Aggregates are one of the important and essential constitutes of concrete. There are different Types of Aggregates used in making 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. Before studying the properties of concrete one should know about the aggregates which constitute major volume in concrete. It is a fact that without studying properties of aggregate the study of concrete is incomplete.
In concrete, the only standard factory-made material that is cement. Another material like aggregate, 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.
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What is Aggregate?
Aggregate can be defined as the crushed stone which used in concrete. Aggregate in concrete occupied about 70 – 80 % of the total volume which provides the main body to concrete. Aggregate makes concrete strong and durable when mixed with cement, sand, and water.
Classification or Types of Aggregates:
Different types of aggregates are classified into the following groups:
1. Types of aggregates according to source.
2. Types of aggregatse according to unit weight.
3. Types of aggregates according to size.
1. Types of Aggregates According to Source:
1. Natural Aggregates:
- Natural aggregates are generally used in 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 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 types of aggregates generally suitable to use in the courses of streets, roads and other areas exposed to traffic.
3. Artificial Aggregates:
- These types of aggregates are made from different waste materials.
- Artificial aggregate are sometimes manufactured for severing special work: –
- To make lightweight concrete: burned clays, artificial cinders, foamed slag, expanded shale’s 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:
These types of recycled aggregate are manufactured from crushing inert construction and demolition waste. It is generally known as recycled concrete aggregate when it 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
Aggregate recycling is a simple process there is no complex method involved. The recycling method includes breaking, removing, and crushing existing concrete into a material with a specified size and quality. Reinforcement and other mixed items in concrete must be removed from aggregate.
Serve precaution must be taken to restrict other material contamination with aggregate that can be troublesome, such as asphalt, soil and clay balls, chlorides, glass, gypsum board, sealants, paper, plaster, wood, and roofing materials.
In general, following are some application of recycled aggregate without processing:
- Many types of general bulk fills
- Bank protection
- Base or fill for drainage structures
- Road construction
- Noise barriers and embankments
Recycled aggregates with new concrete include:
- Pavements, shoulders, median barriers, sidewalks, curbs and gutters
- Bridge foundations
- Concrete bases
- Bituminous concrete.
2. Types of Aggregates According to Unit Weight:
The aggregates are also classified as per different unit weights, shown in Table 1. In this, aggregates are classified on the basis of bulk specific gravity is 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
• additional applications wherea high
mass-to-volume ratio is desired
3. Types of Aggregates According to Size:
The biggest particle size may have diameter around 150 mm, and the smaller aggregate have diameter up to 5 to 10 microns. Aggregates are classified accordingly particle size as follows:
1. Coarse aggregates: The aggregate particle which retains on 4.75 mm sieve is known as Coarse aggregate.
2. Fine aggregates: The aggregate which passed through 4.75 mm sieve know as Fine Aggregate. Fine aggregates are generally occupied the 35% to 45 % volume of total aggregates.
4. Classification of Aggregates Based on Shape:
As we know that aggregate used construction are made from natural occurring rock by blasting or crushing and which makes difficult to gain require shape. But, the shape of aggregate play important role in workability of concrete. Therefore shape of aggregates is 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
1. Rounded Aggregate:
The rounded aggregates are totally nature made shape and available in the form of seashore gravel. This aggregate allows minimum percentages of voids (32 – 33%), hence offers more workability.
They also perform well with a low water-cement ratio. These types of aggregates are not considered suitable for high-strength concrete due to its poor interlocking behavior and weak bond strength.
2. Irregular Aggregates:
The rounded aggregates have partly nature made shape and these are available in the form of pit sands and gravel. These types of 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.
3. Angular Aggregates:
The angular aggregate has well defined edges and these are obtained by crushing the rocks. Types of aggregates offers maximum percentage of voids (38-45%) hence gives less workability.
Angular aggregates give a high specific area compared to rounded aggregates. With a greater specific surface are they show higher bond strength than rounded aggregates. Also, angular aggregate shows the better interlocking property in concrete that contributes to the strength of concrete.
4. Flaky Aggregates:
Flaky aggregate are those which thickness is less compared to length and width or the other, when the least dimension of aggregate is less than the 60% of its mean dimension then that aggregates are called as flaky aggregate.
5. Elongated Aggregates:
Elongated aggregate are those which length is larger to compare to width and thickness or the length of aggregate is greater than 180% of its mean dimension.
6. Flaky and Elongated Aggregates:
If length of aggregate is larger than its width and width is larger than its thickness then it is said to be flaky and elongated aggregates.
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