Classification of Construction Aggregates

What is an Aggregate?

Aggregates are the essential constituents of the concrete which give the body to the concrete and also reduce shrinkage. Aggregates occupy 70 to 80 % of the total volume of concrete. So, we can say that one should know definitely about the aggregates in-depth to study more about concrete. There are many classifications of aggregates when it comes to its use in the construction industry. Classification of aggregates based on various factors such as size, shape, origin, density etc. are discussed here. 

Classification of Aggregates Based on Shape

We know that aggregate is derived from naturally occurring rocks by blasting or crushing etc., so, it is challenging to attain the required shape of aggregate. But, the form of the aggregate will affect the workability of concrete. So, we should take care of the shape of the aggregate. This care is not only applicable to parent rock but also the crushing machine used.

Aggregates are classified according to shape into the following types.

• Rounded aggregates
• Irregular or partly rounded aggregates
• Angular aggregates
• Flaky aggregates
• Elongated aggregates
• Flaky and elongated aggregates

Rounded Aggregate

The rounded aggregates are entirely shaped by attrition and available in the form of seashore gravel. Rounded aggregates result in the minimum percentage of voids (32 – 33%) hence gives more workability. They require a lesser amount of water-cement ratio. They are not considered for high strength concrete because of poor interlocking behavior and weak bond strength.

Irregular Aggregates

The irregular or partly rounded aggregates are somewhat shaped by attrition, and these are available in the form of pit sands and gravel. Irregular aggregates may result in 35- 37% of voids. These will give lesser workability when compared to rounded aggregates. The bond strength is slightly higher than rounded aggregates but not as required for high strength concrete.

Angular Aggregates

The angular aggregates consist of well-defined edges formed at the intersection of roughly planar surfaces, and these are obtained by crushing the rocks. Angular aggregates result in the maximum percentage of voids (38-45%) hence gives less workability. They offer 10-20% more compressive strength due to the development of a stronger aggregate-mortar bond. So, these are useful in high strength concrete manufacturing.

Flaky Aggregates

When the aggregate thickness is small when compared with the width and length of that aggregate it is said to be flaky aggregate. Or in the other, when the least dimension of aggregate is less than the 60% of its mean size, then it is said to be flaky aggregate.

Elongated Aggregates

When the length of aggregate is larger than the other two dimensions then it is called elongated aggregate or the length of aggregate is greater than 180% of its mean dimension.

Flaky and Elongated Aggregates

When the aggregate length is larger than its width and width is larger than its thickness then it is said to be flaky and elongated aggregates. The above three types of aggregates are not suitable for concrete mixing. These are generally obtained from the poorly crushed rocks.

Classification of Aggregates Based on Size

Aggregates are available in nature in different sizes. The size of aggregate used may be related to the mix proportions, type of work, etc. the size distribution of aggregates is called grading of aggregates.

Following are the classification of aggregates based on size:

Aggregates are classified into two types according to size

  1. Fine aggregate

  2. Coarse aggregate

Fine Aggregate

When the aggregate is sieved through 4.75mm sieve, the aggregate passed through it called as fine aggregate. Natural sand is generally used as fine aggregate, silt and clay also come under this category. The soft deposit consisting of sand, silt, and clay is termed as loam. The purpose of the fine aggregate is to fill the voids in the coarse aggregate and to act as a workability agent.

Coarse Aggregate

When the aggregate is sieved through 4.75mm sieve, the aggregate retained is called coarse aggregate. Gravel, cobble and boulders come under this category. The maximum size aggregate used may be dependent upon some conditions. In general, 40mm size aggregate used for normal strengths and 20mm size is used for high strength concrete. The size range of various coarse aggregates given below

Classification of Aggregates Based on the origin

There are three types based on Origin.

(i) Natural:

These include all those types of fine and coarse aggregates, that are available in almost ready to use the form, from natural resources.

Examples are sands from river beds, pits and beaches,  and gravels from river banks.

(ii) By-product:

These include materials obtained as wastes from some industrial and metallurgical engineering operations, which possess suitable properties for being used as aggregate.

Examples: Cinder obtained from burning of coal in locomotives and kilns and Slag is obtained from blast furnaces as Scum is the best example from this category.

(iii) Processed:

These form a special class in Aggregate. They are specifically manufactured for use in making Quality concrete.

Examples: They include burnt clay, Shales, vermiculite and perlite. They are essential Ingredients of Lightweight Concrete.

Classification of Aggregates Based on Density

Three types of aggregates are distinguished based on their weight per unit volume.

(i) Standard or Normal:

These types of aggregates give strength and weighting to the Concrete of around  2300 to 2500 kg/m3.

Gravels, Sand and Crushed stone, are all classed as Standard or Normal Aggregates.

(ii) High-Density Aggregates:

These are that type of Aggregates, which is used in standard proportions yield in heavyweight concretes. Such concretes are especially useful as shields against X-rays and radiations in the atomic power plant.

Examples: Barile – a natural mineral with a specific gravity of 4.3 is an example. Concretes with such aggregate usually weight above 4000 kg/m3.

(iii) Lightweight Aggregate:

 They consist of natural and artificial materials of very low density so that the resulting concrete is also quite light in weight, generally within a range of 350 to 750 kg/m3. They are specially used in soundproofing and fireproofing constructions. They are also used extensively in the manufacture of lightweight precast concrete blocks.

How does it work in concrete?

An aggregate is an essential part of making concrete and is used for many different reasons. The benefits include; reduction of cracking, improve the structure, and help with water drainage. If the size of the aggregates used is small, there is a requirement for more binding material which increases the cost. Conversely, more aggregate use will result in a wastage of the binding material. As a result, most concrete features a mixture of coarse and fine aggregate to eliminate the problems mentioned above. This also creates a premium product that is competitive on price. For a good concrete mix, aggregates need to be clean, hard, strong particles free of absorbed chemicals or coatings of clay and other excellent materials that could cause the deterioration of concrete. Aggregates, which account for 60 to 75 percent of the total volume of concrete, are divided into two distinct categories–fine and coarse. Fine aggregates generally consist of natural sand or crushed stone with most particles passing through a 3/8-inch sieve. Coarse aggregates are any particles greater than 0.19 inches but typically range between 3/8 and 1.5 inches in diameter. Gravels constitute the majority of coarse aggregate used in concrete with crushed stone making up most of the remainder.

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Modern additives modify precisely that timespan; however, the amount of additive added to the mix is very important. Virtue Industries manufactures ready-mix concrete under high-end expert supervision to ensure high-quality end products.