sieve size, generally plotted on a logarithmic scale.

· Vertical axis: Percentage passing or percentage retained.

The curve shows how the aggregate particles are distributed according to size.

Typical Interpretation

8. Interpretation of Grading Chart

A. Well-Graded Aggregate

A well-graded aggregate contains a wide range of particle sizes.

Its grading curve is relatively smooth and extends over a broad range of sizes.

Advantages:

· Better particle packing.

· Fewer voids.

· Lower cement-paste requirement.

· Better workability.

· Higher density.

· Generally improved economy and durability.

B. Uniformly Graded Aggregate

A uniformly graded aggregate contains particles that are approximately of the same size.

Its grading curve tends to be steep.

Characteristics:

· Larger void content.

· Requires more paste to fill voids.

· May be useful for specific applications where uniform particle size is desired.

C. Gap-Graded Aggregate

In a gap-graded aggregate, one or more intermediate particle sizes are missing or present in very small quantities.

The grading curve shows a relatively flat or limited section corresponding to the missing sizes.

Such grading must be carefully controlled because it can affect workability, segregation and packing.

9. Importance of Proper Grading

Proper aggregate grading is essential for producing quality concrete.

10. Conclusion

Grading of aggregates is an important aspect of concrete technology and construction materials engineering. It describes the distribution of aggregate particles according to their size. Aggregates may be classified as coarse, fine and all-in-one aggregates, depending on their particle-size distribution.

The fineness modulus provides a convenient numerical indication of the relative fineness or coarseness of an aggregate, while the grading curve provides a graphical representation of particle-size distribution.

A properly graded aggregate provides better packing, reduces voids and paste requirements, improves workability and contributes to the strength, durability and economy of concrete. Therefore, sieve analysis and proper control of aggregate grading are essential in the design and production of good-quality concrete.