Aggregates can be defined as crushed stones used in concrete. Aggregate in concrete occupied about 70 – 80 % of the total volume which provides the main body of concrete. Aggregate makes concrete strong and durable when mixed with cement, sand, and water.
Aggregates are one of the essential constituents of concrete. Aggregate provides the body to the concrete, reduces shrinkage, and affects the economy. Aggregate makes concrete strong and durable.
Generally, aggregate in concrete occupies around 70-80 percent of the volume of concrete, their impact on various characteristics and properties of concrete is, undoubtedly considerable.
In concrete, the only standard factory-made material is cement. Other materials like aggregate, and cement is natural organic materials and their properties can vary to some extent. An in-depth study of aggregate is essentially required to study the properties of concrete.
Types of Aggregates
Different types of aggregates are classified into the following groups:
- Types of aggregates according to a source.
- Types of aggregates according to the unit weight.
- Types of aggregates according to size.
A. Types of Aggregates According to Source
1. Natural Aggregates
Natural aggregates are generally used in a natural state having rock fragments or are used after mechanical processing such as crushing, washing, and sizing.
Some natural aggregate deposits known as pit-run gravel consist of gravel and sand that can be easily used in concrete after minimal processing.
Natural sand and gravel are generally dug or dredged from a pit, river, lake, or seabed
2. Crushed Rock Aggregates
Crushed rock aggregate is generally excavated or quarried stone that has been crushed and screened to the desired standard particle size and distribution.
Crushed aggregate particles are completely crushed. This gives the aggregate good compaction and load-bearing properties.
These aggregates are generally suitable to use in the courses of streets, roads, and other areas exposed to traffic.
3. Artificial Aggregates
These aggregates are made from different waste materials.
Artificial aggregate is sometimes manufactured for severing special work: –
- To make lightweight concrete: burned clays, artificial cinders, foamed slag, expanded shales, and slate, sintered fly ash exfoliated vermiculite is used.
- To make high-density concrete: steel rivet punching and iron ore (Magnetite) have been used.
4. Recycled Aggregates
Recycled aggregates are manufactured from crushing inert construction and demolition waste. It is generally known as recycled concrete aggregate when it is mainly made from crushed concrete and recycled aggregate when it contains substantial quantities of materials other than crushed concrete.
Currently, only recycled aggregate made from construction or demolition waste is recommended for use in new concrete construction.
The Recycled aggregates properties may be different accordingly types of concrete because the parent concrete was designed for its purposes such as permeable, durable, and high-strength concrete
B. Types of Aggregates According to Unit Weight
The aggregates are also classified per different unit weights, shown in Table 1. In this, aggregates are classified on the basis of bulk specific gravity as lightweight, normal-weight, and heavy-weight aggregates.
| Category | Bulk Specific Gravity | Examples | Typical Applications |
| Ultra-lightweight | Vermiculite, Ceramic | Can be sawed or nailed, Also used for its insulating properties | |
| Lightweight | Gs < 2.4 | Expanded perlite, shale, or slate Burned clay Crushed brick | Structural lightweight concrete 1350 to 1850 kg/m3 Masonry units Also used for its insulating properties |
| Normal weight | 2.4 < Gs < 2.8 | Crushed limestone, Sand, River gravel, Crushed recycled concrete | Used for normal concrete projects Produce normal-weight concrete 2200 to 2400 kg/m3 |
| Heavyweight | Gs > 2.8 | Steel or iron shot Steel or iron pellets | Produce high-density concrete up to 6400 kg/m3 • Radiation shielding • counterweights • additional applications where high the mass-to-volume ratio is desired |
C. Types of Aggregates According to Size
The biggest particle size may have a diameter of around 150 mm, and the smaller aggregate has a diameter of up to 5 to 10 microns. Aggregates are classified according to particle size as follows:
5. Coarse Aggregates
The aggregate particle that retains on a 4.75 mm sieve is known as Coarse aggregate.
6. Fine aggregates
Aggregate which passes through a 4.75 mm sieve is known as Fine Aggregate. Fine aggregates are generally occupied 35% to 45 % of the volume of total aggregates.
4. Classification of Aggregates Based on Shape
As we know aggregate used construction is made from naturally occurring rock by blasting or crushing which makes it difficult to gain the required shape.
However, the shape of aggregate plays an important role in the workability of concrete. Therefore the shape of aggregates is an important factor to be considered.
Types of aggregates according to shape are classified as under,
- Rounded aggregates
- Irregular or partly rounded aggregates
- Angular aggregates
- Flaky aggregates
- Elongated aggregates
- Flaky and elongated aggregates
7. Rounded Aggregate
The rounded aggregates are totally nature-made shapes and available in the form of seashore gravel. This aggregate allows minimum percentages of voids (32–33%), hence offering more workability.
They also perform well with a low water-cement ratio. These aggregates are not considered suitable for high-strength concrete due to their poor interlocking behavior and weak bond strength.
8. Irregular Aggregates
The rounded aggregates have partly nature-made shapes and these are available in the form of pit sands and gravel. These aggregates may have around 35- 37% of voids.
Irregularly shaped aggregate offers lesser workability when compared to rounded aggregates. It has bond strength slightly good compared to rounded aggregate but not as required for high-strength concrete.
9. Angular Aggregates
The angular aggregate has well-defined edges and these are obtained by crushing the rocks. These aggregates offer a maximum percentage of voids (38-45%) and hence give less workability.
Angular aggregates give a high specific area compared to rounded aggregates. With a greater specific surface, they show higher bond strength than rounded aggregates. Also, angular aggregate shows a better interlocking property in concrete that contributes to the strength of concrete.
10. Flaky Aggregates
A flaky aggregate is one whose thickness is less compared to length and width or other, when the least dimension of aggregate is less than 60% of its mean dimension then that aggregate is called flaky aggregate.
11. Elongated Aggregates:
Elongated aggregates are those whose length is larger compared to width and thickness or the length of aggregate is greater than 180% of its mean dimension.
12. Flaky and Elongated Aggregates
If the length of the aggregate is larger than its width and the width is larger than its thickness then it is said to be a flaky and elongated aggregate.
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