All About Coarse Aggregate: Types & Properties

The aggregates are generally acquired by blasting in stone quarries or breaking them by hand or by crusher machines. The aggregates manufactured by machine-crushed consist of stones of various sizes whereas hand-broken aggregates consist of only single-size stones.

Materials that are larger to be retained on a 4.75 mm sieve size are called coarse aggregate, and their maximum size can be up to 63 mm.

In coarse aggregate, foreign materials like coal, lignite, soft fragments, and clay lumps should not exceed 5 percent of their actual weight.

What Is Coarse Aggregate?

Coarse aggregate is a term used in construction to describe a type of material that is commonly used in the production of concrete. It is typically made up of various materials such as gravel, crushed stone, and recycled concrete. The size of the coarse aggregate can vary, but it is typically between 3/8 inch and 1.5 inches in diameter.

Coarse aggregate is an essential component of concrete that provides strength and durability to the structure.

It works in combination with cement and fine aggregate to create a strong and durable concrete mixture that can withstand various environmental and physical stresses.

Concrete Mix is produced by many ingredients or components but is mostly made up of a material called Coarse Aggregates and which are one of the essential components of concrete and occupy large volumes in the concrete mix. Aggregate which has a size bigger than 4.75 mm or is retrained on a 4.75 mm IS Sieve is known as Coarse aggregate.

Read More: Flakiness and Elongation Index Test of Coarse Aggregate

Properties of Coarse Aggregates

There are some properties of characteristics of coarse aggregates which influence the properties of the resulting concrete mix.

The properties of the coarse aggregates are as follows,

1. Composition
2. Size and Shape
3. Surface Texture
4. Specific Gravity
5. Bulk Density
6. Voids
7. Porosity and Absorption
8. Bulking of Sand
9. Fineness Modulus of Aggregate
10. Surface Index of Aggregate
11. Deleterious Materials

1. Composition

Aggregate consisting of such materials that can chemically react with alkalis in cement and cause excessive expansion, cracking, and deterioration of concrete mix should never be used.

Therefore it is necessary to test aggregates to ensure whether there is the presence of any such constituents in aggregate or not.

2. Size and Shape

The size and shape of the aggregate particles greatly influence the quantity of cement required in concrete mix and hence ultimately the economy of concrete. IS: 456 recommended the below choose the maximum size of coarse aggregate to be used in the PCC and RCC mix.

The maximum size of coarse aggregate in concrete making should be less than,

• 1/4th of the minimum dimension of the RCC member.
• 1/5th of the minimum dimension of the RCC member.

3. Surface Texture

The development of hard bond strength between coarse aggregate and cement paste depends upon the surface roughness, surface texture, and porosity of coarse aggregate.

In case the surface is porous, the maximum bond strength will develop in concrete. In porous surface aggregates, the bond strength of aggregate increase as cement paste start setting.

4. Specific gravity

The ratio of the weight of oven-dried aggregate which is kept for 24 hours at a temperature of 100 to 100°C, to the weight of an equal volume of water displaced by saturated dry surface aggregate is called the specific gravity of aggregates.

Specific gravity is mainly of two types:

• Apparent specific gravity
• Bulk specific gravity

The specific gravity of major aggregates falls within the range of 2.6 to 2.9.

5. Bulk Density

The bulk density of aggregate can be defined as the weight of coarse aggregate required to fill the unit volume of the container. It is generally expressed in kg/liter.

The bulk density of aggregates particles depends upon the following 3 factors which are:

• Degree of compaction
• Grading of aggregates
• The shape of aggregate particles

6. Voids

The empty spaces left between coarse aggregate particles are known as voids. The volume of voids equals the difference between the total volume of the aggregate mass & the volume occupied by the particles alone.

7. Porosity and Absorption

The holes produced in the rocks at the time of the solidification of the molten magma, due to air bubbles, are known as pores.

Water absorption may be defined as the difference between the weight of very dry aggregates and the weight of the saturated aggregates with the surface dry condition.

8. Bulking of Sand

It can be defined as an increase in the bulk volume of the quantity of sand in a moist condition over the volume of the same quantity of dry or completely saturated sand.

The proportion/ratio of the volume of moist sand due to the volume of sand when dry is called a bulking factor.

9. Fineness Modulus

The Fineness modulus is commonly utilized to get an idea of how coarse or fine the aggregate is. More fineness modulus value indicates that the aggregate is a coarse sand small value of fineness modulus indicated that the aggregate is finer.

The Fineness modulus is a factor obtained by adding the cumulative percentages of aggregate retained on each of the sieves ranging from 80 mm to 150 mm microns and dividing this by 100.

10. Specific Surface Area of Aggregate

The total surface area of aggregate particles per unit weight of the material is called the specific surface. This is an indirect measure of aggregate grading. The specific surface rises with the reduction in the size of aggregate particles.

The specific surface area of the fine aggregate is very much lesser than that of the coarse aggregate.

11. Deleterious Materials

Aggregate should not contain any harmful material in such a quantity to affect the durability and strength of the concrete.

Deleterious materials might cause one of the following effects on cement or concrete mix:

• To interfere with the hydration of cement
• To prevent the development of proper bond
• To reduce strength and durability
• To modify setting times

Types Of Coarse Aggregates

There are various types of course aggregates considering different parameters like shape, origin, texture, density, and grain size, etc.

A) Types Of Aggregates Based On Shape

The shape of aggregates is one of the most effective ways of differentiating particles. The shape of your chosen aggregates will have a significant effect on the workability of the concrete. The different shapes of aggregates are:

1. Rounded Aggregates

The rounded aggregates are mostly available in the seashore or riverbank area. It offers fewer voids and high workability to the concrete mix, but this is not recommended to use for high-strength concrete elements because of its poor interlocking characteristics.

Primarily it is used in PCC and lintel concrete and also as a filler material.

2. Angular Aggregates

The angular aggregates offer low workability. But it is used for high-strength concrete members because of its angularity and it provides high strength. Due to this aggregate having angularity, it properly interconnects with other aggregates and offers fewer voids in concrete.

3. Irregular Aggregates

The irregular aggregates offer lower compressive strength compared to the angular aggregates. The irregular shapes of these aggregates are formed because of friction between the aggregates.

This aggregate offers lower bonding strength due to its irregular shape and also it offers lower workability for concrete.

4. Elongated Aggregates

Elongates aggregate, generally has its length more than its width. It offers low compressive strength and is not recommended to use in concrete.

In case they are used in concrete, the void content and ratio in concrete become too high compared to the other aggregates.

5. Flaky Aggregates

Flaky aggregates have a width more than their length and they are too flat in shape as they can easily crack.

This aggregate offers too low workability and it used not recommended to be used in concrete making also it quickly gets broken due to its minor thickness.

6. Flaky & Elongated Aggregates

The flaky and elongated aggregate has more length and low thickness. It is not recommended to use in concrete making because of its lower compressive strength.

B) Types of Aggregates Based on Grain Size

Depending upon the size of the aggregate they can be classified into two categories as fine aggregates and coarse aggregates.

1. Fine Aggregates

The size of the fine aggregates is lesser than 4.75 mm. That is aggregates particles that passed through 4.75 mm. fine aggregates include things such as sand, silt, and clay. Machine-crushed stone & crushed gravel might also fall under this category. Typically, fine aggregates are used to improve the workability of a concrete mix.

2. Coarse Aggregates

The coarse aggregate size is measured above the 4.75 mm limit. These are more likely to be natural stone or gravel that has been crushed or processed.

These will reduce the amount of water needed for a concrete mix, which may also reduce workability but improve its inner strength.

C)bTypes of aggregates based on Density

The aggregates are mainly classified into three categories according to density.

1. Lightweight Aggregates

The lightweight aggregate generally has a high water absorption value compared to other types of aggregates. Thus the lower slump results in lightweight concrete instantly due to the absorption of water.

They are used as a filler material in flooring work, deck slabs, insulating the fire line pipes, etc. The density of lightweight aggregates is generally between 800 to 1100 kg/cum.

Read More: Specific Gravity and Water Absorption Test of Coarse Aggregate

2. Normal Weight Aggregates

The normal-weight aggregates like gravel and slab, are commonly used in construction. The normal-weight aggregates are commonly used for the construction of column, beam & slab concrete.

The density of normal-weight aggregates is varied between 1520 to 1680 kg/cum.

3. Heavy Weight Aggregates

The heavyweight aggregates are used for heavy construction such as to protect the radiation room. The major disadvantage of heavy-weight aggregate is it offers very low workability concrete.

The density of heavy-weight aggregates generally varies between 2100 to 2900 kg/cum.

D) Types Of Aggregates Based On Geographical Origin

Depending on the source of aggregate it can be categorized into two types, namely natural aggregates, and manufactured aggregates.

1. Natural Aggregates

Natural aggregates are generally found in seashores, riverbanks, and pit mines. Natural aggregates can be used in construction work after performing some quality tests. The river sand and gravel are an example of naturally available aggregates.

2. Manufactured Aggregates

All aggregates are developed from natural aggregates only. But the manufactured aggregates are nothing but forms of natural aggregates which is given new sizes and shape which can be used for various construction activities.

Characteristics Requirements For Good Quality Coarse Aggregate

The aggregates used in building construction must possess and fulfill the following requirement. The quality of concrete used for concrete making affect the grade of the concrete.

• Aggregate must be strong and hard enough to resist the crushing action.
• They should not have the cover of organic materials, clay, and dust otherwise it will affect the bonding strength of concrete and aggregate.
• The aggregates used for concrete must be durable.
• Coarse aggregates for concrete should be chemically inactive.
• They should not contain an excessive amount of angular, sharp, and hard particles.
• The aggregate shape should be ideally spherical or cubical.
• It must be chemically inert material.
• They should be free from any hygroscopic slat.
• Aggregates should not have water absorption of more than 5% of their actual weight.
• They should be soft and porous in nature.
• The ideal size of coarse aggregates should be such that they should pass the through IS 63 mm sieve and retains on the 4.75 mm IS sieve.
• Aggregate used for construction must be free from any disintegrated pieces, alkalis, vegetable matter, etc.

Uses Of Coarse aggregates In Construction Work

In the construction industry, aggregates are used in concrete considering their economic factor and the strength which it is provided to concrete.

Coarse aggregates are used in different ways in the construction industry. The aggregates are also used as ballast in road and railway works to resist the overall load, distribute the load properly to the soil base, and drain off the rainwater

They also play a major role in sewage treatment processes and water filtration. quickly from the road.

The coarse aggregate has a major effect on concrete properties such as abrasion resistance, hardness, elastic modulus, and another characteristic like durability, strength, and cheaper. Other applications of coarse aggregate are to backfills, fills, drainage, and filtration work.

Aggregate can also be used to create the braking moisture and vapor barriers under the slab. Coarse aggregates are primly used to prepare bases for driveways and roadways. The coarse aggregates are also utilized in water filtration and sewage treatment processes.

The uses of aggregates can be summarized into the following three categories:

• As a bearing material
• As a filling material
• As an infiltrating material

Uses Of Aggregate In Concrete

The aggregate is one of the important ingredients of concrete mix,

• Aggregate acquires 60-65% volume of concrete.
• It used to provide a rigid structure to concrete.
• Aggregate helps in reducing shrinkage and surface cracking of concrete.
• Coarse aggregate is used in concrete to build megastructures for example different components of a building, bridges, foundations, etc.
• As high the coarse aggregate size lesser the surface area and therefore it requires lesser binding material (cement), which results in low-cost construction.
• A mix of coarse and fine aggregate in concrete is used to make it denser, harder, and more durable against atmospheric action.

Uses Of Aggregates In Roads

• Aggregates are used in road construction for the base, sub-base, and surface of roads in various forms.
• Aggregates are used in the soil stabilization process using cementitious materials (blends of cement, fly ash, slag, and lime).
• They are also used in the stabilization process with bituminous materials (bitumen or tar).
• Stabilized with other materials (resins, fibers, geo-synthetics).
• Recycled aggregate.
• On roads, aggregate help in the equal distribution of the load and direct rainwater running off from the road surface.

Grading of Coarse Aggregate

The coarse aggregate used in making concrete contains aggregates of different sizes. This particle size distribution of the aggregate is termed “Gradation”. The sieve analysis was conducted to determine this particle size distribution.

The grading pattern is evaluated by sieving a sample successively through all the sieves mounted one over the other in order of size, with a larger sieve on the top.

The material preserved on each IS sieve after shaking represents the fraction of aggregate coarser than the sieve and finer than the sieve above. Complete gradation of coarse aggregates is one of the most important factors in producing a workable concrete mix.

Correct gradation makes sure that a sample consists of all standard fractions of aggregate in the required proportion such that the sample contains minimum voids. A sample of the well-graded aggregate consisting of lesser voids will require minimum paste to fill up the voids in the aggregates.

There are three range categories of aggregate grading:

1. Well graded

Well-graded aggregates have a gradation of particle size that fairly evenly spans the size from the finest to the coarsest. A core of well-graded aggregate concrete shoes a packed field of many different particle sizes.

2. Poor Graded

This aggregate is characterized by a small variation in size. It includes aggregate particles that are near the same size. This means that the particles of the aggregate pack together, leaving relatively large voids in the concrete.

3. Gap Graded

Gap-graded aggregate consists of aggregate particles in which some intermediate-size particles are missing. A core of gap-graded concrete shows a field of small-seized aggregate interspersed with slightly large aggregate pieces embedded in a small-sized aggregate.

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